The ASCRS Textbook of Colon and Rectal Surgery, Second Edition

The ASCRS Textbook of Colon and Rectal Surgery

The ASCRS Textbook of Colon and Rectal Surgery Second Edition Editors

David E. Beck, md, facs, fascrs Patricia L. Roberts, md, facs, fascrs Theodore J. Saclarides, md, facs, fascrs Anthony J. Senagore, md, facs, fascrs Michael J. Stamos, md, facs, fascrs Steven D. Wexner, md, facs, fascrs, frcs, frcs(ed)

Editors David E. Beck, MD, FACS, FASCRS Department of Colon and Rectal Surgery Louisiana State University Health Sciences Center New Orleans, Louisiana, USA [email protected]

Anthony J. Senagore, MD, FACS, FASCRS Spectrum Health Department of Surgery Grand Rapids, Michigan, USA [email protected]

Patricia L. Roberts, MD, FACS, FASCRS Department of Colorectal Surgery Lahey Clinic, Tufts University School of Medicine Burlington, Massachusetts, USA [email protected]

Michael J. Stamos, MD, FACS, FASCRS Department of Surgery UCI Medical Center Orange, California, USA [email protected]

Theodore J. Saclarides, MD, FACS, FASCRS Section of Colon and Rectal Surgery, Rush Medical Center, Chicago, Illinois, USA [email protected]

Steven D. Wexner, MD, FACS, FASCRS, FRCS, FRCS(Ed) Cleveland Clinic Florida Department of Colorectal Surgery, Weston, Florida, USA [email protected]

ISBN 978-1-4419-1581-8 e-ISBN 978-1-4419-1584-9 DOI 10.1007/978-1-4419-1584-9 Springer New York Dordrecht Heidelberg London Library of Congress Control Number: 2011926885 © Springer Science+Business Media, LLC 2011 All rights reserved. This work may not be translated or copied in whole or in part without the written permission of the publisher (Springer Science+Business Media, LLC, 233 Spring Street, New York, NY 10013, USA), except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights. While the advice and information in this book are believed to be true and accurate at the date of going to press, neither the authors nor the editors nor the publisher can accept any legal responsibility for any errors or omissions that may be made. The publisher makes no warranty, express or implied, with respect to the material contained herein. Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com)

Foreword, 1st Edition

This text was developed under the aegis of the American Society of Colon and Rectal ­Surgeons (ASCRS). It represents an attempt to cover the field of colon and rectal surgery with input from expert surgeons who have, in one way or another, shown special interest or expertise in specific areas of the specialty. The book will hopefully serve as a source of useful information and perhaps even guidance to surgeons whose practice is confined to the specialty of colon and rectal surgery, and also to general surgeons, surgery residents, and medical students with an interest in surgery. The finished product represents significant efforts from authors who have taken time from their busy schedules to set into writing their often unique perspectives. I know for certain that no author of any chapter in this book has a light schedule, but that fact validates each author’s selection for authorship. Special acknowledgment is due the editors, Bruce Wolff, David Beck, John Pemberton, and Steven Wexner. This project simply would not have come together without their efforts on many levels. Finally, Jim Fleshman must be singled out for special recognition. The idea of an ASCRSsponsored text began with Jim – an idea that he advocated, developed, nurtured, and forced until it became realized in the substance you now hold. Robert Fry, MD Emilie and Roland deHellebranth Professor of Surgery Chief, Division of Colon and Rectal Surgery The Hospital of the University of Pennsylvania

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Foreword, 2nd Edition

The American Society of Colon and Rectal Surgeons (ASCRS) Textbook was originally conceived as a means of providing state of the art information to residents in training and fully trained surgeons seeking maintenance of certification and to support the mission of the ASCRS to continue to be the world’s most established authority on colon and rectal disease. The second edition perpetuates these goals. This textbook has been significantly reorganized in response to the numerous important changes in our specialty. All chapters were rewritten and updated and new chapters have been added. While several authors from the first edition were retained, for the majority of chapters, new authors were selected. Most of these internationally acclaimed contributors are active members of the ASCRS who routinely contribute to our literature. These respected individuals have carefully crafted chapters which provide a comprehensive summary of the subject with an appropriate mixture of detailed personal experience and comprehensive literature review. This approach provides the reader with a very open-minded, evidence-based approach to all aspects of colorectal disease. At the inception of the first edition, the editorial board was designed to be a rotating group of experts selected by the ASCRS Executive Council. As the two senior editors of this edition, it has been our distinct pleasure and honor to work with this edition’s editorial board. They have sacrificed time and energy to achieve what we believe continues to be the standard in colon and rectal surgery. For the sake of ease of use and consistency, the Table of Contents correlates with the Core Curriculum established by the Association of Program Directors in Colon and Rectal Surgery and the key topics used by the American Board of Colon and Rectal Surgery. Furthermore, the Practice Parameters developed by the ASCRS Standards Committee have been incorporated into the text appendix. As occurred with the first edition, the proceeds from this textbook and any related publications will be utilized by the ASCRS Executive Council to sponsor the research and education efforts of the society. The second edition continues to provide what the fellows and members of the ASCRS and trainees at all levels agree is the definitive source of knowledge in colon and rectal surgery. We are honored to have been a part of this ongoing monumental achievement and thank the leaders of the ASCRS for their continued support of the textbook. We thank our predecessors Bruce Wolf and Jim Fleshman, whose significant commitments as the co-senior editors of the first edition ensured the success of the textbook. We gratefully acknowledge our coeditors for their major contributions and rest comfortably as we leave the editorial board, knowing that future editions will continue to benefit from their dedication and expertise. We thank each chapter author and coauthor for their devotion to this task and to the mission of the ASCRS as without their individual and collective efforts the second edition would not be at such a tremendously high standard. We are grateful to the authors and coauthors of the first edition upon whose labors the foundation of this edition was easily laid. We also thank Sharon Beck for her support during this effort and our Developmental Editor Elektra McDermott for her extraordinary efforts which ensured the timely and professional completion of this second edition. Finally, we want to specially acknowledge the loss of our friend and colleague, W. Douglas Wong,

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Foreword, 2nd Edition

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past president of the ASCRS. During the production of this edition, Doug passed away after a ­courageous battle with a terrible disease. He made numerous major contributions to our specialty and to this textbook. Although Doug will be missed, his innovations and teachings will endure and continue to benefit surgeons and patients throughout the world. David E. Beck, MD, FASCRS Steven D. Wexner, MD, FASCRS April 2011

Preface

Drs. David Beck and Steven Wexner (Senior Editors) as well as the other members of the editorial board, Drs. Patricia Roberts, Theodore Saclarides, Michael Stamos, and Anthony Senagore, are to be congratulated on their successful completion of the second edition of the American Society of Colon and Rectal Surgeons (ASCRS) Textbook. The table of contents has been changed to reflect recent changes in the Core Curriculum for training Residents in colon and rectal surgery and format changes have been made to improve the presentation of the material. At least one third of the material has been completely renewed to reflect advances in the science of our specialty. The ASCRS Textbook has rapidly become a major resource for training, read by almost all trainees interested in our specialty. The purpose of the text was originally to provide a standardized reference for evidence-based recommendations, referenced to ASCRS practice parameter guidelines were available. The first edition has been downloaded onto the ­Web-based Colon and Rectal Education System as the reference for all the educational efforts  of the ASCRS Continuing Medical Education Committee. Our society, as owners and editors of the ASCRS Textbook, can be proud to be one of the few national organizations with its own comprehensive textbook from which certification and recertification materials can be drawn. Using an entire society as the pool of authors, allows the ASCRS Textbook, as much as possible, to avoid personal bias in the materials presented. The ASCRS Manual for Residents in training at all levels has also been released this past year. The current Editorial Board has condensed the material from each chapter of the first Edition of the ASCRS Textbook to provide students and residents rapid access to a succinct, distilled version of the ASCRS Textbook. The production of these two educational resources adds evidence that the ASCRS is “the” authority on colorectal disease and has within its membership individuals who can claim the status of “expert.” Their contribution has made the ASCRS Textbook a success and should be considered a major selfless contribution to our society and specialty. As time passes, our specialty and the body of knowledge surrounding it certainly change. The existence of this ASCRS Textbook and the mechanism which perpetuates it guarantees that we always have the most up-to-date information available to our members and trainees. The proceeds from the sale of the ASCRS Textbook also flow into the Education Foundation of the ASCRS to bring independence from outside sponsorship to our national meeting. The Executive Council of the ASCRS is fully supportive of this effort by the Editorial Board of the ASCRS Textbook and gratefully recognizes their contribution and that of the group of experts who provide new and updated chapters for each successive edition.

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Preface

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Please remember the sacrifice of time and effort that made this publication possible. Strive to add to the knowledge which one day changes our practice and our specialty. Use this ASCRS Textbook to show the world that the ASCRS is THE authority on colorectal disease as we improve patient care. James W. Fleshman, MD Senior Editor of the First Edition of the ASCRS Textbook of Colon and Rectal Surgery Immediate Past President of the American Society of Colon and Rectal Surgeons Professor of Surgery Washington University in St, Louis

Special Acknowledgement

Senior Editors from First Edition Bruce G. Wolff James W. Fleshman David E. Beck John H. Pemberton Steven D. Wexner

Associate Editors from the First Edition James M. Church Julio Garcia-Aguilar Patricia L. Roberts Theodore J. Saclarides Michael J. Stamos

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Contents

Foreword, 1 st edition by Robert Fry...............................................................................

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Foreword, 2 nd edition by David E. Beck and Steven D. Wexner.................................. vii Preface...............................................................................................................................

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Contributors...................................................................................................................... xvii   1  Anatomy and Embryology....................................................................................... José Marcio Neves Jorge and Angelita Habr-Gama

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  2  Colonic Physiology.................................................................................................... 23 Ursula M. Szmulowicz and Tracy L. Hull   3  Anorectal Physiology................................................................................................ 41 Richard E. Karulf   4  Physiologic Testing.................................................................................................... 49 Anders F. Mellgren   5  Endoscopy.................................................................................................................. 63 Charles B. Whitlow   6  Radiology................................................................................................................... 77 Jaime L. Bohl and Alan E. Timmcke   7  Endoluminal Ultrasound.......................................................................................... 107 Donald G. Kim and W. Douglas Wong †   8  Preoperative Management....................................................................................... 125 Janice F. Rafferty   9  Postoperative Management...................................................................................... 137 Sharon L. Stein and Conor P. Delaney 10  Postoperative Complications.................................................................................... 157 David W. Dietz 11  Hemorrhoids.............................................................................................................. 175 Marc Singer

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12  Anal Fissure............................................................................................................... 203 Rocco Ricciardi, Sharon L. Dykes, and Robert D. Madoff 13  Anorectal Abscess and Fistula................................................................................. 219 Carol-Ann Vasilevsky 14  Benign Anorectal and Rectovaginal Fistulas.......................................................... 245 David A. Etzioni and Ann C. Lowry 15  Pilonidal Disease and Hidradenitis Suppurativa................................................... 261 Harry T. Papaconstantinou and J. Scott Thomas 16  Dermatology and Pruritus Ani................................................................................ 277 Charles O. Finne and John R. Fenyk 17  Sexually Transmitted Diseases................................................................................. 295 Charles B. Whitlow, Lester Gottesman, and Mitchell A. Bernstein 18  Fecal Incontinence.................................................................................................... 309 Dana R. Sands and Mari A. Madsen 19  Pelvic Floor Disorders.............................................................................................. 323 Patrick Y. H. Lee and Guillaum Meurette 20  Anal Cancer............................................................................................................... 337 Mark Lane Welton and Nalini Raju 21  Presacral Tumors...................................................................................................... 359 Eric J. Dozois and Maria Dolores Herreros Marcos 22  Diverticular Disease.................................................................................................. 375 Alan G. Thorson and Jennifer S. Beaty 23  Colonic Volvulus........................................................................................................ 395 Jan Rakinic 24  Lower Gastrointestinal Hemorrhage...................................................................... 407 Craig A. Reickert and Melissa Times 25  Endometriosis............................................................................................................ 421 Michael J. Snyder 26  Trauma of the Colon and Rectum........................................................................... 435 David B. Hoyt and Michael E. Lekawa 27  IBD: Diagnosis and Evaluation................................................................................ 449 Walter A. Koltun 28  IBD: Medical Management...................................................................................... 463 Bruce E. Sands 29  Ulcerative Colitis: Surgical Management............................................................... 479 Zuri Murrell and Phillip Fleshner

Contents

Contents

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30  Crohn’s Disease: Surgical Management................................................................. 499 Scott A. Strong 31  Intestinal Stomas....................................................................................................... 517 Laurence R. Sands and Floriano Marchetti 32  Constipation and Functional Bowel Disorders....................................................... 535 Madhulika G. Varma and Brooke H. Gurland 33  Rectal Prolapse.......................................................................................................... 549 Steven Mills 34  Other Benign Colorectal Disorders......................................................................... 565 Justin A. Maykel and Scott R. Steele 35  Advanced Laparoscopic Colorectal Surgery.......................................................... 597 Tonia Young-Fadok 36  Polyps......................................................................................................................... 625 Paul E. Wise 37  Hereditary Colorectal Cancer.................................................................................. 643 James Church 38  Colorectal Cancer: Epidemiology, Etiology, and Molecular Basis....................... 669 Harvey G. Moore, Nancy N. Baxter, and Jose G. Guillem 39  Colorectal Cancer: Screening.................................................................................. 691 Jason F. Hall and Thomas E. Read 40  Colon Cancer Evaluation and Staging.................................................................... 703 Eric G. Weiss 41  Surgical Management of Colon Cancer.................................................................. 711 Matthew Mutch and Christina Cellini 42  The Preoperative Staging of Rectal Cancer........................................................... 721 Susan L. Gearhart and Jonathan E. Efron 43  Local Excision of Rectal Cancer.............................................................................. 731 Peter A. Cataldo 44  Surgical Treatment of Rectal Cancer...................................................................... 743 Ronald Bleday and Nelya Brindzei 45  Rectal Cancer: Locally Advanced and Recurrent................................................. 761 Robert R. Cima 46  Colorectal Cancer: Adjuvant Therapy................................................................... 773 Kelli Bullard Dunn and Judith L. Trudel 47  Colorectal Cancer: Metastatic (Palliation)............................................................. 783 Elisabeth C. McLemore and Sonia Ramamoorthy

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48  Colorectal Cancer: Surveillance.............................................................................. 803 Nadav Dujovny and Jon S. Hourigan 49  Miscellaneous Neoplasms......................................................................................... 813 Robin P. Boushey and Husein Moloo 50  Pediatric Colorectal Disorders................................................................................. 825 Marc A. Levitt and Alberto Peña 51  Health Care Economics............................................................................................ 843 David A. Margolin and Lester Rosen 52  Ethical and Legal Considerations........................................................................... 853 Ira J. Kodner, Mark Siegler, Daniel M. Freeman, and William T. Choctaw 53  Legal Considerations................................................................................................ 883 Michael J. Meehan 54  Surgical Education.................................................................................................... 893 David J. Schoetz 55  Continuing Medical Education................................................................................ 901 Martin Luchtefeld 56  Quality........................................................................................................................ 907 Formosa Chen, Hiroko Kunitake, Elise Lawson, Joan Ryoo, and Clifford Y. Ko Index................................................................................................................................... 927

Contents

Contributors

Nancy N. Baxter, MD, PhD Associate Professor of Surgery, Department of Surgery, St. Michael’s Hospital, University of Toronto, Toronto, ON, Canada Jennifer S. Beatty, MD Assistant Professor of Surgery, Division of Colon and Rectal Surgery, Creighton University School of Medicine, Omaha, NE, USA Mitchell A. Bernstein, MD, FACS, FASCRS Assistant Professor of Clinical Surgery; Senior Attending Department of Surgery, Program Director, Colon and Rectal Surgery Fellowship, Columbia University College of Physicians and Surgeons, New York, NY, USA; St. Luke’s Roosevelt Hospital Center, New York NY, USA Ronald Bleday, MD Chair, Section of Colorectal Surgery, Department of Surgery, Brigham and Women’s Hospital, Boston MA, USA Jaime L. Bohl, MD Staff Surgeon, Department of Colon and Rectal Surgery, Ochsner Clinic, New Orleans, LA, USA Robin P. Boushey, BSc, MD, PhD, CIP, FRCSC Assistant Professor of Surgery, Director, General Surgery Research, Site Lead, Colorectal Cancer Program Champlain LHIN, Investigator, Ottawa Regional Cancer Center, Affiliate Investigator, Ottawa Hospital Research Institute, Department of Surgery, The Ottawa Hospital – General Campus, Ottawa, ON, Canada Nelya Brindzei, MD Department of Surgery, Brigham and Women’s Hospital, Boston, MA, USA Peter A. Cataldo, MD Professor of Surgery, Department of Surgery, Fletcher Allen Healthcare, Burlington, VT, USA Christina Cellini, MD Assistant Professor of Surgery, Division of Colorectal Surgery, University of Rochester, Strong Memorial Hospital, Rochester, NY, USA Formosa Chen, MD Department of Surgery, Ronald Regan UCLA Medical Center, Los Angeles, CA, USA

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William T. Choctaw, MD, JD Past Chief of Surgery, Department of Surgery, Citrus Valley Medical Center, Covina, CA, USA James Church, MBChB, MMedSci, FRACS Victor W. Fazio Chair in Colorectal Surgery, Department of Colorectal Surgery, Digestive Diseases Institute, Cleveland Clinic Foundation, Cleveland, OH, USA Robert R. Cima, MD Consultant, Associate Professor of Surgery, Mayo Clinic, Rochester, MN, USA Conor P. Delaney, MD, MCh, PhD, FRCSI, FACS, FASCRS Professor and Chief of Colorectal Surgery, Department of Surgery, University Hospital Case Medical Center, Cleveland, OH, USA David W. Dietz, MD Staff Surgeon, Department of Digestive Disease, Cleveland Clinic, Cleveland, OH, USA Eric J. Dozois, MD Associate Professor of Surgery, Department of Colon and Rectal Surgery, Mayo Clinic, Rochester, MN, USA Nadav Dujovny, MD Assistant Professor of Surgery, Ferguson Clinic, Spectrum Health Medical Group, Michigan State University, 4100 Lake Drive #205, Grand Rapids, MI 49546, USA Kelli Bullard Dunn, MD Associate Professor of Surgery, Roswell Park Cancer Institute and the University of Buffalo, State University of New York, Buffalo, NY, USA Sharon L. Dykes, MD Adjunct Instructor of Surgery, Division of Colorectal Surgery, University of Minnesota, Minneapolis, MN, USA Jonathan E. Efron, MD Associate Professor and Chair of the Ravitch Division, Department of Surgery, Johns Hopkins Hospital, Baltimore, MD, USA David A. Etzioni, MD, MSHS Associate Professor, Mayo Clinic College of Medicine, Senior Associate Consultant, Mayo Clinic Arizona, Department of Surgery, Phoenix, AZ, USA John R. Fenyk Jr, MD Professor of Medicine, Department of Dermatology, University of Minnesota Medical School, Minneapolis, MN, USA Charles O. Finne III, MD Adjunct Professor of Surgery, Division of Colon and Rectal Surgery, Department of Surgery, University of Minnesota Medical School, Minneapolis, MN, USA Phillip Fleshner, MD, FACS, FASCRS Director, Colorectal Surgery Residency; Clinical Professor of Surgery, Cedars-Sinai Medical Center, Los Angeles, CA, USA; UCLA School of Medicine, Los Angeles, CA, USA

Contributors

Contributors

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Daniel M. Freeman, AB, JD Academic Director, Public Law, Washington Semester Program, American University, Washington, DC, USA Susan L. Gearhart, MD Assistant Professor of Colorectal Surgery, Department of Surgery, Johns Hopkins Medical Institution, Baltimore, MD, USA Lester Gottesman, MD, FACS, FASCRS Associate Professor of Clinical Surgery, Columbia School of Physicians and Surgeons, St. Luke’s Roosevelt, Department of Colorectal Surgery, New York, NY, USA Jose G. Guillem, MD, PhD Attending Surgeon, Department of Colorectal Surgery, Memorial Sloan-Kettering Cancer Center, New York, NY, USA Brooke H. Gurland, MD Staff Colon and Rectal Surgeon, Department of Colorectal Surgery, Cleveland Clinic Foundation, Cleveland, OH, USA Angelita Habr-Gama, MD, PhD, FACS (Hon), ASA (Hon), ESA (Hon) Professor of Surgery; Director, University of São Paulo, Sao Paolo, Brazil; Angelita & Joaquim Gama Institute, Sao Paolo, Brazil Jason F Hall, MD Assistant Professor of Surgery; Staff Surgeon, Department of Colon and Rectal Surgery, Tufts University School of Medicine, Boston, MA, USA; Lahey Clinic, Burlington, MA, USA Maria Dolores Herreros Marcos, MD, PhD Department of Colon and Rectal Surgery, Mayo Clinic, Rochester, MN, USA Jon S. Hourigan, MD Assistant Professor of Surgery, Section of Colorectal Surgery, Department of Surgery, University of Kentucky, 800 Rose Street, C220 Lexington, KY 40536, USA David B. Hoyt, MD, FACS Executive Director, American College of Surgeons, Chicago, IL, USA Tracy L. Hull, MD Professor of Surgery, Department of Colon and Rectal Surgery, Cleveland Clinic Foundation, Cleveland, OH, USA Richard E. Karulf, MD Clinical Professor, Division of Colon and Rectal Surgery, Department of Surgery, University of Minnesota, Edina, MN, USA José Marcio Neves Jorge, MD, PhD Associate Professor, Department of Gastroenterology, Colorectal Division, University of Sao Paolo, Sao Paolo, Brazil Donald G. Kim, MD Clinical Assistant Professor, Department of Surgery; Staff Colon and Rectal Surgeon, Michigan State University College of Human Medicine, Lansing, MI, USA; Ferguson Clinic, Grand Rapids, MI, USA

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Clifford Y. Ko, MD, MS, MSHS, FACS, FASCRS Professor of Surgery, Robert and Kelly Day Chair in Surgical Outcomes, Director, Center for Surgical Outcomes and Quality, Department of Surgery, Los Angeles, CA, USA Ira J. Kodner, MD Solon and Bettie Gershmen Professor of Colon and Rectal Surgery, Department of Surgery, Washington University School of Medicine, St. Louis, MO, USA Walter A. Koltun, MD, FACS, FASCRS Peter and Marshla Carlino Chair in Inflammatory Bowel Disease, Professor of Surgery, Chief, Division of Colon and Rectal Surgery, Department of Surgery, Penn State Milton S. Hershey Medical Center, Hershey, PA, USA Hiroko Kunitake, MD Department of Surgery, Massachusetts General Hospital, Boston, MA, USA Elise Lawson, MD Department of Surgery, Ronald Regan UCLA Medical Center, Los Angeles, CA, USA Patrick Y. H. Lee, MD Associate Clinical Professor; Consultant, Surgical Speciality/ The Colon and Rectal Clinic, Oregon Health Sciences University, Portland, OR, USA Michael E. Lekawa, MD, FACS Professor and Chief, Division of Trauma and Critical Care, Director, Trauma Services, University of California, Irvine Medical Center, Orange, CA, USA Marc A. Levitt, MD Associate Professor, Colorectal Center for Children, Associate Professor, Division of Pediatric Surgery, Cincinanati Children’s Hospital Medical Center, Cincinnati, OH, USA Ann C. Lowry, MD Adjunct Professor, Department of Surgery, Division of Colon and Rectal Surgery, University of Minnesota, St Paul, MN, USA Martin Luchtefeld, MD Chief of Colon and Rectal Surgery, Spectrum Health, Michigan Medical PC – Ferguson Clinic, Grand Rapids, MI, USA Elisabeth C. McLemore, MD Assistant Professor of Surgery, Colon & Rectal Surgery, Moores Cancer Center, University of California, San Diego, La Jolla, CA, USA Robert D. Madoff, MD, FACS Professor of Surgery, Department of Surgery, University of Minnesota, Minneapolis, MN, USA Mari A. Madsen, MD Clinical Chief, Division of Colon and Rectal Surgery, Cedars-Sinai Medical Center, West Hollywood, CA, USA

Contributors

Contributors

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Floriano Marchetti, MD Assistant Professor of Clinical Surgery, Program Director, Colon and Rectal Surgery Residency Program, Department of Surgery, University of Miami Miller School of Medicine, Miami, FL, USA David A. Margolin, MD Director, Colon and Rectal Surgery Research, Staff, Colon and Rectal Surgeon, Department of Colon and Rectal Surgery, Ochsner Clinic Foundation, New Orleans, LA, USA Justin A. Maykel, MD Assistant Professor of Surgery, Chief, Division of Colon and Rectal Surgery, Department of Surgery, University of Massachusetts Medical Center, Worcester, MA, USA Michael J. Meehan, BA, JD Senior Director, Regional Hospitals Group, Cleveland Clinic Law Department, Cleveland Clinic Foundation, Cleveland, OH, USA Anders F. Mellgren, MD, PhD Adjunct Professor of Surgery; Director, Department of Colon and Rectal Surgery, University of Minnesota, Minneapolis, MN, USA; Pelvic Floor Center, Minneapolis, MN, USA Guillaum Meurette, MD Chirurgie Digestive et Endocrine, Institut des Maladies de l’appareil Digestif, Centre Hospitalo-Universitaire, Nantes, France Steven Mills, MD, FASCRS Assistant Clinical Professor of Surgery, Department of Surgery, University of California, Irvine, Orange, CA, USA Husein Moloo, MD, MSc, FRCSC Assistant Professor of Surgery; Clinical Investigator, Ottawa Hospital Research Institute, University of Ottawa, Ottawa, ON, Canada; Department of Surgery, The Ottawa Hospital, Ottawa, ON, Canada Harvey G. Moore, MD, FACS, FASCRS Assistant Professor of Surgery, Department of Surgery, New York University School of Medicine, New York, NY, USA Zuri Murrell, MD, FACS Attending Surgeon, Division of Colorectal Surgery, Cedars-Sinai Medical Center, Los Angeles, CA, USA Matthew Mutch, MD Associate Professor of Surgery, Department of Surgery, Section of Colon and Rectal Surgery, Barnes Jewish Hospital/Washington University School of Medicine, St. Louis, MO, USA Harry T. Papaconstantinou, MD Assistant Professor, Chief Section of Colon and Rectal Surgery, Scott & White Memorial Hospital and Clinic, Temple, TX, USA; Department of Surgery, Texas A & M University Health System, Temple, TX, USA

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Alberto Peña, MD Director, Colorectal Center for Children, Professor, Division of Pediatric Surgery Cincinnati Children’s Hospital Medical Center, Cincinnati, OH, USA Janice F. Rafferty, MD Professor of Surgery, Chief, Division of Colon and Rectal Surgery, Department of Surgery, University of Cincinnati College of Medicine, Cincinnati, OH, USA Nalini Raju, MD General Surgery, Section of Colon and Rectal Surgery, Stanford University School of Medical, Stanford, CA, USA Jan Rakinic, MD Associate Professor of Surgery, Chief, Section of Colorectal Surgery, Department of Surgery, Southern Illinois University School of Medicine, Springfield, IL, USA Sonia Ramamoorthy, MD Assistant Professor of Surgery, Colon and Rectal Surgery, Rebecca and John Moores Cancer Center, La Jolla, CA, USA Thomas E. Read, MD, FACS, FASCRS Professor of Surgery; Staff Surgeon, Department of Colon and Rectal Surgery, Program Director, Colon and Rectal Surgery Residency, Tufts University School of Medicine, Boston, MA, USA; Lahey Clinic Medical Center, Burlington, MA, USA Craig A. Reickert, MD Assistant Professor of Surgery, Wayne State University School of Medicine, Detroit, MI, USA; Department of Colon and Rectal Surgery, Henry Ford Hospital, Detroit, MI, USA Dana R. Sands, MD, FACS, FASCRS Director, Colorectal Physiology Center, Staff Surgeon, Department of Colorectal Surgery, Cleveland Clinic Florida, Weston, FL, USA Rocco Ricciardi, MD, MPH Staff Surgeon, Assistant Professor of Surgery, Department of Colon and Rectal Surgery, Lahey Clinic, Burlington, MA, USA Lester Rosen, MD Attending Surgeon, Department of Colon and Rectal Surgery, Cleveland Clinic Florida, Weston, FL, USA Joan Ryoo, MD Department of General Internal Medicine/Health Services Research, Ronald Regan UCLA Medical Center, Los Angeles, CA, USA Bruce E. Sands, MD, MS Associate Professor of Medicine; Acting Chief, Gastrointestinal Unit, Medical Co-director, MGH Crohn’s and Colitis Center, Harvard School of Medicine, Boston, MA, USA; Massachusetts General Hospital, Boston, MA, USA

Contributors

Contributors

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Laurence R. Sands, MD, MBA Associate Professor of Clinical Surgery, Chief, Division of Colon and Rectal Surgery, Department of Surgery, University of Miami, Miller School of Medicine, Miami, FL, USA David J. Schoetz Jr, MD Chairman Emeritus, Department of Colon and Rectal Surgery; Professor of Surgery, Lahey Clinic, Burlington, MA, USA; Tufts University School of Medicine, Boston, MA, USA Mark Siegler, MD Professor, Department of Medicine and Surgery, Lindy Bergman Distinguished Service Professor, Director, MacLean Center for Clinical Medical Ethics, Department of Medicine Ethics Center, University of Chicago, Chicago, IL, USA Marc Singer, MD Department of Surgery, Section of Colon and Rectal Surgery, NorthShore University Health System, Evanston, IL, USA Michael J. Snyder, MD Program Director, The University of Texas Health Sciences Center at Houston, Houston, TX, USA; Department of Colon and Rectal Surgery, Methodist Hospital, Houston, TX, USA Scott R. Steele, MD Chief, Colon and Rectal Surgery Department of Surgery, Madigan Army Medical Center, Tacoma, WA, USA Sharon L. Stein, PhD Department of General Surgery, University Hospital Case Medical Center, Cleveland, OH, USA Scott A. Strong, MD Department of Colon and Rectal Surgery, Cleveland Clinic Foundation, Cleveland, OH, USA Ursula M. Szmulowicz, MD Associate Staff, Department of Colorectal Surgery, Cleveland Clinic Foundation, Cleveland, OH, USA J. Scott Thomas, MD Assistant Professor, Department of Surgery, Section of Colon and Rectal Surgery, Scott & White Memorial Hospital and Clinic, Temple, TX, USA; Texas A & M University Health System, Temple, TX, USA Alan G. Thorson, MD Clinical Associate Professor of Surgery; Program Director, Colon and Rectal Surgery, Creighton University School of Medicine, Omaha, NE, USA; University of Nebraska College of Medicine, Omaha, NE, USA Alan E. Timmcke, MD Gastroenterologist, 1514 Jefferson Hwy, Jefferson, LA70121 USA

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Melissa Times, MD Assistant Professor of Surgery, Wayne State University School of Medicine, Detroit, MI, USA; Department of Colon and Rectal Surgery, Henry Ford Hospital, Detroit, MI, USA Judith L. Trudel, MD, MSc, MHPE, FRCSC, FACS Clinical Professor of Surgery, Department of Surgery, University of Minnesota, St. Paul, MN, USA Madhulika G. Varma, MD Associate Professor, Chief, Section of Colorectal Surgery, Department of Surgery, University of California, San Francisco, San Francisco, CA, USA Carol-Ann Vasilevsky, MD, FRCSC, FACS Assistant Professor of Surgery and Oncology, Division of Colorectal Surgery, Jewish General Hospital, McGill University, Montreal, Quebec Eric G. Weiss, MD Vice Chairman, Colorectal Surgery, DIO/Chairman of Graduate Medical Education and Program Director, Colorectal Surgery, Director of Surgical Endoscopy, Cleveland Clinic Florida, Weston, FL, USA Mark Lane Welton, MD, MHCM Division of General Surgery, Section of Colon and Rectal Surgery, Stanford University School of Medicine, Stanford, CA, USA Charles B. Whitlow, MD Staff Surgeon and Residency Program Director, Department of Colon and Rectal Surgery, Ochsner Medical Center, New Orleans, LA, USA Paul E. Wise, MD Assistant Professor of Surgery, Director, Vanderbilt Hereditary Colorectal Cancer Registry, Department of General Surgery, Vanderbilt University Medical Center, Nashville, TN, USA W. Douglas Wong†, MD, FACS, FRSC(C) Professor of Surgery; Chief, Colorectal Service, Weill Medical College of Cornell University, New York, NY, USA; Memorial Sloan-Kettering Cancer Center, New York, NY, USA Tonia Young-Fadok, MD, MS, FACS, FASCRS Professor of Surgery, Mayo Clinic College of Medicine, Chair, Division of Colon and Rectal Surgery, Mayo Clinic Scottsdale, Phoenix, AZ, USA

Contributors

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Anatomy and Embryology José Marcio Neves Jorge and Angelita Habr-Gama

Although much of our fundamental understanding of the anatomy of the colon, rectum, and anus comes from the efforts of researchers of the nineteenth and early twentieth centuries, comprehensive observations of this region had been made as early as 1543 by Andreas Vesalius through anatomic dissections.1 However, anatomy of this region, especially that of the rectum and anal canal, is so intrinsically related to its physiology that much can be appreciated only in the living. Thus, it is a region in which the surgeon has an advantage over the anatomist through in vivo dissection, physiologic investigation, and endoscopic examination. However, anatomy of the pelvis is also challenging to the surgeon: the pelvis is a narrow space, packed with intestinal, urologic, gynecologic, vascular, and neural structures, all confined within a rigid and deep osseous–muscular cage. Therefore, detailed anatomy of this region is difficult to learn in the setting of an operating room and it demands not only observations in vivo, but historical reviews, anatomy laboratory studies, including dissections of humans and animals, with in-depth descriptions and drawings and sometimes associated with physiologic evaluation. Based on these studies, some controversial concepts of the anatomy, especially of the rectum and anal canal, have been actually changed.2–8 In addition, virtual reality models have been designed to improve visualization of three-dimensional structures and more properly teach anatomy, pathology, and surgery of the anorectum and pelvic floor.9

Anatomy Anus and Rectum Anal Canal Structure, Anus, and Anal Verge The anal canal is anatomically peculiar and has a complex physiology, which accounts for its crucial role in continence and, in addition, its susceptibility to a variety of diseases. The anus or anal orifice is an anteroposterior cutaneous slit

that, along with the anal canal, remains virtually closed at rest, as a result of tonic circumferential contraction of the sphincters and the presence of anal cushions. The edge of the anal orifice, the anal verge or margin (anocutaneous line of Hilton), marks the lowermost edge of the anal canal and is sometimes the level of reference for measurements taken during sigmoidoscopy. Others favor the dentate line as a landmark because it is more precise. The difference between the anal verge and the dentate line is usually 1–2 cm. The epithelium distal to the anal verge acquires hair follicles, glands, including apocrine glands, and other features of normal skin, and is the source of perianal hidradenitis suppurativa, inflammation of the apocrine glands.

Anatomic Versus Surgical Anal Canal Two definitions are found describing the anal canal (Fig­ ure 1-1). The “anatomic” or “embryologic” anal canal is only 2.0  cm long, extending from the anal verge to the dentate line, the level that corresponds to the proctodeal membrane. The “surgical” or “functional” anal canal is longer, extending for approximately 4.0 cm (in men) from the anal verge to the anorectal ring (levator ani). This “long anal canal” concept was first introduced by Milligan and Morgan10 and has been considered, despite not being proximally marked by any apparent epithelial or developmental boundary, useful both as a physiologic and surgical parameter. The anorectal ring is at the level of the distal end of the ampullary part of the rectum and forms the anorectal angle, and the beginning of a region of higher intraluminal pressure. Therefore, this definition correlates with digital, manometric, and sonographic examinations.

Anatomic Relations of the Anal Canal Posteriorly, the anal canal is related to the coccyx and anteriorly to the perineal body and the lowest part of the posterior vaginal wall in the female, and to the urethra in the male. The ischium and the ischiorectal fossa are situated on either side. The fossa ischiorectal contains fat and the ­inferior ­rectal

D.E. Beck et al. (eds.), The ASCRS Textbook of Colon and Rectal Surgery: Second Edition, DOI 10.1007/978-1-4419-1584-9_1, © Springer Science+Business Media, LLC 2011

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Figure 1-1. Anal canal.

v­ essels and nerves, which cross it to enter the wall of the anal canal.

Muscles of the Anal Canal The muscular component of the mechanism of continence can be stratified into three functional groups: lateral compression from the pubococcygeus, circumferential closure from the internal and external anal sphincter, and angulation from the puborectalis (Figure 1-2). The internal and external anal sphincters, and the conjoined longitudinal are intrinsically related to the anal canal, and are addressed here.

Internal Anal Sphincter The internal anal sphincter represents the distal 2.5- to 4.0-cm condensation of the circular muscle layer of the rectum. As a consequence of both intrinsic myogenic and extrinsic autonomic neurogenic properties, the internal anal sphincter is a smooth muscle in a state of continuous maximal contraction, and represents a natural barrier to the involuntary loss of stool and gas. The lower rounded edge of the internal anal sphincter can be felt on physical examination, about 1.2 cm distal to the dentate line. The groove between the internal and external anal sphincter, the intersphincteric sulcus, can be visualized or easily palpated. Endosonographically, the internal anal

sphincter is a 2- to 3-mm-thick circular band and shows a uniform hypoechogenicity.

External Anal Sphincter The external anal sphincter is the elliptical cylinder of striated muscle that envelops the entire length of the inner tube of smooth muscle, but it ends slightly more distal than the internal anal sphincter. The external anal sphincter was initially described as encompassing three divisions: subcutaneous, superficial, and deep.10 Goligher et  al.11 described the external anal sphincter as a simple, continuous sheet that forms, along with the puborectalis and levator ani, one funnel-shaped skeletal muscle. The deepest part of the external anal sphincter is intimately related to the puborectalis muscle, which can actually be considered a component of both the levator ani and the external anal sphincter muscle complexes. Others considered the external anal sphincter as being subdivided into two parts, deep (deep sphincter and puborectalis) and superficial (subcutaneous and superficial sphincter).6,12,13 Shafik14 proposed the three U-shaped loop system, but clinical experience has not supported this schema. The external anal sphincter is more likely to be one muscle unit, attached by the anococcygeal ligament posteriorly to the coccyx, and anteriorly to the perineal body, not divided into layers or laminae. Nevertheless, differences in

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Figure 1-2. Muscles of the anal canal.

the arrangement of the external anal sphincter have been described between the genders.15 In the male, the upper half of the external anal sphincter is enveloped anteriorly by the conjoined longitudinal muscle, whereas the lower half is crossed by it. In the female, the entire external anal sphincter is encapsulated by a mixture of fibers derived from both longitudinal and internal anal sphincter muscles. Endosonographically, the puborectalis and the external anal sphincter, despite their mixed linear echogenicity, are both predominantly hyperechogenic, with a mean thickness of 6 mm (range, 5–8 mm). Distinction is made by position, shape, and topography. Recently, both anal endosonography and endocoil magnetic resonance imaging have been used to detail the anal sphincter complex in living, healthy subjects.16–19 These tests provide a three-dimensional mapping of the anal sphincter; they help to study the differences in the arrangement of the external anal sphincter between the sexes and uncover sphincter disruption or defect during vaginal deliveries. In addition, there is some degree of “anatomical asymmetry” of the external anal sphincter, which accounts for both radial and longitudinal “functional asymmetry” observed during anal manometry.20 The automatic continence mechanism is formed by the resting tone, maintained by the internal anal sphincter, magnified by voluntary, reflex, and resting external anal sphincter contractile activities. In response to conditions of threatened incontinence, such as increased intraabdominal pressure and rectal distension, the external anal sphincter and ­puborectalis

reflexively and voluntarily contract further to prevent fecal leakage. Because of muscular fatigue, maximal voluntary contraction of the external anal sphincter can be sustained for only 30–60 s. However, the external anal sphincter and the pelvic floor muscles, unlike other skeletal muscles, which are usually inactive at rest, maintain unconscious resting electrical tone through a reflex arc at the cauda equina level. Histologic studies have shown that the external anal sphincter, puborectalis, and levator ani muscles have a predominance of type I fibers, which are a peculiarity of skeletal muscles connecting tonic contractile activity.21

Conjoined Longitudinal Muscle Whereas the inner circular layer of the rectum gives rise to the internal anal sphincter, the outer longitudinal layer, at the level of the anorectal ring, mixes with fibers of the levator ani muscle to form the conjoined longitudinal muscle. This muscle descends between the internal and external anal sphincter, and ultimately some of its fibers, referred to as the corrugator cutis ani muscle, traverse the lowermost part of the external anal sphincter to insert into the perianal skin. Some of these fibers may enter the fat of the ischiorectal fossa.22 Other sources for the striated component of the conjoined longitudinal muscle include the puborectalis and deep external anal sphincter, the pubococcygeus and top loop of the external anal sphincter, and the lower fibers of the ­puborectalis.7,23,24 In its descending course, the

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conjoined longitudinal muscle may give rise to medial extensions that cross the internal anal sphincter to contribute the smooth muscle of the submucosa (musculus canalis ani, sustentator tunicae mucosae, Treitz muscle, musculus submucosae ani).25 Possible functions of the conjoined longitudinal muscle include attaching the anorectum to the pelvis and acting as a skeleton that supports and binds the internal and external sphincter complex together.22 Haas and Fox26 consider that the meshwork formed by the conjoined longitudinal muscle may minimize functional deterioration of the sphincters after surgical division and act as a support to prevent hemorrhoidal and rectal prolapse. In addition, the conjoined longitudinal muscle and its extensions to the intersphincteric plane divide the adjacent tissues into subspaces and may actually have a role in the septation of thrombosed external hemorrhoids and containment of sepsis.7 Finally, Shafik23 ascribed to the conjoined longitudinal muscle the action of shortening and widening of the anal canal as well as eversion of the anal orifice, and proposed the controversial term evertor ani muscle. In addition to this primary function during defecation, a limited role in anal continence, specifically a potentialization effect in maintaining an anal seal, has also been proposed.23

Epithelium of the Anal Canal The lining of the anal canal consists of an upper mucosal (endoderm) and a lower cutaneous (ectoderm) segment ­(Figure 1-1). The dentate (pectinate) line is the “sawtoothed” junction between these two distinct origins of venous and lymphatic drainage, nerve supply, and epithelial lining. Above this level, the intestine is innervated by the sympathetic and parasympathetic systems, with venous, arterial, and lymphatic drainage to and from the hypogastric vessels. Distal to the dentate line, the anal canal is innervated by the somatic nervous system, with blood supply and drainage from the inferior hemorrhoidal system. These differences are important when the classification and treatment of hemorrhoids are considered. The pectinate or dentate line corresponds to a line of anal valves that represent remnants of the proctodeal membrane. Above each valve, there is a little pocket known as an anal sinus or crypt. These crypts are connected to a variable number of glands, in average 6 (range, 3–12).27,28 The anal glands first described by Chiari in 187829 are more concentrated in the posterior quadrants. More than one gland may open into the same crypt, whereas half the crypts have no communication. The anal gland ducts, in an outward and downward route, enter the submucosa; two-thirds enter the internal anal sphincter, and half of them terminate in the intersphincteric plane.28 Obstruction of these ducts, presumably by accumulation of foreign material in the crypts, may lead to perianal abscesses and fistulas.30 Cephalad to the dentate line, 8–14 longitudinal folds, known as the rectal columns (columns of Morgagni), have their bases connected in pairs to each valve at the dentate line. At the lower end of the columns are the

anal papillae. The mucosa in the area of the columns consists of several layers of cuboidal cells and has a deep purple color because of the underlying internal hemorrhoidal plexus. This 0.5- to 1.0-cm strip of mucosa above the dentate line is known as the anal transition or cloacogenic zone. Cephalad to this area, the epithelium changes to a single layer of columnar cells and macroscopically acquires the characteristic pink color of the rectal mucosa. The cutaneous part of the anal canal consists of modified squamous epithelium that is thin, smooth, pale, stretched, and devoid of hair and glands. The terms pecten and pecten band have been used to define this segment.31 However, as pointed out by Goligher, the round band of fibrous tissue called pecten band, which is divided in the case of anal fissure (pectenotomy), probably represents the spastic internal anal sphincter.11,32

Rectum Both proximal and distal limits of the rectum are controversial: the rectosigmoid junction is considered to be at the level of the third sacral vertebra by anatomists but at the sacral promontory by surgeons, and likewise, the distal limit is regarded to be the muscular anorectal ring by surgeons and the dentate line by anatomists. The rectum measures 12–15 cm in length and has three lateral curves: the upper and lower are convex to the right and the middle is convex to the left. These curves correspond intraluminally to the folds or valves of Houston. The two left-sided folds are usually noted at 7–8 cm and at 12–13  cm, respectively, and the one on the right is generally at 9–11 cm. The middle valve (Kohlrausch’s plica) is the most consistent in presence and location and corresponds to the level of the anterior peritoneal reflection. Although the rectal valves do not contain all muscle wall layers from a clinical point of view, they are a good location for performing a rectal biopsies because they are readily accessible with minimal risk of perforation.13,33 The valves of Houston must be negotiated during proctosigmoidoscopy; they are absent after mobilization of the rectum, and this is attributed to the 5-cm length gained after complete surgical dissection. The rectal mucosa is smooth, pink, and transparent, which allows visualization of small and large submucosal vessels. This characteristic “vascular pattern” disappears in inflammatory conditions and in melanosis coli. The rectum is characterized by its wide, easily distensible lumen, and the absence of taeniae, epiploic appendices, haustra, or a well-defined mesentery. The prefix “meso,” in gross anatomy, refers to two layers of peritoneum that suspend an organ. Normally, the rectum is not suspended but entirely extraperitoneal on its posterior aspect, and closely applied to the sacral hollow. Consequently, the term “mesorectum” is anatomically inaccurate. An exception, however, is that a peritonealized mesorectum may be noted in patients with procidentia. But, the word “mesorectum” has gained widespread popularity among surgeons to address the ­perirectal areolar tissue, which is thicker posteriorly, containing ­terminal

1. Anatomy and Embryology

branches of the inferior mesenteric artery and enclosed by the fascia propria.34,35 The “mesorectum” may be a metastatic site for a rectal cancer and is removed during surgery for rectal cancer without neurologic sequelae because no functionally significant nerves pass through it. The upper third of the rectum is anteriorly and laterally invested by peritoneum; the middle third is covered by peritoneum on its anterior aspect only. Finally, the lower third of the rectum is entirely extraperitoneal because the anterior peritoneal reflection occurs at 9.0–7.0 cm from the anal verge in men and at 7.5–5.0 cm from the anal verge in women.

Anatomic Relations of the Rectum The rectum occupies the sacral concavity and ends 2–3 cm anteroinferiorly from the tip of the coccyx. At this point, it angulates backward sharply to pass through the levators and becomes the anal canal. Anteriorly, in women, the rectum is closely related to the uterine cervix and posterior vaginal wall; in men, it lies behind the bladder, vas deferens, seminal vesicles, and prostate. Posterior to the rectum lie the median sacral vessels and the roots of the sacral nerve plexus.

Fascial Relationships of the Rectum The parietal endopelvic fascia lines the walls and floor of the pelvis and continues on the internal organs as a visceral pelvic fascia (Figure 1-3A, B). Thus, the fascia propria of the rectum is an extension of the pelvic fascia, enclosing the rectum, fat, nerves, the blood, and lymphatic vessels. It is more evident in the posterior and lateral extraperitoneal aspects of the rectum.

Figure 1-3. Fascial relationships of the rectum: A male, B female.

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The lateral ligaments or stalks of the rectum are distal c­ ondensations of the pelvic fascia that form a roughly triangular structure with a base on the lateral pelvic wall and an apex attached to the lateral aspect of the rectum.32 Still a subject of misconception, the lateral stalks comprise essentially connective tissue and nerves, and the middle rectal artery does not traverse them. Branches, however, course through in approximately 25% of cases.36 Consequently, division of the lateral stalks during rectal mobilization is associated with a 25% risk of bleeding. Although the lateral stalks do not contain important structures, the middle rectal artery and the pelvic plexus are both closely related, running, at different angles, underneath it.37 One theoretical concern in ligation of the stalks is leaving behind lateral mesorectal tissue, which may limit adequate lateral or mesorectal margins during cancer surgery.34,35,38 The presacral fascia is a thickened part of the parietal endopelvic fascia that covers the concavity of the sacrum and coccyx, nerves, the middle sacral artery, and presacral veins. Operative dissection deep to the presacral fascia may cause troublesome bleeding from the underlying presacral veins. Presacral hemorrhage occurs as frequently as 4.6–7.0% of resections for rectal neoplasms, and despite its venous nature, can be life threatening.39–41 This is a consequence of two factors: the difficulty in securing control because of retraction of the vascular stump into the sacral foramen and the high hydrostatic pressure of the presacral venous system. The presacral veins are avalvular and communicate via basivertebral veins with the internal vertebral venous system. The adventitia of the basivertebral veins adheres firmly to the sacral periosteum at the level of the ostia of the sacral foramina, mainly at the level of S3-4. With the patient in the lithotomy

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position, the presacral veins can attain hydrostatic pressures of 17–23 cm H2O, two to three times the normal pressure of the inferior vena cava.40 The rectosacral fascia is an anteroinferiorly directed thick fascial reflection from the presacral fascia at the S-4 level to the fascia propria of the rectum just above the anorectal ring.42 The rectosacral fascia, classically known as the fascia of Waldeyer, is an important landmark during posterior rectal dissection.2,42 The visceral pelvic fascia of Denonvilliers is a tough fascial investment that separates the extraperitoneal rectum anteriorly from the prostate and seminal vesicles or vagina.43 Therefore, three structures lie between the anterior rectal wall and the seminal vesicles and prostate: anterior mesorectum, fascia propria of the rectum, and Denonvilliers’ fascia. A consensus has generally been reached about the anatomy of the plane of posterior and lateral rectal dissection, but anteriorly, the matter is more controversial. The anterior plane of rectal dissection may not necessarily follow the same plane of posterior and lateral dissection, and the use of the terms close rectal, mesorectal, and extramesorectal have been recently suggested to describe the available anterior planes.44 The close rectal or perimuscular plane lies inside the fascia propria of the rectum, and therefore it is more difficult and bloody than the mesorectal plane. The mesorectal plane represents the continuation of the same plane of posterior and lateral dissection of the rectum. This is a natural anatomic plane and consequently more appropriate for most rectal cancers. Finally, the extramesorectal plane involves resection of the Denonvilliers’ fascia, with the exposure of prostate and seminal vesicles, and is associated with high risk of mixed parasympathetic and sympathetic injury because of damage of the periprostatic plexus.

Urogenital Considerations Identification of the ureters is advisable to avoid injury to their abdominal or pelvic portions during colorectal operations. On both sides, the ureters rest on the psoas muscle in their inferomedial course; they are crossed obliquely by the spermatic vessels anteriorly and the genitofemoral nerve posteriorly. In its pelvic portion, the ureter crosses the pelvic brim in front of or a little lateral to the bifurcation of the common iliac artery, and descends abruptly between the peritoneum and the internal iliac artery. Before entering the bladder in the male, the vas deferens crosses lateromedially on its superior aspect. In the female, as the ureter traverses the posterior layer of the broad ligament and the parametrium close to the side of the neck of the uterus and upper part of the vagina, it is enveloped by the vesical and vaginal venous plexuses and is crossed above and lateromedially by the uterine artery.

J.M.N. Jorge and A. Habr-Gama

level of S-3 and then to the posterior aspect of the rectum. In 80% of cases, it bifurcates into right, usually wider, and left terminal branches; multiple branches are present in 17%.45 These divisions, once within the submucosa of the rectum, run straight downward to supply the lower rectum and the anal canal. Approximately five branches reach the level of the rectal columns, and condense in capillary plexuses, mostly at the right posterior, right anterior, and left lateral positions, corresponding to the location of the major internal hemorrhoidal groups. The superior and inferior hemorrhoidal arteries represent the major blood supply to the anorectum. In addition, it is also supplied by the internal iliac arteries. The contribution of the middle hemorrhoidal artery varies with the size of the superior hemorrhoidal artery; this may explain its controversial anatomy. Some authors report the absence of the middle hemorrhoidal artery in 40–88%46,47, whereas others identify it in 94–100% of specimens.45 It originates more frequently from the anterior division of the internal iliac or the pudendal arteries, and reaches the rectum. The middle hemorrhoidal artery reaches the lower third of the rectum anterolaterally, close to the level of the pelvic floor and deep to the levator fascia. It therefore does not run in the lateral ligaments, which are inclined posterolaterally.2 The middle hemorrhoidal artery is more prone to be injured during low anterior resection, when anterolateral dissection of the rectum is performed close to the pelvic floor and the prostate and seminal vesicles or upper part of the vagina are being separated.37 The anorectum has a profuse intramural anastomotic network, which probably accounts for the fact that division of both superior and middle hemorrhoidal arteries does not result in necrosis of the rectum. The paired inferior hemorrhoidal arteries are branches of the internal pudendal artery, which in turn is a branch of the internal iliac artery. The inferior hemorrhoidal artery arises within the pudendal canal and is throughout its course entirely extrapelvic. It traverses the obturator fascia, the ischiorectal fossa, and the external anal sphincter to reach the submucosa of the anal canal, ultimately ascending in this plane. Klosterhalfen et  al.4 performed postmortem angiographic, manual, and histologic evaluations and demonstrated that in 85% of cases the posterior commissure was less well ­perfused than were the other sections of the anal canal. In addition, the blood supply could be jeopardized by contusion of the vessels passing vertically through the muscle fibers of the internal anal sphincter with increased sphincter tone. The resulting decreased blood supply could lead to ischemia at the posterior commissure, in a pathogenetic model of ­primary anal fissure.

Arterial Supply of the Rectum and Anal Canal

Venous Drainage and Lymphatic Drainage of the Rectum and Anal Canal

The superior hemorrhoidal artery is the continuation of the inferior mesenteric artery, once it crosses the left iliac vessels. The artery descends in the sigmoid mesocolon to the

The anorectum also drains, via middle and inferior hemorrhoidal veins, to the internal iliac vein and then to the inferior vena cava. Although it is still a controversial subject,

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the presence of communications among these three venous systems may explain the lack of correlation between portal hypertension and hemorrhoids.48 The paired inferior and middle hemorrhoidal veins and the single superior hemorrhoidal vein originate from three anorectal arteriovenous plexuses. The external hemorrhoidal plexus, situated subcutaneously around the anal canal below the dentate line, constitutes when dilated the external hemorrhoids. The internal hemorrhoidal plexus is situated submucosally, around the upper anal canal and above the dentate line. The internal hemorrhoids originate from this plexus. The perirectal or perimuscular rectal plexus drains to the middle and inferior hemorrhoidal veins. Lymph from the upper two-thirds of the rectum drains exclusively upward to the inferior mesenteric nodes and then to the paraaortic nodes. Lymphatic drainage from the lower third of the rectum occurs not only cephalad, along the superior hemorrhoidal and inferior mesentery arteries, but also laterally, along the middle hemorrhoidal vessels to the internal iliac nodes. Studies using lymphoscintigraphy have failed to demonstrate communications between inferior mesenteric and internal iliac lymphatics.49 In the anal canal, the dentate line is the landmark for two different systems of lymphatic drainage: above, to the inferior mesenteric and internal iliac nodes, and below, along the inferior rectal lymphatics to the superficial inguinal nodes, or less frequently along the inferior hemorrhoidal artery. In the female, drainage at 5 cm above the anal verge in the lymphatic may also spread to the posterior vaginal wall, uterus, cervix, broad ligament, fallopian tubes, ovaries, and cul-de-sac, and at 10 cm above the anal verge, spread seems to occur only to the broad ligament and cul-de-sac.50

Innervation of the Rectum and Anal Canal Innervation of the Rectum The sympathetic supply of the rectum and the left colon arises from L-1, L-2, and L-3 (Figure 1-4A, B). Preganglionic fibers, via lumbar sympathetic nerves, synapse in the preaortic plexus, and the postganglionic fibers follow the branches of the inferior mesenteric artery and superior rectal artery to the left colon and upper rectum. The lower rectum is innervated by the presacral nerves, which are formed by fusion of the aortic plexus and lumbar splanchnic nerves. Just below the sacral promontory, the presacral nerves form the hypogastric plexus (or superior hypogastric plexus). Two main hypogastric nerves, on either side of the rectum, carry sympathetic innervation from the hypogastric plexus to the pelvic plexus. The pelvic plexus lies on the lateral side of the pelvis at the level of the lower third of the rectum, adjacent to the lateral stalks. The parasympathetic fibers to the rectum and anal canal emerge through the sacral foramen are called the nervi erigentes (S-2, S-3, and S-4). They pass laterally, forward, and upward to join the sympathetic hypogastric nerves at the pelvic plexus. From the pelvic plexus, combined ­postganglionic

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parasympathetic and sympathetic fibers are distributed to the left colon and upper rectum via the inferior mesenteric plexus, and directly to the lower rectum and upper anal canal. The periprostatic plexus, a subdivision of the pelvic plexus situated on Denonvilliers’ fascia, supplies the prostate, seminal vesicles, corpora cavernosa, vas deferens, urethra, ejaculatory ducts, and bulbourethral glands. Sexual function is regulated by cerebrospinal, sympathetic, and parasympathetic components. Erection of the penis is mediated by both parasympathetic (arteriolar vasodilatation) and sympathetic inflow (inhibition of vasoconstriction). All pelvic nerves lie in the plane between the peritoneum and the endopelvic fascia and are in danger of injury during rectal dissection. Permanent bladder paresis occurs in 7–59% of patients after abdominoperineal resection of the rectum51; the incidence of impotence is reported to range from 15 to 45%, and that of ejaculatory dysfunction from 32 to 42%.52 The overall incidence of sexual dysfunction after proctectomy has been reported to reach 100% when wide dissection is performed for malignant disease53–55; however, this kind of procedure is unnecessary and these rates are much lower for benign conditions, such as inflammatory bowel disease (0–6%).53,54,56,57 Dissections performed for benign conditions are undertaken closer to the bowel wall, thus reducing the possibility of nerve injury.58 Trauma to the autonomic nerves may occur at several points. During high ligation of the inferior mesenteric artery, close to the aorta, the sympathetic preaortic nerves may be injured. Division of both superior hypogastric plexus and hypogastric nerves may occur also during dissection at the level of the sacral promontory or in the presacral region. In such circumstances, sympathetic denervation with intact nervi erigentes results in retrograde ejaculation and bladder dysfunction. The nervi erigentes are located in the posterolateral aspect of the pelvis, and at the point of fusion with the sympathetic nerves are closely related to the middle hemorrhoidal artery. Injury to these nerves completely abolishes erectile function.56 The pelvic plexus may be damaged either by excessive traction on the rectum, particularly laterally, or during division of the lateral stalks when this is performed close to the lateral pelvic wall. Finally, dissection near the seminal vesicles and prostate may damage the periprostatic plexus, leading to a mixed parasympathetic and sympathetic injury. This can result in erectile impotence as well as a flaccid, neurogenic bladder. Sexual complications after rectal surgery are readily evident in men but are probably underdiagnosed in women.59 Anal Canal The internal anal sphincter is supplied by sympathetic (L-5) and parasympathetic nerves (S-2, S-3, and S-4) following the same route as the nerves to the rectum. The external anal sphincter is innervated on each side by the inferior rectal branch of the pudendal nerve (S-2 and S-3) and by the perineal branch of S-4. Despite the fact that the puborectalis

Figure 1-4. A, B Innervation of the colon, rectum, and anal canal.

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and external anal sphincter have somewhat different ­innervations, these muscles seem to act as an indivisible unit.14 After unilateral transection of a pudendal nerve, external anal sphincter function is still preserved because of the crossover of the fibers at the spinal cord level. Anal sensation is carried in the inferior rectal branch of the pudendal nerve and is thought to have a role in maintenance of anal continence. The upper anal canal contains a rich profusion of both free and organized sensory nerve endings, especially in the vicinity of the anal valves.60 Organized nerve endings include Meissner’s corpuscles (touch), Krause’s bulbs (cold), Golgi-Mazzoni bodies (pressure), and genital corpuscles (friction).

Anorectal Spaces The potential spaces of clinical significance in close relation to the anal canal and rectum include: ischiorectal, perianal, intersphincteric, submucosal, superficial postanal, deep postanal, supralevator, and retrorectal spaces (Figure 1-5A, B). The ischiorectal fossa is subdivided by a thin horizontal fascia into two spaces: the perianal and ischiorectal. The ischiorectal space comprises the upper two-thirds of the ischiorectal fossa. It is pyramid-shaped, situated on both sides between the anal canal and the lower part of the rectum medially, and the side wall of the pelvis laterally.61 The apex is at the origin of the levator ani muscle from the obturator fascia; the base is the perianal space. Anteriorly, the fossa is bounded by the urogenital diaphragm and transversus perinei muscle. Posterior to the ischiorectal fossa is the sacrotuberous ligament and the inferior border of the gluteus maximus. On the superolateral wall, the pudendal nerve and the internal pudendal vessels run in the pudendal canal (Alcock’s canal). The ischiorectal fossa contains fat and the inferior rectal vessels and nerves. The perianal space surrounds the lower part of the anal canal and contains the external hemorrhoidal plexus, the subcutaneous part of the external anal sphincter, the lowest part of the internal anal sphincter, and fibers of the longitudinal muscle. This space is the typical site of anal hematomas, perianal abscesses, and anal fistula tracts. The perianal space is continuous with the subcutaneous fat of the buttocks laterally and extends into the intersphincteric space medially. The intersphincteric space is a potential space between the internal and external anal sphincters. It is important in the genesis of perianal abscess because most of the anal glands end in this space. The submucous space is situated between the internal anal sphincter and the mucocutaneous lining of the anal canal. This space contains the internal hemorrhoidal plexus and the muscularis submucosae ani. Above, it is continuous with the submucous layer of the rectum, and, inferiorly, it ends at the level of the dentate line. The superficial postanal space is interposed between the anococcygeal ligament and the skin. The deep postanal space, also known as the retro-sphincteric space of ­Courtney,

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is situated between the anococcygeal ligament and the ­anococcygeal raphe. Both postanal spaces communicate posteriorly with the ischiorectal fossa and are the sites of horseshoe abscesses. The supralevator spaces are situated between the peritoneum superiorly and the levator ani inferiorly. Medially, these bilateral spaces are limited by the rectum, and laterally by the obturator fascia. Supralevator abscesses may occur as a result of upward extension of a cryptoglandular infection or develop from a pelvic origin. The retrorectal space is located between the fascia propria of the rectum anteriorly and the presacral fascia posteriorly. Laterally are the lateral rectal ligaments and inferiorly the rectosacral ligament, and above the space is continuous with the retroperitoneum. The retrorectal space is a site for embryologic remnants and rare presacral tumors.

Pelvic floor Musculature The muscles within the pelvis can be divided into three categories: (1) the anal sphincter complex; (2) pelvic floor muscles; and (3) muscles that line the sidewalls of the osseous pelvis.61 Muscles in this last category form the external boundary of the pelvis and include the obturator internus and piriform. These muscles, compared with the other two groups, lack clinical relevance to anorectal diseases; however, they provide an open communication for pelvic infection to reach extrapelvic spaces. For example, infection from the deep postanal space, originated from posterior midline glands, can track along the obturator internus fascia and reach the ischiorectal fossa. The anal sphincter and pelvic floor muscles, based on phylogenetic studies, derive from two embryonic cloaca groups, respectively, sphincteric and lateral compressor.62 The sphincteric group is present in almost all animals. In mammals, this group is divided into ventral (urogenital) and dorsal (anal) components.63 In primates, the latter components form the external anal sphincter. The lateral compressor or pelvicaudal group connects the rudimentary pelvis to the caudal end of the vertebral column. This group is more differentiated and subdivided into lateral and medial compartments only in reptiles and mammals. The homolog of the lateral compartment is the ischiococcygeus, and of the medial, pelvicaudal compartment, the pubococcygeus and ileococcygeus. In addition, most primates possess a variably sized group of muscle fibers close to the inner border of the medial pelvicaudal muscle, which attaches the rectum to the pubis. In humans, the fibers are more distinct and known as the puborectalis muscle.

Levator Ani The levator ani muscle, or pelvic diaphragm, is the major component of the pelvic floor. It is a pair of broad, ­symmetric sheets composed of three striated muscles: ileococcygeus, pubococcygeus, and puborectalis (Figure 1-6A, B). A ­variable

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Figure 1-5. Paraanal and pararectal spaces. A Frontal view. B Lateral view.

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1. Anatomy and Embryology

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Figure 1-6. Levator ani muscle. A Superior. B Inferior surface.

fourth component, the ischiococcygeus or coccygeus, is ­rudimentary in humans and represented by only a few muscle fibers on the surface of the sacrospinous ligament. The levator ani is supplied by sacral roots on its pelvic surface (S-2, S-3, and S-4) and by the perineal branch of the pudendal nerve on its inferior surface. The puborectalis muscle receives additional innervation from the inferior rectal nerves.

The ileococcygeus muscles arise from the ischial spine and posterior part of the obturator fascia and course inferiorly and medially to insert into the lateral aspects of S-3 and S-4, the coccyx, and the anococcygeal raphe. The pubococcygeus arises from the posterior aspect of the pubis and the anterior part of the obturator fascia; it runs dorsally alongside the anorectal junction to decussate with fibers of the opposite

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side at the anococcygeal raphe and insert into the anterior surface of the fourth sacral and first coccygeal segments. The pelvic floor is “incomplete” in the midline where the lower rectum, urethra, and either the dorsal vein of the penis in men, or the vagina in women, pass through it. This defect is called the levator hiatus and consists of an elliptic space situated between the two pubococcygeus muscles. The hiatal ligament, originating from the pelvic fascia, keeps the intrahiatal viscera together and prevents their constriction during contraction of the levator ani. A possible (but controversial) dilator function has been attributed to the anococcygeal raphe because of its crisscross arrangement.14 The puborectalis muscle is a strong, U-shaped loop of striated muscle that slings the anorectal junction to the posterior aspect of the pubis (Figure 1-7). The puborectalis is the most medial portion of the levator ani muscle. It is situated immediately cephalad to the deep component of the external sphincter. Because the junction between the two muscles is indistinct and they have similar innervation (pudendal nerve), the puborectalis has been regarded by some authors as a part of the external anal sphincter and not of the levator ani complex.14,15 Anatomic and phylogenetic studies suggest that the puborectalis may be a part of the levator ani63 or of the external anal sphincter.24,62 Embryologically, the puborectalis has a common primordium with the ileococcygeus and pubococcygeus muscles, and it is never connected with the external anal sphincter during the different stages of development.6 In addition, neurophysiologic studies have implied that the innervation of these muscles may not be the same because stimulation of the sacral nerves results in ­electromyographic activity

J.M.N. Jorge and A. Habr-Gama

in the ­ipsilateral puborectalis muscle but not in the external anal sphincter.64 Currently, because of this controversy, the puborectalis has been considered to belong to both muscular groups, the external anal sphincter and the levator ani.65

The Anorectal Ring and the Anorectal Angle Two anatomic structures of the junction of the rectum and anal canal are related to the puborectalis muscle: the anorectal ring and the anorectal angle. The anorectal ring, a term coined by Milligan and Morgan10 is a strong muscular ring that represents the upper end of the sphincter, more precisely the puborectalis, and the upper border of the internal anal sphincter, around the anorectal junction. Despite its lack of embryologic significance, it is an easily recognized boundary of the anal canal appreciated on physical examination, and it is of clinical relevance because division of this structure during surgery for abscesses or fistula inevitably results in fecal incontinence. The anorectal angle is thought to be the result of the anatomic configuration of the U-shaped sling of puborectalis muscle around the anorectal junction. Whereas the anal sphincters are responsible for the closure of the anal canal to retain gas and liquid stool, the puborectalis muscle and the anorectal angle are designed to maintain gross fecal continence. Different theories have been postulated to explain the importance of the puborectalis and the anorectal angle in the maintenance of fecal continence. Parks et al.66 opined that increasing intraabdominal pressure forces the anterior rectal wall down into the upper anal canal, occluding it by

Figure 1-7. The anteriorly directed pull of the puborectalis contributes to the angulation between the rectum and anal canal, the anorectal angle.

1. Anatomy and Embryology

a type of flap valve mechanism that creates an effective seal. ­Subsequently, it has been demonstrated that the flap mechanism does not occur. Instead, a continuous sphincteric occlusion-like activity that is attributed to the puborectalis is noted.67,68

Colon General Considerations The colon is a capacious tube that roughly surrounds the loops of small intestine as an arch. Named from the Greek koluein (“to retard”), the colon is variable in length, averaging approximately 150 cm, which corresponds to one-quarter the length of the small intestine. Its diameter can be substantially augmented by distension – it gradually decreases from 7.5 cm at the cecum to 2.5 cm at the sigmoid. In humans, the colon is described to be somewhere between the short, straight type with a rudimentary cecum, such as that of the carnivores, and a long sacculated colon with a capacious cecum, such as that of the herbivores. Anatomic differences between the small and large intestines include position, caliber, degree of fixation, and, in the colon, the presence of three distinct characteristics: the taeniae coli, the haustra, and the appendices epiploicae. The three taeniae coli, anterior (taenia libera), posteromedial (taenia mesocolica), and posterolateral (taenia omentalis), represent bands of the outer longitudinal coat of muscle that traverse the colon from the base of the appendix to the rectosigmoid junction, where they merge. The muscular longitudinal layer is actually a complete coat around the colon, although it is considerably thicker at the taeniae.69 The haustra or haustral sacculations are outpouchings of bowel wall between the taeniae; they are caused by the relative shortness of the taeniae, about one-sixth shorter than the length of the bowel wall.13 The haustra are separated by the plicae semilunares or crescentic folds of the bowel wall, which give the colon its characteristic radiographic appearance when filled with air or barium. The appendices epiploicae are small appendages of fat that protrude from the serosal aspect of the colon.

Cecum The cecum is the sacculated segment (Latin caecus, “blind”) of the large bowel that projects downward as a 6- to 8-cm blind pouch below the entrance of the ileum. Usually situated in the right iliac fossa, the cecum is almost entirely or at least in its lower half, invested with peritoneum. However, its mobility is usually limited by a small mesocecum. The ileum terminates in the posteromedial aspect of the cecum; the angulation between these two structures is maintained by the superior and inferior ileocecal ligaments. These ligaments, along with the mesentery of the appendix, form three pericecal recesses or fossae: superior ileocecal, inferior ileocecal, and retrocecal. Viewed from the cecal lumen, the ileocecal junction is represented by a narrow, transversely situated,

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slit-like opening known as the ileocecal valve or the valve de Bauhin. At either end, the two prominent semilunar lips of the valve fuse and continue as a single frenulum of mucosa. A circular sphincter, the ileocecal sphincter, originates from a slight thickening of the muscular layer of the terminal ileum. A competent ileocecal valve is related to the critical closed-loop type of colonic obstruction. However, ileocecal competence is not always demonstrated on barium enema studies. Instead of preventing reflux of colonic contents into the ileum, the ileocecal valve regulates ileal emptying. The ileocecal sphincter seems to relax in response to the entrance of food into the stomach.70 As in the gastroesophageal junction, extrasphincteric factors such as the ileocecal angulation apparently have a role in the prevention of reflux from the colon to the ileum.71

Appendix The vermiform appendix is an elongated diverticulum that arises from the posteromedial aspect of the cecum about 3.0 cm below the ileocecal junction. Its length varies from 2 to 20 cm (mean, 8–10 cm), and it is approximately 5 mm in diameter. The appendix, because of its great mobility, may occupy a variety of positions, possibly at different times in the same individual. It has been estimated that in 85–95% of cases, the appendix lies posteromedial on the cecum toward the ileum, but other positions include retrocecal, pelvic, subcecal, preileal and retroileal.72–74 The confluence of the three taeniae is a useful guide in locating the base of the appendix. The mesoappendix, a triangular fold attached to the posterior leaf of the mesentery of the terminal ileum, usually contains the appendicular vessels close to its free edge.

Ascending Colon The ascending colon is approximately 15 cm long. It ascends, from the level of the ileocecal junction to the right colic or hepatic flexure, laterally to the psoas muscle and anteriorly to the iliacus, the quadratus lumborum, and the lower pole of the right kidney. The ascending colon is covered with peritoneum anteriorly and on both sides. In addition, fragile adhesions between the right abdominal wall and its anterior aspect, known as Jackson’s membrane, may be present. Like the descending colon on its posterior surface, the ascending colon is devoid of peritoneum, which is instead replaced by an areolar tissue (fascia of Toldt) resulting from an embryologic process of fusion or coalescence of the mesentery to the posterior parietal peritoneum. In the lateral peritoneal reflection, this process is represented by the white line of Toldt, which is more evident at the descending-sigmoid junction. This line serves as a guide for the surgeon when the ascending, descending, or sigmoid colon is mobilized. At the visceral surface of the right lobe of the liver and lateral to the gallbladder, the ascending colon turns sharply medially and slightly caudad and ventrally to form the right colic (hepatic) flexure. This flexure is supported by the ­nephrocolic

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ligament and lies immediately ventral to the lower part of the right ­kidney and over the descending duodenum.

Transverse Colon The transverse colon is approximately 45 cm long, the longest segment of the large bowel. It crosses the abdomen, with an inferior curve immediately caudad to the greater curvature of the stomach. The transverse colon is relatively fixed at each flexure, and, in between, it is suspended by a 10- to 15-cmwide area which provides variable mobility; the nadir of the transverse colon may reach the hypogastrium. The transverse colon is completely invested with peritoneum, but the greater omentum is fused on its anterosuperior aspect. The left colic or splenic flexure is situated beneath the lower angle of the spleen and firmly attached to the diaphragm by the phrenocolic ligament, which also forms a shelf to support the spleen. Because of the risk of hemorrhage, mobilization of the splenic flexure should be approached with great care, preceded by dissection upward along the descending colon and medially to laterally along the transverse colon toward the splenic flexure. This flexure, when compared with the hepatic flexure, is more acute, higher, and more deeply situated.

comprising the last 5–8  cm of sigmoid and the uppermost 5 cm of the rectum.32,80 However, others have considered it a clearly defined segment because it is the narrowest portion of the large intestine; in fact, it is usually characterized endoscopically as a narrow and sharply angulated segment.81 According to a study in human cadavers, the rectosigmoid junction, macroscopically identified as the point where the taenia libera and the taenia omentalis fuse to form a single anterior taenia and where both haustra and mesocolon terminate, is situated 6–7 cm below the sacral promontory.5 With microdissection, this segment is characterized by conspicuous strands of longitudinal muscle fibers and the presence of curved interconnecting fibers between the longitudinal and circular muscle layers, resulting in a delicate syncytium of smooth muscle that allows synergistic interplay between the two layers. The rectosigmoid does not fit the anatomic definition of a sphincter as “a band of thickened circular muscle that closes the lumen by contraction and of a longitudinal muscle that dilates it”; however, this segment may be regarded as a functional sphincter because mechanisms of active dilation and passive “kinking” occlusion do exist.81

Blood Supply Descending Colon The descending colon courses downward from the splenic flexure to the brim of the true pelvis, a distance of approximately 25  cm.32 Similarly to the ascending colon, the descending colon is covered by peritoneum only on its anterior and lateral aspects. Posteriorly, it rests directly against the left kidney and the quadratus lumborum and transversus abdominis muscles. However, the descending colon is narrower and more dorsally situated than the ascending colon.

Sigmoid Colon The sigmoid colon is commonly a 35- to 40-cm-long, mobile, omega-shaped loop completely invested by peritoneum; however, it varies greatly in length and configuration. The mesosigmoid is attached to the pelvic walls in an inverted V shape, resting in a recess known as the intersigmoid fossa. The left ureter lies immediately underneath this fossa and is crossed on its anterior surface by the spermatic, left colic, and sigmoid vessels. Both the anatomy and function of the rectosigmoid junction have been matters of substantial controversy. As early as 1833, it was postulated that the sigmoid could have a role in continence as the fecal reservoir, based on the observation that the rectum is usually emptied and contracted.74 Since then, a thickening of the circular muscular layer between the rectum and sigmoid has been described and diversely termed the sphincter ani tertius, rectosigmoid sphincter, and pylorus sigmoidorectalis, and it has probably been mistaken for one of the transverse folds of the rectum.75–79 The rectosigmoid junction has been frequently regarded by surgeons as an indistinct zone, a region

The superior and inferior mesenteric arteries nourish the entire large intestine, and the limit between the two territories is the junction between the proximal two-thirds and the distal third of the transverse colon. This represents the embryologic division between the midgut and the hindgut. The superior mesenteric artery originates from the aorta behind the superior border of the pancreas at L-1 and supplies the cecum, appendix, ascending colon, and most of the transverse colon. After passing behind the neck of the pancreas and anteromedial to the uncinate process, the superior mesenteric artery crosses the third part of the duodenum and continues downward and to the right along the base of the mesentery. From its left side arises a series of 12–20 jejunal and ileal branches. From its right side arise the colic branches: middle, right, and ileocolic arteries. The ileocolic, the most constant of these vessels, bifurcates into a superior or ascending branch, which communicates with the descending branch of the right colic artery, and an inferior or descending branch, which gives off the anterior cecal, posterior cecal, and appendicular and ileal divisions.82 The right colic artery may also arise from the ileocolic or middle colic arteries and is absent in 2–18% of specimens.45,82,83 It supplies the ascending colon and hepatic flexure through its ascending and descending branches, both of them joining with neighboring vessels to contribute to the marginal artery. The middle colic artery is the highest of the three colic branches of the superior mesenteric artery, arising close to the inferior border of the pancreas. Its right branch supplies the right transverse colon and hepatic flexure, anastomosing with the ascending branch of the right colic artery. Its left branch supplies the distal half of the transverse colon. ­Anatomic variations of this artery include the absence in

1. Anatomy and Embryology

4–20% of cases and the presence of an accessory middle colic artery in 10%; the middle colic artery can be the main supply to the splenic flexure in about 33% of cases.82,84 The inferior mesenteric artery originates from the left anterior surface of the aorta, 3–4  cm above its bifurcation at the level of L2-3, and runs downward and to the left to enter the pelvis. Within the abdomen, the inferior mesenteric artery branches into the left colic artery and two to six sigmoidal arteries. After crossing the left common iliac artery, it acquires the name superior hemorrhoidal artery (superior rectal artery). The left colic artery, the highest branch of the inferior mesenteric artery, bifurcates into an ascending branch, which runs upward to the splenic flexure to contribute to the arcade of Riolan, and a descending branch, which supplies most of the descending colon. The sigmoidal arteries form arcades within the sigmoid mesocolon, resembling the small-bowel vasculature, and anastomose with branches of the left colic artery proximally, and with the superior hemorrhoidal artery distally. The marginal artery terminates within the arcade of sigmoidal arteries. The superior hemorrhoidal artery is the continuation of the inferior mesenteric artery, once it crosses the left iliac vessels. The artery descends in the sigmoid mesocolon to the level of S-3 and then to the posterior aspect of the rectum. In 80% of cases, it bifurcates into right and left terminal branches; multiple branches are present in 17%.45 These divisions, once within the submucosa of the rectum, run straight downward to supply the lower rectum and the anal canal. The venous drainage of the large intestine basically follows its arterial supply. Blood from the right colon, via the superior mesenteric vein, and from left colon and rectum, via the inferior mesenteric vein, reaches the intrahepatic capillary bed through the portal vein.

Collateral Circulation The anatomy of the mesenteric circulation is still a matter of controversy, and this may in part be related to the inherent confusion of the use of eponyms. The central anastomotic artery connecting all colonic mesenteric branches, first described by Haller85 in 1786, later became known as the marginal artery of Drummond because this author was the first to demonstrate its surgical significance (1913).86,87 Subsequently, discontinuity of the marginal artery has been shown at the lower ascending colon, and especially at the left colic flexure and the sigmoid colon. This potential hypovascularity is a source of concern during colonic resection. The splenic flexure comprises the watershed between midgut and hindgut supplies (Griffiths’ critical point); this anastomosis is of variable magnitude, and it may be absent in about 50% of cases.88 For this reason, ischemic colitis usually affects or is most severe near the splenic flexure.89,90 Another potential area of discontinuity of the marginal artery is the Sudeck’s critical point, situated between the lowest sigmoid and the superior hemorrhoidal arteries; however, surgical experience

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and radiological studies have both demonstrated adequate communications between these vessels.91 There is also a collateral network involving middle hemorrhoidal, internal iliac, and external iliac arteries which could potentially prevent gangrene of the pelvis and even the lower extremities in case of occlusion of the distal aorta.92,93 The term arc of Riolan was vaguely defined as the communication between superior and inferior mesenteric arteries in the author’s original work. Later, the eponym marginal artery of Drummond confused the subject.94 In 1964, Moskowitz et al.95 proposed another term, meandering mesenteric artery, and differentiated it from the marginal artery of Drummond. The meandering mesenteric artery is a thick and tortuous vessel that makes a crucial communication between the middle colic artery and the ascending branch of the left colic artery, especially in advanced atherosclerotic disease.94 The presence of the meandering mesenteric artery indicates severe stenosis of either the superior mesenteric artery (retrograde flow) or inferior mesenteric artery (antegrade flow).

Lymphatic Drainage The submucous and subserous layers of the colon and rectum have a rich network of lymphatic plexuses, which drain into an extramural system of lymph channels and follow their vascular supply.50 Colorectal lymph nodes are classically divided into four groups: epiploic, paracolic, intermediate, and principal.96 The epiploic group lies on the bowel wall under the peritoneum and in the appendices epiploicae; they are more numerous in the sigmoid and are known in the rectum as the nodules of Gerota. The lymphatic drainage from all parts of the colon follows its vascular supply. The paracolic nodes are situated along the marginal artery and on the arcades; they are considered to have the most numerous filters. The intermediate nodes are situated on the primary colic vessels, and the main or principal nodes on the superior and inferior mesenteric vessels. The lymph then drains to the cisterna chyli via the paraaortic chain of nodes. Colorectal carcinoma staging systems are based on the neoplastic involvement of these various lymph node groups.

Innervation The sympathetic and parasympathetic components of the autonomic innervation of the large intestine closely follow the blood supply. The sympathetic supply of the right colon originates from the lower six thoracic segments. These thoracic splanchnic nerves reach the celiac, preaortic, and superior mesenteric ganglia, where they synapse. The postganglionic fibers then course along the superior mesenteric artery to the small bowel and right colon. The parasympathetic supply comes from the right (posterior) vagus nerve and celiac plexus. The fibers travel along the superior mesenteric artery, and finally synapse with cells in the autonomic plexuses within the bowel wall. The sympathetic supply of the left colon and rectum arises from L-1, L-2, and L-3.

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Preganglionic fibers, via lumbar sympathetic nerves, synapse in the preaortic plexus, and the postganglionic fibers follow the branches of the inferior mesenteric artery and superior rectal artery to the left colon and upper rectum.

Embryology Anus and Rectum The distal colon, rectum, and the anal canal above the dentate line are all derived from the hindgut. Therefore, this segment is supplied by the hindgut (inferior mesenteric) artery, with corresponding venous and lymphatic drainage. Its parasympathetic outflow comes from S-2, S-3, and S-4 via splanchnic nerves. The dentate line marks the fusion between endodermal and ectodermal tubes, where the terminal portion of the hindgut or cloaca fuses with the proctodeum, an ingrowth from the anal pit. The cloaca originates at the portion of the rectum below the pubococcygeal line, whereas the hindgut originates above it. Before the fifth week of development, the intestinal and urogenital tracts terminate in conjunction with the cloaca. During the sixth to eighth weeks of fetal life, the urorectal septum or fold of Tourneux migrates caudally and divides the cloacal closing plate into an anterior urogenital plate and a posterior anal plate (Figure 1-8). Any slight posterior shift in the position of the septum during its descent reduces the size of the anal opening, giving rise to anorectal defects. The cloacal part of the anal canal, which has both endodermal and ectodermal elements, forms the anal transitional zone after breakdown of the anal membrane.73 During the tenth week, the anal tubercles, a pair of ectodermal swellings around the proctodeal pit, fuse dorsally to form a horseshoeshaped structure and anteriorly to create the perineal body. The cloacal sphincter is separated by the perineal body into urogenital and anal portions (external anal sphincter). The internal anal sphincter is formed later (6th to 12th week) from enlarging fibers of the circular layer of the rectum.6,97 In the female, the fused Müllerian ducts that form the uterus and vagina move downward to reach the urogenital sinus about the sixteenth week. In the male, the site of the urogenital membrane is obliterated by fusion of the genital folds and the sinus becomes incorporated into the urethra. The sphincters apparently migrate during their development; the external sphincter grows cephalad and the internal sphincter moves caudally. Concomitantly, the longitudinal muscle descends into the intersphincteric plane.6

Anorectal Malformations

Figure 1-8. A Embryology of the large intestine. I. Sagittal section of early embryo with the primitive tube at the third week of development. II. Normal development of intestine. IIa: Midgut loop within the umbilical cord (physiologic herniation); IIb: midgut rotation and return to the abdomen; IIc: rotation complete with wide retroperitoneal fixation of small bowel mesentery as well as ascending and descending colon. III. Development of the anus and rectum. IIIa: The hindgut, tailgut, and the allantois form the cloaca; IIIb: at the sixth week, the urogenital septum grows to separate the hindgut posteriorally and the allantois anteriorally; IIIc: the rectum with the persistent anal membrane has been separated from the urogenital structures. B Malformations of the digestive systems. I, Nonrotation; II, incomplete rotation; III, reversed rotation.

The anorectal malformations can be traced to developmental arrest at various stages of normal maturation. The Duhamel’s theory of “syndrome of caudal regression” is supported by the high incidence of spinal, sacral, and lower limb

defects associated with these anomalies.98 In fact, associated anomalies, most frequently skeleton and urinary defects, may occur in up to 70%.99 Digestive tract, particularly

1. Anatomy and Embryology Table 1-1. Classification of anorectal malformations A.  Anal defects (“low” defects) 1. Anal stenosis 2. Membranous atresia (rare) 3. Anal agenesis a. Without fistula b. With fistula (ectopic anus) B.  Anorectal defects (“high” defects) 1. Anorectal agenesis a. Without fistula b. With fistula 2. Rectal atresia (“high” atresia) C.  Persistent cloaca 1. Rectal duplication 2. Developmental cysts

tracheoesophageal fistula or esophageal stenosis, cardiac, and abdominal wall defects may also occur in patients with anorectal anomalies. There is evidence for familial occurrence of anorectal defects; the estimated risk in a family of a second occurrence of some form of imperforate anus is up to 50 times the normal chance.100 The proposed classification systems for the congenital malformations of the anorectal region are usually either incomplete or complex. The most comprehensive system has been proposed by Gough101 and Santulli102 and takes into consideration whether the rectum terminates above (anorectal defects) or below (anal defects) the puborectalis sling (Table 1-1).

Anal Defects Anal Stenosis Some degree of stricture of the rectum is present in 25–39% of infants, and only about 25% of these shows some degree of disordered evacuation, but spontaneous dilation occurs between 3 and 6 months of age in the vast majority of patients.103 Although stenosis has been attributed to excessive fusion of the anal tubercles, probably the cause is a posterior shift in the position of the urorectal septum during its descent at the sixth week of fetal life.101

Membranous Atresia This defect, also known as “covered anus,” is very rare. It is characterized by the presence of a thin membrane of skin between the blind end of the anal canal and the surface. Most cases occur in males and probably represent excessive posterior closure of the urogenital folds.101

Anal Agenesis The rectum extends below the puborectalis and ends, either blindly, or more often, in an ectopic opening or fistula to the perineum anteriorly, to the vulva, or urethra. Regardless of the location of the ectopic orifice, the sphincter is present at the normal site.

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Anorectal Defects Anorectal Agenesis Anorectal agenesis more often affects males and represents the most common type of “imperforate anus.” The rectum ends well above the surface, the anus is represented by a dimple, and the anal sphincter is usually normal. This malformation is the result of excessive obliteration of the embryonic tailgut and the adjacent dorsal portion of the cloaca. The descending urorectal septum reaches the dorsal wall of the diminished cloaca, leaving a blindly ending colon above and an isolated rectal membrane below. In most cases, there is a fistula or fibrous remnant connecting the rectal ending to the urethra or vagina. Fistulae represent areas in the septum where the lateral ridges have joined but failed to unite, although the more caudal union is complete. High fistulae, vaginal and urethral, with anorectal agenesis originate as early as the sixth or seventh week, whereas low fistulae (perineal) of anal ectopia originate in the eighth or ninth week of fetal life.

Rectal Atresia or “High Atresia” Although considered clinically as an anorectal defect, embryologically this is the most caudal type of atresia of the large intestine. The rectum and anal canal are separated from each other by an atretic portion.

Persistent Cloaca This is a rare condition that occurs only in female infants. It results from the total failure of the urorectal septum to descend, and therefore occurs at a very early stage of development (10-mm stage).

Colon and Small Bowel The primitive gut tube develops from the endodermal roof of the yolk sac. At first, the primitive intestine is a straight tube suspended in a sagittal plane on a common mesentery. At the beginning of the third week of development, it can be divided into three regions: the foregut in the head fold, the hindgut with its ventral allantoic outgrowth in the smaller tail fold, and, between these two portions, the midgut, which at this stage opens ventrally into the yolk sac (Figure 1-8). The normal embryologic process of rotation of the intestinal tract includes three stages, as outlined below.

First Stage: Physiologic Herniation of the Primitive Digestive Tube The first stage of rotation begins between the sixth and eighth weeks of intrauterine life, when the primitive intestinal tube elongates on its mesenteric around the superior mesenteric artery and bulges through the umbilical cord as a temporary physiologic herniation. This intraumbilical loop moves, at the eighth week of embryologic development, counterclockwise

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90° from the sagittal to the horizontal plane. The anomalies of this stage are rare and include situs inversus, inverted duodenum, and extroversion of the cloaca.

Second Stage: Return of the Midgut to the Abdomen The second stage of gut rotation occurs at the tenth week of intrauterine life. During this stage, the midgut loop returns to the peritoneal cavity from the umbilical herniation, and simultaneously rotates 180° counterclockwise around the pedicle formed by the mesenteric root. The prearterial segment of the midgut or duodenojejunal loop returns first to the abdomen, as the gut rotates counterclockwise. The duodenum comes to lie behind the superior mesenteric artery. The postarterial segment or cecocolic loop also reduces and comes to lie in front of the superior mesenteric artery. Anomalies of the second stage are relatively more common than the ones originated from the first stage and include nonrotation, malrotation, reversed rotation, internal hernia, and omphalocele.

Third Stage: Fixation of the Midgut The third stage of gut rotation starts after return of the gut to the peritoneal cavity and ends at birth. The cecum, initially in the upper abdomen, descends, migrating to the right lower quadrant, as counterclockwise rotation continues to 270°. After completion of the sequential rotation of the gastrointestinal tract, in the latter weeks of the first trimester, the process of fixation initiates. Gradually, fusion of parts of the primitive mesentery occurs, with fixation of the duodenum, and the ascending and descending parts of the colon to the posterior abdominal wall in their final position. Anomalies of this stage are common and include mobile cecum, subhepatic or undescended cecum, hyperdescent of the cecum, and persistent colonic mesentery. The midgut progresses below the major pancreatic papilla to form the small intestine, the ascending colon, and the proximal two-thirds of the transverse colon. This segment is supplied by the midgut (superior mesenteric) artery, with corresponding venous and lymphatic drainage. The neuroenteric ganglion cells migrate from the neural crest to the upper end of the alimentary tract and then follow vagal fibers caudad. The sympathetic innervation of the midgut and likewise the hindgut originates from T-8 to L-2, via splanchnic nerves and the autonomic abdominopelvic plexuses. The parasympathetic outflow to the midgut is derived from the tenth cranial nerve (vagus) with preganglionic cell bodies in the brain stem. The distal colon (distal third of the transverse colon), the rectum, and the anal canal above the dentate line are all derived from the hindgut. Therefore, this segment is supplied by the hindgut (inferior mesenteric) artery, with corresponding venous and lymphatic drainage. Its parasympathetic outflow comes from S-2, S-3, and S-4 via splanchnic nerves.

Abnormalities of Rotation Nonrotation In this condition, the midgut loop returns to the peritoneal cavity without the process of rotation, and, consequently, the entire small bowel locates on the right side of the abdomen, and the left colon is on the left side. This condition may be entirely asymptomatic and constitute a finding at laparotomies. However, it may complicate with volvulus affecting the entire small bowel. The twist of the entire midgut loop on its pedicle can occur, usually at the level of the duodenojejunal junction and the midtransverse colon because of the defective fixation of the mesenteric root.

Malrotation In malrotation, the cecum fails to complete the 360° rotation around the superior mesenteric, and does not complete the migration process. As a result of this failure in the migration process, the malrotated cecum locates in the right upper quadrant and is fixed by lateral bands or adhesions. These bands can overlie the distal part of the duodenum and cause extrinsic compression.

Reversed Rotation In this condition, the midgut rotates clockwise instead of counterclockwise; consequently, the transverse colon locates posteriorly and the duodenum anteriorly, in relation to the mesenteric artery.

Omphalocele Omphalocele is the retention of the midgut in the umbilical sac as a result of failure of the gut to return to the peritoneal cavity.

Incomplete Attachment of Cecum and Mesentery Under normal conditions, the cecum is almost entirely or at least in its lower half invested with peritoneum. However, its mobility is usually limited by a small mesocecum. In approximately 5% of individuals, the peritoneal covering is absent posteriorly; it then rests directly on the iliacus and psoas major muscles.32 Alternatively, an abnormally mobile cecum-ascending colon, resulting from an anomaly of fixation, can be found in 10–22% of individuals.104 In this case, a long mesentery is present, and the cecum may assume varied positions. This lack of fixation may predispose to the development of volvulus.

Internal Hernias Around the Ligament of Treitz Both internal hernias and congenital obstructive bands or adhesions are causes of congenital bowel obstruction, and result from an anomaly during the process of fixation. This failure may occur when the fusion of mesothelial layers is incomplete or if it occurs between structures that are

1. Anatomy and Embryology

abnormally rotated. Retroperitoneal hernias can occur in any intraperitoneal fossae, particularly paraduodenal, paracecal, and intersigmoid. The most common internal hernias resulting from abnormal fixation of the colon are right and left paraduodenal hernias.103

Other Congenital Malformations of the Colon and Small Intestine Proximal Colon Duplications Duplication of the colon comprises three general groups of congenital abnormalities: mesenteric cysts, diverticula, and long colon duplication.105 Mesenteric cysts, similarly to the duplication cysts found at the retroperitoneum and the mediastinum, are lined by intestinal epithelium and a variable amount of smooth muscle. These cysts lie in the mesentery of the colon or behind the rectum, may be separable or inseparable from the bowel wall, and usually present, as the size increases, either as a palpable mass or intestinal obstruction. Diverticula are blind ending pouches of variable lengths and arise either from the mesenteric or the antimesenteric border of the bowel. They may have heterotopic gastric mucosa or pancreatic-type tissue. Long colon duplication or tubular duplication of the colon is the rarest form of duplication. Almost invariably, the two parts lie parallel, sharing a common wall throughout most of their length; frequently, it involves the entire colon and rectum. Often, there is an association of pelvic genitourinary anomalies.

Meckel’s Diverticulum Meckel’s diverticulum is a remnant of the vitelline or omphalomesenteric duct, arising from the antimesenteric border of the terminal ileum, usually within 50  cm of the ileocecal valve.73 Associated abnormalities include persistence of a fibrous band connecting the diverticulum to the umbilicus or a patent omphalomesenteric duct, the presence of ectopic mucosa or aberrant pancreatic tissue (in more than half of asymptomatic diverticula), and herniation of the diverticulum in an indirect inguinal hernia (Littré’s hernia). In most people, Meckel’s diverticulum is asymptomatic, and according to autopsy series, it exists in 1–3% of the general population.106 Surgical complications are more frequent in infants and children and include hemorrhage from ectopic gastric mucosa, intestinal obstruction resulting from associated congenital bands or ileocolic intussusception, diverticulitis, perforation, and umbilical discharge from a patent omphalomesenteric duct.

Atresia of the Colon Colonic atresia is a rare cause of intestinal obstruction; it represents only 5% of all forms of gastrointestinal atresia. It is probably caused by a vascular accident occurring during intrauterine life.104 Colonic atresia can be classified in

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three basic types: (1) incomplete occlusion of the lumen by a membranous diaphragm; (2) proximal end distal colonic segments that end blindly and are joined by a cord-like remnant of the bowel; and (3) complete separation of the proximal anal distal blind segments with the absence of a segment of megacolon.107 Colonic atresia may be variable in length and can occur at any site in the colon, and its association with Hirschsprung’s disease has been reported.104

Hirschsprung’s Disease Congenital megacolon is one of the most distressing of nonlethal anomalies, and was promptly attached to Hirschsprung’s name after his description of autopsies of two infants who died from this condition in 1888.108 However, it was recognized as early as 1825 in adults, and, in 1829, in infants.109 This disease results from the absence of ganglion cells in the myenteric plexus of the colon caused by the interruption of migration of neuroenteric cells from the neural crest before they reach the rectum. The physiologic obstruction, more insidious than an anatomic atresia, results in proximal dilation and hypertrophy of the colon above. The extent of the aganglionosis is variable. The internal anal sphincter is involved in all cases, and the entire rectum in most cases. The disease is more common in males and its severity is related to the length of the aganglionic segment. Although most patients reach surgery before they are a year old, many are older, and a few reach adulthood. This topic is further discussed in Chap. 50.

References 1. Vesalii Bruxellensis Andreae, de Humani corporis fabrica. De Recti intestini musculis. 1st ed. 1543:228. 2. Church JM, Raudkivi PJ, Hill GL. The surgical anatomy of the rectum – a review with particular relevance to the hazards of rectal mobilisation. Int J Colorectal Dis. 1987;2:158–66. 3. Shafik A. A concept of the anatomy of the anal sphincter mechanism and the physiology of defecation. Dis Colon Rectum. 1987;30:970–82. 4. Klosterhalfen B, Vogel P, Rixen H, Mitterman C. Topography of the inferior rectal artery. A possible cause of chronic, primary anal fissure. Dis Colon Rectum. 1989;32:43–52. 5. Stoss F. Investigations of the muscular architecture of the rectosigmoid junction in humans. Dis Colon Rectum. 1990; 33:378–83. 6. Levi AC, Borghi F, Garavoglia M. Development of the anal canal muscles. Dis Colon Rectum. 1991;34:262–6. 7. Lunniss PJ, Phillips RKS. Anatomy and function of the anal longitudinal muscle. Br J Surg. 1992;79:882–4. 8. Tjandra JJ, Milsom JW, Stolfi VM, et  al. Endoluminal ultrasound defines anatomy of the anal canal and pelvic floor. Dis Colon Rectum. 1992;35:465–70. 9. Dobson HD, Pearl RK, Orsay CP, et  al. Virtual reality: new method of teaching anorectal and pelvic floor anatomy. Dis Colon Rectum. 2003;46:349–52. 10. Milligan ETC, Morgan CN. Surgical anatomy of the anal canal: with special reference to anorectal fistulae. Lancet. 1934;2:1150–6.

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11. Goligher JC, Leacock AG, Brossy JJ. The surgical anatomy of the anal canal. Br J Surg. 1955;43:51–61. 12. Garavoglia M, Borghi F, Levi AC. Arrangement of the anal striated musculature. Dis Colon Rectum. 1993;36:10–5. 13. Nivatvongs S, Gordon PH. Surgical anatomy. In: Gordon PH, Nivatvongs S, editors. Principle and practice of surgery for the colon, rectum and anus. St. Louis, MO: Quality Medical Publishing; 1992. p. 3–37. 14. Shafik A. A new concept of the anatomy of the anal sphincter mechanism and the physiology of defecation. II. Anatomy of the levator ani muscle with special reference to puborectalis. Invest Urol. 1975;12:175–82. 15. Oh C, Kark AE. Anatomy of the external anal sphincter. Br J Surg. 1972;59:717–23. 16. Bollard RC, Gardiner A, Lindow S, Phillips K, Duthie GS. Normal female anal sphincter: difficulties in interpretation explained. Dis Colon Rectum. 2002;45:171–5. 17. Fritsch H, Brenner E, Lienemann A, Ludwikowski B. Anal sphincter complex: reinterpreted morphology and its clinical relevance. Dis Colon Rectum. 2002;45:188–94. 18. Morren GL, Beets-Tan RGH, van Engelshoven JMA. Anatomy of the anal canal and perianal structures as defined by phased-array magnetic resonance imaging. Br J Surg. 2001;88: 1506–12. 19. Williams AB, Bartram CI, Halligan S, Marshall MM, Nicholls RJ, Kmiot WA. Endosonographic anatomy of the normal anal compared with endocoil magnetic resonance imaging. Dis Colon Rectum. 2002;45:176–83. 20. Jorge JMN, Habr-Gama A. The value of sphincter asymmetry index in anal incontinence. Int J Colorectal Dis. 2000;15: 303–10. 21. Swash M. Histopathology of pelvic floor muscles in pelvic floor disorders. In: Henry MM, Swash M, editors. Coloproctology and the pelvic floor. London: Butterworth-Heinemann; 1992. p. 173–83. 22. Courtney H. Anatomy of the pelvic diaphragm and anorectal musculature as related to sphincter preservation in anorectal surgery. Am J Surg. 1950;79:155–73. 23. Shafik A. A new concept of the anatomy of the anal sphincter mechanism and the physiology of defecation. III. The longitudinal anal muscle: anatomy and role in sphincter mechanism. Invest Urol. 1976;13:271–7. 24. Lawson JON. Pelvic anatomy. II. Anal canal and associated sphincters. Ann R Coll Surg Engl. 1974;54:288–300. 25. Roux C. Contribution to the knowledge of the anal muscles in man. Arch Mikr Anat. 1881;19:721–3. 26. Haas PA, Fox TA. The importance of the perianal connective tissue in the surgical anatomy and function of the anus. Dis Colon Rectum. 1977;20:303–13. 27. Gordon PH. Anorectal anatomy and physiology. Gastroenterol Clin North Am. 2001;30:1–13. 28. Lilius HG. Investigation of human fetal anal ducts and intramuscular glands and a clinical study of 150 patients. Acta Chir Scand Suppl. 1968;383:1–88. 29. Chiari H. Über die Nalen Divertik Fel der Rectum-schleimhaut und Ihre Beziehung zu den anal fisteln. Wien Med Press. 1878;19:1482. 30. Parks AG. Pathogenesis and treatment of fistula-in-ano. BMJ. 1961;1:463–9. 31. Abel AL. The pecten band: pectenosis and pectenectomy. Lancet. 1932;1:714–8.

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1. Anatomy and Embryology 52. Orkin BA. Rectal carcinoma: treatment. In: Beck DE, Wexner SD, editors. Fundamentals of anorectal surgery. New York: McGraw-Hill; 1992. p. 260–369. 53. Balslev I, Harling H. Sexual dysfunction following operation for carcinoma of the rectum. Dis Colon Rectum. 1983; 26:785. 54. Danzi M, Ferulano GP, Abate S, Califano G. Male sexual function after abdominoperineal resection for rectal cancer. Dis Colon Rectum. 1983;26:665–8. 55. Weinstein M, Roberts M. Sexual potency following surgery for rectal carcinoma. A follow-up of 44 patients. Ann Surg. 1977;185:295–300. 56. Bauer JJ, Gerlent IM, Salky B, Kreel I. Sexual dysfunction following proctectomy for benign disease of the colon and rectum. Ann Surg. 1983;197:363–7. 57. Walsh PC, Schlegel PN. Radical pelvic surgery with preservation of sexual function. Ann Surg. 1988;208:391–400. 58. Lee ECG, Dowling BL. Perimuscular excision of the rectum for Crohn’s disease and ulcerative colitis. A conservative technique. Br J Surg. 1972;59:29–32. 59. Metcalf AM, Dozois RR, Kelly KA. Sexual function in women after proctocolectomy. Ann Surg. 1986;204:624–7. 60. Duthie HL, Gairns FW. Sensory nerve endings and sensation in the anal region in man. Br J Surg. 1960;47:585–95. 61. Kaiser AM, Ortega AE. Anorectal anatomy. Surg Clin North Am. 2002;82:1125–38. 62. Wendell-Smith CP. Studies on the morphology of the pelvic floor [Ph.D. thesis]. University of London; 1967. 63. Paramore RH. The Hunterian lectures on the evolution of the pelvic floor in non-mammalian vertebrates and pronograde mammals. Lancet. 1910;1(1393–1399):1459–67. 64. Percy JP, Swash M, Neill ME, Parks AG. Electrophysiological study of motor nerve supply of pelvic floor. Lancet. 1981;1:16–7. 65. Russell KP. Anatomy of the pelvic floor, rectum and anal canal. In: Smith LE, editor. Practical Guide to anorectal testing. New York: Igaku-Shoin; 1991. p. 744–7. 66. Parks AG, Porter NH, Hardcastle J. The syndrome of the descending perineum. Proc R Soc Med. 1966;59:477–82. 67. Bannister JJ, Gibbons C, Read NW. Preservation of faecal continence during rises in intra-abdominal pressure: is there a role for the flap valve? Gut. 1987;28:1242–5. 68. Bartolo DCC, Roe AM, Locke-Edmunds JC, Virjee J, Mortensen NJ. Flap-valve theory of anorectal continence. Br J Surg. 1986;73:1012–4. 69. Fraser ID, Condon RE, Schulte WJ, Decosse JJ, Cowles VE. Longitudinal muscle of muscularis externa in human and nonhuman primate colon. Arch Surg. 1981;116:61–3. 70. Guyton AC, editor. Textbook of medical physiology. Philadelphia: WB Saunders; 1986. p. 754–69. 71. Kumar D, Phillips SF. The contribution of external ligamentous attachments to function of the ileocecal junction. Dis Colon Rectum. 1987;30:410–6. 72. Wakeley CPG. The position of the vermiform appendix as ascertained by an analysis of 10, 000 cases. J Anat. 1983;67: 277–83. 73. Skandalakis JE, Gray SW, Ricketts R. The colon and rectum. In: Skandalakis JE, Gray SW, editors. Embryology for surgeons. The embryological basis for the treatment of congenital anomalies. Baltimore: Williams & Wilkins; 1994. p. 242–81. 74. O’Beirne J, editor. New views of the process of defecation and their application to the pathology and treatment of diseases of

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the stomach, bowels and other organs. Dublin: Hodges and Smith; 1833. 75. Hyrtl J. Handbuch der topographischen anatomie und ihrer praktisch medicinisch-chirurgischen anwendungen. II. Band, 4. Aufl. Wien: Braumüller; 1860. 76. Mayo WJ. A study of the rectosigmoid. Surg Gynecol Obstet. 1917;25:616–21. 77. Cantlie J. The sigmoid flexure in health and disease. J Trop Med Hyg. 1915;18:1–7. 78. Otis WJ. Some observations on the structure of the rectum. J Anat Physiol. 1898;32:59–63. 79. Balli R. The sphincters of the colon. Radiology. 1939; 33:372–6. 80. Ewing MR. The significance of the level of the peritoneal reflection in the surgery of rectal cancer. Br J Surg. 1952;39: 495–500. 81. Stelzner F. Die Verschlubsysteme am Magen-Darm-Kanal und ihre chirurgische Bedeutung. Acta Chir Austriaca. 1987;19:565–9. 82. Sonneland J, Anson BJ, Beaton LE. Surgical anatomy of the arterial supply to the colon from the superior mesenteric artery based upon a study of 600 specimens. Surg Gynecol Obstet. 1958;106:385–98. 83. Steward JA, Rankin FW. Blood supply of the large intestine. Its surgical considerations. Arch Surg. 1933;26:843–91. 84. Griffiths JD. Surgical anatomy of the blood supply of the distal colon. Ann R Coll Surg Engl. 1956;19:241–56. 85. Haller A. The large intestine. In: Cullen W, editor. First Lines of physiology. A reprint of the 1786 edition (Sources of Science 32). New York: Johnson; 1966. p. 139–40. 86. Drummond H. Some points relating to the surgical anatomy of the arterial supply of the large intestine. Proc R Soc Med Proctol. 1913;7:185–93. 87. Drummond H. The arterial supply of the rectum and pelvic colon. Br J Surg. 1914;1:677–85. 88. Meyers CB. Griffiths’ point: critical anastomosis at the splenic flexure. Am J Roentgenol. 1976;126:77. 89. Landreneau RJ, Fry WJ. The right colon as a target organ of non-occlusive mesenteric ischemia. Arch Surg. 1990;125: 591–4. 90. Longo WE, Ballantyne GH, Gursberg RJ. Ischemic colitis: patterns and prognosis. Dis Colon Rectum. 1992;35:726–30. 91. Sudeck P. Über die Gefassversorgung des Mastdarmes in Hinsicht auf die Operative Gangran. Münch Med Wochenschr. 1907;54:1314. 92. Griffiths JD. Extramural and intramural blood supply of the colon. BMJ. 1961;1:322–6. 93. Lindstrom BL. The value of the collateral circulation from the inferior mesenteric artery in obliteration of the lower abdominal aorta. Acta Chir Scand. 1950;1:677–85. 94. Fisher DF, Fry WI. Collateral mesenteric circulation. Surg Gynecol Obstet. 1987;164:487–92. 95. Moskowitz M, Zimmerman H, Felson H. The meandering mesenteric artery of the colon. Am J Roentgenol. 1964;92: 1088–99. 96. Jameson JK, Dobson JF. The lymphatics of the colon. Proc R Soc Med. 1909;2:149–72. 97. Nobles VP. The development of the human anal canal. J Anat. 1984;138:575. 98. Duhamel B. From the mermaid to anal imperforation: the syndrome of caudal regression. Arch Dis Child. 1961;36:152–5.

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99. Moore TC, Lawrence EA. Congenital malformations of rectum and anus. II. Associated anomalies encountered in a series of 120 cases. Surg Gynecol Obstet. 1952;95:281. 100. Anderson RC, Reed SC. The likelihood of congenital ­malformations. J Lancet. 1954;74:175–9. 101. Gough MH. Congenital abnormalities of the anus and rectum. Arch Dis Child. 1961;36:146–51. 102. Santulli TV, Schullinger JN, Amoury RA. Malformations of the anus and rectum. Surg Clin North Am. 1965;45: 1253–71. 103. Brown SS, Schoen AH. Congenital anorectal stricture. J Pediatr. 1950;36:746–50. 104. Romolo JL. Congenital lesions: intussusception and volvulus. In: Zuidema GD, editor. Shackelford’s surgery of

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Colonic Physiology Ursula M. Szmulowicz and Tracy L. Hull

The human colon is a dynamic organ, involved in a vast array of functions, including the absorption of water and electrolytes, the salvage of unabsorbed nutrients, and the transport of luminal contents as feces. While not an organ essential for life, the colon still plays a major role in maintaining the overall health of the human body. Understanding these physiologic principles is integral to the successful treatment – both medical and surgical – of colonic disease.

Embryology The embryology of the colon informs its anatomy. In the third and fourth weeks of gestation, the primitive intestine arises from the cephalocaudal and lateral folding of the dorsal surface of the endoderm-lined yolk sac, forming a straight tube situated posteriorly in the embryo.1–3 Although the mucosa originates from the endoderm of the yolk sac, the muscular wall, connective tissue, and serosa have a mesodermal etiology.4 By the fourth week of gestation, the gut tube develops into three distinct regions: the foregut, midgut, and hindgut.4 The midgut begins immediately distal to the confluence of the common bile duct and the duodenum, extending to include the proximal two-thirds of the transverse colon. At the midgut, the primitive intestine maintains its connection to the yolk sac via the vitelline duct; failure of the vitelline duct to obliterate ultimately results in a Meckel’s diverticulum or a vitelline cyst or fistula.2,5 The hindgut reaches from the distal third of the transverse colon to the anal canal proximal to the dentate line. The midgut receives its vascular supply from the superior mesenteric artery and the hindgut from the inferior mesenteric artery.6 The colon gains its ultimate position by means of a series of rotations of the embryonic gut. During the fifth week of gestation, the midgut, particularly the future ileum, rapidly enlarges beyond the capacity of the abdominal cavity, culminating in its physiologic herniation through the umbilicus in the sixth week.1 While exteriorized, this hairpin-shaped primary intestinal loop rotates 90° counterclockwise around

an axis comprised of the superior mesenteric artery, as viewed from the front. Although the cranial limb of the herniated primary intestinal loop – the future small intestine – continues to elongate and organize into loops, the caudal limb – the proximal colon – remains mainly unaltered, its only adjustment involving the development of a cecal bud, a bulge from its antimesenteric border.3 In the tenth week of gestation, the herniated intestine commences its return to the abdominal cavity, in the process completing an additional 180° counterclockwise rotation to finalize the disposition of the embryonic proximal jejunum on the left and the primitive colon on the right.2 The cecum – the last component to reenter the abdomen – initially is located in the right upper quadrant but then migrates inferiorly to the right iliac fossa as the dorsal mesentery suspending the ascending colon shortens and then recedes.4 As the cecal bud descends, the appendix appears as a narrow diverticulum.2–7 The loss of the dorsal mesentery of the ascending and descending colon produces their retroperitoneal fixation, absent in the cecum, transverse colon, and sigmoid colon.2

Innervation Colonic innervation emanates from two sources: the extrinsic and the intrinsic nerves. Extrinsic innervation involves the sympathetic and parasympathetic nerves of the autonomic nervous system, which are responsible for colonic motility and sensation. Intrinsic innervation arises from the poorly understood enteric nervous system. The parasympathetic nerves primarily exert an excitatory affect upon colonic motility. The main parasympathetic neurotransmitters include acetylcholine and tachykinins such as substance P. The parasympathetic supply to the proximal colon originates from the posterior division of the vagus nerve – the tenth cranial nerve.8,9 These parasympathetic fibers reach the colon by following the branches of the superior mesenteric artery: the ileocolic and middle colic arteries. The distal colon, however, receives its parasympathetic

D.E. Beck et al. (eds.), The ASCRS Textbook of Colon and Rectal Surgery: Second Edition, DOI 10.1007/978-1-4419-1584-9_2, © Springer Science+Business Media, LLC 2011

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input from the second to fourth sacral nerves, with the third sacral nerve the most dominant.8,9 These pelvic splanchnic nerves – the nervi erigentes – emerge from the lateral horns of S2–4.8,9 After exiting the spinal column, the preganglionic parasympathetic fibers travel superiorly and laterally, deep to the peritoneum, to join the inferior hypogastric plexus, located on the anterolateral pelvic wall, adjacent to the lateral ligaments at the level of the lower third of the rectum.10 From the inferior hypogastric plexus, these parasympathetic fibers are distributed to the pelvic organs and to the distal colon as far proximal as the splenic flexure.11 The preganglionic parasympathetic fibers ultimately synapse in the bowel wall at the ganglia within the myenteric plexus of Auerbach and Meissner’s plexus.12 Unlike the parasympathetic nerves, the sympathetic system effects a tonic inhibition of the nonsphincteric colonic muscle and, thus, of colonic peristalsis.11 Additionally, sympathetic fibers prevent epithelial secretion and reduce splanchnic blood flow.9 However, the sympathetic supply is excitatory to sphincter muscles, particularly to the ileocecal junction and the internal anal sphincter.9,11,13 The primary sympathetic neurotransmitter is norepinephrine.8,11 The inhibition of colonic tone, although not fully understood, is believed to be mediated via a2 adrenergic receptors.14 In one study in humans, the a2 agonist clonidine was  determined to decrease fasting colonic tone, while the a2 antagonist yohimbine increased it.15 Stimulation of  the  a2 adrenoreceptor also blocked the release of acetylcholine from the hyperpolarized parasympathetic neurons in the myenteric and pelvic plexi, thus arresting parasympathetic function.11,15 In contrast, the in  vivo administration of the a1 agonist phenylephrine and the b2 agonist ritodrine at the highest acceptable dosages had no impact upon colonic tone.15 Yet, in an in vitro study, agonists of the b1, b2, and b3 colonic adrenoceptors relaxed the human taenia coli; the circular muscular tone maximally diminished after treatment with the b1 agonist.16 The preganglionic sympathetic effector cells are derived from the intermediolateral cell column (lateral horns) of the thoracic (T1–12) and, in particular, the lumbar spine (L1–2 or 3–4). The sympathetic nerve fibers leave the spinal column via the ventral spinal nerve roots (rami). These myelinated preganglionic fibers then exit from the ventral spinal nerve root as a white ramus communicans (“communicating branches”) to merge with the paired sympathetic trunks (paravertebral ganglia), found along the entire length of the vertebral column. Once it has joined a sympathetic trunk, the preganglionic sympathetic nerve fiber either immediately synapses at that ganglion or it ascends or descends along the trunk before synapsing at another vertebral level, from the first cervical vertebra to the first coccygeal vertebra, along the sympathetic trunk.17 A single preganglionic fiber synapses with 30 or more postganglionic fibers.8 After synapsing, the unmyelinated postganglionic fibers depart the sympathetic trunk as a gray ramus communicans, reuniting

U.M. Szmulowicz and T.L. Hull

with the ventral spinal ramus, after which it is distributed to the sweat glands and arrector pili muscles of the body wall and to the smooth muscle of the blood vessels throughout the body.18 Yet, most preganglionic fibers – those innervating the abdominopelvic viscera – pass through the sympathetic trunk without synapsing, instead proceeding to the interconnected collateral (preaortic or prevertebral) ganglia that lie anterior to the aorta at its junction with its main vascular branches as part of a specific splanchnic (“visceral”) nerve: the greater (T5–T9 or 10), lesser (T10–T11), least (T12), or lumbar (L1–2) splanchnic nerves.12,17,19 While the midgut receives its sympathetic input from the lesser splanchnic nerve, the hindgut is supplied by the lumbar splanchnic nerve.18 Once these preganglionic fibers synapse at a collateral ganglion, the postganglionic fibers follow the vasculature to the intestine, concluding in the enteric ganglia.20 Some of the postganglionic fibers terminate instead on intestinal epithelial cells, whereby intestinal secretion is inhibited.20 The midgut derivatives are primarily innervated by postganglionic fibers from the superior mesenteric ganglion and the hindgut structures, from the inferior mesenteric ganglion, although commingling between the ganglia is common.8 The intrinsic innervation of the colon – the enteric ­nervous system – is uniquely able to mediate reflex behavior independent of input from the brain or spinal cord.21 This intrinsic network regulates the majority of colonic motility.9 However, the activity of the enteric nervous system is impacted by the extrinsic nerves: the sympathetic, parasympathetic, and visceral afferent nerves.8,11 This “little brain” employs the same modulators and neuro transmitters present in the central nervous system, including the excitatory acetylcholine, substance P, and neurokinin A and the inhibitory nitric oxide, adenosine triphosphate (ATP), vasoactive intestinal polypeptide (VIP), and pituitary adenyl cyclaseactivating peptide.9,13,21,22 The enteric nervous system acts through many different types of neurons, with their cell bodies amassed within neuronal plexi positioned either between the circular and longitudinal muscles (the myenteric plexus of Auerbach) or in the submucosal layer.9 The submucosal plexi are comprised of Meissner’s plexus, situated adjacent to the mucosa, and Schabadasch’s plexus, which lies near the circular muscle.9 Notably, the number of neurons in the enteric nervous system greatly exceeds that of the entire autonomic nervous system.9 The myenteric plexus directs smooth muscle function while the submucosal plexus modulates mucosal ion transport and absorptive functions.11 Moreover, the enteric network influences blood flow to the colon.20 Control of colonic motor function via the enteric nervous system remains poorly understood at this time. However, a reflex arc is initiated by a mechanical (e.g., stretch), chemical, or other noxious stimulus, which activates an enteric primary afferent neuron; the impulse is carried to an enteric motor neuron – the effector cell – via an enteric interneuron, producing an excitatory or inhibitory effect.9,20

2. Colonic Physiology

Colonic Function Salvage, Metabolism, and Storage Although digestion and absorption primarily take place in the stomach and small intestine, the colon still plays a major role in these operations. The colon processes various complex carbohydrates and, to a lesser extent, proteins that prove resistant to digestion and absorption in the more proximal intestine.23,24 Unlike the small intestine, the colon salvages nutrients from these products via fermentation. Fermentation occurs by means of the saccarolytic and proteolytic members of the over 400 species of bacteria, the majority of which are obligate anaerobes, present within the colon.25 Approximately 10% of ingested carbohydrates enter the cecum as undigested material.11 Among the diverse end products of the bacterial fermentation of complex carbohydrates – mainly the soluble plant residues (fiber) – are the short-chain fatty acids, represented principally by butyrate (15%), propionate (25%), and acetate (60%).26 Ingestion of a diet higher in complex carbohydrates, beans, resistant starches, and soluble fiber leads to a greater output of short-chain fatty acids than that of insoluble fibers. The composition of the bacterial microenvironment also influences the amount of synthesized short-chain fatty acids.11,27 The nondigestible carbohydrate inulin – an extract of chicory – has been studied as a prebiotic, a food that selectively alters the admixture of the colonic bacterial flora; although its addition to the diet increased the proportion of beneficial bifidobacteria in the feces in various studies, its impact – whether healthful or harmful – upon other bacterial species could not be well gauged.28 Bacterial fermentation of the complex carbohydrates primarily transpires in the ascending and proximal transverse colon. In contrast, the undigested dietary proteins that reach the colon, as well as proteins from mucous and sloughed epithelial cells, are fermented in the distal colon, primarily because the carbohydrates – the preferred nutrient of most bacteria – were previously exhausted in the proximal colon; the concentration of the short-chain fatty acids produced in the distal colon is 30% less than in the proximal colon.27,29,30 However, a diet that includes prebiotics such as inulin results in a greater degree of saccarolytic fermentation in the distal colon due to the greater availability of these slowly fermentable, highly polymerized carbohydrates.27 The fermented proteins are converted into short-chain fatty acids, branched chain fatty acids, and amines. In addition, the bacterial fermentation of undigested proteins generates ammonia, phenols, indoles, and sulfurs; these possibly toxic substances are considered potential etiologic agents for such diseases as colon cancer and ulcerative colitis.27 Some of these proteolytic metabolites become a nitrogen source for bacterial growth.29,31 The residual products of the bacterial fermentation of complex carbohydrates and proteins are absorbed or, like carbon dioxide, hydrogen, and methane, passed with the feces.28 The dietary fats that reach the colon likely are not recovered in the colon but are expelled with the stool.25

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The short-chain fatty acids occupy an integral position in colonic health. More than 95% of the short-chain fatty acids are created in and are immediately appropriated by the colon, with very little excreted in the feces.25,27,32 An average of 400 mmol/day, with a range of 150–600 mmol/day, of short-chain fatty acids are produced in the colon.33,34 This reclamation of undigested matter in the colon as short-chain fatty acids provides 5–15% of the total caloric needs of an individual.27 These weak acids – the preeminent colonic anions – mainly remain dissociated in the colonic lumen until absorbed either in exchange for bicarbonate via a SCFA/ HCO3− transport channel; by an active transport mechanism such as the sodium-coupled monocarboxylate transporter (SMCT1) or the monocarboxylate transporter isoform 1 (MCT1); or by diffusion in their lipid soluble form.27 The sodium-coupled monocarboxylate transporter facilitates the conservation of sodium, chloride, and water in the colon.27 Furthermore, the short-chain fatty acids are incorporated as the basic elements for mucin synthesis, lipogenesis, gluconeogenesis, and protein production. In particular, propionate combines with other three-carbon compounds in the liver to participate in gluconeogenesis. Acetate is used by the liver as a component to fashion longer-chain fatty acids and by the muscle as sustenance.23,24 Although the least abundant of the short-chain fatty acids, butyrate has the greatest import in colonic homeostasis. This short-chain fatty acid acts as the primary energy source for the colonocyte, supplying 70–90% of its energy requirements; these epithelial cells receive their nourishment solely from luminal substrates, not from the bloodstream.20,23,24 Of the short-chain fatty acids, butyrate best promotes the absorption of water, sodium, and chloride from the colon, acting as an antidiarrheal agent.27 This short-chain fatty acid also advances colonic cell proliferation and differentiation, repair, and immune function.11,27,35 Butyrate has been shown to influence colon carcinogenesis: studies have revealed that fewer butyrate transporters were present in human colonic adenocarcinomas, resulting in a decrease in the utilization of the trophic butyrate in the malignant cells.27 Moreover, in vitro studies of cancer cell lines identified apoptosis, nonproliferation, and differentiation after the administration of butyrate.27 One method by which butyrate modulates gene expression and, thus, cancer growth likely arises from its ability to suppress histone deacetylase, thus encouraging the union of various transcription factors with nuclear DNA; to modify intracellular kinase signaling; and to inhibit nuclear factor-kB.27 In the colon, many metabolic processes are influenced by functional food components. These foods – the pre- and probiotics – alter the colonic microenvironment, adding to the impact of environmental factors and genetics.33 As previously discussed, prebiotics – primarily nondigestable oligosaccharides (NDOs) – are slowly fermentable foods that selectively propagate microbial proliferation and/or ­activity.36 These products are completely metabolized in the

26

colon into short-chain fatty acids, energy, and lactic acid, leaving no nondigestable oligosaccharides in the stool.33 In contrast, probiotics represent active bacterial cultures that benefit the host by replenishing the colonic microenvironment.37 Synbiotics combine the action of pre- and probiotics.33 Investigations of these functional foods have focused on the lactobacilli and bifidobacteria, the growth of which transforms the colonic milieu, augmenting the immune function of the gut-associated lymphoid tissue (GALT).31,33 The effect of these supplements is thought to be attributable to an increased production of butyrate, changes in mucin production, or interference in the binding of pathogenic bacteria to the colonic mucosa.27,38 Prebiotics are particularly associated with an elevation of the concentration of short-chain fatty acids.33 To combat the rising incidence of antibiotic-resistant bacteria in hospitals, the World Health Organization has recommended the use of microbial interference therapies – nonpathogenic bacteria that eradicate pathogens – such as probiotics.34 Currently, probiotics are prescribed in cases of disturbed microbial balance, such as antibiotic-associated diarrhea. In the future, pre- and probiotics may become important supplements regularly administered to patients to promote health and to prevent illness. These functional foods further present the possibility of reducing the potential of carcinogens to form cancers.31,38 While the colon is one organ, it demonstrates regional differences. As noted, the proximal and distal colon have different embryological origins, derived from the mid- and hindgut, respectively. In appearance, the proximal colon is more saccular and the distal colon, more tubular.39 The short-chain fatty acids are principally synthesized in the more acidic environment of the proximal colon. The proximal colon serves as a reservoir, in contrast to the distal colon, which mainly performs as a conduit.40 Yet, this truism is ­disputed by studies in which radiopaque markers were determined to have the same dwell time of approximately 11 h in the proximal, distal, and middle colonic segments, suggesting that the proximal colon does not preferentially operate as a receptacle for stool.9 Also, the character of the luminal contents impacts transit times. Large volumes of liquid quickly pass through the ascending colon but remain within the transverse colon for as long as 20–40  h; in contrast, a solid meal is retained by the cecum and ascending colon for longer periods than a liquid diet.9,41,42 The salvage of water and electrolytes is primarily accorded to the proximal colon, although the distal colon and rectum contribute to this task, albeit to a lesser extent.43,44 In the event that a large amount of chyme is delivered to the colon, the distal colon and rectum may assist in the absorption of enough fluid to produce a solid stool.41 The regional heterogeneity of the colon exhibits adaptability. After a right hemicolectomy, the transverse colon adjusts to become a neo-proximal colon; 6 months after the surgery, the progress of an ingested isotope from the small intestine through the shortened colon returns to the preoperative baseline.45 The assumption of this role may

U.M. Szmulowicz and T.L. Hull

ensue due to the greater concentrations of short-chain fatty acids to which the transverse colon is exposed following a right hemicolectomy, producing such changes as an increase in water and sodium retention.30

Transport of Water and Electrolytes Among its roles in preserving intestinal homeostasis, the colon is also integral to water and electrolyte transport. The colon maintains an appropriate hydration and electrolyte balance by means of the absorption and secretion of intestinal water and electrolytes. The mucosal surface area available for these processes amounts to approximately 2,000 cm2.30 While the surface epithelial cells in the colon are primarily responsible for absorption, the crypt cells are involved in fluid secretion; however, crypt cells have been found to some degree to contribute to absorption.11,30,46 After the liquid effluent from the small intestine has traversed the colon, fluid and electrolyte absorption and secretion as well as bacterial activity produce about 200 g of solid feces per day.20 The colon is extremely efficient at conserving intestinal water.13 Water is passively absorbed along an osmotic gradient, enabled by a luminal sodium concentration lower than that of the epithelial cells; as such, the salvage of intestinal water relies upon sodium conservation.11,30 Normally, the colon is presented with 1.5–2 L of water daily, as compared to the 9–10 L that pass through the small intestine; of this amount, 1.5–2 L are ingested, with the remainder originating from salivary, biliary, pancreatic, and intestinal secretions.24,30 Approximately 90% of this water is reclaimed by the colon, leaving 100–150 mL in the feces.20 The absorption of water primarily follows a paracellular pathway, although a transcellular route involves various protein channels: aquaporins 3, 4, 5, 8, and 9.20 The ascending colon demonstrates the greatest absorptive capability, as the chyme resides within this segment the longest, thus maximizing its contact with the mucosa. As a consequence, diarrhea more consistently ensues after a right, as opposed to a left, hemicolectomy. Fluid retention is promoted by the antidiuretic hormone.43 When challenged, the proximal colon, with the additional contribution of the sigmoid colon and rectosigmoid, is able to save a further 5–6 L of intestinal water daily.47–49 Yet this facility is contingent upon the composition, rate of flow (less than 1–2 mL/min), and amount of the effluent. In the case that the absorptive capacity of the colon is exceeded, diarrhea results. Fluid secretion in the colon only transpires in the presence of diverse secretagogues, such as laxatives, bacterial endotoxins, hormones (e.g., VIP), and endogenous substances (e.g., bile acids).43 The colon is essential to the recovery of sodium. Under normal conditions, the colon principally absorbs sodium and chloride but secretes bicarbonate and potassium. The liquid chyme delivered to the colon contains 130–140 mmol/L of sodium whereas the concentration in stool is 40  mmol/L;

27

2. Colonic Physiology

approximately 95% of the sodium transported into the colon is conserved.29,43 If required, the colon is able to increase its salvage of sodium to 800 mmol/L/day.43 The normal colon can prevent hyponatremia even despite a diet containing as little as 1 mEq of sodium daily; in contrast, the absence of a colon encourages dehydration and hyponatremia.43 The transport mechanisms for sodium absorption, located on the luminal surface of the epithelial cells, vary throughout the colon: a Na+/H+ exchange channel in the proximal colon and an electrogenic sodium-specific channel (ENaC) in the distal colon and rectum.20 The Na+/H+ exchange channels (NHE 2 and 3) are coupled to the Cl−-HCO3− exchange channels.20 The activity of the electrogenic sodium-specific channel in the distal colon and rectum is requisite for the desiccation of stool.20 These two types of transport channel allow for the passive diffusion of sodium into the colonic epithelial cells along an electrochemical gradient, consisting of a low intracellular sodium concentration (