4-3 Colorectal Anatomic Correlates

James Doty and Eric G.Weiss

An understanding of the anorectal anatomy and its rela- The attachment forms a broad sheet, laterally extending tionship to the pelvic floor is essential to understanding the from the cephalad parametrium, which attaches the uterus pathophysiology of pelvic floor dysfunction and thus how to the sidewall, to the inferior paracolpium, which attaches to evaluate and manage its disorders. the vagina to the side wall at the level of the levators. The The pelvic floor is composed of the pelvic floor muscu- cephalic paracolpium is lengthy and attaches the vagina to lature, the fascia of the pelvic floor, the associated viscera the pelvic walls. More caudally the attachment is more that pass through, and the blood vessels and nerves that direct. It is this attachment that stretches the vagina supply these structures. The major components of the between the rectum and the bladder. Support of the pelvic floor from cephalic to caudal are the peritoneum, bladder is dependent on the attachment of the bladder to viscera, endopelvic fascia, levator ani muscles, and external the vagina posteriorly and the support of the vagina by the genital muscles. more caudal paracolpium. Similarly, the posterior vaginal wall and rectovaginal fascia form a barrier to the anterior bulging of the rectum and thus prevent formation of a Support of the Pelvic Floor rectocele. In the most distal vagina, the vaginal wall is attached directly to surrounding structures without a para- The pelvic organs when removed from the body are a limp colpium. Anteriorly, it is fused to the urethra, posteriorly and formless mass. Their shape and position in vivo are with the perineal body and laterally with the levator ani dependent on their various attachments to the bony skele- muscles. Damage to the upper supports of the vagina ton through the pelvic muscles and connective tissue. results in vaginal and uterine prolapse whereas damage to These tissues can have an active or passive role in pelvic the lower supports results in a cystocele and/or rectocele visceral support. The passive support structures are the formation. sacrum, coccyx, pubic rami, parietal fascia, endopelvic Posterior to the rectum is the mesorectum, which con- fascia, and levator tendons. The primary active support tains both blood vessels and lymphatics that supply and structures are the levator ani muscles. drain the rectum. This is loosely bound down the front of the sacrum and coccyx by connective tissue known as the Fascia and Ligaments of fascia propria. The lateral ligaments, which attach the rectum to the pelvic walls, are condensations of the fascia the Pelvic Floor propria and contain loose areolar tissue, nerves, and small blood vessels. Thus, the mesorectum can be mobilized by The walls and floor of the pelvis are lined by the parietal dissection in the “mesorectal plane” leaving the mesorec- endopelvic fascia, which continues on the internal organs tum invested in this thin layer of fascia. The sacrum and as visceral fascia and serves to attach the pelvic organs to coccyx are also covered in a thicker fascia, which extends the pelvic walls (Figure 4-3.1). Unlike fascia in the abdom- downward and forward, just superficial to the anococ- inal wall, which contains regularly arranged collagen cygeal ligament known as Waldeyer’s fascia. Anteriorly the bundles, this fascia has variable amounts of collagen, rectum is covered with a layer of visceral fascia that elastin, and fibrovascular elements. Much of the strength of extends from the anterior peritoneal reflection to the uro- this endopelvic fascia is derived from the walls of arteries genital diaphragm. This is Denonvilliers fascia and lies and veins that run within it. between the rectum and vagina (or prostate in men). In the female, on each side of the pelvis, the endopelvic Nerves important to bladder control and male sexual func- fascia connects the cervix and vagina to the pelvic wall. tion pass through this fascia. The hiatal ligament, originat- 89 90 Anatomic Correlates

ischiococcygeus (see Figure 4-2.4, Chapter 4-2). The ilio- and ischiococcygeus originate from the ischial spine and posterior obturator fascia and insert into the anococcygeal raphe, the coccyx, and the sacrum, forming a shelf on which the pelvic organs may lie. The pubococcygeus arises from the posterior pubis and anterior obturator and inserts into the anococcygeal raphe, the sacrum, and coccyx. Various muscle subdivisions have been assigned to the medial portion of the pubococcygeus depending on its attachments. These include puborectalis, pubovaginalis, and pubourethralis. The puborectalis originates from the pubis and inserts into the anococcygeal raphe. It is the medial and inferior portion of the pubococcygeus. The puborectalis is a U- shaped muscle that originates from the pubic bones and passes behind the rectum forming a sling. The puborectalis passes beside the vagina to which it is attached laterally (here named the pubovaginalis) and then passes posterior to the anorectal junction. It provides support for the rectum and indirect support for the vagina, bladder, and urethra by drawing these structures anterior toward the pubic bone. Indirect elevation of the anterior vaginal wall and urethrovesical neck is provided by the bulk of the pub- Figure 4-3.1. Fascial layers of the female posterior pelvic floor. (Reprinted from orectalis muscle as it draws the rectum and posterior Fundamentals of Anorectal Surgery, 2nd ed, DE Beck, SD Wexner, page 3, © 1996, with permission vaginal wall forward with contraction. The tonic contrac- from Elsevier) tion of the puborectalis closes the urogenital hiatus, con- tributes to the posterior curve of the vagina, and reduces ing from the pelvic fascia,surrounds the rectum and vagina pressure on the pelvic outlet. When its tone is lax, the uro- and maintains their patency during levator contraction. genital hiatus opens, the anorectal angle becomes obtuse, In addition to the bony support of the pelvis, there are and the levator plate sags. two true tendons. The arcus tendineus fascia pelvis (ATFP) The levator muscles maintain constant tone and, pro- and arcus tendineus levator ani (ATLA). These are dense vided they are functioning, the supportive ligaments and aggregations of connective tissue, predominantly collagen, fascia are under no tension. When the pelvic floor muscles that provide lateral passive pelvic support. These tendons relax or are damaged, the intraabdominal pressures are are condensations of the obturator and levator ani fascia. applied to the pelvic organs and ligaments. The ligaments The ATLA inserts anteriorly at the pubic rami and posteri- function well for short periods under this stress but will orly at the ischial spine. The ATFP lies medial to the ATLA stretch and weaken over time, eventually leading to organ at the anterior insertion of the pubic rami and inserts prolapse and problems with incontinence. posteriorly at the ischial spine. These tendons provide anchoring sites for the levators and the vagina and thus are key to the support of the pelvic floor. Rectal and Anal Muscles Muscles of the Pelvic Floor The rectal muscles, from mucosa to serosa are the muscu- laris mucosae, an inner circular layer followed by an outer and Perineum longitudinal layer (Figure 4-3.2). The inner muscular layer forms the rectal valves and transitions into the internal The pelvic diaphragm, composed of the levator muscles anal sphincter (IAS). The outer longitudinal layer extends and their fascia, form a muscular sheet through which the from the sigmoid colon where it envelops the circumfer- pelvic visceral structures (lower rectum and vagina) pass. ence but is thickest at the taenia coli. This muscle splays It functions to support the pelvic viscera and helps to and becomes confluent at the rectosigmoid junction maintain urinary and fecal continence. descending down the rectum to the anorectal junction. Fibers from this muscle descend into the intersphincteric groove where they splay out and may cross both the IAS Levator Ani and external anal sphincter (EAS) and ultimately insert on the perineal and perianal skin. Some of the fibers above the The levator ani muscular sling is composed of three anorectal junction insert into the perineal body and muscles: the pubococcygeus, the iliococcygeus, and the the coccyx. Colorectal Anatomic Correlates 91

the anal canal. The upper third of the rectum is peri- tonealized anteriorly and laterally, the middle third only anteriorly, and the lower third is retroperitoneal. The distal third of the rectum is related anteriorly to the vagina and uterus (prostate and seminal vesicles in men) and forms the rectouterine (or rectovesical) pouch or pouch of Douglas. The anterior peritoneal reflection lies approxi- mately 5 to 8cm from the perineal skin. The rectum has three folds, two on the left at 7 to 8cm and 12 and 13cm, and one on the right at 9 to 11cm. The middle valve in the rectum corresponds to the level of the anterior peri- toneal reflection. There are also three lateral curves. The upper and lower curves are convex to the left, and the middle is convex to the right. The rectum sits in a hollow anterior to the sacrum passing downward and posteriorly and then down and anteriorly to become the anal canal at the level of the pelvic floor. The anorectal ring (palpated as the puborectalis) is at the junction of the IAS and the levators.

Figure 4-3.2. Muscular anatomy of the anus. (Reprinted from Fundamentals of Anorectal Anal Canal Surgery, 2nd ed, DE Beck, SD Wexner, page 7, © 1996, with permission from Elsevier) The anal canal is approximately 4cm in length extending from the top of the EAS (or anorectal ring) to the anal The EAS is a cylindrical-shaped voluntary skeletal verge. This definition is clinical. Histologically, the anal muscle that lies outside the IAS. Proximally it abuts the canal mucosa extends from the anal verge to approximately puborectalis at the anorectal ring and extends distally 1cm above the dentate line. The anal canal, similar to its beyond the level of the ZAS. The muscle is attached poste- sphincters, is related anteriorly to the perineal body and riorly to the anococcygeal ligament and the perineal body the lower posterior part of the vagina. Posterior to the anteriorly. anal canal is the presacral fascia, the anococcygeal liga- ment, the anococcygeal raphe (an extension of the iliococ- cygeus), and the posterior extension of the puborectalis Perineal Muscles and external sphincter musculature, which inserts into the coccyx. The anal canal is separated from the urogenital organs by Skin from the buttock is continuous with the anal the perineum,which contains the perineal body (see Figure margin and continues to the lower border of the IAS. This 4-2.6, Chapter 4-2). The perineal body, from superficial to epithelium is keratinized stratified squamous with hair fol- deep, contains the superficial and deep transverse perineii licles, sweat glands, and sebaceous glands. Proximal to the muscles and the anterior extension of the external sphinc- level of the dentate line, the epithelium is nonkeratinized ter, which inserts upon the bulbocavernosus. Below the squamous with no dermal appendages. There is a transi- pelvic diaphragm is the triangular urogenital diaphragm. tion zone where squamous and columnar epithelium are It lies at the level of the hymenal ring and attaches to the mixed and then the columnar epithelium of the rectum urethra, vagina, and perineal body and to the ischiopubic predominates. Vertical mucosal folds known as anal rami. Just above the urogenital diaphragm membrane are columns are found at the upper anal canal just above the compressor muscles for the urethra and vagina. dentate line. Anal valves connect these folds at the inferior margins. Above each valve is the anal pit or sinus, which drains on average eight anal glands. Pelvic Viscera The Rectum Blood Supply

The rectum is identified at the level of the sacral promon- Arterial Supply tory, distinguished by loss of complete peritoneal covering, absence of appendiceal epiploicae, absence of a true meso- The superior and inferior hemorrhoidal arteries supply colon, and divergence of the three taenia coli. The rectum the rectum and anal canal (Figure 4-3.3). The superior is approximately 15 to 20cm in length and ends distally at hemorrhoidal artery is a continuation of the inferior 92 Anatomic Correlates Innervation Levator Ani

Motor innervation of the levator muscles is from the 2nd, 3rd, and 4th sacral roots from above the muscle and the pudendal nerve from below. Controversial in the literature, the puborectalis is reported to be innervated by the 4th sacral root, the pudendal, or both. Some authors suggest that the puborectalis belongs more to the external sphinc- ter than to the levator muscle group.

External Sphincter

The EAS is a voluntary muscle innervated by the inferior rectal branch of the pudendal nerve. The pudendal nerve arises from sacral roots 2–4, leaves the pelvis through the greater sciatic foramen, crosses the ischial spine, and con- tinues in the pudendal canal. This nerve innervates the EAS, as well as the penis and clitoris.

Internal Sphincter

Sympathetic innervation is derived from the hypogastric and pelvic plexus. Parasympathetic innervation is from sacral roots 1–3 via the pelvic plexus. Sympathetic tone is Figure 4-3.3. Anorectal vascular anatomy. (Reprinted from Fundamentals of Anorectal believed to be excitatory, but it is unclear if parasympa- Surgery, 2nd ed, DE Beck, SD Wexner, page 6, © 1996, with permission from Elsevier) thetic is inhibitory or excitatory.

Anal Canal mesenteric artery. The inferior hemorrhoidal artery is a The anal canal’s sensation to touch, sharp, and temperature branch of the pudendal artery, which in turn is a branch is present from the anal verge to about 1cm above the from the internal iliac artery. The inferior hemorrhoidal dentate line. This sensation is mediated by the inferior artery, after branching off the pudendal, crosses the rectal branch of the pudendal nerve, which is derived from ischiorectal fossa and goes through the EAS to enter the sacral roots 2–4. This branch of the pudendal and the per- submucosa of the anal canal and ascend in this plane. ineal nerve are the two pudendal branches that arise within The middle hemorrhoidal artery may or may not be Alcock’s canal in the ischiorectal fossa. present depending on the size of the superior hemor- rhoidal artery. It originates from the internal iliac or prox- Rectum imal pudendal and goes anterolateral at the level of the levators. It can be injured at this level when dissecting the The rectum is only sensitive to distension. This sensation upper vagina from the rectum. is thought to be a combination of “sensors” in the rectal wall as well as surrounding structures such as the pelvic floor muscles and pelvic fascia. Rectal sensation is para- Venous Drainage sympathetic through the pelvic plexus sacral roots 2–4.

The three venous plexuses drain through their respective veins. The inferior hemorrhoid plexus drains via the infe- Sympathetic and Parasympathetic Innervation rior rectal (or hemorrhoidal) vein into the pudendal vein to the Pelvis and then into the internal iliac vein. The internal hemor- rhoid plexus above the dentate line drains into the middle Sympathetic innervation originates from lumbar roots L1 rectal vein. The perirectal plexus drains into the single to L3. These fibers pass through the sympathetic ganglion superior rectal vein. and leave as lumbar sympathetic nerves that join the Colorectal Anatomic Correlates 93 preaortic plexus. This extends caudally as the mesenteric The ischiorectal space is lateral to the perianal space. Its plexus and then the superior hypogastric plexus. This apex is formed by the joining of the levator and obturator plexus is formed both by the lumbar sympathetic nerves muscles while the inferior boundary is the skin of the per- through the plexus and separately by lumbar splanchnic ineum. Anteriorly, it is bound by the transverse perineal nerves (from the sympathetic chains). The plexus then muscles and posteriorly by the gluteal skin. This space con- divides into the two hypogastric nerves descending along tains the pudendal vessels and pudendal nerve in Alcock’s the iliacs until joined by the parasympathetic nervi eri- canal. Alcock’s canal, or the pudendal canal, is on the gentes to form the pelvic or inferior hypogastric plexus. medial side of the obturator internus in the ischiorectal Parasympathetic nerve fibers arise from the sacral nerve fossa. roots and emerge as pelvic splanchnic nerve or nervi eri- The intersphincteric space is between the anal sphincters gentes, which join the sympathetic nerve to form a pair of and communicates with the perianal space. Most of the inferior hypogastric (or pelvic) plexuses on each of the anal glands are in the intersphincteric space. pelvic side walls. Fibers from this pelvic plexus (sympa- The superficial postanal space connects the perianal thetic and parasympathetic) then branch out and innervate spaces with each other below the level of the anococcygeal the bladder, ureters, corpora cavernosa, and rectum. ligament. The deep postanal space, the right and left Somatic fibers also pass through the pelvic plexus to inner- ischioanal/rectal spaces communicate with this space pos- vate the levators and ureteral striated muscles. These pelvic teriorly deep to the anococcygeal ligament.Infection in this plexuses run anterolateral to the rectum necessitating close space leads to a horseshoe abscess. rectal dissection to avoid nerve injury.

Anorectal Spaces References

Knowledge of the anorectal spaces is important in the 1. Aigner F, Zbar AP,Ludwikowski B, Kreczy A, Kovacs P,Fritsch H. The understanding of anorectal and pelvic floor disorders and rectogenital septum: morphology,function,and clinical relevance.Dis to avoid injury to important structures during surgical Colon Rectum 2004;47(2):131–140. therapy. 2. Beck DE,Wexner SD, eds. Fundamentals of Anorectal Surgery. 2nd ed. London: WB Saunders; 1998. The perianal space is that area surrounding the anal 3. Corman ML, ed. Colon and Rectal Surgery. 4th ed. Philadelphia: canal in the immediate area of the anal verge. Laterally it Lippincott-Raven; 1998. is continuous with the fat of the buttocks. Cephalically, it is 4. Gordon PH, Nivatvongs S, eds. Principles and Practice of Surgery for continuous with the intersphincteric space and it contains the Colon, Rectum and Anus. St. Louis: Quality Medical-Publishing; the distal EAS, branches of the inferior rectal vessels, 1992. 5. Pemberton JH, Swash M, Henry MM, eds. The Pelvic Floor: Its Func- nerves, and lymphatics. The external hemorrhoid plexus tion and Disorders. London: WB Saunders; 2002. lies in the perianal space and communicates with the inter- 6. Strohbehn K. Urogynecology and pelvic floor dysfunction. Obstet nal hemorrhoid plexus at the dentate line. Gynecol Clin 1998;25(4):684.