Anorectal Anatomy

Surg Clin N Am 82 (2002) 1125–1138

Anorectal anatomy Andreas M. Kaiser, MD*, Adrian E. Ortega, MD Division of Colon and Rectal Surgery, Department of Surgery, Keck School of Medicine, University of Southern California, 450 San Pablo Street, Suite 5400, Los Angeles, CA 90033, USA

Anatomic landmarks and epithelia The anorectum is the terminal portion of the gastrointestinal tract. It is embedded in the osseous pelvis and surrounded by urogenital organs as well as muscular, ligamentous, and connective tissue structures. It is the functional unit that maintains fecal continence by providing both a stopperequipped reservoir and a controlled expulsion mechanism for feces. The rectum is the last and partially extraperitoneal segment of the large intestine. It starts at the rectosigmoid junction and continues through the pelvic floor into the anal canal. The nonmobilized rectum is characterized by three distinct endoluminal curves. The resulting folds that are seen on endoscopy are referred to as the valves of Houston. Definitions of where the sigmoid colon ends and the rectum begins include: (1) a distance of fifteen centimeters above the anal verge, (2) the position of the peritoneal reflection, and (3) the level of the sacral promontory. We maintain that the most useful landmark from a functional as well as surgical viewpoint is the confluence of the teniae coli at the rectosigmoid junction [1]. Because this anatomic reference point cannot be visualized endoscopically, the National Cancer Institute has recently defined the rectum as the last twelve centimeters above the anal verge for the purpose of uniformity in clinical trials [2]. This definition is useful in preparing for a low anterior resection versus a sigmoid resection, particularly when measured with a rigid sigmoidoscope. Definitions of the anal canal also vary among surgeons and anatomists [3]. The surgical anal canal is approximately 4 cm long and extends from the anal verge to the anorectal ring, which is defined as the proximal level of the levator-external anal sphincter complex (Fig. 1) [4]. This clinical description correlates with either a digital or a sonographic examination but does not * Division of Colon and Rectal Surgery, Department of Surgery, Keck School of Medicine, University of Southern California, 1450 San Pablo Street, Suite 5400, Los Angeles, CA 90033. E-mail address: [email protected] (A.M. Kaiser). 0039-6109/02/$ - see front matter Ó 2002, Elsevier Science (USA). All rights reserved. PII: S 0 0 3 9 - 6 1 0 9 ( 0 2 ) 0 0 0 5 6 - 7

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Fig. 1. The surgical anal canal is depicted in terms of the components of the sphincter mechanism, the dentate line, anoderm, and transitional zone.

correspond to the histologic architecture along the canal. As the rectum narrows into the anal canal, the smooth mucosal lining changes into a plicated appearance. The columns of Morgagni represent longitudinal folds, which alternate with pockets [5]. The bases of the columns form the anal valves, creating an undulating demarcation line referred to as the dentate line. It translates into a distance of about 2 cm from the anal verge. The dentate line marks the point of embryologic fusion and morphologic transition from endodermal (intestinal) to ectodermal (skin) tissue. This transition is important for the understanding of the differences in the linings, innervation, arterial and venous blood supply, and the lymphatic drainage of the anal canal. The bottom of the anal columns represents the origin of the cryptoglandular complex [6]. Four to eight anal glands empty into the anal canal via anal ducts at each crypt. Anoderm covers the last 1 cm to 1.5 cm of the distal anal canal below the dentate line and consists of modified squamous epithelium. The intersphincteric groove between the internal and external anal sphincter can be felt on clinical examination at the same level. On external inspection, the normal anus appears to be a virtual orifice. As a result of the tonic circumferential sphincter contraction, it remains closed at rest. The normal position of the anus is in the midline, approximately six tenths of the distance from the coccyx to the vagina in females or to the beginning of the scrotal raphe in males. An ectopic anus is typically anteriorly displaced and has a reduced perineal body [7]. The lining of the anorectum consists of three types of epithelium at different levels [8]. It is characterized by a gradual transition from the colonic-

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type intestinal mucosa with its unbranched columnar crypts, to the skin-type squamous cell epithelium [9]. The anal transition zone (ATZ) above the dentate line combines columnar, transitional-cuboidal, and squamous epithelium [5,8,10,11]. It is also referred to as the cloacogenic zone and is important for the classification of neoplasms. The anoderm is located between the dentate line and the anal verge and is characterized by a squamous epithelium that lacks skin adnexal tissues. Outside the anal verge, the skin texture is arranged in radiating folds around the anus. The skin lining becomes thicker, pigmented, and contains hair follicles and glands. This region is referred to as the anal margin. The teleological function of the cryptoglandular complexes (crypt, duct, and anal gland) is unknown. They vary in number between four and eight and are composed of apocrine glandular elements and anal ducts [12]. The glands are typically located in the intersphincteric space traversing the internal anal sphincter. Less frequently, glands may even extend into the external anal sphincter [6]. Because the anal ducts form a one-way conduit for contamination of the perianal and perirectal tissues, their anatomy forms the bases for cryptoglandular infections and their sequelae, ie, fistula in ano (Fig. 2).

Fig. 2. Four to eight anal glands empty into the anal canal at the base of the anal columns. Most anal glands penetrate the internal anal sphincter to varying degrees but can also traverse the external sphincter. Cryptoglandular infections may follow a path of least resistance to five potential spaces: perianal, ischioanal, intersphincteric, submucosal, and supralevator compartments.

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Muscles, fascias, and spaces Muscles Muscular structures within the pelvis can be divided into three categories: (1) muscles that line the sidewalls of the osseous pelvis, (2) muscles of the pelvic floor, and (3) muscles of the anal sphincter complex. The obturator internus and the piriformis muscle form the external boundary of the pelvis. Neither one is of importance with regard to anorectal diseases except that they provide an open communication for pelvic infections to reach extrapelvic tissues. In particular, the posterior midline cryptoglandular complex can produce infection in the deep postanal space. From this position, infection can track along the obturator internus fascia to reach the ischioanal space, unilaterally or bilaterally. The pelvic floor (pelvic diaphragm) is a funnel-shaped musculotendineous termination of the pelvic outlet [13–16]. It is innervated by branches of the ventral primary rami of spinal nerves S3–S4. The pelvic floor supports the abdominal and pelvic organs, but allows the anorectal and urogenital viscera to pass through via two hiatal openings [15]. The levator ani muscles form a symmetrical array of paired striated muscles that originate from the continuous arcus tendineus of the obturator fascia. The latter fascial structure extends anteroposteriorly from the pubic bone to the ischial spine at the level S3-S4. Separate units of the levator ani complex are identified as the ischiococcygeus, iliococcygeus, pubococcygeous muscle, and puborectalis muscle. The anococcygeal raphe is a fibrous condensation of the iliococcygeus muscle in the posterior midline and contains fibers that cross over from one side to the other (Fig. 3). The puborectalis muscle is the most medial portion of the levator ani complex. It is seated cephalad to the deep component of the external anal sphincter muscle. Like the external anal sphincter, the puborectalis muscle is innervated by the inferior rectal nerve, a branch of the pudendal nerve. Because of its synergistic function, proximity, and shared innervation with the external anal sphincter, it has been questioned whether the puborectalis muscle should be considered more a subunit of the sphincter complex rather than of the levator ani [17]. In effect, the puborectalis serves both functions as part of the sphincter mechanism and levator floor. The puborectalis muscle is a strong, U-shaped sling of striated muscle that pulls the anorectal junction anteriorly to the posterior aspect of the pubis [4]. The resulting effect is an angulation between rectum and anal canal (anorectal angle) [18]. Between the two symmetric limbs of the pubococcygeus muscles, an elliptic space remains open in the anterior midline. This so-called levator hiatus allows the rectum, vagina, urethra, and the dorsal vein of the penis to pass through the pelvic diaphragm [4,19]. The puborectalis ‘‘sling’’ relaxes during defecation, thereby widening the anorectal angle and straightening the rectum. Contraction at the rest of the levator ani complex elevates

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Fig. 3. The pelvic diaphragm supports the urogenital organs and the anorectum, exiting the pelvis through their respective openings. The levator muscles converge from the pelvic sidewalls to the anococcygeal raphe in the midline. The anococcygeal ligament supports the sphincter mechanism posteriorly and defines the superficial and deep pos-anal spaces. The inferior pudendal nerves and vessels exit the pelvis via the pudendal canal to terminate on the anal sphincter as the interior rectal nerves and vesels.

the pelvic floor and leads to a widening of the levator hiatus. The perineal body anterior to the anus is formed by the superficial and deep transverse perinei muscles, as well as some fibers of the external sphincter muscle that fuse with the bulbocavernosus muscle in a tendinous intersection in support of the pelvic floor [20]. The anal sphincter complex consists of the internal and the external sphincter muscles. Even though they form a unit, they are distinct in both structure and function. The internal anal sphincter (IAS) is a specialized smooth muscle condensation in continuation of the circular muscles of the rectum [4,21]. On endorectal ultrasound, it appears as a uniform

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circumferential hypoechogenic ring of 2 mm to 3 mm in thickness. The IAS is innervated by autonomic sympathetic and parasympathetic nerves. It remains in a state of continuous contraction and accounts for 50% to 85% of the resting anal tone [4]. In contrast, the external anal sphincter (EAS) consists of striated skeletal muscle, which is innervated by the inferior rectal branch of the pudendal nerve. The EAS forms a muscular cylinder that surrounds the anal canal, including the internal sphincter muscle, on its entire length [4,22]. The distal interface between the IAS and the EAS forms the intersphincteric groove, which can be palpated approximately 1 cm below to the dentate line. The classic anatomic viewpoint divides the EAS into subcutaneous, superficial, and deep components [23]. From a surgical viewpoint, these distinctions are less relevant. The anococcygeal ligament is a dense connective tissue structure that attaches the superficial portion of the EAS to the tip of the coccyx. The deep portion of the external sphincter does not have posterior attachments. Its fibers are intimately related to the puborectalis muscle, and insert anteriorly into the perineal body [24,25]. On endorectal ultrasound, the EAS appears as a hyperechogenic structure [26]. In contrast with other skeletal muscles, the EAS maintains an unconscious resting electrical tone through a reflex arc at the cauda equina level, and thus generates 25% to 30% of the resting anal sphincter tone [4]. Reflexive or voluntary contraction of the EAS/puborectalis complement each other to prevent fecal leakage. An active contraction can only be sustained for 30 to 60 seconds, however [4]. The longitudinal muscle of the rectum fuses with striated fibers of the levator ani and puborectalis muscles at the level of the anorectal ring to form the conjoined longitudinal muscle [27]. The fibers descend between the internal and external anal sphincters. Continuation of these fibers through the lower portion of the external sphincter forms the corrugator cutis ani, which inserts in the perianal skin [28].

Fascial structures and spaces Defined planes of tense fibrous tissue delineate several virtual compartments in the pelvis. These spaces may be filled with adipose and fine areolar connective tissue as well as blood vessels, nerves, and lymphatics. The fascial planes are important anatomic landmarks because they define the course of the surgical dissection [29]. They also represent routes for extension of diseases—abscesses in particular. At the level of the rectum, the fascial planes are important barriers between compartments in which rectal neoplasms primarily extend. Knowledge of the fascial landmarks is crucial for the complete excision of rectal tumors [29]. Damage to these natural planes results in a loss of the compartmental integrity and allows cancer cells to remain unexcised or contaminate the operative field, thus increasing the risk of recurrence [30].

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The pelvis is invested by the endopelvic fascia, which has two components: a visceral and a parietal layer (Fig. 4). The visceral layer of the endopelvic fascia (fascia propria of the rectum) lines the rectum. It is a thin, transparent layer that maintains the integrity of the mesorectum. The parietal layer of the endopelvic fascia (presacral fascia) covers the sacrum. Violation of this layer exposes the sacral veins and represents a potential source of bleeding during the mobilization of the rectum [29]. Upon opening the peritoneal reflection at the level of the sacral promontory, fine areolar tissue can be appreciated between the anterior surface of the parietal layer and the posterior surface of the visceral layer of the endopelvic fascia. The two layers fuse a few centimeters above the coccyx to form Waldeyer’s fascia [31]. Complete mobilization of the rectum to the level of the levator floor requires division of Waldeyer’s fascia behind the rectum. Laterally, the rectum is bounded by the hypogastric and pelvic nerves and plexi, in addition to the hypogastric blood vessels medial to the pelvic sidewalls [32]. Anteriorly, Denonvillier’s fascia is interposed between the bladder and rectum. It can be thought of as a virtual space rather than a distinct fascial structure. Below the peritoneal reflection, the anterior rectum

Fig. 4. A sagital view of the anal canal and rectum is depicted. The confluence of the teniae coli define the upper rectum. The fascial planes serve as the basis for guiding surgical dissection and understanding potential routes for the spread of infectious processes.

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is separated from bladder, prostate, and seminal vesicles or vagina by neurovascular bundles originating in the pelvic plexus and hypogastric circulation. The levator ani divides the pelvis into the supralevator space, situated between the peritoneum and the pelvic diaphragm, and the extrapelvic infralevator spaces. The two compartments are almost completely separate from one another. The supralevator space can communicate with the ischioanal space via the fascia of the obturator internus, medial to the ischial spine. Therefore, the fascia of the obturator internus may serve as a conduit for supralevator infections into extrapelvic sites. Below the levator ani, there are the ischioanal, perianal, intersphincteric, submucous, and the superficial and deep postanal spaces. The term ‘‘ischiorectal fossa’’ is used synonymously with ischioanal fossa, but this name is misleading because the rectum is not part of its boundaries. On cross section, the ischioanal fossa appears to have a pyramid shape [33], but actually it extends around the anal canal and is bounded anteriorly by the urogenital diaphragm as well as the transversus perinei muscle. Furthermore, it is defined superiorly by the fascia of the levator ani muscle, and medially by the external anal sphincter complex at the level of the anal canal. The lateral border is formed by the obturator fascia, and inferiorly a thin transverse fascia separates the ischioanal fossa from the perianal space. Apart from fat, the ischioanal fossa contains neurovascular structures, including the pudendal nerve and the internal pudendal vessels, which enter through the pudendal (Alcock’s) canal [34]. Located behind the anal canal are the superficial and the deep postanal spaces of Courtney. Both communicate on either side with the ischioanal fossa, thus providing the route for the formation of a horseshoe abscess [35]. The superficial postanal space is situated between the anococcygeal ligament and the skin. The deep postanal space is bounded inferiorly by the anococcygeal ligament and superiorly by the anococcygeal raphe. The intersphincteric space is located between the internal and external sphincter muscles [4,35]. It is of significance because most of the anal glands are found in this location. Distally, the intersphincteric space communicates with the perianal space, which surrounds the lower part of the anal canal and extends laterally to the subcutaneous fat of the buttocks. Within the perianal space are the external hemorrhoidal plexus and the internal hemorrhoidal plexus, which communicate at the dentate line. The perianal space also contains the most distal part of the EAS and IAS, as well as the fibers of the corrugator ani muscle [4,35]. By acting like septa, the latter divides the perianal space into compact and inelastic subcompartments. Because perianal hematomas or abscesses have little room to expand in this relatively confined space, they cause a rapid increase of pressure as the basis for rather dramatic pain in the region [4,35]. The submucosal space between the IAS and the rectal mucosal lining starts at the dentate and continues into the submucosal layer of the distal rectum [4,35]. The important structures in this space are the internal hemorrhoidal plexus and the muscularis mucosa.

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Innervation, vascular supply, lymphatic drainage Innervation The colon and the rectum are innervated by sympathetic and parasympathetic nerves, whereas the external anal sphincter and the lining of the anal canal are supplied by somatic nerves (Fig. 5). Pelvirectal autonomic neuroanatomy consists of two important sets of nerves—the hypogastric and pelvic neural complexes [32,36]. Postganglionic fibers of the thoracolumbar sympathetic ganglia course retroperitoneally just anterior to the abdominal aorta. These fibers are known as the inferior mesenteric nerve at the level of

Fig. 5. Pelvirectal autonomic neuroanatomy consists of sympathetic inflow from the hypogastric nerves, parasympathetic input from S2–S4, and extrapelvic somatic innervation to the sphincters and perineal structures also from S2–S4.

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the distal aorta, near the origin of the artery with the same name. The inferior mesenteric nerve bifurcates into two hypogastric nerves at the level of the aortic bifurcation. The inferior mesenteric and hypogastric nerves are most readily identified in the presacral space upon opening the peritoneal reflection and gently tracing them back to the origin of the inferior mesenteric artery. Care should be taken to avoid injury to these neural complexes in the course of division of the superior rectal or inferior mesenteric arteries. The hypogastric nerves follow a course behind the rectums toward the lateral pelvic sidewalls into each hypogastric region, where they become enmeshed with the pelvic plexi. They continue anterolaterally to the prostate, seminal vesicles, and urethra. Parasympathetic visceral preganglionic fibers arise from the center of the sacrum (ie, S2, S3, S4). Upon exiting the sacral foramina bilaterally, they form the pelvic plexus of nerves—also known as the nervi errigentes. Each pelvic plexus is located near the root of the middle rectal artery on the medial side of the hypogastric blood vessels. The pelvic nerves are joined by the hypogastric nerves, forming the pelvic plexi. This combination of neural pathways terminates on the rectum, bladder, seminal vesicles, and urethra. Ejaculatory function depends on the sympathetic component, whereas erectile function and bladder emptying depend on parasympathetic input from the pelvic plexus and nerves. It is also important to note that the true hindgut receives its parasympathetic input in a retrograde fashion via pelvic nerves, plexus, hypogastric nerves, and ultimately the thoracolumbar plexuses. The pudendal nerve arises from S2–S4 [37]. It becomes extrapelvic by passing through the pudendal canal (Alcock’s canal) formed by fascia on the medial surface of the obturator internus muscle to the ischiorectal fossa [34,38]. There, it gives rise to the inferior rectal nerve, later the perineal nerve, and continues as the dorsal nerve of the penis or clitoris. Motor innervation to the pelvic floor muscles is provided on its superior, pelvic surface by the sacral roots (S2–S4), and on its inferior surface by the perineal branch of the pudendal nerve. The puborectalis muscle receives fibers from the inferior rectal nerves. The innervation to the external anal sphincter comes from the inferior rectal branch of the pudendal nerve (S2, S3), and from the perineal branch of S4. As stated previously, the internal anal sphincter is the extension of the circular smooth muscle of the rectum [21] Hence, it shares the same innervation as the rectum: sympathetic (L5) and parasympathetic nerves (S2–S4). Sensory innervation to the anal canal has a demarcation line between the dentate line and approximately 0.3 cm to 1.5 cm above that point [35]. The rectum proximal to this reference point is only sensitive to distension, which is conveyed in afferent fibers along parasympathetic nerves and the pelvic plexus to S2–S4. Anal sensation is considered to play a role in the anal continence mechanism and includes sensations to touch, pinprick, heat, and cold. They are transmitted by the inferior rectal branch of the pudendal nerve [4].

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Arterial and venous blood supply The blood supply to the rectum is supplied by two sources: the superior and middle rectal arteries. The superior rectal artery (superior hemorrhoidal artery) is the terminal branch of the inferior mesenteric artery. It forms a rich reticular anastomotic network in the rectal submucosa with the middle rectal artery (middle hemorrhoidal artery). This abundant interconnecting network of dual blood supplies gives the rectal mucosa its distinct reticular mucosal vascular pattern appreciated in hindgut endoscopy. The middle rectal arteries originate from the hypogastric (internal iliac) or the inferior vesicle arteries. They course from the lateral pelvic sidewalls to reach the distal rectum bilaterally above the level of the levator muscles (pubococcygeus, iliococcygeus). The middle rectal arteries run a lateral-to-medial course in the pelvis, intertwined with the nervi errigentes. Some surgical anatomists have emphasized the median sacral artery. It arises from the posterior surface of the aorta and descends behind the rectum, reaching it at the level of the tip of the coccyx. On rare occasions, the blood vessel can be the source of bleeding in the posterior mobilization of the rectum. The venous drainage of the rectum follows the arterial anatomy. The inferior and middle hemorrhoidal (rectal) veins ultimately drain into the inferior vena cava via the internal pudendal and hypogastric veins. The superior rectal vein drains into the portal circulation via the inferior mesenteric vein. The anorectum receives its major blood supply from the superior and inferior hemorrhoidal arteries and to a lesser degree from the middle hemorrhoidal artery, forming a wide intramural network of collaterals [39]. The internal iliac artery gives off a branch to form the extrapelvic pudendal artery. It accompanies the pudendal nerve through the pudendal canal to continue as the inferior hemorrhoidal artery. The venous blood from the anorectum collects in the arteriovenous plexuses. It drains through the bilateral middle and inferior hemorrhoidal veins and the single superior hemorrhoidal vein, to the internal iliac vein, and ultimately is a tributary to the inferior vena cava. The external hemorrhoidal plexus, the correlate for external hemorrhoids, is located in the perianal space below the dentate line. It communicates with the internal hemorrhoidal plexus—the basis for internal hemorrhoids. It is found in the submucosal space above the dentate line at the level of the upper anal canal. Lymphatic drainage Both the rectum and the anal canal have a rich network of lymphatic plexuses that drain into an extramural system of lymph channels and nodes [40]. The dentate line represents the interface between the two different systems of lymphatic drainage. Above the dentate line, lymph drains to the inferior mesenteric and internal iliac nodes; below the dentate line, lymph

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drainage proceeds to the superficial inguinal lymph nodes. Lymph from the upper two thirds of the rectum drains exclusively to the inferior mesenteric and para-aortic nodes, whereas the lower third of the rectum drains in two directions [40]. One is along the superior hemorrhoidal and inferior mesenteric arteries; the other follows the middle hemorrhoidal vessels laterally to the internal iliac lymph nodes. Clinical focus The evolution of surgical techniques in the treatment of rectal neoplasms requires increasingly precise clinical physical diagnosis. Descriptions of lesions as a distance from the anal verge are inherently imprecise because the length of the anal canal is quite variable. The exception to this precept is evaluation of lesions using rigid sigmoidoscopy. Those lesions measured within 15 cm from the anal verge can be considered rectal, and therefore amenable to neoadjuvant treatment prior to surgery. The challenge in physical diagnosis of rectal neoplasms is in describing mid and distal rectal cancers in relation to the sphincter mechanism. There are two key reference points. Lesions should be described posterolaterally in relation to the top of the puborectalis muscle. At its upper edge, the examining finger will ‘‘fall’’ into the rectal vault at the top edge of the puborectalis muscle. If a reasonable distance is palpated between the lower edge of a rectal cancer (2–3 cm) and the top of the puborectalis muscle, sphincter preservation is generally possible. Because the puborectalis is a U-shaped sling around the anorectal junction, it is not useful in describing tumors in the anterior sector. Assessment of the top of the external sphincter is generally not difficult in females. Similar criteria can be applied with respect to the feasibility of sphincter preservation. In males, the prostate gland serves as a useful reference point in the anterior quadrant. Lesions located above the mid-prostate are good candidates for sphincter-preserving surgery. Those located below the midportion of the prostate gland will almost invariably require abdominoperineal resection. We suggest that the description of distal and mid-rectal tumors should be based on their relationship to these structures. References [1] Stoss F. Investigations of the muscular architecture of the rectosigmoid junction in humans. Dis Colon Rectum 1990;33:378–83. [2] Nelson H, Petrelli N, Carlin A, et al. Guidelines 2000 for colon and rectal cancer surgery. J Natl Cancer Inst 2001;93:583–96. [3] Wendell-Smith CP. Anorectal nomenclature: fundamental terminology. Dis Colon Rectum 2000;43:1349–58. [4] Wexner SD, Jorge JMN. Anatomy and embryology of the anus, rectum, and colon. In: Corman ML, editor. Colon and rectal surgery. Philadelphia: Lippincott-Raven; 1998. p. 1–26.

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