Anorectal Anatomy

Anorectal anatomy

Andreas M. Kaiser, MD*, Adrian E. Ortega, MDDivision 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 func-

tional unit that maintains fecal continence by providing both a stopper-

equipped 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 floorinto the anal canal. The nonmobilized rectum is characterized by three dis-

tinct 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 pointcannot 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 thelevator-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 � 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

Surg Clin N Am 82 (2002) 1125–1138

correspond to the histologic architecture along the canal. As the rectum nar-

rows into the anal canal, the smooth mucosal lining changes into a plicated

appearance. The columns of Morgagni represent longitudinal folds, whichalternate 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 impor-

tant for the understanding of the differences in the linings, innervation, arte-

rial and venous blood supply, and the lymphatic drainage of the anal canal.

The bottom of the anal columns represents the origin of the cryptoglandularcomplex [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 sixtenths 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 ante-

riorly displaced and has a reduced perineal body [7].

The lining of the anorectum consists of three types of epithelium at differ-

ent levels [8]. It is characterized by a gradual transition from the colonic-

Fig. 1. The surgical anal canal is depicted in terms of the components of the sphincter

mechanism, the dentate line, anoderm, and transitional zone.

1126 A.M. Kaiser, A.E. Ortega / Surg Clin N Am 82 (2002) 1125–1138

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 squamousepithelium [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 squa-

mous 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 inter-

nal 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.

1127A.M. Kaiser, A.E. Ortega / Surg Clin N Am 82 (2002) 1125–1138

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 thatthey provide an open communication for pelvic infections to reach extra-

pelvic tissues. In particular, the posterior midline cryptoglandular complex

can produce infection in the deep postanal space. From this position, infec-

tion can track along the obturator internus fascia to reach the ischioanal

space, unilaterally or bilaterally.

The pelvic floor (pelvic diaphragm) is a funnel-shaped musculotendine-

ous 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 supportsthe 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 struc-

ture 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 iliococcy-geus 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 puborectalismuscle 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 levatorhiatus 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 straight-

ening the rectum. Contraction at the rest of the levator ani complex elevates


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 speciali-

zed smooth muscle condensation in continuation of the circular muscles

of the rectum [4,21]. On endorectal ultrasound, it appears as a uniform

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.


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 sur-

rounds 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 subcuta-

neous, 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 pubo-

rectalis muscle, and insert anteriorly into the perineal body [24,25]. On endo-

rectal ultrasound, the EAS appears as a hyperechogenic structure [26]. In

contrast with other skeletal muscles, the EAS maintains an unconscious rest-

ing electrical tone through a reflex arc at the cauda equina level, and thusgenerates 25% to 30% of the resting anal sphincter tone [4]. Reflexive or vol-

untary contraction of the EAS/puborectalis complement each other to pre-

vent 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 throughthe 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 compart-

ments in the pelvis. These spaces may be filled with adipose and fine areolarconnective 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 dis-

eases—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 toremain unexcised or contaminate the operative field, thus increasing the risk

of recurrence [30].


The pelvis is invested by the endopelvic fascia, which has two compo-

nents: a visceral and a parietal layer (Fig. 4). The visceral layer of the endo-

pelvic fascia (fascia propria of the rectum) lines the rectum. It is a thin,transparent layer that maintains the integrity of the mesorectum. The pari-

etal layer of the endopelvic fascia (presacral fascia) covers the sacrum. Vio-

lation 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.


is separated from bladder, prostate, and seminal vesicles or vagina by neuro-

vascular bundles originating in the pelvic plexus and hypogastric circulation.

The levator ani divides the pelvis into the supralevator space, situatedbetween the peritoneum and the pelvic diaphragm, and the extrapelvic infra-

levator 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 synonymouslywith 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 fas-

cia, and inferiorly a thin transverse fascia separates the ischioanal fossa fromthe perianal space. Apart from fat, the ischioanal fossa contains neurovas-

cular 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 super-

ficial postanal space is situated between the anococcygeal ligament and the

skin. The deep postanal space is bounded inferiorly by the anococcygealligament 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 hemor-

rhoidal plexus, which communicate at the dentate line. The perianal spacealso 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 peri-

anal 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 distalrectum [4,35]. The important structures in this space are the internal hemor-

rhoidal plexus and the muscularis mucosa.


Innervation, vascular supply, lymphatic drainage

Innervation

The colon and the rectum are innervated by sympathetic and parasympa-

thetic nerves, whereas the external anal sphincter and the lining of the anal

canal are supplied by somatic nerves (Fig. 5). Pelvirectal autonomic neuro-

anatomy consists of two important sets of nerves—the hypogastric and pel-

vic neural complexes [32,36]. Postganglionic fibers of the thoracolumbarsympathetic 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 hypo-

gastric nerves, parasympathetic input from S2–S4, and extrapelvic somatic innervation to the

sphincters and perineal structures also from S2–S4.


the distal aorta, near the origin of the artery with the same name. The infe-

rior mesenteric nerve bifurcates into two hypogastric nerves at the level of

the aortic bifurcation. The inferior mesenteric and hypogastric nerves aremost readily identified in the presacral space upon opening the peritoneal

reflection and gently tracing them back to the origin of the inferior mesen-

teric 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 pros-

tate, seminal vesicles, and urethra. Parasympathetic visceral preganglionicfibers 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 ves-

sels. The pelvic nerves are joined by the hypogastric nerves, forming the pel-

vic 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 emptyingdepend 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 perinealnerve, 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 con-

tinence mechanism and includes sensations to touch, pinprick, heat, andcold. They are transmitted by the inferior rectal branch of the pudendal

nerve [4].


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 hemorrhoidalartery) 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 rec-

tal arteries originate from the hypogastric (internal iliac) or the inferior

vesicle arteries. They course from the lateral pelvic sidewalls to reach the dis-

tal 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 rec-

tum, 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. Theinferior and middle hemorrhoidal (rectal) veins ultimately drain into the in-

ferior vena cava via the internal pudendal and hypogastric veins. The

superior rectal vein drains into the portal circulation via the inferior mesen-

teric vein.

The anorectum receives its major blood supply from the superior and

inferior hemorrhoidal arteries and to a lesser degree from the middle hem-

orrhoidal artery, forming a wide intramural network of collaterals [39]. The

internal iliac artery gives off a branch to form the extrapelvic pudendalartery. 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 plex-

uses. It drains through the bilateral middle and inferior hemorrhoidal veins

and the single superior hemorrhoidal vein, to the internal iliac vein, and ulti-

mately 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 hemorrhoi-dal plexus—the basis for internal hemorrhoids. It is found in the submu-

cosal 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 sys-tems of lymphatic drainage. Above the dentate line, lymph drains to the

inferior mesenteric and internal iliac nodes; below the dentate line, lymph


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 twodirections [40]. One is along the superior hemorrhoidal and inferior mesen-

teric 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 thereforeamenable to neoadjuvant treatment prior to surgery. The challenge in phys-

ical diagnosis of rectal neoplasms is in describing mid and distal rectal can-

cers 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 reason-

able distance is palpated between the lower edge of a rectal cancer (2–3 cm)

and the top of the puborectalis muscle, sphincter preservation is generallypossible. Because the puborectalis is a U-shaped sling around the anorectal

junction, it is not useful in describing tumors in the anterior sector. Assess-

ment 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 mid-

portion of the prostate gland will almost invariably require abdominoperi-neal resection. We suggest that the description of distal and mid-rectal

tumors should be based on their relationship to these structures.

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Anorectal Anatomy