Anal Canal Anatomy

The anal canal is the most terminal part of the lower gastrointestinal (GI) tract/large intestine, which lies between the anal verge (anal orifice, anus) in the perineum below and the rectum above. It extends from the pelvic diaphragm to the anal orifice. ^[1]  The description in this topic is from below upwards as that is how this region is usually examined in clinical practice. Images depicting the anal canal can be seen below. ^{[2, 3]}

Rectum and anal canal anatomy. Courtesy of Wikimedia Commons (https://commons.wikimedia.org/wiki/File:Rectum_anatomy_de_01.svg; author Armin Kübelbeck).

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Coronal section of rectum and anal canal.

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Coronal section through the anal canal.

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The pigmented, keratinized perianal skin of the buttocks (around the anal verge) has skin appendages (e.g., hair, sweat glands, sebaceous glands), but the anal canal skin above the anal verge, which is also pigmented and keratinized, does not have skin appendages. ^[4]

The demarcation between the rectum above and the anal canal below is the anorectal ring or anorectal flexure, where the puborectalis muscle forms a sling around the posterior aspect of the anorectal junction, kinking it anteriorly.

The anal canal is completely extraperitoneal. The length of the anal canal is about 4 cm (range, 3-5 cm), with two thirds of this being above the pectinate line (also known as the dentate line) and one third below the pectinate line.

The epithelium of the anal canal between the anal verge below and the pectinate line above is variously described as anal mucosa or anal skin. This area appears like (pigmented) skin, is sensitive like skin (why a fissure-in-ano is very painful), and is keratinized like skin (but does not have skin appendages), therefore calling it anal skin (anoderm) would more closely align its structure to function, albeit both are used.

The pectinate line is the site of transition of the proctodeum below and the postallantoic gut above. It is a scalloped demarcation formed by the anal valves (transverse folds of mucosa) at the inferiormost ends of the anal columns. Anal glands open above the anal valves into the anal sinuses. The pectinate line is not seen on inspection in clinical practice, but under anesthesia, the anal canal descends, and the pectinate line can be seen on slight retraction of the anal canal skin.

The anal canal just above the pectinate line for about 1-2 cm is called the anal pecten or transitional zone. Above this transitional zone, the anal canal is lined with columnar epithelium (which is insensitive to cutting). Anal columns (of Morgagni) are 6-10 longitudinal (vertical) mucosal folds in the upper part of the anal canal.

At the bottom of these columns are anal sinuses or crypts, into which open the anal glands and anal papillae. Infection of the anal glands is likely the initial event in the causation of perianal abscess and fistula-in-ano. Three of these columns (left lateral, right posterior, and right anterior, at 3-, 7-, and 11-o'clock positions in the supine position) are prominent; they are called anal cushions and contain branches and tributaries of the superior rectal (hemorrhoidal) artery and vein. When prominent, veins in these cushions form the internal hemorrhoids.

The anorectal junction or anorectal ring is situated about 5 cm from the anus. At the anorectal flexure or angle, the anorectal junction is pulled anterosuperiorly by the puborectal sling to continue below as the anal canal.

Levator ani and coccygeus muscles form the pelvic diaphragm. Lateral to the anal canal are the pyramidal ischioanal (ischiorectal) fossae (one on either side), below the pelvic diaphragm and above the perianal skin. The paired ischioanal fossae communicate with each other behind the anal canal. The anterior relations of the anal canal are the seminal vesicles, prostate, and urethra in males and the cervix and vagina with perineal body in between in females. In front of (anterior to) the anal canal is the rectovesical fascia (of Denonvilliers) and behind (posterior) is the presacral endopelvic fascia (of Waldeyer), under which lies a rich presacral plexus of veins. Posterior to the anal canal lies the tip of the coccyx (joined to it by the anococcygeal ligament) and lower sacrum.

The anal canal is surrounded by several perianal spaces: subcutaneous, submucosal, intersphincteric, ischioanal (rectal), and pelvirectal.

Blood supply and lymphatics

The blood supply and lymphatics of the anal canal can be divided based on the pectinate (dentate) line as the anatomical landmark. ^{[1, 5]}

Arterial supply:

• The anal canal above the pectinate line is supplied by the terminal branches of the superior rectal (hemorrhoidal) artery, which is the terminal branch of the inferior mesenteric artery.
• The middle rectal artery (a branch of the internal iliac artery) and the inferior rectal artery (a branch of the internal pudendal artery) supply blood to the lower anal canal.

This vascular differentiation plays a role in various clinical conditions, particularly those affecting the anal and rectal regions, such as hemorrhoids. ^{[1, 5]}

Venous drainage:

Above the pectinate line, venous blood drains into the portal venous system via the internal hemorrhoidal (rectal) plexus. This connection is clinically significant as conditions such as liver cirrhosis may cause portal hypertension that can result in the formation of rectal varices. The anal canal is therefore an important area of portosystemic venous connection (the others being the esophagogastric junction, and paraumbilical veins). ^{[1, 5]}

Beneath the pectinate line lies the external hemorrhoidal plexus of veins, which drains into systemic veins. Venous drainage occurs through the inferior rectal veins into the internal pudendal veins, which then drain into the internal iliac vein. This pathway connects to systemic circulation, highlighting an important portosystemic anastomosis within the anal canal. ^{[1, 5]}

Lymphatic drainage:

• Lymphatics from the anal canal drain into the superficial inguinal group of lymph nodes.
• Lymphatic drainage from the anal canal above the pectinate line primarily flows into the internal iliac lymph nodes or inferior mesenteric lymph node. Below this line, the lymph drains into the superficial inguinal lymph nodes. ^{[1, 5]}

Summary ^{[1, 5]} (Open Table in a new window)

Nerve supply:

The dual nerve supply allows for distinct physiological responses in each segment of the anal canal. For example, while the upper portion is sensitive mainly to stretch due to its autonomic innervation, the lower portion is responsive to pain and temperature due to somatic innervation. ^{[1, 5]}

Physiology

Anorectal sphincter tone can be assessed during digital rectal examinations (DREs) when the patient is asked to squeeze the examining finger.

Perianal ultrasound can also be used as an additional tool to visualize the anal canal and sphincter muscles, offering a more comfortable and reproducible alternative to DRE. ^{[6, 7]}

Anorectal manometry measures the pressures: resting and squeezing. Anorectal manometry remains the gold standard for quantitatively measuring anorectal pressures. This technique is crucial for evaluating anorectal function, particularly in cases of fecal incontinence. Manometry provides detailed information on the sphincter tone and rectal sensitivity, aiding in the diagnosis and management of various anorectal disorders. ^{[6, 7]}

High-resolution anorectal manometry (HR-ARM) utilizes multiple closely spaced sensors to provide a detailed pressure map along the anal canal. HR-ARM records pressures at rest, during contraction maneuvers, and during simulated defecation. This technology allows for improved assessment of rectoanal coordination and can identify functional disorders such as fecal incontinence and constipation more effectively than conventional manometry. ^{[6, 7]}

Three-dimensional high-definition ARM can also offer even greater detail by depicting circumferential pressure distribution around the anal canal, enhancing and improving diagnostic capabilities. ^{[6, 7]}

Embryology

The anal canal below the pectinate line develops from the proctodeum (ectoderm), while that above the pectinate line develops from the endoderm of the hindgut.

The formation of the anal canal follows the rupture of the cloacal membrane around the eighth week of development, separating the urogenital sinus from the anorectal canal. This separation is achieved by the growth of the urorectal septum. The pectinate line marks the transition between these two embryonic origins, influencing the vasculature, lymphatic drainage, and innervation. ^{[6, 8, 9, 10]}

The perianal skin is keratinized, stratified squamous epithelium with skin appendages (e.g., hair, sweat glands, sebaceous glands, and somatic nerve endings that are sensitive to pain). The anal canal skin (anoderm) is also keratinized, stratified squamous epithelium and has somatic nerve endings (sensitive to pain), but without skin appendages. The anal canal mucosa is cuboidal in the transitional zone and columnar above it; it is insensitive to pain. The rectal mucosa above the anorectal junction is lined by pinkish red, insensitive columnar epithelium.

The anorectal flexure is formed by the puborectalis (the innermost fibers of levator ani muscle, which extends from the pubic bone, obturator fascia, and ischial spine to the coccyx and anococcygeal ligament) and the upper ends of the external and internal anal sphincters. Puborectalis plays a much more important role in continence than the internal and external sphincters. The involuntary autonomous internal anal sphincter is the lowermost continuation of the inner, circular smooth muscle layer of the rectum. The external longitudinal muscle layer continues as the corrugator cutis ani. The external anal sphincter has three parts: subcutaneous, superficial, and deep. The external anal sphincter is composed of skeletal muscle, is under voluntary control, and is supplied by pudendal nerves (S2-S4).

The internal anal sphincter is innervated by autonomic nerves, both sympathetic and parasympathetic, while the external anal sphincter is primarily supplied by somatic nerves from the pudendal nerve. The interplay between these neural pathways contributes significantly to fecal continence. ^{[6, 8, 9, 10]}

Pathophysiologic anal variants include the following:

• Anal atresia
• Ectopic anus
• Persistent cloaca

Anal atresia (imperforate anus) is a low anorectal malformation in which the anus is either atretic (absent) or narrowed and the colon and rectum are normal. If the proctodeum and the postallantoic gut fail to unite, an imperforate anus results. Anal atresia is commonly diagnosed at birth, with symptoms such as failure to pass stool or abdominal distension. Anal atresia occurs in approximately one in 5,000 live births, with a significant proportion of cases presenting with associated fistulas most commonly rectourethral fistulas in males and rectovestibular fistulas in females. Surgical intervention, such as posterior sagittal anorectoplasty, is often required to relocate the anus to its proper position, enabling normal bowel function. Associated malformations, like VACTERL syndrome (vertebral, anal, cardiac, tracheal, esophageal, renal, and limb defects), are common and require further evaluation of other organs. ^{[11, 12, 13]}

In ectopic anus, the anus is misplaced, usually anteriorly in the perineum (in males) or in the vagina (in females). It may result in difficulties in bowel movement, delayed passage of the meconium, and possible infections. Diagnosis is usually made at birth through clinical examination and imaging studies. Treatment involves repositioning the anus through surgery to restore normal anatomy. ^{[11, 12, 13]}

Persistent cloaca is a complex congenital anomaly where the lower gastrointestinal tract (rectum), lower urinary tract (bladder or urethra), and lower genital tract in females (vagina) converge into a single common channel. The incidence of this condition is approximately one in 25,000 live births. It is often identified during prenatal ultrasounds or at birth due to the absence of distinct genital and anal openings. This condition requires intricate surgical intervention to separate the tracts and restore normal function. Persistent cloaca requires a multidisciplinary approach due to its association with other anomalies in the spine, kidneys, and genitalia. ^{[11, 12, 13]}

Perianal lesions encompass a variety of conditions that affect the area surrounding the anus. These lesions are often classified based on their anatomical location and clinical presentation, and they can significantly impact the patient's quality of life. The formation of a perianal abscess typically begins with infection of an anal gland, leading to subsequent complications such as fistula-in-ano. Fissure-in-ano, characterized by painful ulcers in the sensitive anal canal skin, remains a common complaint among patients. ^{[14, 15, 16]}

The location of perianal lesions is described in relation to a clock (as seen in the supine position), e.g., 2 o'clock, 7 o'clock. The sites of perianal lesions include the following:

• Perianal skin - Abscess, hematoma (erroneously called thrombosed external hemorrhoids), external opening of fistula-in-ano, skin tag (in chronic fissure-in-ano)
• Anal canal skin (anoderm, below dentate line) - Fissure-in-ano, external hemorrhoids, cancer
• Anal canal mucosa (above pectinate line) - Internal hemorrhoids, cancer

The pectinate line cannot be felt on rectal examination but is seen on anoproctoscopy; under anesthesia, the pectinate line can be seen on retraction of the perianal skin. The anorectal flexure can be palpated on rectal examination (but not under anesthesia when the muscles relax).

Infection of an anal gland is considered the initial event in the formation of a perianal abscess and then a fistula-in-ano. Fissure-in-ano is an ulcer in the sensitive anal canal skin and is a very painful condition. Fistula-in-ano can be intersphincteric, trans-sphincteric, or suprasphincteric. The internal opening of the fistula-in-ano can be in the anal canal or rectum.

Factors associated with a higher prevalence of perianal lesions include: ^{[14, 15, 16]}

• Male sex
• Age under 40 years
• Ileocolonic disease location
• Studies showing that the prevalence and implications of perianal lesions is particularly high in patients with Crohn's disease (CD).

The presence of these lesions is linked to increased morbidity and can significantly affect health-related quality of life due to pain and functional impairments. Treatments generally include topical therapies, drainage, fistulotomy, or placement of setons. Treatments for CD-related perianal disease include biologics like antitumor necrosis factor agents. ^{[14, 15, 16]}

External hemorrhoids are located below the pectinate line on the sensitive anal canal skin and are painful, while internal hemorrhoids are located above the pectinate line in the insensitive anal canal mucosa and are painless (unless complicated). For the same reason, internal hemorrhoids can be treated (injected with sclerosant or ligated with rubber band) without anesthesia.

During posterior or lateral sphincterotomy for fissure-in-ano, it is only the internal sphincter that is divided.

A cruciate incision in the perianal skin lateral to the anal verge provides easy and direct access to ischioanal fossae for the drainage of an abscess.

Surgical considerations

Intersphincteric resection (ISR) of the rectum (e.g., for ulcerative colitis) follows the plane between the external and internal sphincters; the external anal sphincter, levator ani, and puborectalis muscles are preserved. Preserving these muscles helps to minimize postoperative complications, including incontinence and sexual dysfunction. For patients with tumors not deeply infiltrating the rectal muscle layers, ISR, sometimes accompanied by chemoradiotherapy, has shown promising outcomes with good oncologic safety and acceptable continence levels. ^{[19, 17, 18, 19]}

In hand-sewn ileal pouch anal anastomosis (IPAA), also called restorative proctocolectomy for ulcerative colitis, the ileal pouch is anastomosed to the pectinate line, which is exposed perianally. For stapled IPAA, the surgical anal canal is divided 1-2 cm above the pectinate line using a linear stapler; the ileal pouch is then anastomosed to the anal canal stump using a circular stapler.

Cancers of anal canal below the pectinate line are usually squamous cell carcinoma (or basal cell carcinoma and melanoma), whereas those of the anal canal above the pectinate line (and of the rectum) are adenocarcinoma. Anal canal and low rectal cancers can infiltrate the anorectal ring and cause incontinence — a contraindication for sphincter preservation (by chemoradiotherapy for squamous cell carcinoma and low-anterior resection for adenocarcinoma). Anal canal cancer (or rectal cancer infiltrating into the anal canal) spreads to the superficial inguinal lymph nodes.

The rising incidence of squamous cell carcinoma of the anus (SCCA) has been linked to human papillomavirus infection, which accounts for approximately 90% of cases. ^[20] Management strategies emphasize early detection and treatment of precancerous lesions, particularly in high-risk populations such as people living with human immunodeficiency virus (HIV). Studies show that treating high-grade squamous intraepithelial lesions significantly reduces the risk for anal cancer in this demographic group. ^{[19, 17, 18, 19]}

Treatment of anal cancer is focused on individualized approaches that consider tumor biology and patient-specific factors. Immunotherapy is a promising option for patients with advanced SCCA, particularly those whose tumors exhibit specific biomarkers such as programmed death-ligand 1 expression. Additionally, intensity-modulated radiotherapy is preferred for its ability to minimize treatment-related toxicities while effectively targeting the locoregional disease. The integration of chemoradiotherapy remains a cornerstone for managing locoregional SCCA, with combination therapies showing improved outcomes. ^{[19, 17, 18, 19]}

Imaging considerations

Magnetic resonance imaging (MRI) has become the imaging modality of choice for delineation of anal and perianal anatomy in diseases such as fistula-in-ano, incontinence, and anorectal cancer, among others.

Various imaging modalities, including computed tomography (CT), endoanal ultrasound (EAUS), transperineal ultrasound (TPUS), and fistulography, complement MRI in evaluating anal and perianal conditions such as fistula-in-ano, incontinence, and anorectal cancer. CT is useful for detecting deep abscesses and malignancies but lacks soft tissue detail, while EAUS provides high-resolution images of the anal sphincter, aiding in the assessment of sphincter integrity and defects. TPUS offers a noninvasive option for dynamic pelvic floor assessments. Fistulography helps in visualizing fistulous tracts but is being increasingly replaced by MRI, which provides comprehensive imaging. Techniques such as dynamic contrast-enhanced MRI and defecography enhance evaluations of disease activity and pelvic floor disorders. Together, these modalities allow for a tailored diagnostic approach, improving the management of complex clinical scenarios. These imaging techniques can be selected based on specific clinical scenarios, providing a comprehensive approach to diagnosing and managing conditions such as fistula-in-ano, incontinence, and anorectal cancer. ^{[21, 22, 23]}