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Anal triangle

The anal triangle is the posterior subdivision of the perineum, a diamond-shaped region located between the thighs and below the pelvic floor, containing the anal canal and associated structures essential for defecation.^[1]^[2] It is separated from the anterior urogenital triangle by an imaginary line connecting the ischial tuberosities and plays a critical role in supporting pelvic organs while facilitating the voluntary and involuntary control of bowel movements.^[3] The boundaries of the anal triangle are well-defined: anteriorly by the line between the ischial tuberosities or the posterior margin of the perineal membrane; posteriorly by the tip of the coccyx; laterally by the sacrotuberous ligaments; and superiorly by the levator ani muscle.^[1]^[2] Within this region lie the ischiorectal (ischioanal) fossae, wedge-shaped spaces filled with adipose tissue that provide cushioning and allow expansion during defecation.^[1] Key contents of the anal triangle include the anal canal, which extends from the anal verge to the anorectal junction and features a transition in epithelial lining from columnar above the pectinate line to stratified squamous below it, along with anal columns, valves, and glands.^[1] The internal anal sphincter, a smooth muscle continuation of the rectal circular muscle, provides involuntary tone, while the external anal sphincter, a skeletal muscle divided into subcutaneous, superficial, and deep parts, enables voluntary control.^[1]^[2] Innervation is provided by the inferior rectal nerve (a branch of the pudendal nerve) for somatic sensory and motor functions below the pectinate line, with visceral innervation from the inferior pelvic plexus above it; blood supply comes from the superior rectal artery (from the inferior mesenteric artery) superiorly and inferior rectal arteries (from the internal pudendal artery) inferiorly.^[1]^[2]

Anatomy

Location and boundaries

The anal triangle constitutes the posterior subdivision of the perineum, forming a diamond-shaped region that houses the anal canal and the external anal opening. It is situated inferior to the pelvic diaphragm and between the thighs, serving as the posterior compartment of the perineal space.^[1]^[4] The boundaries of the anal triangle are precisely defined as follows: anteriorly by a transverse line connecting the ischial tuberosities, posteriorly by the tip of the coccyx, laterally by the sacrotuberous ligaments on each side, and superiorly by the levator ani muscle. The perineum itself is divided into the anterior urogenital triangle and the posterior anal triangle by the transverse perineal muscles and the perineal body, which approximate this interischial line.^[4]^[2]^[1] Surface anatomy landmarks for identifying the anal triangle during clinical examination include the central anal aperture and the surrounding perianal skin, with the ischioanal fossae palpable laterally as soft, fat-filled depressions extending toward the pelvic floor muscles such as the levator ani.^[4]^[2]

Relations to adjacent structures

The anal triangle, forming the posterior subdivision of the perineum, relates posteriorly to the gluteal cleft—also termed the natal cleft—which is the midline groove between the buttocks that extends from the sacrum to the anus, positioning the anal opening at its deepest point.^[5] Anteriorly, it adjoins the urogenital triangle via the perineal body, a central fibromuscular mass that serves as the junction between the anal and urogenital regions, ensuring separation while providing structural continuity across the perineum.^[6] Laterally, the triangle extends into the ischiorectal fossae (also known as ischioanal fossae), bilateral wedge-shaped spaces filled with adipose tissue that flank the anal canal and accommodate expansions during defecation.^[1] Superiorly, the anal triangle connects to the pelvic cavity through the pelvic diaphragm, primarily the levator ani muscle, which acts as a muscular shelf separating the perineal surface from the pelvic contents and allowing passage of the anal canal.^[1] In clinical practice, the anal triangle is exposed inferiorly in the lithotomy position, where the legs are elevated and abducted to provide access to the perineum, highlighting the perineal body's role in preserving regional integrity against mechanical stresses.^[1] Embryologically, the anal triangle arises from the division of the cloacal membrane by the descending urorectal septum around the 7th week of gestation, partitioning the primitive cloaca into a dorsal anorectal component (forming the rectum and anal canal) and a ventral urogenital sinus, thereby establishing the spatial distinction between anal and urogenital domains.^[7] This septation process lays the foundational adjacency between the resulting anal and urogenital regions, mirrored in the adult perineal subdivision by an imaginary transverse line across the ischial tuberosities.^[1]

Musculature

The musculature of the anal triangle primarily consists of sphincteric and pelvic floor muscles that encircle and support the anal canal, contributing to fecal continence through coordinated contraction and maintenance of the anorectal angle.^[1] These structures form a functional complex that ensures voluntary and involuntary control of defecation while providing structural stability to the posterior perineum.^[8] The external anal sphincter is a striated skeletal muscle that forms a ring around the anus, divided into three parts: subcutaneous, superficial, and deep.^[1] It originates from the coccyx and perineal body, with fibers inserting around the anal canal to encircle it completely.^[9] This muscle enables voluntary contraction to close the anus and maintain continence, allowing conscious control over defecation.^[1] In contrast, the internal anal sphincter is an involuntary smooth muscle layer that represents a downward continuation of the circular muscle of the rectum.^[1] It originates from the rectal wall and inserts along the internal surface of the anal canal, providing a thickened ring that maintains resting anal tone through constant tonic contraction.^[10] This structure relaxes reflexively during defecation to facilitate passage of stool while preserving baseline continence.^[1] The levator ani muscle complex forms the superior boundary of the anal triangle and serves as the primary pelvic floor support, comprising the puborectalis, pubococcygeus, and iliococcygeus components.^[1] The puborectalis originates from the posterior aspect of the pubic bone and forms a U-shaped sling around the anorectal junction, inserting into the anococcygeal raphe and posterior rectum to maintain the puborectalis angle essential for continence.^[11] The pubococcygeus arises from the posterior superior pubic ramus and obturator fascia, inserting onto the anococcygeal ligament and coccyx to elevate and support the pelvic viscera, including the rectum.^[12] The iliococcygeus originates from the tendinous arch of the levator ani and ischial spine, inserting into the coccyx and anococcygeal raphe to provide lateral and posterior pelvic floor reinforcement.^[8] Collectively, these components create a dynamic sling that resists intra-abdominal pressure and aids in anorectal closure.^[11] The superficial transverse perineal muscle is a paired striated structure that spans the posterior aspect of the urogenital triangle, adjacent to the anal triangle.^[6] It originates from the ischial tuberosity and inserts into the perineal body, a central fibromuscular node that anchors multiple perineal muscles.^[13] This muscle stabilizes the perineal body and provides indirect support to the anal canal by maintaining perineal integrity during straining.^[6] The coccygeus muscle lies posterior to the levator ani, forming part of the pelvic diaphragm.^[1] It originates from the ischial spine and sacrospinous ligament, inserting into the lateral margins of the coccyx and lowest sacral segments.^[14] Through its contraction, the coccygeus elevates the coccyx and supports the posterior pelvic floor, contributing to overall stability around the anal triangle.^[1] The ischiorectal fossae, located laterally within the anal triangle, contain expansive fat pads that cushion the sphincteric muscles and anal canal.^[1] These pyramidal fat accumulations, bounded by the levator ani superiorly and external anal sphincter medially, permit distension of the anal canal during defecation while providing passive support to maintain continence under normal conditions.^[1]

Vascular supply

The vascular supply of the anal triangle primarily supports the anal canal and associated structures, such as the internal and external anal sphincters. The arterial supply is derived from branches of the internal iliac and internal pudendal arteries, ensuring robust perfusion to the region.^[1] The main arterial supply to the lower anal canal and sphincters below the pectinate line comes from the inferior rectal artery, a branch of the internal pudendal artery, which arises in the perineum and travels through the ischioanal fossa to reach the anal triangle. The middle rectal artery, originating from the internal iliac artery, contributes to the vascularization of the upper portions of the anal canal, anastomosing with the inferior rectal artery to form a collateral network. Additionally, the superior rectal artery (from the inferior mesenteric artery) provides supply to the uppermost anal canal above the pectinate line, with extensive anastomoses among the superior, middle, and inferior rectal arteries forming a submucosal arterial plexus that enhances redundancy in blood flow.^[15]^[16]^[17] Venous drainage follows a similar segmental pattern, with portosystemic anastomoses occurring at the level of the anal columns within the hemorrhoidal plexuses. Below the pectinate line, the inferior rectal veins drain the external hemorrhoidal plexus into the internal pudendal vein, which joins the internal iliac vein and ultimately the inferior vena cava, representing systemic circulation. Above the pectinate line, drainage is via the superior rectal vein into the inferior mesenteric vein and portal system, while the middle rectal veins contribute to the upper anal canal drainage toward the internal iliac vein; these venous networks interconnect, creating critical portosystemic shunts.^[1]^[18]^[17] Lymphatic drainage of the anal triangle varies by the pectinate line, reflecting the embryological origins of the region. The lower anal canal below the pectinate line drains to the superficial inguinal lymph nodes, while the upper portion above the line drains to the internal iliac lymph nodes, providing a pathway for lymphatic return to the systemic circulation.^[1]^[15]

Neural supply

The neural supply of the anal triangle encompasses somatic, autonomic, and sensory components that regulate motor function, sensation, and reflex activity in the region, primarily targeting the external and internal anal sphincters as well as the perianal skin and lower anal canal.^[1] The pudendal nerve, arising from the ventral rami of sacral spinal nerves S2-S4, provides the primary somatic innervation to the anal triangle. This nerve exits the pelvis through the greater sciatic foramen, re-enters via the lesser sciatic foramen, and travels within the pudendal canal along the lateral wall of the ischioanal fossa. Its first branch, the inferior rectal nerve, supplies motor innervation to the external anal sphincter, enabling voluntary contraction for fecal continence, and delivers somatic sensory fibers to the perianal skin and the lower anal canal below the pectinate line (approximately 1-1.5 cm in length), conveying sensations of pain, touch, and temperature.^[19]^[1] Autonomic innervation to the anal triangle derives from the inferior hypogastric plexus and modulates the internal anal sphincter, a smooth muscle structure that maintains basal tone for involuntary continence. Sympathetic fibers, originating from the superior and inferior hypogastric plexuses (with preganglionic input from T10-L2 via lumbar splanchnic nerves), exert an excitatory effect on the internal anal sphincter, promoting contraction and sustaining its resting tone to prevent leakage. In contrast, parasympathetic fibers from the pelvic splanchnic nerves (nervi erigentes, arising from S2-S4) provide inhibitory innervation, relaxing the internal anal sphincter tone and facilitating peristaltic propulsion during defecation. These autonomic pathways integrate within the inferior hypogastric plexus before distributing to the anal canal above the pectinate line.^[20]^[1] Sensory innervation in the anal triangle follows a dual pattern based on the region's embryological origins. The perianal skin and external anal region receive somatic sensory input via the inferior rectal nerve branches of the pudendal nerve, corresponding to sacral dermatomes S3-S5, which detect light touch, pinprick, and temperature. Above the pectinate line, the upper anal canal and internal anal sphincter are supplied by visceral afferents traveling with the pelvic splanchnic nerves and pelvic plexus, relaying stretch and distension sensations to the sacral spinal cord without precise dermatomal localization.^[20]^[1]^[21] The anorectal reflex, essential for coordinating defecation, involves sacral spinal segments S2-S4 and operates through a polysynaptic pathway. Afferent signals from rectal distension or perianal stimulation travel via pudendal nerve fibers to the sacral cord, synapsing in the dorsal horn before efferent motor output returns through the inferior rectal nerve to contract the external anal sphincter and relax the internal sphincter, facilitating expulsion. This reflex arc underscores the integrated neural control of the anal triangle's sphincteric mechanisms.^[22]

Clinical significance

Common disorders

Hemorrhoids represent one of the most prevalent disorders affecting the anal triangle, characterized by the engorgement and swelling of veins in the hemorrhoidal plexuses. Internal hemorrhoids, located above the dentate line and covered by insensate anorectal mucosa, typically present with painless rectal bleeding but may prolapse if enlarged. External hemorrhoids, situated below the dentate line and covered by perianal skin, are often painful, particularly when thrombosed, due to the formation of a blood clot within the dilated vein. The etiology primarily involves increased intra-abdominal pressure leading to venous congestion, commonly precipitated by chronic constipation, straining during defecation, pregnancy, or prolonged sitting. Anal fissures are linear tears in the anoderm of the anal canal, most frequently occurring in the posterior midline due to the relative ischemia in that region. These acute or chronic lesions are often idiopathic and solitary, with symptoms including severe pain during and after defecation, sometimes accompanied by minor bleeding. They are closely associated with hypertonia of the internal anal sphincter, which elevates resting pressure and impairs local blood flow, perpetuating the fissure and contributing to a cycle of spasm and poor healing. Anal fistulas and abscesses arise predominantly from cryptoglandular infections originating in the anal glands at the dentate line, where obstructed glands lead to suppuration and abscess formation in the ischiorectal fossa. Anorectal abscesses may progress to fistulas in up to 40% of cases, forming an abnormal epithelialized tract connecting the anal canal's internal opening to an external perianal opening. Goodsall's rule aids in predicting the fistula tract's course: external openings posterior to an imaginary transverse line through the anus typically curve posteriorly to meet the midline internal opening, while anterior openings extend more directly forward. Fecal incontinence, the involuntary leakage of stool, frequently stems from damage to the anal sphincters or pudendal neuropathy, disrupting the mechanisms of continence. Sphincter injury, often from obstetric trauma during vaginal delivery, weakens the external anal sphincter and can cause immediate or delayed incontinence. Pudendal nerve damage, resulting from stretch injury or compression during childbirth, further exacerbates the condition by impairing sensory and motor function to the pelvic floor.

Surgical and diagnostic considerations

The digital rectal examination (DRE) is a fundamental diagnostic procedure for evaluating the anal triangle, allowing assessment of anal sphincter tone, detection of masses, and palpation of the anal canal walls for irregularities.^[23] During the examination, the clinician inserts a gloved, lubricated finger to appreciate the resting tone of the external anal sphincter and instruct the patient to squeeze, thereby evaluating voluntary contraction strength.^[24] This method also facilitates identification of nodules, tenderness, or enlargements within the anal canal, aiding in the preliminary diagnosis of conditions such as abscesses or tumors.^[25] Anoscopy and proctoscopy provide direct visualization of the anal triangle structures, essential when DRE findings are inconclusive. Anoscopy involves insertion of a short, rigid anoscope to inspect the anus, anal canal, and distal internal sphincter, enabling detailed examination of mucosal surfaces for lesions or bleeding sources.^[26] Proctoscopy extends this view to the lower rectum using a longer instrument, allowing assessment of the rectal ampulla and proximal anal canal for abnormalities like polyps or inflammation.^[27] These procedures are typically performed in an outpatient setting with minimal sedation and are particularly useful for evaluating disorders such as fistulas.^[26] Imaging modalities play a critical role in non-invasive assessment of the anal triangle. Magnetic resonance imaging (MRI) excels in delineating complex fistula tracts, identifying their origin, course, and relation to the sphincters with high-resolution T2-weighted sequences that highlight hyperintense tracts against surrounding tissues.^[28] Endoanal ultrasound serves as the gold standard for evaluating sphincter integrity, providing real-time imaging of the internal and external anal sphincters to detect defects, tears, or thinning that may contribute to incontinence.^[29] These techniques offer complementary insights, with endoanal ultrasound preferred for superficial sphincter evaluation and MRI for deeper perianal extensions.^[30] Surgical approaches to the anal triangle often utilize perineal access for procedures targeting its contents. Hemorrhoidectomy involves excision of hemorrhoidal tissue through a perineal incision, typically performed under regional anesthesia to minimize bleeding and ensure precise removal.^[31] Fistulotomy employs a similar perineal route to lay open and drain anal fistulas, promoting healing while preserving sphincter function where possible.^[31] Sphincterotomy, used for chronic anal fissures, entails partial incision of the internal anal sphincter via perineal access to relieve spasm and reduce pressure.^[31] The lithotomy position is commonly adopted for optimal exposure during these operations, positioning the patient supine with legs elevated to facilitate access to the perineum and anal verge.^[32] In obstetrics, episiotomy carries specific risks to the anal triangle, particularly involving the perineal body. An episiotomy incision, intended to enlarge the vaginal outlet, can extend unintendedly into the anal sphincter if not precisely placed, leading to obstetric anal sphincter injury and subsequent incontinence.^[33] Factors such as episiotomy depth less than 16 mm or midline placement increase the likelihood of obstetric anal sphincter injuries, potentially compromising the integrity of the anal triangle.^[34] Routine episiotomy is now discouraged due to these risks, with emphasis on selective use to avoid perineal trauma.^[33]

References

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