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External anal sphincter

The external anal sphincter is a cylindrical-shaped striated muscle that encircles the anus, forming part of the pelvic floor and contributing to the maintenance of fecal continence through both voluntary and involuntary mechanisms.^[1] It consists predominantly of slow-twitch muscle fibers capable of prolonged contraction to sustain resting tone, while also allowing for rapid voluntary responses.^[1] Anatomically, it envelops the internal anal sphincter and extends approximately 1 cm beyond it caudally, attaching posteriorly to the coccyx via the anococcygeal ligament and anteriorly to the perineal body, with a typical thickness of 5–8 mm as measured by endosonography.^[1] Innervated primarily by the inferior rectal branch of the pudendal nerve (originating from sacral segments S2–S4), the external anal sphincter receives somatic motor input that enables conscious control over defecation and reflex contractions in response to rectal distension or increased intra-abdominal pressure.^[2] Its function is integral to the anorectal continence mechanism, where it generates and maintains anal canal pressure to prevent involuntary leakage, working in synergy with the puborectalis muscle and levator ani to form a functional unit during rest and defecation.^[2] Maximal voluntary contractions can be sustained for 30–60 seconds, supporting activities like postponing defecation, while subconscious resting tone ensures baseline closure.^[1] Damage to the external anal sphincter, often from childbirth or trauma, can lead to fecal incontinence, highlighting its clinical significance in pelvic floor disorders.^[1] In males and females, subtle anatomical differences exist, such as variations in the encasement by surrounding muscle fibers, which influence surgical approaches to repair or reconstruction.^[1]

Anatomy

Structure

The external anal sphincter is a cylindrical or oval-shaped skeletal muscle that encircles the anal canal, forming the outer layer surrounding the internal anal sphincter. It extends approximately 3 cm in length along the anal canal and measures approximately 0.5 to 1 cm (5-10 mm) in thickness, with regional variations such as greater posterior thickness (up to 10 mm) compared to the anterior portion (around 5-7 mm). These dimensions contribute to its role in providing structural support to the distal anorectum, as observed in anatomical studies using ultrasound and MRI imaging.^[3]^[4]^[5] The muscle is divided into three functional layers based on gross anatomical dissection and imaging: the subcutaneous layer, which is the most distal and encircles the anal verge; the superficial layer, surrounding the lower portion of the anal canal; and the deep layer, located proximally and blending seamlessly with the puborectalis muscle of the levator ani. Each layer consists of striated muscle fibers that enable voluntary control, with the overall structure maintaining a flattened oval cross-section in the transverse plane. This layered organization was detailed in early histological and surgical anatomy descriptions, highlighting the progressive thickening from distal to proximal regions.^[6]^[7] Composed of type I (slow-twitch) and type II (fast-twitch) striated fibers, the external anal sphincter features a triple-loop configuration that ensures circumferential compression of the anal canal. In this arrangement, fibers form an anterior loop and left and right posterior loops, interconnecting to create a robust, overlapping network rather than a simple ring. This model, proposed in Shafik's seminal 1975 study based on dissections of human cadavers, provides mechanical efficiency for closure and has been corroborated by subsequent 3D imaging analyses.^[8]^[9] Anatomical variations in structure occur in up to 10% of individuals, including asymmetry, incomplete fiber rings (such as abbreviated posterior patterns), or double-loop configurations instead of the conventional triple-loop setup, as identified through endoanal ultrasound and MRI in population studies. These variations can influence surgical planning but do not typically impair baseline anatomy in healthy individuals.^[10]

Attachments

The external anal sphincter (EAS) originates from the perianal skin and fascia, forming a cylindrical sheath that encircles the distal anal canal, with its fibers attaching anteriorly to the perineal body and blending with the superficial transverse perineal muscles.^[11]^[12] This anterior attachment provides structural continuity with the central tendon of the perineum, facilitating integrated pelvic floor mechanics. Posteriorly, the EAS connects to the anococcygeal raphe (also known as the anococcygeal ligament), which anchors to the coccyx, while some superficial fibers may indirectly relate to the sacrotuberous ligament through fascial extensions in the posterior perineum.^[11]^[13]^[14] Laterally, the EAS blends with the walls of the ischiorectal fossae, allowing space for fat and neurovascular structures, and integrates with the levator ani muscle group, particularly the puborectalis component, which forms a sling-like support around the anorectal junction.^[11]^[12] Proximally, the deep portion of the EAS fuses with the puborectalis fibers of the levator ani, contributing to the formation of the anorectal ring, a palpable constriction at the proximal anal canal that maintains anorectal angulation.^[15]^[12] Distally, the EAS extends approximately 1-2 cm below the dentate line, reaching the anal verge and adhering to the surrounding skin.^[16]^[17] These attachments collectively enhance pelvic floor support in conjunction with the levator ani.^[18]

Innervation

The external anal sphincter receives its primary somatic innervation from the inferior rectal nerve, a branch of the pudendal nerve derived from the ventral rami of spinal nerves S2-S4.^[19] This innervation enables voluntary control of the sphincter muscle, allowing for conscious contraction to maintain continence.^[20] Additional motor contributions arise from the perineal nerve (a branch of the pudendal nerve at S4 level) to the anterior fibers of the sphincter, while direct branches from the sacral nerves (particularly S4) supply the deeper portions in some cases.^[21]^[22] Sensory innervation is provided by the inferior rectal nerve, which conveys proprioceptive feedback from the sphincter muscle itself and pain sensation from the perianal skin and lower anal canal below the pectinate line.^[19] Motor pathways originate from alpha motor neurons located in Onuf's nucleus within the ventral horn of the sacral spinal cord (S2-S4), facilitating voluntary contractions via the pudendal nerve; reflex arcs mediated by the same nerve support rapid squeeze responses to perianal stimulation.^[23]^[19] In cases of denervation due to cauda equina lesions affecting the S2-S4 roots, the external anal sphincter undergoes atrophy, leading to weakened tone and potential fecal incontinence.^[24] The somatic innervation of the external sphincter coordinates with autonomic inputs to the internal anal sphincter to regulate overall anal function.^[19]

Blood supply and lymphatic drainage

The arterial supply to the external anal sphincter is primarily provided by the inferior rectal arteries, which are branches of the internal pudendal artery arising within the pudendal canal.^[15] These arteries course through the ischioanal fossa to reach the sphincter muscle, supplying its external and internal components along with the lower anal canal below the pectinate line.^[25] Anastomoses with the middle rectal arteries, which originate from the internal iliac artery, contribute additional blood flow, forming a collateral network around the anorectal region.^[16] Venous drainage from the external anal sphincter occurs via the inferior rectal veins, which accompany the corresponding arteries and empty into the internal pudendal vein.^[16] The internal pudendal vein then converges with the internal iliac vein, directing deoxygenated blood into the systemic circulation.^[15] Lymphatic drainage of the external anal sphincter follows the inferior rectal vessels to the internal iliac and sacral lymph nodes, while distal perianal components may route to the superficial inguinal nodes.^[26] This dual pathway reflects the sphincter's position in the transitional zone of the anal canal below the dentate line.^[15] The rich anastomotic connections between the inferior and middle rectal arteries provide collateral circulation that helps prevent ischemic complications, particularly relevant during surgical interventions in the perianal region.^[27] Anatomical variations, including absence or hypoplasia of the inferior rectal artery on one or both sides, occur in a subset of individuals, in which case reliance shifts to the middle rectal supply for adequate perfusion.^[28]

Histology

The external anal sphincter is composed exclusively of skeletal (striated) muscle fibers, lacking any smooth muscle components, which distinguishes it from the internal anal sphincter.^[29] These fibers are organized into a cylindrical structure surrounding the anal canal, with a predominance of type I slow-twitch fibers (approximately 70-80%), which are fatigue-resistant and suited for sustained contraction, alongside a smaller proportion of type II fast-twitch fibers for rapid responses.^[30]^[31] Embedded within the muscle are proprioceptive structures, including muscle spindles that detect changes in muscle length and Golgi tendon organs that sense tension, enabling fine-tuned control and feedback during contraction.^[32] The endomysium, the connective tissue sheath surrounding individual muscle fibers, is rich in collagen and elastin, providing structural support and elasticity to accommodate repeated deformations without rupture.^[33] With advancing age, particularly after 40 years, the external anal sphincter undergoes progressive histological changes, including muscle fiber atrophy and increased fibrosis, where connective tissue replaces functional muscle, leading to diminished contractility and heightened risk of incontinence.^[34]^[35] These alterations involve upregulation of fibrogenic proteins such as collagen I and transforming growth factor-β, contributing to a stiffer, less responsive tissue matrix.^[35]

Embryological development

The external anal sphincter originates from mesodermal tissue surrounding the cloacal membrane during weeks 4 to 7 of gestation, forming as part of the external cloacal sphincter that encircles the cloaca.^[36] This mesodermal contribution derives from splanchnic mesoderm, with striated muscle differentiation occurring under the influence of somitic mesoderm migrating into the perineal region.^[37] The cloacal sphincter initially appears as a mesenchymal condensation, subdividing into distinct portions, including the precursors of the external anal and urogenital sphincters.^[36] During weeks 7 to 8, the urorectal septum, formed from mesoderm, partitions the cloaca into the urogenital sinus anteriorly and the anorectal canal posteriorly, facilitating the formation of the anal canal.^[38] As the anal membrane ruptures around week 8, the external anal sphincter encircles the definitive anus, establishing its position at the inferior end of the anal canal.^[38] Disruptions in this process, such as failed canalization or incomplete partitioning of the cloaca, can lead to congenital anomalies like imperforate anus, which affects sphincter development and occurs in approximately 1 in 5,000 live births.^[39]

Function

Role in continence

The external anal sphincter (EAS) plays a critical role in maintaining fecal continence by providing voluntary squeeze pressure that augments the tone of the internal anal sphincter, thereby preventing leakage of gas or stool during episodes of increased intra-abdominal pressure, such as coughing or sneezing.^[40] This voluntary contraction can generate maximum squeeze pressures ranging from 90–159 mmHg in women and 218–238 mmHg in men, significantly enhancing the overall anal canal pressure to preserve continence.^[40] The EAS achieves this through somatic innervation from the pudendal nerve (S2–S4), originating in Onuf's nucleus, which enables conscious control over sphincter contraction.^[41] In addition to voluntary efforts, the EAS contributes to basal resting tone, accounting for approximately 15–20% of the total anal resting pressure (typically 40–80 mmHg overall), or approximately 6–16 mmHg, through continuous low-level motor unit firing from Onuf's nucleus.^[40] This tonic activity helps sustain a high-pressure zone in the distal anal canal, supporting passive continence even without active effort.^[42] During the sampling reflex, where the internal anal sphincter briefly relaxes in response to rectal distension to allow sensory assessment of contents, the EAS maintains its pressure to prevent inappropriate expulsion, contracting further if the contents are deemed unsuitable for release.^[42] The EAS works in synergy with the puborectalis muscle to form the anorectal angle (approximately 60°–105° at rest), which mechanically occludes the anal canal and serves as a key barrier to fecal leakage.^[40] This coordinated action ensures effective closure, with the EAS providing additional striated muscle support to the puborectalis sling.^[2] Gender differences influence this function, as women often exhibit weaker EAS pressures, and vaginal childbirth poses a significant risk of trauma, with up to 35% of primiparous women sustaining occult injuries to the sphincter or pudendal nerve, leading to reduced continence over time.^[42]

Role in defecation

During defecation, the external anal sphincter undergoes voluntary relaxation in response to rectal distension, which triggers the rectoanal inhibitory reflex and signals the urge to defecate, allowing coordinated fecal expulsion.^[43] This relaxation is under conscious control via somatic innervation, enabling the individual to choose the timing of defecation while the reflex primarily facilitates internal sphincter relaxation.^[43] As a result, intra-anal pressure drops significantly, permitting the passage of stool without resistance.^[44] This process involves precise coordination with the relaxation of the internal anal sphincter and lengthening of the puborectalis muscle, which straightens the anorectal angle to facilitate smooth expulsion; the external sphincter's attachments to the puborectalis support this angle adjustment briefly during the act.^[43] Immediately following the passage of feces, the external anal sphincter contracts to restore the high-pressure zone in the anal canal, reestablishing continence and preventing leakage. The external anal sphincter's actions are mediated by the pudendal nerve, integrating reflex signals from rectal distension with the ability to override for voluntary retention if defecation is postponed.^[45] This pudendal-mediated control allows for efficient function during brief bursts of relaxation and contraction, supported by the muscle's predominance of slow-twitch type I fibers that provide fatigue resistance and low energy expenditure for sustained readiness.^[46]

Clinical significance

Fecal incontinence

Fecal incontinence refers to the involuntary leakage of stool, often resulting from dysfunction of the external anal sphincter (EAS), which impairs the ability to maintain continence during daily activities.^[47] This condition arises primarily from structural or neurological impairments to the EAS, leading to compromised anal closure pressure and reduced barrier function against fecal passage.^[48] The prevalence of fecal incontinence affects approximately 8% of community-dwelling adults worldwide (as of 2023), with rates increasing significantly in the elderly to as high as 15-20%.^[49]^[50]^[51] Women experience a slightly higher overall prevalence than men, estimated at around 8-9% compared to 7-8%.^[51] Key risk factors for fecal incontinence linked to EAS dysfunction include female gender, advanced age over 65 years, and chronic constipation accompanied by straining during defecation.^[52]^[53] Female gender elevates risk due to anatomical vulnerabilities and obstetric history, while aging contributes through progressive muscle weakening and comorbidities.^[52] Chronic constipation with straining can exacerbate EAS strain, potentially leading to pudendal nerve stretch and subsequent dysfunction.^[54] Common causes of EAS-related fecal incontinence include obstetric trauma, anal surgery, and neuropathy. Obstetric trauma, particularly during vaginal delivery, results in anal sphincter tears in 1-6% of cases overall, rising to 5-10% in primiparous or instrumental deliveries, directly damaging the EAS and leading to leakage.^[55] Anal surgery, such as fistulotomy for perianal fistulas, causes incontinence in 6-40% of patients by dividing portions of the EAS, with higher rates in procedures involving more extensive sphincter transection.^[56] Neuropathy from conditions like diabetes or multiple sclerosis impairs EAS innervation, reducing muscle tone and contributing to incontinence through weakened voluntary control.^[57]^[58] In pathophysiology, EAS defects encompassing more than 90 degrees of the sphincter circumference significantly reduce resting and squeeze pressures, failing to generate adequate barrier function for continence.^[59] Denervation of the EAS, often from pudendal nerve injury, promotes muscle atrophy over time, further diminishing contractility and leading to passive leakage even without overt defects.^[59] These changes disrupt the normal sampling reflex and voluntary contraction mechanisms essential for deferring defecation.^[48] Symptoms of EAS-related fecal incontinence are commonly assessed using the Wexner (Cleveland Clinic) score, a validated scale ranging from 0 (perfect continence) to 20 (complete incontinence), evaluating frequency of episodes.^[60] This includes involuntary passage of solid stool (scored 0-4 based on frequency), liquid stool, mucus or soiling, flatus, and urgency requiring lifestyle alterations like pad use.^[61] Higher scores correlate with more severe impact, encompassing passive soiling, urgent leakage, and full loss of control over bowel movements.^[60]

Imaging and diagnosis

Endoanal ultrasound serves as the gold standard for detecting structural defects in the external anal sphincter, providing high-resolution imaging that delineates muscle thickness, integrity, and tears in three dimensions. It exhibits a sensitivity of 90% and positive predictive value of 85% for identifying external anal sphincter defects in patients with fecal incontinence, outperforming other modalities in routine assessment of sphincter morphology.^[62] Magnetic resonance imaging (MRI), including endoanal and pelvic approaches, offers detailed visualization of the external anal sphincter, puborectalis muscle, and associated nerve damage, making it particularly valuable for evaluating complex cases involving multiple pelvic floor structures. Endoanal MRI demonstrates comparable accuracy to endoanal ultrasound for detecting external anal sphincter defects, with a sensitivity of 81% and positive predictive value of 89%, while providing superior soft-tissue contrast for assessing atrophy and puborectalis involvement.^[62]^[63] Anorectal manometry evaluates the functional integrity of the external anal sphincter by measuring squeeze pressures, which reflect voluntary contraction strength, as well as resting pressures and endurance during sustained contractions. This technique quantifies maximal squeeze pressure as a primary indicator of external sphincter function, with lower values correlating to impaired continence mechanisms, and is essential for assessing overall anorectal physiology beyond structural imaging.^[64] Pudendal nerve terminal motor latency testing assesses neuropathy affecting the external anal sphincter by measuring the conduction time from pudendal nerve stimulation to muscle response, with prolongation beyond 2.2 ms indicating damage and potential contribution to sphincter dysfunction. This electrophysiological measure helps differentiate neuropathic from myopathic causes of weakness, particularly in cases of chronic incontinence.^[65] Electromyography of the external anal sphincter detects denervation through the identification of fibrillation potentials and other spontaneous activities, providing evidence of neuronal injury or reinnervation patterns in the muscle. Concentric needle electromyography is particularly sensitive for confirming pudendal neuropathy or sacral root involvement, complementing imaging by revealing subclinical neuromuscular changes.^[66]

Surgical considerations

The external anal sphincter is frequently involved in surgical interventions aimed at restoring continence in cases of fecal incontinence due to trauma or obstetric injury. Overlapping sphincteroplasty is a primary surgical technique for repairing tears in the external anal sphincter, involving end-to-end approximation or overlapping of the muscle ends to reconstruct the sphincter ring. This procedure has demonstrated long-term success rates of approximately 60-70% for continence improvement at 5 years, with marked functional gains in about 46% of patients based on standardized scoring systems like the St. Mark's incontinence score.^[67] Sacral nerve stimulation represents another key surgical option, particularly for neurogenic fecal incontinence where direct sphincter repair is not feasible; it involves implanting a device to modulate pudendal nerve firing and enhance sphincter function. Long-term outcomes show symptom improvement in 74% of patients, with at least 50% reduction in incontinence episodes sustained over 5 years, and quality-of-life enhancements across multiple domains.^[68] Surgical procedures carry inherent risks, including postoperative fecal incontinence in 10-20% of cases, wound infections (up to 45% in some series), and potential worsening of sphincter function due to inadvertent pudendal nerve injury during dissection, which can compromise innervation to the external sphincter.^[67]^[69] In surgeries such as hemorrhoidectomy or anal fistula treatment, lateral internal sphincterotomy is often performed to relieve hypertonicity, intentionally sparing the external anal sphincter; however, extension beyond the intended internal layer risks inadvertent division of the external sphincter, leading to incontinence rates of 15-45% depending on the extent of disruption.^[70]^[71] While the focus remains on surgical approaches, non-operative adjuncts like biofeedback therapy and injectable bulking agents can complement outcomes by strengthening sphincter control or augmenting volume, though they are not substitutes for repair in structural defects. Emerging therapies, including stem cell-based repairs in clinical trials (as of 2025), show promise for regenerating sphincter tissue in cases unsuitable for traditional repair.^[72]

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Anal sphincter atrophy has more recently been identified as an explanation for late-onset fecal incontinence in the absence of an anal sphincter defect [3]. It ...
[58]
Diagnostic tools for fecal incontinence: Scoring systems are ... - NIH
It consists of 5 items regarding types of leakage (solid, liquid, gas), the necessity to wear hygiene pads, and lifestyle alterations. Every item has points ...<|separator|>
[59]
External anal sphincter defects in patients with fecal incontinence
Sensitivity and positive predictive value were 81% and 89%, respectively, for endoanal MR imaging and 90% and 85%, respectively, for endoanal US. Conclusion: In ...Missing: ultrasound | Show results with:ultrasound
[60]
Endosonography in anorectal disease: an overview - PubMed
Studies comparing AE with endoanal magnetic resonance imaging (MRI) have shown that both methods are equally good for demonstrating defects in the external ...<|control11|><|separator|>
[61]
Survey of Anal Sphincter Dysfunction Using Anal Manometry in ...
Resting anal pressure and maximal squeeze pressure were considered as measures of internal anal sphincter and external anal sphincter function respectively.
[62]
Pudendal neuropathy in evacuatory disorders - PubMed
Pudendal neuropathy was defined as a pudendal nerve terminal motor latency greater than 2.2 ms. Results: Patients were a mean age of 60.7 (range, 17-88) ...
[63]
External Anal- and Urethral-Sphincter Electromyography for ... - NIH
Electromyography (EMG) of the anal and urethral sphincters is a robust approach for demonstrating denervation and renervation changes suggestive of neuronal ...
[64]
Long-term functional results of transvaginal anal sphincter repair for ...
Results: An improvement was seen in 69.3% of women with a marked improvement in 46.5%. Furthermore, 70% said they would recommend the procedure to a friend, if ...<|separator|>
[65]
Long-term outcome of sacral nerve stimulation for fecal incontinence
At least 50 percent improvement in continence was achieved in 74 percent of the patients, and at least 70 percent improvement (median value) was achieved in 50 ...
[66]
Unilateral pudendal neuropathy. Impact on outcome of anal ...
We conclude that both pudendal nerves must be intact to achieve normal continence after sphincter repair. Patients with unilateral pudendal neuropathy are ...Missing: risks | Show results with:risks
[67]
Long-term results of lateral internal sphincterotomy for chronic anal ...
Ninety-eight percent of patients were satisfied with the outcome of surgery, but some degree of fecal incontinence occurred in fully 45 percent of patients at ...Missing: external | Show results with:external
[68]
Mild and severe anal incontinence after lateral internal sphincterotomy
In our opinion, the threat for incontinence is unpredictable; however, vaginal delivery history may increase the risk of severe incontinence.