External Anal Sphincter

Overview

The external anal sphincter is a voluntary muscle that surrounds the lower part of the anal canal and plays a crucial role in maintaining fecal continence. It functions in coordination with the internal anal sphincter and pelvic floor muscles to control the passage of feces and gas. Unlike the internal anal sphincter, which is under involuntary control, the external sphincter is consciously controlled and essential for social continence.

Location

The external anal sphincter is located in the perineal region, surrounding the lower third of the anal canal. It:

• Extends from the anococcygeal ligament posteriorly to the perineal body anteriorly

• Encircles the anal canal beneath the skin, superficial to the internal anal sphincter

• Lies within the ischioanal fossa, in close relation to the levator ani muscles

Structure

The external anal sphincter is composed of skeletal muscle and is under voluntary control. It has three identifiable parts:

• Subcutaneous part: The most superficial portion, encircling the anal orifice just beneath the skin

• Superficial part: Arises from the coccyx and inserts into the perineal body, forming a sling around the anal canal

• Deep part: Closely associated with the internal anal sphincter and the puborectalis muscle, forming a continuous ring of muscle fibers

The muscle is innervated by the inferior rectal nerve, a branch of the pudendal nerve (S2–S4), and is supplied by the inferior rectal artery.

Function

The external anal sphincter is responsible for:

• Voluntary contraction: Allows conscious control of defecation and gas expulsion

• Baseline tone: Maintains resting tone in the anal canal to support continence

• Reflex response: Contracts reflexively during increases in intra-abdominal pressure (e.g., coughing, sneezing)

It works in conjunction with the internal anal sphincter and puborectalis muscle to create a functional closure mechanism.

Physiological Role(s)

The external anal sphincter plays a critical role in anorectal physiology:

• Maintaining continence: Ensures that stool and gas are retained until voluntary release is desired

• Responding to rectal distension: Contracts in response to rectal filling, preventing involuntary leakage

• Supporting anorectal angle: Works with pelvic floor muscles to maintain the anorectal angle, essential for continence

• Protective barrier: Guards against accidental loss of contents due to abdominal pressure or sudden movements

Clinical Significance

Disorders affecting the external anal sphincter can lead to significant functional impairment:

• Fecal incontinence: Often caused by damage to the external sphincter from obstetric trauma, surgical injury, or neurological conditions

• Anal sphincter tears: Can occur during childbirth or anal surgery, resulting in weakness and incontinence

• Pudendal neuropathy: Damage to the pudendal nerve may impair voluntary control over the sphincter

• Neuromuscular disorders: Conditions like multiple sclerosis or spinal cord injury can affect sphincter coordination

• Biofeedback therapy: Often used to retrain the sphincter in patients with incontinence or incomplete evacuation

• Surgical repair (sphincteroplasty): Performed in cases of structural damage to restore continence

Assessment of external sphincter function is typically performed using digital rectal examination, anorectal manometry, endoscopic ultrasound, or electromyography (EMG).