external anal sphincter

**Anatomy** - Striated skeletal muscle forming the outer sphincter of the anal canal; continuous superiorly with the puborectalis (levator ani). Traditionally divided into subcutaneous, superficial, and deep parts. - Lies external/distal to the internal anal sphincter (IAS), which is a continuation of the circular smooth muscle of the rectum. The intersphincteric groove (white line of Hilton) marks the border between IAS and EAS and is palpable on exam. - Blood supply: inferior rectal artery (from the internal pudendal artery). - Innervation: inferior rectal nerve, a branch of the pudendal nerve (S2–S4, "S2, 3, 4 keeps the poop off the floor"). Voluntary (somatic) control. - Venous and lymphatic drainage: below the pectinate line, drains via inferior rectal veins to the internal pudendal vein, then the internal iliac vein (systemic). Lymph goes to the superficial inguinal nodes. - Sensation of the anal canal below the pectinate line is carried by the inferior rectal nerve (somatic, pain-sensitive). This is why external hemorrhoids are painful and internal ones are not.

**Physiology** - IAS: involuntary, smooth muscle, tonically contracted, accounts for ~70–80% of resting anal tone. Parasympathetic input (S2–S4 pelvic splanchnic nerves) causes relaxation during defecation. Sympathetic input (hypogastric nerve) maintains contraction. - EAS: voluntary, accounts for ~20% of resting tone, and is responsible for the maximal squeeze pressure. - Rectoanal inhibitory reflex: rectal distension causes reflex IAS relaxation. The EAS contracts to maintain continence (the guarding reflex) until defecation is socially appropriate. Voluntary relaxation of the EAS and puborectalis permits defecation. - Continence also depends on the puborectalis sling (maintains the anorectal angle), rectal compliance, stool consistency, and sensation.

**Clinical significance** - **Fecal incontinence:** Most commonly caused by obstetric injury (a third- or fourth-degree perineal laceration), anorectal surgery, pudendal neuropathy (prolonged labor, chronic straining), neurologic disease (spinal cord injury, MS, diabetic neuropathy), or radiation. - Obstetric laceration grades: 3rd degree = involves the anal sphincter complex (3a <50% EAS, 3b >50% EAS, 3c EAS + IAS); 4th degree = also involves the anorectal mucosa. Risk factors include forceps or vacuum delivery, median episiotomy, macrosomia, occiput posterior position, and a first vaginal delivery. - Evaluation: anorectal manometry (resting and squeeze pressures), endoanal ultrasound (defects), pudendal nerve terminal motor latency, and defecography. - **Anal fissure:** Usually in the posterior midline. Caused by trauma that leads to IAS hypertonia and ischemia. Treatment includes fiber, topical nifedipine or diltiazem, nitroglycerin, or botulinum toxin; lateral internal sphincterotomy is used for refractory cases. The EAS is not cut because that would risk incontinence. - **Anorectal fistula/abscess:** Classified by relation to the sphincters (Parks classification): intersphincteric, transsphincteric, suprasphincteric, extrasphincteric. Crohn disease is a classic association. Surgical fistulotomy risks incontinence if too much EAS is divided. - **Hirschsprung disease:** Failure of neural crest migration causes aganglionosis. The IAS fails to relax (absent rectoanal inhibitory reflex), leading to a transition zone and a narrowed distal segment. - **Cauda equina/conus medullaris syndrome:** Loss of the anal wink reflex (S2–S4), decreased anal tone, and fecal incontinence or retention. - **Anal wink reflex:** Stroking perianal skin causes EAS contraction (afferent and efferent: pudendal nerve). It is absent in lower motor neuron lesions of S2–S4. - **Pudendal nerve injury or block:** Pudendal block (ischial spine landmark) during labor anesthetizes the perineum and EAS. - **Hemorrhoids:** Internal ones are above the pectinate line (visceral innervation, painless, bleed). External ones are below the line (somatic innervation, painful when thrombosed).

**High-yield pearls** - EAS = skeletal muscle, voluntary, pudendal nerve (S2–S4). IAS = smooth muscle, involuntary. - IAS gives resting tone, EAS gives squeeze tone. - Pectinate line: above = visceral innervation, internal iliac nodes, portal drainage (superior rectal vein) and below = somatic innervation, superficial inguinal nodes, systemic drainage. - Fecal continence is lost with EAS disruption, classically from obstetric injury (disrupted ring identifiable on endoanal ultrasound). - Repair of obstetric anal sphincter injury: end-to-end or overlapping sphincteroplasty.