The external anal sphincter is the voluntary component of anal continence, producing the squeeze pressure measured on anorectal manometry. Its pudendal nerve supply makes it vulnerable to obstetric and perineal trauma.
| Origin | Anococcygeal ligament and coccyx posteriorly; Perineal body anteriorly |
|---|---|
| Insertion | Encircles the anal canal — a complete ring in its superficial and deep parts |
| Nerve Supply | Pudendal nerve — inferior rectal branch (S2, S3, S4) |
| Blood Supply | Inferior rectal artery |
| Actions | Voluntary anal continence — the squeeze component; Maintains active anal closure during coughing, straining, and activity |
|---|
Obstetric anal sphincter injuries (OASI) involving the EAS are classified by grade — 3rd degree (EAS partial or complete tear) and 4th degree (with rectal mucosa involvement). Primary repair of OASI produces better long-term continence than secondary repair. Pudendal neuropathy from difficult delivery reduces EAS squeeze pressure even without direct sphincter injury.
Assessed by squeeze pressure on anorectal manometry and by endoanal ultrasound for structural integrity.
Third or fourth degree perineal tear disrupting the external anal sphincter during vaginal delivery, requiring immediate primary repair and long-term follow-up for faecal continence outcomes.
