The external anal sphincter is the voluntary striated muscle sphincter surrounding the anal canal, providing the conscious control component of faecal continence. Working alongside the smooth muscle internal anal sphincter and the puborectalis sling, it maintains continence by compressing the anal canal closed. Its innervation by the pudendal nerve and S4 perineal branches makes it vulnerable to obstetric perineal tears and pudendal nerve injury from childbirth.
| Origin | Three portions: subcutaneous (surrounding the anal canal), superficial (attached to the coccyx posteriorly and perineal body anteriorly), and deep (encircling the upper anal canal continuous with the puborectalis) |
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| Insertion | Perineal body anteriorly and anococcygeal raphe posteriorly |
| Nerve Supply | Inferior rectal branch of the pudendal nerve (S2, S3, S4) and perineal branch of S4 |
| Blood Supply | Inferior rectal artery |
| Actions | Voluntary closure of the anal canal for continence control; Maintains resting anal tone alongside the internal sphincter; Contracts during coughing and straining to prevent incontinence |
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The external sphincter can be voluntarily contracted to defer defaecation when socially inconvenient, and its ability to sustain this contraction for 40 to 60 seconds is the time window for reaching appropriate facilities. Beyond this, fatigue forces defaecation.
Obstetric anal sphincter injury (third and fourth degree perineal tears) is the most common cause of faecal incontinence in women, with a sphincter tear rate of approximately 3 percent in vaginal deliveries. Sphincteroplasty (overlapping repair) and sacral neuromodulation are the primary surgical treatments for faecal incontinence from sphincter injury. Endoanal ultrasound and anorectal manometry assess sphincter integrity and function.
The external anal sphincter is assessed by digital rectal examination, feeling for the ring of voluntary sphincter tone and the voluntary squeeze response around the examining finger.
Third or fourth degree perineal tear disrupting the EAS during childbirth producing faecal incontinence managed with primary repair and pelvic floor physiotherapy, with sphincteroplasty or neuromodulation for persistent incontinence.
