The internal anal sphincter (IAS) is the involuntary smooth muscle ring surrounding the anal canal for its proximal two-thirds, responsible for approximately 85% of resting anal sphincter tone. It is under autonomic control and is continuously contracted at rest, relaxing reflexly when the rectum fills.
| Origin | Continuation of the circular smooth muscle of the rectum at the anorectal junction |
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| Insertion | Intersphincteric groove — fuses with the external sphincter distally |
| Nerve Supply | Inferior hypogastric plexus — autonomic: sympathetic (L1-L2) maintains tone; parasympathetic (S2-S4) relaxes during defaecation |
| Blood Supply | Superior rectal artery; Middle rectal artery |
| Actions | Maintains anal continence at rest through sustained tonic contraction (85% of resting anal tone); Relaxes reflexly in response to rectal distension (rectoanal inhibitory reflex) |
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IAS dysfunction is assessed by anorectal manometry measuring resting pressure. IAS injury from obstetric trauma or internal sphincterotomy produces passive faecal incontinence — leakage without urgency. Internal lateral sphincterotomy, the treatment for chronic anal fissure, must be limited to avoid disrupting more than 30% of IAS to prevent incontinence. Botulinum injection into the IAS temporarily relaxes it for fissure treatment.
Palpable as a firm ring on digital rectal examination in the upper anal canal, distinct from the external sphincter by its smooth texture and involuntary tone.
Deliberate lateral internal sphincterotomy for chronic anal fissure risks passive faecal incontinence if more than 30% of the IAS is divided, particularly in elderly women with pre-existing sphincter weakness.
