Nerve Supply and Surgical Considerations for the External Anal Sphincter

Primary Innervation Pattern and Clinical Relevance

The external anal sphincter is primarily innervated by the inferior rectal branches of the pudendal nerve (S2-S4), with additional direct supply from the S4 sacral nerve root in approximately 31% of individuals 1

Surgeons must exercise extreme caution when debriding the lower sacral segments (below S3/S4) in patients with pelvic osteomyelitis, as the S2-S4 nerve roots innervating the anal sphincter can be damaged, resulting in permanent fecal incontinence in non-paraplegic patients 1
The inferior rectal branches of the pelvic plexus are vulnerable during intersphincteric dissections for low rectal cancer 2
The inferior rectal branches of the pelvic plexus are also vulnerable during fistula surgery involving the intersphincteric space 3
Aggressive sacral debridement procedures can damage the nerves innervating the external anal sphincter 1