The S4-S5 dermatome forms the innermost ring of the saddle area, the skin immediately around the anus and the perineum. In standard exams S4 and S5 are tested together. Sensation here is carried mainly by branches of the pudendal nerve and the coccygeal nerves.
| Nerve root | S4-S5 |
|---|---|
| Area | Perianal skin and perineum |
| How to test | Light touch and pinprick are tested at the ISNCSCI key point in the perianal area, less than 1 cm lateral to the mucocutaneous junction, and deep anal pressure is checked on rectal exam. |
| Nerve Supply | Pudendal nerve (inferior anal and perineal nerves); Anococcygeal nerves |
Carries touch, pain and temperature sensation from this strip of skin to the spinal cord through the S4-S5 dorsal root.
Preserved sensation at S4-S5, or deep anal pressure, defines a spinal cord injury as incomplete in the ISNCSCI system, which strongly affects the chance of recovery. Loss of perianal sensation with back pain is a red flag for cauda equina syndrome. Dermatome maps differ between sources (Foerster, Keegan and Garrett, and modern evidence-based maps disagree), and neighbouring dermatomes overlap, so borders are approximate.
Compression of the lowest sacral roots causing perianal numbness, urinary retention and loss of anal tone; it needs emergency treatment.
Absence of all sensory and motor function at S4-S5 classifies an injury as complete (AIS A).
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