The S2 dermatome is a long strip running down the back of the thigh and the popliteal fossa into the upper calf. It also includes part of the buttock and some genital skin. Its thigh portion is supplied by the posterior cutaneous nerve of thigh.
| Nerve root | S2 |
|---|---|
| Area | Strip down the posterior thigh and popliteal fossa to the upper calf, plus part of the buttock |
| How to test | Light touch and pinprick are compared side to side at the ISNCSCI key sensory point: in the midline of the popliteal fossa. |
| Nerve Supply | Posterior cutaneous nerve of thigh; Pudendal nerve |
Carries touch, pain and temperature sensation from this strip of skin to the spinal cord through the S2 dorsal root.
S2 testing matters in spinal cord injury and cauda equina syndrome, where loss of sacral sensation changes the prognosis. Isolated S2 radiculopathy is rare, so loss of sensation in this strip should prompt testing of the other sacral levels and of bladder and bowel function. 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 several sacral roots, usually by a large central disc, causing saddle numbness, leg symptoms and bladder or bowel dysfunction; it is a surgical emergency.
Compression of the S2 root, for example by a sacral nerve root cyst or tumour, causing posterior thigh pain and numbness.
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