The S1 dermatome covers the outer (lateral) border of the foot, the little toe, the heel and much of the sole, with a strip up the back of the calf. It is the dermatome involved in sciatica from an L5-S1 disc. The lateral heel is its key sensory point.
| Nerve root | S1 |
|---|---|
| Area | Lateral border of the foot, little toe, heel, much of the sole and the back of the calf |
| How to test | Light touch and pinprick are compared side to side at the ISNCSCI key sensory point: on the lateral heel (calcaneus). |
| Nerve Supply | Sural nerve; Lateral plantar nerve; Tibial nerve (calcaneal branches) |
Carries touch, pain and temperature sensation from this strip of skin to the spinal cord through the S1 dorsal root.
S1 radiculopathy causes pain down the back of the leg to the outer foot and sole, weak plantarflexion and a reduced or absent ankle reflex. Sural nerve lesions mimic the sensory loss on the outer foot but cause no weakness or reflex change. 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 S1 root, usually by an L5-S1 disc herniation, causing pain into the outer foot, calf weakness and a reduced ankle reflex.
Injury to the sural nerve leaves numbness along the outer foot and heel without weakness.
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