The C8 dermatome covers the little finger, the ulnar (inner) side of the hand and the lower part of the medial forearm. It overlaps heavily with the ulnar nerve territory in the hand. The ring finger is often shared between C7 and C8.
| Nerve root | C8 |
|---|---|
| Area | Little finger, ulnar side of the hand and distal medial forearm |
| How to test | Light touch and pinprick are compared side to side at the ISNCSCI key sensory point: on the dorsal surface of the proximal phalanx of the little finger. |
| Nerve Supply | Ulnar nerve (palmar and dorsal cutaneous branches); Medial cutaneous nerve of forearm |
Carries touch, pain and temperature sensation from this strip of skin to the spinal cord through the C8 dorsal root.
C8 radiculopathy, usually from a C7-T1 disc or foraminal narrowing, gives little finger numbness with weak finger flexion and grip. Ulnar neuropathy at the elbow is the main mimic, but it typically splits the ring finger and spares the medial forearm, which is supplied by the medial cutaneous nerve of forearm. A Pancoast tumour at the lung apex can involve C8 and T1. 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 C8 root at C7-T1 causing numbness of the little finger and medial hand with grip weakness.
Ulnar nerve compression at the elbow produces numbness of the little and half of the ring finger, but the forearm skin is usually spared.
A lung apex tumour can invade the lower brachial plexus, causing C8 and T1 sensory loss, hand weakness and sometimes Horner syndrome.
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