The T1 dermatome runs along the medial (inner) side of the forearm from about the wrist to just above the elbow. It is the lowest dermatome that extends into the arm. Its skin is supplied mainly by the medial cutaneous nerve of forearm.
| Nerve root | T1 |
|---|---|
| Area | Medial (ulnar) side of the forearm, up to just above the elbow |
| How to test | Light touch and pinprick are compared side to side at the ISNCSCI key sensory point: on the medial (ulnar) side of the antecubital fossa, just above the medial epicondyle. |
| Nerve Supply | Medial cutaneous nerve of forearm; Medial cutaneous nerve of arm |
Carries touch, pain and temperature sensation from this strip of skin to the spinal cord through the T1 dorsal root.
T1 root lesions are uncommon because disc herniations at T1-T2 are rare, so medial forearm numbness should prompt a search for lower brachial plexus causes such as thoracic outlet syndrome or an apical lung tumour. A T1 lesion may also involve sympathetic fibres and produce Horner 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 lower trunk of the brachial plexus (C8-T1) by a cervical rib or band, causing medial forearm and hand symptoms.
Traction of the arm upward (Klumpke palsy) can damage C8-T1, causing medial forearm numbness and weakness of the small hand muscles.
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