The C4 dermatome is a cape-like area over the lower neck, the top of the shoulder and the upper chest down to about the level of the second rib. It is supplied by the supraclavicular nerves. On the front of the chest it borders T2 directly, because C5 to T1 are drawn out into the arm.
| Nerve root | C4 |
|---|---|
| Area | Lower neck, top of the shoulder over the acromioclavicular joint and upper chest above about the second rib |
| How to test | Light touch and pinprick are compared side to side at the ISNCSCI key sensory point: over the acromioclavicular joint. |
| Nerve Supply | Supraclavicular nerves |
Carries touch, pain and temperature sensation from this strip of skin to the spinal cord through the C4 dorsal root.
Pain felt over the top of the shoulder can be referred from C4, which explains why diaphragm irritation (phrenic nerve, C3-C5) can be felt as shoulder tip pain. A C4 level spinal cord injury spares this area but leaves the arms without sensation. Dermatome maps differ between sources (Foerster, Keegan and Garrett, and modern evidence-based maps disagree), and neighbouring dermatomes overlap, so borders are approximate.
Irritation of the diaphragm by blood, pus or gas (for example after laparoscopy or a ruptured spleen) is felt over the C4 skin of the shoulder.
Compression of the C4 root at the C3-C4 level causing pain and numbness over the top of the shoulder and base of the neck.
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