The T4 dermatome is a band around the chest at the level of the nipples, which is the classic surface landmark for this level in men and in most people lying flat. It runs from the spine round to the sternum on each side. Its skin is supplied by the fourth intercostal nerve.
| Nerve root | T4 |
|---|---|
| Area | Band around the chest at the level of the nipples (fourth intercostal space) |
| How to test | Light touch and pinprick are compared side to side at the ISNCSCI key sensory point: in the midclavicular line at the fourth intercostal space (nipple line). |
| Nerve Supply | Fourth intercostal nerve (lateral and anterior cutaneous branches); Posterior rami of thoracic spinal nerves |
Carries touch, pain and temperature sensation from this strip of skin to the spinal cord through the T4 dorsal root.
The nipple line is used as a quick check of a T4 sensory level in spinal cord injury and during regional anaesthesia, where a block to T4 is commonly sought for caesarean section. The nipple is a less reliable landmark in women with larger breasts, where the fourth intercostal space is better. Dermatome maps differ between sources (Foerster, Keegan and Garrett, and modern evidence-based maps disagree), and neighbouring dermatomes overlap, so borders are approximate.
Reactivated varicella zoster virus in the T4 dorsal root ganglion causes a painful band of blistering rash confined to this dermatome, sometimes followed by postherpetic neuralgia.
Injury at around T4 leaves sensation intact above the nipple line and absent below it, and affects trunk and leg function.
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