The L1 dermatome covers the groin, the skin over the inguinal ligament and the upper front of the thigh just below it, along with part of the external genitalia. In the back it reaches the upper buttock over the iliac crest. Its skin is supplied mainly by the iliohypogastric, ilioinguinal and genitofemoral nerves.
| Nerve root | L1 |
|---|---|
| Area | Groin and skin over the inguinal ligament, upper anterior thigh and part of the external genitalia |
| How to test | Light touch and pinprick are compared side to side at the ISNCSCI key sensory point: on the upper anterior thigh, midway between the T12 point (midpoint of the inguinal ligament) and the L2 point. |
| Nerve Supply | Iliohypogastric nerve; Ilioinguinal nerve; Genitofemoral nerve |
Carries touch, pain and temperature sensation from this strip of skin to the spinal cord through the L1 dorsal root.
Groin pain can come from the L1 root, from the ilioinguinal or genitofemoral nerves (often after hernia surgery) or from the hip joint, so the pattern of numbness helps sort these out. The cremasteric reflex also uses L1-L2. Dermatome maps differ between sources (Foerster, Keegan and Garrett, and modern evidence-based maps disagree), and neighbouring dermatomes overlap, so borders are approximate.
Trapping or injury of the ilioinguinal nerve, often after hernia repair, causing burning groin pain spreading into the scrotum or labia.
Compression of the L1 root, for example by a high lumbar disc or foraminal stenosis, causing groin pain and numbness.
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