Electrophysiological mapping of the segmental innervation of the saphenous and sural nerves

Delineation of an area of sensory loss is often helpful in localizing a lesion at the spinal or root level. We have studied the segmental innervation of two cutaneous nerves, the saphenous and the sural, during selective posterior rhizotomy. Each nerve was stimulated electrically, and recordings were made from dorsal roots L-3 to S-2 in 30 patients. We found that both nerves received innervation through at least three spinal levels. The level of maximum innervation was approximately equally divided between L-3 and L-4 for the saphenous nerve. The sural nerve received maximum innervation through S-1 in 80% of the cases, but the remainder came through L-5 or S-2. These findings provide electrophysiologic evidence that these cutaneous nerves have multiple segmental supply, and the sensory area they supply does not lie solely within a single dermatome.