Compound nerve action potentials of the medial and lateral plantar nerves through the tarsal tunnel.

The purpose of this study was to determine and standardize an easy and convenient method of studying the orthodromic latencies of the medial and lateral plantar nerves for clinical purposes using conventional equipment. Forty-one able-bodied adult subjects ranging in age from 20 to 76 years were tested orthodromically to establish normal values over fixed distances. The temperatures of the feet ranged from 26C to 32C. A standard TECA bipolar surface electrode fixed in a plastic mount was placed on the posterior tibial nerve proximal to the flexor retinaculum. The medial plantar nerve was stimulated with a bipolar surface stimulator at a distance of 10, 14, and 18cm, and at the great toe. The lateral plantar nerve was stimulated at a distance of 14 and 18cm, and at the little toe. Orthodromic latencies for the medial plantar nerve for distances of 10, 14 and 18cm, and the great toe were 2.4 +/- SD 0.15msec, 3.2 +/- SD 0.26msec, 4.0 +/- SD 0.22msec, and 5.0 +/- SD 0.38msec, respectively. Lateral plantar nerve latencies for 14 and 18cm segments were 3.2 +/- SD 0.25msec and 4.0 +/- SD 0.27msec, respectively. These standard values should allow more accurate assessment of tarsal tunnel syndrome (TTS) and peripheral neuropathies because the orthodromic method involves a mixed motor and sensory nerve action potential and should be more sensitive to early changes than a pure motor nerve action potential.