Motor nerve latencies through the tarsal tunnel in normal adult subjects: standard determinations corrected for temperature and distance.

A standardized method of obtaining distal motor latencies of the medial and lateral plantar nerves over fixed distances with temperature correction in 37 normal subjects is presented. The posterior tibial nerve was supramaximally stimulated 6, 8, 10, 12 and 14cm proximal to the active electrode over the motor point of the abductor hallucis. These same stimulation points were then used when recording from the abductor digiti minimi. Distance was measured with both flexible tape and calipers. Surface skin temperature was recorded at 3 different skin sites on the foot. Distal motor latency for the medial plantar nerve using a flexible tape with no temperature correction for an 8cm segment was 3.4 +/- SD 0.5msec and 3.8 +/- 0.5msec for a 10cm segment. With the same 8cm stimulation point, a flexible tape, but no temperature correction, the lateral plantar nerve distal motor latencies were 3.6 +/- SD 0.5; for the 10cm point, they were 3.9 +/- SD 0.5. Nontemperature corrected data for 6, 12, and 14cm segments using both calipers and flexible tape are presented, as are the data from 8 and 10cm segments of both the medial and lateral plantar nerves with temperature correction between 28 and 32C. These standard vales allow more accurate assessment of the tarsal tunnel syndrome (TTS). A case of bilateral tarsal tunnel is included. It is noted that early changes in TTS may not be relfected in prolonged latency measurements, and that the emg may be a more sensitive indicator of the process. Both latencies and emgs are used for the diagnosis.