Across-tarsal-tunnel motor-nerve conduction technique.

Tarsal tunnel syndrome is a commonly considered compression of the tibial nerve and its plantar divisions as the nerve curves behind the medial malleolus underneath the flexor retinaculum. Motor, sensory, and/or mixed-nerve conduction studies are used to confirm or exclude the presence of compression of the posterior tibial nerve and its plantar divisions. In previous studies, stimulation has been done either proximal to the tunnel or distally in the sole of the feet or in the toes. Thus, differentiation between compression of the nerve within the proximal tarsal tunnel, as distinguished from compression of the plantar nerves in the distal tarsal tunnel or distal to the tunnel, has not been feasible. In addition, onset latency is frequently difficult to measure, and peak latencies have not been reported for the motor-evoked action potential. This study reports across-tarsal-tunnel latencies and amplitude decrements for both the medial and the lateral plantar nerves. For the medial plantar nerve with active electrodes placed over the medial head of the flexor pollicis brevis, the calculated mean + 2SD across tunnel onset latency is 3.2msec, peak latency is 2.9msec, and amplitude decrement is 29.3%. For the lateral plantar division, the calculated across-tunnel onset latency is 3.2msec, peak latency is 2.9msec, and amplitude decrement is 27.2%. Medial plantar nerve latency distal to the tarsal tunnel for the mean + 2SD is 5.9msec to onset and 9.5msec to peak, and the lateral plantar nerve latency is onset 5.9msec and peak 9.7msec.