Lower tibial than peroneal compound muscle action potential amplitude in neuromuscular diseases.

The pattern of findings on nerve conduction studies is an important component of an electrodiagnostic evaluation to assess for peripheral neuromuscular disorders. The aim of this study was to determine the extent to which a lower tibial compound muscle action potential (CMAP) amplitude compared with the peroneal CMAP amplitude is more indicative of specific neuromuscular disorders such as S1 radiculopathies, sciatic neuropathies, or peripheral neuropathies. The electromyographic (EMG) findings of 921 patients who had undergone an EMG of the lower extremity and in whom the EMG study was interpreted as normal or a single neuromuscular diagnosis was identified were retrospectively reviewed to determine the frequency of an absolutely lower tibial than peroneal CMAP amplitude. Thirty-five (7%) healthy subjects had a lower tibial than peroneal CMAP amplitude (i.e., the absolute value of the tibial CMAP was lower than the absolute value of the peroneal CMAP), despite both values being in normal range. The finding on nerve conduction study of an absolutely lower tibial than peroneal CMAP occurred in a significantly higher proportion of patients with a diffuse polyneuropathy (24%) and S1 radiculopathies (21%) compared with controls. The findings suggest that in subjects in whom lower extremity nerve conduction study demonstrates an absolutely lower tibial than peroneal CMAP amplitude, neuromuscular disorders such as a polyneuropathy or S1 radiculopathy should be considered.