F- and M-wave conduction velocity in amyotrophic lateral sclerosis.

The F-wave was used to estimate motor nerve conduction velocity (MNCV) along the proximal segment of the fastest axons of the deep peroneal nerve (anterior-horn cells to knee) in 13 patients with amyotrophic lateral sclerosis (ALS) and in 21 age- and sex-matched control subjects. Measurements were based on the shortest latency of 100 F-waves obtained after supramaximal stimulation at the ankle and knee. The MNCV was estimated with the F-wave and conventional M-response methods in the distal segment (knee to ankle) of the nerve. Our results showed that proximal-nerve conduction is faster than distal conduction both in control subjects and in ALS patients. In ALS, there was a moderate reduction of MNCV, which was of equal degree in both the proximal and distal segments of the deep peroneal nerve. In control subjects and in ALS patients, no statistically significant difference was found between the MNCV estimated for the knee-to-ankle segment by conventional means and that obtained by the F-wave method. This indicates that the F-wave of shortest latency is conducted by the fastest surviving motor fibers, as in the initial components of the M-response--demonstrating that it is legitimate to make direct comparison of F- and M-latencies in estimating the proximal MNCV.