F-waves in neurogenic atrophy.

F-waves were determined in the peroneal nerves of 17 normal subjects, 11 amyotrophic lateral sclerosis (ALS) patients, and 13 patients with other disorders causing neurogenic atrophy (NA) in extensor digitorum brevis muscle. F-waves were examined with regard to latency and waveform. Latency was increased above normal for both ALS and NA patients (P less than 0.024 and P less than 0.003, respectively). In normal subjects, the mean number of repeated responses (same latency and waveform) was 1.9 (r = 1.0-5.0), where 1.0 corresponds to no repeated response. In ALS patients, repeated responses were 2.9 (r = 1.2-7.8), which is not significantly different from normal (P less than 0.099). In NA patients, repeated responses were 6.5 (r = 1.6-23), which is significantly above normal (P less than 0.041). A dominant waveform occurred in 21.5% (mean) of responses in normal subjects (r = 4%-58%). In ALS and NA, corresponding values were 40.3% (r = 10%-88%) and 44.1% (r = 17%-100%), both of which were significantly above normal (P less than 0.014 and P less than 0.012, respectively). The distribution of F-wave latencies is often nonrandom in normal, ALS, and NA subjects. The dominant F-wave that was present in normal subjects and significantly increased in ALS and NA subjects may result from decreased numbers of motoneurons capable of contributing to the F-response and would seem to be a useful additional measure of abnormality.