Occurrence of A-waves in F-wave studies of healthy nerves.

A-waves are compound muscle action potentials that follow the M-wave with a constant shape and latency; usually they are detected during F-wave studies. A large reference value database for F-wave parameters from the median, ulnar, peroneal, and tibial nerves was collected from 121-196 subjects aged 14 to 95 years without known pathology involving the studied nerves. From this material, we studied retrospectively the occurrence of A-waves. To be included, an A-wave had to be clearly discriminated from the baseline in at least 8 of 20 traces with a jitter of less than 0.5 ms. A-waves occurred in 25% of the tibial nerves studied and 14% of the peroneal nerves but in only 2% of median and ulnar nerves. In the peroneal and tibial nerves, the frequency of A-waves increased with age, suggesting that A-waves may be related to normal age-related mild neuropathic changes of alpha motor neurons. Thus, A-waves are frequently found in lower-extremity nerves in healthy subjects and less commonly in upper-extremity nerves. Their presence must therefore be interpreted with caution and cannot be taken as necessarily indicative of abnormality.