[Isolated extensor digitorum brevis involvement in the population of normal systemic nerve conduction velocities]

To investigate the characteristics of isolated atrophy in extensor digitorum brevis(EDB), we analysed 262 patients whose polyneuropathy index(PNI) was 90% or more than the normal mean value. The PNI was determined as the mean percentage of the normal in 12 indices concerning to the velocity or long distance latency in motor nerve conduction studies. Latencies were adjusted to 160 cm height. Amplitude of compound muscle action potential (CMAP) in EDB showed no correlation with the patient's age. Similar result was obtained as well when studied in 115 patients whose PNI level was 95% or more than the mean normal value. In 18(7%) out of 262 patients CMAP amplitude in EDB was 1 mV or less; larger prevalence(p < 0.05) in women(10%) than in men(4%). The number of motor units which innervate EDB decreases along with the age, but this age-related change could be compensated by the magnification of each motor unit. In Western reports isolated EDB palsy has a predilection for emaciated men. Instead, our results showed the predominance in women. We may have some factors other than in Western countries, for example customs to sit directly on the mat for a long time, in the occurrence of isolated EDB palsy. In conclusion, amplitude reduction in EDB CMAP may reflect the following two factors; neuropathy-related factor and another factor independent of age or neuropathy.