Nerve refractory period in early polyneuropathies

The refractory period of the sural nerve was evaluated in 115 patients with suspected polyneuropathy of various origin. They were asymptomatics or with minor, mainly sensitive, disturbances. In all the conventional neurophysiologic examination (motor and sensory velocities, distal latencies and F-waves of the peroneal, ulnar and sural nerves) was normal. The absolute refractory period (ARP) was defined as the shortest interstimulus interval at which a response to the second stimulus could be recorded. The relative refractory period (RRP) was defined as the interstimulus interval at which the responses to the first and to the second stimulus have equal latencies. The ARP was prolonged in 13 patients and the RRP in 28. In all, the refractory period was prolonged in 38 patients (33%). In any but three patients the ARP and the RRP were prolonged together. Thus the refractory period is more sensitive than the conventional neurophysiological examination in detecting early neuropathy. Moreover the ARP was modified in 2/9 patients with Steinert's myotonic dystrophy without clinical or electrophysiological signs of peripheral neuropathy.