Repetitive after-discharges are more common in acquired demyelinating polyneuropathies

Repetitive after-discharges (RAD) following compound muscle action potential (CMAP) have been increasingly observed during F-wave studies in the diagnosis of demyelinating polyneuropathy. RAD was defined as repetitive potentials occurring within 500 ms after the initial CMAP at 1 mV/div sensitivity. EMG traces from patients with demyelinating polyneuropathy (n = 91), sensorimotor axonal polyneuropathy (n = 119) and normal subjects (n = 84) were reviewed to identify RAD during F-wave studies. The temporal relationship between RAD and the mixed nerve silent period (MNSP) was also assessed. RAD was detected in 90.1% of patients with demyelinating polyneuropathy, 29.4% of patients with sensorimotor axonal polyneuropathy, and 2.3% of normal subjects. Among patients with hereditary demyelinating polyneuropathy, RAD was present in 19.4%. In 84.7% of traces with obtainable F-waves, the initial RAD components appeared during the first period of MNSP. RAD may serve as a useful electrophysiological indicator of demyelinating polyneuropathy and may help distinguish it from sensorimotor axonal polyneuropathy and healthy individuals. It may also contribute to differentiating hereditary from acquired demyelinating polyneuropathies. The occurrence of RAD during the first period of MNSP, when voluntary motor activity is suppressed, suggests that these signals are not due to incomplete relaxation or voluntary discharges.