Motor neuron disease: decremental responses to repetitive nerve stimulation.

We found significant decrement of the compound muscle action potential recorded from the abductor digiti minimi muscle in response to repetitive nerve stimulation in patients with rapidly developing motor neutron disease (MND), but not in patients with slowly developing disease when measured before exercise (7.6 plus or minus 1.3% versus 1.6 plus or minus 1.6%) and 2 minutes after exercise (13.6 plus or minus 1.2% versus 0.5 plus or minus 0.5%). All patients showed repair of decrement immediately after exercise. The mean amplitude of the compound muscle action potential was significantly lower in both the rapid-disease (2.9 plus or minus 0.9 mV) and slow-disease (5.2 plus or minus 1.6 mV) groups than in controls (14.5 plus or minus 1.2 mV). For individual patients, there was no correlation between amplitude of the compound muscle action potential and decrement. These data indicate that decrement in MND relates to activity of disease of the motor neuron.