M wave changes with temperature in amyotrophic lateral sclerosis and disorders of neuromuscular transmission.

Local cooling of the muscle resulted in significant increases in M wave surface areas in patients with ALS, myasthenia gravis, the Lambert-Eaton myasthenic syndrome, and also in controls. The most striking increases were seen in patients with early ALS who had minimal lower motor neuron involvement and/or little defect on neuromuscular transmission and patients with the myasthenic syndrome. Patients with myasthenia gravis had intermediate increases between these groups and the controls; there was a larger increase in M wave surface area in myasthenia gravis compared with controls but this could be accounted for by improvement in neuromuscular transmission. The large increase in M wave surface area in early ALS could be the result of temporal summation of individual muscle fibers in motor units with increased fiber densities. The increase in both ALS and the myasthenic syndrome could result also from the loss of a trophic factor causing changes at the muscle membrane.