Early electrophysiologic changes in conduction block

Fibrillation potentials and positive sharp waves were recorded in the paralyzed muscles of 31 patients with a conduction block of more than 14 days' duration. In 75% of the muscles, these effects were attributed to axonal damage in addition to the local demyelinating block, this conclusion being based on changes in the properties of the motor unit found electromyographically when recovery had taken place. In the remaining 25% of the muscles, the denervation-like activity was thought to be an effect of the local demyelinating block alone. The delay of the sensory action potential recorded above the presumed site of nerve compression most likely arises from a block of the largest myelinated nerve fibers. This assumption is based on the fact that the amplitudes of the fastest- and the slower-conducted components in the patients differed little from corresponding components in the normal nerve.