Proximal and Distal Conduction Velocities in Neuropathies

Abstract

Conventional methods for measurements of motor and sensory conduction have proved to be useful for evaluating function of motor and sensory nerve fibres in distal segments of peripheral nerves. The results, expressed as conduction velocity in meters per second and distal latency in milliseconds, provide numerical data which are of considerable clinical interest. The combination of needle electromyography and conventional nerve conduction studies provide useful information which is helpful in differentiating “neurogenic” disorders from primary muscle diseases. One of the most useful applications of nerve conduction studies has been in the diagnosis of peripheral nerve entrapments such as the carpal tunnel syndrome; appropriate slowing of nerve conduction in a specific segment of a peripheral nerve helps in localizing and documenting a peripheral nerve entrapment. Changes in the amplitude, duration and configuration of the compound muscle action potential (CMAP) evoked by stimulation of the peripheral nerve below and above the entrapment can provide information about the conduction “block” and slowing of conduction in the nerve at the site of entrapment. In some instances, when the conduction velocity values show significant slowing (greater than 40% of normal values), it may be possible to distinguish neuropathies in which the primary pathology is segmental demyelination versus primary axonal degeneration.