Carpal tunnel syndrome: a system for categorizing and grading electrophysiologic abnormalities

A system for classifying and grading electrophysiologic abnormalities in carpal tunnel syndrome is described. It is based on criteria for assessing severity of: 1) focal nerve conduction slowing; 2) conduction block or dispersion in the responses; and 3) axonal loss. These criteria are applied separately to each component (sensory or motor) of the median nerve. The focal sensory conduction velocity (CV) slowing across the carpal tunnel is measured from ratios of the wrist-to-palm CVs to palm-to-finger sensory CVs; and motor conduction slowing, from ratios of the wrist-to-palm CVs to elbow-to-wrist motor CVs. The amount of amplitude loss due to dispersion or conduction block at the carpal tunnel is calculated from the amplitude ratios of the responses (sensory or motor) evoked at the wrist to those evoked at the palm. Axonal loss is estimated from reduction in the absolute amplitudes of sensory or motor responses evoked at the palm. This classification scheme is relatively simple, and it not only quantifies the abnormality, but it also suggests the pathophysiology.