|Nerve conduction studies (NCS) assess peripheral nerve function analagous to measuring current flow through a water pipe or a continuity test of an electric wire. NCS are conducted by recording evoked responses to stimulation of the nerve. Time between stimulus and onset of the action potential recorded distally is utilized to evaluate for nerve conduction slowing or blockage associated with pathologic nerve impingement.Motor NCS require stimulating a nerve and recording the compound muscle action potential (CMAP) that accompanies the distal mechanical muscle twitch. For example, median motor NCS are performed by stimulating the nerve at a topographic site at the wrist and ante cubital fossa while recording the CMAP from the abductor pollicis brevis. The electrical CMAP event is recorded with surface electrodes, amplified, and conveyed to an oscilloscope. A measuring device is used to calculated the time in milliseconds between onset of the stimulus and onset of the action potential (CMAP), referred to as a latency value. Motor nerve conduction velocity (NCV) is calculated by factoring distance and conduction time between two stimulation sites. (conduction time calculated by subtracting 1st latency from 2nd).
NCV= distance between stim. sites in mm./ conduction time between 2 stim. sites.
Sensory NCS are performed by using a strictly sensory site for either stimulation or recording. For example, median sensory NCS can be calculated by stimulating the median nerve at the same locations as with motor NCS, but with recordings made with ring electrodes over an autonomous zone, such as with ring electrodes on the index or middle fingers. NCS performed in this fashion are referred to as anti-dromic studies. Orthodromic(normal flow of sensory fibers) study could also be performed by stimulating the autonomous sensory fibers from the digit, with recordings made from mixed (motor+ sensory) nerve fibers at the wrist or ante-cubital fossa.