label: Compound motor action potential abnormality
label_lc: compound motor action potential abnormality
description: "An abnormal finding in a compound motor action potential measurement in EMG. Nerve conduction studies involve the application of a depolarising square wave electrical pulses to the skin over a peripheral nerve producing: (1) a propagated nerve action potential (NAP) recorded at a distant point over the same nerve: and (2) a compound muscle action potential (CMAP) arising from the activation of muscle fibers in a target muscle supplied by the nerve. In both cases these may be recorded with surface or needle electrodes. The CMAP is a summated voltage response from the individual muscle fiber action potentials. The shortest latency of the CMAP is the time from stimulus artifact to onset of the response and is a biphasic response with an initial upward deflection followed by a smaller downward deflection. The CMAP amplitude is measured from baseline to negative peak (the neurophysiological convention is that negative voltage is demonstrated by an upward deflection) and measured in millivolts (mV)."