[Strategies to interpret late waves in routine neurophysiology]

A number of late waves other than F and H spinal waves are commonly observed during motor nerve conduction studies. Simple criteria are available that allow an understanding of their origin and significance. A late wave can be characterized by 1) its origin, proximal or distal to the stimulator, 2) its position with reference to the spinal waves, 3) its latency, stable or not, 4) its appearance in response to all (constant) or not to all stimuli (inconstant), 5) its persistence or not to intense stimuli, 6) its relative threshold, 7) its behavior to paired stimuli (namely if it is or not evoked twice by paired stimuli). By use of these criteria, one may distinguish the late waves that are evoked proximal to the stimulator (indirect late waves), such as the spinal responses (H and F), the motor axon reflex and the indirect double discharge, from those late waves that originate distally to the stimulator (direct late waves) such as the late potential, the myo-axonal ephaptic response and the direct double discharge. Signification of these late waves in relation to peripheral nerve disorders is discussed. Late potentials, motor axon reflex and ephaptic myo-axonal responses are signs of a prior axonal lesions, whereas indirect double discharges relate to an ongoing demyelinating process.