Basic Principles and Technique

Stimulation of a mixed nerve or its sensory branch leads to a response that can be recorded from the nerve more proximally or distally (the sensory nerve action potential), as well as over the spine and scalp. The scalp response is small and obscured by the ongoing EEG, and for this reason it is often necessary to average up to 2000 responses after stimulation of a nerve in the arm or 4000 responses with stimulation of a nerve in the leg, depending on the amount of background noise, to obtain well-defined responses.

The SEP depends on the functional integrity of the fast-conducting large-diameter group IA muscle afferent fibers and group II cutaneous afferent fibers and on the posterior columns of the cord, although some fibers may follow a different, extralemniscal pathway. It is elicited for clinical purposes by an electrical stimulus that is sufficient to produce a slight muscle twitch when a mixed nerve is stimulated or to generate a sensory nerve action potential that is about 50 percent of maximum. It is generally well tolerated, even when applied with a repetition rate of about 3 or 5 Hz. SEPs can also be elicited by cutaneous nerve or dermatomal stimulation, but the role of these approaches, especially in the evaluation of suspected radiculopathies, is uncertain. SEPs are best recorded with surface electrodes. Over the scalp, either a bipolar derivation (both recording electrodes placed on the scalp) or a referential derivation (involving a noncephalic reference electrode) may be used.

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