Use of somatosensory evoked potentials to evaluate the peripheral nervous system.

Somatosensory evoked potentials (SEPs) are being used increasingly to evaluate peripheral somatosensory pathways. They have been used in patients with plexus lesions but may provide misleading information when multiple lesions are present, demonstrating only the electrophysiologic consequences of the most distal lesion that is present. Ulnar SEPs can be abnormal in neurogenic thoracic outlet syndrome, whereas they are normal in the nonneurogenic variety. SEPs to nerve trunk stimulation are generally not helpful in patients with isolated radiculopathies, and SEPs elicited by dermatomal or cutaneous nerve stimulation have provided conflicting results. SEPs may be important in evaluating conduction along inaccessible proximal segments of limb nerves. Their value in Guillain-Barré syndrome, in which pathology may be predominantly proximal, is unclear. In evaluating peripheral neuropathies, SEPs can sometimes be useful when peripheral sensory nerve action potentials are unobtainable; sometimes, however, SEPs provide misleading information concerning conduction velocity. The SEP findings should not be relied on to determine whether sensory loss is organic or nonorganic, although they are one factor to consider in making this determination.