Somatosensory evoked potentials in chronic acquired demyelinating peripheral neuropathy

We recorded somatosensory evoked potentials (SEPs) over the scalp in eight patients with chronic acquired demyelinating peripheral neuropathy. They were obtained from 15 nerves in which sensory nerve action potentials (SNAPs) were absent or not more than 1 µV, but from which motor responses could be elicited. Motor and sensory (SEP-derived) conduction velocity was determined from the difference in response latency with wrist and elbow stimulation. In 11 nerves, afferent conduction velocity was slowed. In 10, there was relatively equal slowing in sensory and motor axons, whereas in 1 there was disproportionate slowing in afferent fibers. In four nerves, afferent conduction velocity was within the normal range despite slowing of motor conduction. We conclude that SEPs may be useful in evaluating peripheral sensory conduction in the absence of SNAPs, but can provide misleadingly normal data, presumably because of central amplification of an attenuated response arising from a few axons that conduct normally.