Sensory nerve conduction deficit in experimental monoclonal gammopathy of undetermined significance (MGUS) neuropathy

An emerging body of evidence from in vitro studies and in vivo animal model s supports a pathogenic role of antibodies in the development of peripheral neuropathy associated with monoclonal gammopathy of undetermined significa nce (MGUS). Although the assessment of motor and sensory nerve fiber functi on is of clinical importance, it is seldom applied experimentally. We descr ibe the application of an electrophysiologic method for the evaluation of m otor and sensory nerve fiber function using an experimental model of MGUS n europathy. Supramaximal stimulation of the tibial nerve elicited an early m otor response (M-wave, 1.7 +/- 0.1 ms, n = 10) and a late sensory (H-reflex , 7.8 +/- 0.1 ms, n = 10) response that was recorded from the hind foot of anesthetized rats. Intraneural injection of serum antibodies from a MGUS pa tient with sensorimotor polyneuropathy, but not from an age-matched control subject, produced a marked attenuation of the H-reflex (P < 0.01, n = 10) without affecting the M-wave. Light and electron microscopy of affected ner ve showed myelinoaxonal degeneration with sparing of the smaller unmyelinat ed nerve fibers. The combined electrophysiologic and morphologic findings p resented in this study are consistent with a selective sensory conduction d eficit in MGUS neuropathy. Selective injury of afferent nerve fibers by thi s patient's serum antibodies may result from reactivity to neural antigens uniquely expressed by sensory neurons.