Assessment of temporal dispersion in motor nerves with normal conduction velocity

Demyelinated nerves attenuate high-frequency components of propagating acti on potentials. In order to study if there is diagnostic use of this in moto r nerves, the spectral energy above 49Hz, amplitude, area, and duration of the compound muscle action potentials were measured; values after distal an d proximal stimulation of posterior tibial nerves were compared. Normative data were collected in 48 control subjects. The same measurements were made in 20 patients with polyneuropathy and reduced motor nerve conduction velo city, in 21 patients with mild polyneuropathy but normal motor nerve conduc tion velocity, and in 8 patients with myasthenia gravis. Overall, high-freq uency attenuation was closely correlated with amplitude decay (r = 0.63, P < 10(-19)) and with increase of action potential duration (r = 0.34, P = 10 (-5)). In the group of patients with normal NCV, high-frequency attenuation was abnormal in 9 (43%), amplitude decay was abnormal in two (10%), and ar ea decay was abnormal in one (5%) patient. The action potential duration wa s normal in all of these patients. High-frequency attenuation was not influ enced by stimulus intensity, thus it is not changed by conduction block, an d it was not influenced by impaired neuromuscular transmission. Hence, high -frequency attenuation, both sensitively and specifically does indicate abn ormal temporal dispersion. In conclusion, the simple measurement of high-fr equency attenuation markedly improves detection and characterization of dem yelination of human motor fibers. (C) 1999 Elsevier Science Ireland Ltd. Al l rights reserved.