Conduction block and abnormal temporal dispersion--diagnostic criteria.

Conduction block and abnormal temporal dispersion are important electrophysiological parameters for demyelination. As criteria for conduction block a more than 20% drop in the peak-to-peak amplitude with less than a 10% change in the negative-peak duration has been proposed. In order to check the validity of these criteria, the amplitude, duration and area of the compound muscle action potentials (CMAP) were studied in 50 normal persons. In these normal subjects there was a substantial difference in the degree of CMAP amplitude and area reduction and CMAP duration prolongation with proximal stimulation among the various nerves. Particularly the posterior tibial nerve showed the highest values for amplitude drop and for dispersion, which is most likely due to the inherent technical difficulty of stimulation at the popliteal fossa. Thus, different criteria should be used for conduction block or abnormal temporal dispersion for a given nerve. If a single value must be used as a criterion for all nerves, 50% for conduction block and 30% for abnormal temporal dispersion is recommended.