Defining True Motor Conduction Block in Chronic Immune-Mediated Demyelinating Neuropathies: A Temporal Dispersion-Based Electrodiagnostic Approach.

BACKGROUND: Abnormal temporal dispersion (TD) poses a significant challenge to the detection of motor conduction block (CB) in routine electrodiagnostic practice. However, the most recent electrodiagnostic criteria do not explicitly address the relationship between these phenomena. This study aimed to evaluate the impact of motor conduction velocity (MCV) slowing and abnormal TD on motor CB, and to establish TD-based electrodiagnostic criteria for the accurate identification of motor CB. METHODS: Electrophysiological data from 35 patients diagnosed with chronic immune-mediated demyelinating neuropathies, according to the EAN/PNS electrodiagnostic criteria, were retrospectively analyzed. Linear regression models were employed to examine the relationship between MCV changes, TD, and proximal compound muscle action potential (p-CMAP) amplitude reduction. RESULTS: p-CMAP amplitude reduction was significantly associated with TD and MCV changes in the median, ulnar, and peroneal nerves (p < 0.001). Moreover, increases in TD accounted for 46.9% to 56.2% of the variability in p-CMAP amplitude reduction. In contrast, in the tibial nerve, TD explained only 21.8% of the observed variability. The regression analyses demonstrated that true motor CB could not be defined when abnormal TD exceeded 140% in the median nerve, 85% in the ulnar nerve, 185% in the peroneal nerve, and 240% in the tibial nerve. Furthermore, in the absence of abnormal TD, the cut-off values for detecting definite motor CB ranged from 40% to 60%, depending on the nerve examined. CONCLUSIONS: Even a marked decline in p-CMAP amplitude may not necessarily reflect true motor CB. Reliable determination of definite motor CB requires stringent, nerve-specific electrodiagnostic criteria that take abnormal TD into account.