Electrophysiological parameters associated with reponse to IVIg in multifocal motor neuropathy

Introduction: Multifocal motor neuropathy (MMN) is a relatively rare disorder characterized by focal conduction blocks along motor fibres leading to weakness in the territory of individual nerves. Although its pathogenesis is not clear, MMN is thought to arise from dysimmune mechanisms and responds to treatment with intravenous immunoglobulin (IVIg). Objective: To evaluate the effect of the first administration of high dose IVIg on electrophysiological parameters in MMN. Methods: We tested 18 clinically affected nerves from 9 subjects with newly diagnosed MMN before and 7-20 days after the first administration of IVIg (2g/kg). Only nerves with clinical improvement (at least 1 point in the Medical Council Research scale) were included in the analysis. Distal and proximal CMAP amplitude, conduction velocity (CV), distal latency (DL), and conduction block (CB) were evaluated. CB was considered only in distal tract (e.g. elbow-wrist). Results: At baseline 10 nerves had abnormal distal and 13 proximal CMAP amplitudes. No significant change in their amplitude was detected after IVIg administration. Seven nerves had CV below lower normal limits at baseline examination including five with definite and two with possible CB. CV increased in six nerves after IVIg (mean 129.4%±13.5; range 84-190%) and slightly decreased in one nerve with a greater than 90% CB at baseline. CB remained unchanged despite clinical improvement in five nerve, improved by at least 20% in two nerves and slightly worsened (8%) in one. No correlation between CV and CB changes was detected. Only 3 of 17 nerves had abnormal DL at baseline and none improved after therapy. Conclusion: Improvement after IVIg therapy has been inconsistently associated with reduction of CB also because the correlation between clinical and electrophysiological improvement in individual nerves has been seldom analysed. In this study only CV, which is a marker of fast conducting fibres, but not CB, consistently improved in parallel with clinical improvement after IVIg. These findings suggests that the improvement of CV or, as recently reported, temporal dispersion may indicate an improved nerve conduction which may explain response to treatment despite the persistence of CB.