Single fibre electromyography in multifocal motor neuropathy with persistent conduction blocks

OBJECTIVE—To study theprocess of denervation-reinnervation in multifocal motor neuropathywith persistent conduction blocks in clinically affected and unaffected muscles.
METHOD—Volitionalsingle fibre electromyography (SFEMG) was performed in the extensordigitorum communis (EDC) of seven patients. The jitter, the fibredensity, and the mean interpotential interval were determined. Theresults before and after treatment with intravenous immunoglobulin(IVIg) between the unaffected EDC and affected EDC examined during thesame SFEMG session were also compared. In addition the values ofjitter, fibre density, and mean interpotential interval were analysedfor correlation with the strength score on the MRC scale, the durationof the neuropathy, the number of IVIg treatment periods, and the radialnerve conduction block values.
RESULTS—Mean jitter,percentage of jitters>60 µs, and impulse blocking percentage, werehigher than normal in both the affected EDCs and to a lesser degree inunaffected EDCs. Jitter decreased significantly after IVIg andcorrelated only with the MRC score. Fibre density and meaninterpotential interval were higher than normal equally in the affectedEDC and unaffected EDCs, but no correlation was found with strength,duration of the neuropathy, number of treatment periods, and conductionblock values.
CONCLUSION—The majorfinding is the presence of SFEMG abnormalities in clinically unaffectedEDCs. This shows a process of denervation-reinnervation even in theabsence of clinical symptoms, probably more frequent than commonlysupposed in this neuropathy. The rapid clinical improvement after IVIginfusions could be due to remyelination after demyelination and to aninterference of IVIg with the blocking effect of antibodies on theNa⁺ channels at the motor nerve endings.