Activity-dependent conduction block in multifocal motor neuropathy - Magnetic fatigue test

Background: Multifocal motor neuropathy (MMN) is often misdiagnosed as motor neuron disease, especially when overt evidence of conduction block (CB) is lacking. Activity-dependent CB (ADCB), defined as transient CB induced by brief exercise, has been recently found in MMN but not in ALS. Methods: To test the diagnostic utility of ADCB for differentiating MMN from ALS, the authors recorded the compound muscle action potentials (CMAPs) from small hand muscles by magnetically stimulating nerve roots before and after 1 minute of maximal voluntary contraction (magnetic fatigue test). They examined nine patients with MMN with unequivocal clinical responses to IV immunoglobulins (IVIgs), yet lacked CB according to the conventional criteria. Results: Six MMN patients had postexercise CB/temporal dispersion maximum in the immediate postexercise period. ADCB in an MMN patient improved after IVIg. Further analysis revealed that prolongation of the duration from the onset to the positive peak of the CMAP was the most sensitive indicator for MMN, presumably because the phase cancellation obscures the abnormalities of the other parameters. Conclusion: The magnetic fatigue test is useful in detecting mild conduction block presumably located in a proximal nerve segment in patients with multifocal motor neuropathy who do not fulfill its conventional electrodiagnostic criteria.