Multifocal motor neuropathy: clinical and electrophysiological findings

Multifocal motor neuropathy (MMN) can be differentiated from motor neuron disease by electrophysiological evidence of conduction block. To increase the probability of recording conduction block, we studied the whole nerve length including proximal segments in 84 patients with pure motor syndromes, using a special stimulation technique. In 8 patients, the diagnosis of MMN was confirmed by electrophysiological evidence of conduction block or temporal dispersion. The typical clinical picture of MMN with chronic progressive, asymmetrical, marked distal weakness was observed in our patients. Electrophysiological routine tests of distal nerves were usually normal except in nerve segments with conduction block. In 4 patients, conduction block could be recorded only in proximal nerve segments or spinal roots. All patients showed rapid improvement of clinical features and parallel reduction of conduction block during or after high-dose intravenous immunoglobulin (ivIG) therapy, supporting the diagnosis of an immune-mediated neuropathy. Three of them are now in remission without any therapy, whereas 5 still receive a regular ivIG course every 2-12 weeks as long-term treatment. In all patients with pure or predominantly motor syndromes and normal findings in electrophysiological routine tests of distal nerve segments, there should be proximal conduction block studies to avoid overlooking a treatable disorder such as MMN.