CRITERIA FOR EARLY DETECTION OF CONDUCTION BLOCK IN MULTIFOCAL MOTOR NEUROPATHY (MMN) - A STUDY BASED ON CONTROL POPULATIONS AND FOLLOW-UP OF MMN PATIENTS

Motor conduction block (MCB) has been used as the main diagnostic crit erion in multifocal motor neuropathy (MMN). Nonetheless, no agreed def inition of block currently exists; the proposed required percent decre ment of proximal compound muscle action potential (CMAP) amplitude var ies from > 20% to > 50%. The aim of this work was to-evaluate, through a follow-up study of patients with MMN, the behaviour of MCB over tim e. The percent decrement and temporal dispersion of proximal CMAP have also been calculated in normal controls and in patients affected by a myotrophic lateral sclerosis (ALS). The results show that MCB in patie nts with MMN is a dynamic entity which greatly varies over time and th at a > 50% CMAP amplitued reduction may well be preceded by a smaller decrement that is nonetheless indicative of focal myelin damage in the appropriate clinical context. This datum and the results obtained in the control group and in patients with ALS suggest that a reappraisal of the diagnostic criteria for MCB, in cases with clinical and electro physiological data strongly indicative of MMN, should be considered. S ince MMN is a treatable disorder, the use of the proposed less restric tive criteria for the identification of MCB could allow for a promp an d more effective treatment.