P292 Biophysical insights of sensory axons in multifocal motor neuropathy

Abstract

Objective To compare the biophysical properties of motor and sensory axons in multifocal motor neuropathy (MMN) by means of an established mathematical model of the human myelinated axon. Methods We performed excitability tests of motor and sensory axons in affected median nerves of 20 MMN patients and 20 healthy controls. Motor responses were recorded from the thenar muscles and sensory responses from the 3rd digit. Results By fitting the excitability recordings of healthy controls, a control motor and sensory nerve model was constructed. Fitting the control motor model to measured motor excitability in MMN indicated complex interaction of model components of the myelinated axon. Model fits suggested hyperpolarization of motor axons in MMN due to an increased pump current and altered passive components. There was only a small discrepancy between control sensory model and sensory excitability in MMN where fitting indicated an increased pump current and leak conductance in MMN. No common pattern was observed between motor and sensory excitability changes. Discussion Motor and sensory axon excitability recordings in nerves affected by MMN may provide an understanding of the mechanisms underlying the selective involvement of motor axons. The contribution of passive components may suggest the selective structural nodal/paranodal changes of motor axons in MMN. Conclusions Biophysical properties of sensory axons in MMN and their lack of association with excitability abnormalities in motor axons recorded at the same site does not indicate a compensatory mechanism of sensory axons. Significance Sensory axons are unlikely to be targeted in MMN.