O122 Nerve excitability at different target levels in multifocal motor neuropathy

Abstract

Objectives To determine if different types of motor axons in multifocal motor neuropathy (MMN) are affected differently. Methods In 17 MMN patients we performed excitability tests of the median nerve motor axons at three target levels (20%, 40%, and 60% of the maximum CMAP). Motor responses were recorded from the thenar muscle. Nerve conduction findings included motor conduction block, demyelinative slowing, or normal values in the forearm region of the median nerve. Mann-Whitney test was used for statistical analysis between groups. Results Excitability parameters between 20% and 40% target levels did not demonstrate any difference. Depolarizing threshold electrotonus peak (TEdpeak) and S2 accommodation differed significantly between 40% and 60% target levels. Largest differences were observed between 20% and 60% target levels: fanning out of threshold electrotonus ( p = 0.008) was more pronounced at 60% target level, resting ( p = 0.029) and minimal ( p = 0.017) current-voltage slopes were steeper at 20% target level, TEd peak ( p = 0.007) and S2 accommodation ( p = 0.024) were greater at 20% target level. Discussion Discrepancies in the excitability parameters among axons with different thresholds may highlight selective nerve abnormalities in MMN, which renders them more vulnerable to develop axon damage along the course of MMN. Conclusions Current technique showed that motor excitability-abnormalities in MMN differ between axons with different thresholds. Significance Excitability tests applied at different target levels may reveal specific ion-channel disfunction in different types of motor axons in MMN.