HP51 : Motor unit integrity in multifocal motor neuropathy: A systematic evaluation with CMAP scans

Abstract

To systematically study the yield of CMAP scan recordings in a large sample of MMN patients. Introduction: We identified motor unit (MU) integrity in a large cohort of patients with multifocal motor neuropathy (MMN). Methods: We performed compound muscle action potential (CMAP) scans in 40 MMN patients and 24 age-matched healthy controls. We derived the maximum CMAP amplitude (CMAPmax), a MU number estimate (MUNE) and the largest MU amplitude using MScanFit, stimulus current required to elicit 5% (S5), 50% (S50) and 95% (S95) of CMAPmax and relative ranges ([S95-S5]x100/S50). These MU characteristics were compared with clinical, laboratory and nerve conduction study (NCS) results. Results: CMAPmax and MUNE were reduced in MMN patients (both p<0.001). Largest MU amplitude as a percentage of CMAPmax was increased in MMN patients (p<0.001). Neither treatment duration nor presence of anti-GM1 antibodies were associated with MUNE. Relative range was larger in patients with confirmed presence of anti-GM1 antibodies than those without anti-GM1 antibodies (p=0.016) and controls (p<0.001). Largest MU amplitudes were larger in patients without anti-GM1 antibodies than patients with presence of anti-GM1 antibodies (p=0.037) and controls (p=0.044). Conclusion: We found that MU loss is common in MMN and accompanied by enlarged MUs. The presence of anti-GM1 antibodies is associated with an increase in relative range of MU thresholds and a reduction of largest MU amplitude, but not to MU loss. Our findings indicate that CMAP scan complements routine NCS, and may have potential for practical monitoring of treatment efficacy and disease progression.