Introduction/Aims

Progressive axonal loss in multifocal motor neuropathy (MMN) is often assessed with nerve conduction studies (NCS), by recording maximum compound muscle action potentials (CMAPs). However, reinnervation maintains the CMAP amplitude until a significant portion of the motor unit (MU) pool is lost. We therefore performed more informative CMAP scans to study MU characteristics in a large cohort of patients with MMN.

Methods

We derived the maximum CMAP amplitude (CMAPmax), a MU number estimate (MUNE) and the largest MU amplitude, stimulus current required to elicit 5%-50%-95% of CMAPmax (S5, S50, S95) and relative ranges ([S95-S5]x100/S50) from the scans. These metrics were compared with clinical, laboratory and NCS results.

Results

40 MMN patients and 24 healthy controls were included. 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). Disease duration and treatment duration were not associated with MUNE. Relative range was larger in patients with 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 anti-GM1 antibodies (p=0.037) and controls (p=0.044).

Discussion

We found that MU loss is common in MMN and accompanied by enlarged MUs. Presence of anti-GM1 antibodies was associated with increased relative range of MU thresholds and reduction in largest MU amplitude. Our findings indicate that CMAP scans complement routine NCS, and may have potential for practical monitoring of treatment efficacy and disease progression.

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