Motor unit remodelling in multifocal motor neuropathy: The importance of axonal loss

•Axonal degeneration is an integral part of the disease process in MMN.•Axonal loss in MMN may be masked by prominent re-innervation.•Nerve conduction studies are not sensitive to motor unit remodelling in MMN.

ObjectiveTo estimate the degree of axonal loss in patients diagnosed with multifocal motor neuropathy (MMN) using a novel assessment of motor unit numbers and size.MethodsAutomated motor unit number estimation using a compound muscle action potential (CMAP) scan was undertaken in median nerves with conduction block. Results were compared with 30 age-matched healthy controls.ResultsCompared with healthy controls, MMN patients had fewer motor units (MMN: 33 ± 11 vs HC: 93 ± 36 [mean ± SD]; p < 0.0001) and larger 'size of the largest unit' (MMN: 1.2 ± 0.5 mV vs HC: 0.4 ± 0.1 mV; p < 0.0001), despite having normal distal CMAP amplitudes (MMN: 7.6 ± 1.8 mV vs HC: 8.7 ± 2.5 mV; p = 0.24).ConclusionsMMN is associated with marked axonal loss which may be masked by striking re-innervation resulting in preservation of distal CMAP amplitudes.SignificanceAssessment of motor unit properties should be incorporated into assessment of disease progression in MMN, given that nerve conduction studies are insensitive to motor unit remodelling.