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

OBJECTIVE: To estimate the degree of axonal loss in patients diagnosed with multifocal motor neuropathy (MMN) using a novel assessment of motor unit numbers and size.

METHODS: Automated 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.

RESULTS: Compared with healthy controls, MMN patients had fewer motor units (MMN: 33±11vs HC: 93±36 [mean±SD]; p<0.0001) and larger 'size of the largest unit' (MMN: 1.2±0.5mVvs HC: 0.4±0.1mV; p<0.0001), despite having normal distal CMAP amplitudes (MMN: 7.6±1.8mVvs HC: 8.7±2.5mV; p=0.24).

CONCLUSIONS: MMN is associated with marked axonal loss which may be masked by striking re-innervation resulting in preservation of distal CMAP amplitudes.

SIGNIFICANCE: Assessment 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.