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

Publication date: Available online 7 August 2017
Source:Clinical Neurophysiology
Author(s): Nidhi Garg, James Howells, Con Yiannikas, Steve Vucic, Arun V. Krishnan, Judith Spies, Hugh Bostock, Emily K. Mathey, John D. Pollard, Susanna B. Park, Matthew C. Kiernan
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 ± 0.5mV vs HC: 0.4 ± 0.1mV; p<0.0001), despite having normal distal CMAP amplitudes (MMN: 7.6 ± 1.8mV vs HC: 8.7 ± 2.5mV; 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.



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