Monitoring the short-term effect of intravenous immunoglobulins in multifocal motor neuropathy using motor unit number index

•MUNIX was used to test therapeutic response to intravenous immunoglobulins (IVIg) in multifocal motor neuropathy (MMN) patients.•MUNIX sum-score was lower in MMN patients than in healthy controls.•MUNIX sum-score improved quickly after intravenous immunoglobulin treatment in MMN.

ObjectiveTo determine whether motor unit number index (MUNIX) is pertinent to monitor the effect of intravenous immunoglobulins (IVIg) in multifocal motor neuropathy (MMN).MethodsMUNIX was assessed longitudinally in 7 MMN patients and 17 healthy controls in the abductor pollicis brevis (APB) and abductor digiti minimi (ADM) muscles. A MUNIX sum-score and a compound muscle action potential (CMAP) sum-score were calculated by summing up the scores of APB and ADM. MMN patients were evaluated on the first day of IVIg infusion, 5 MMN patients were evaluated 22 days after IVIg infusion, and 3 MMN patients were evaluated 1 month after two IVIg infusions.ResultsIntraclass correlation coefficient of the MUNIX sum-score in healthy controls was 0.85, showing good test-retest reproducibility. MUNIX and CMAP sum-scores were lower in MMN patients than in healthy controls (p < 0.01 and 0.02, respectively). MUNIX sum-score improved in three of the five patients 22 days after IVIg infusion and in two of the three patients 1 month after 2 IVIg infusions, whereas CMAP sum-score improved in only one patient in both evaluations.ConclusionsIn this preliminary study, MUNIX seems to be a reliable and sensitive tool to monitor the short-term efficiency of IVIg in MMN.SignificanceMUNIX can help monitor IVIg treatment in MMN.