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

Objective

To determine whether motor unit number index (MUNIX) is pertinent to monitor the effect of intravenous immunoglobulins (IVIg) in multifocal motor neuropathy (MMN).

Methods

MUNIX 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.

Results

Intraclass 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.

Conclusions

In this preliminary study, MUNIX seems to be a reliable and sensitive tool to monitor the short-term efficiency of IVIg in MMN.

Significance

MUNIX can help monitor IVIg treatment in MMN.