Patterns of motor nerve demyelination and their prognostic significance in multifocal motor neuropathy

Multifocal motor neuropathy (MMN) is a rare immune-mediated neuropathy characterized by motor nerve involvement and, typically, a good response to intravenous immunoglobulin (IVIg). However, a subgroup of patients shows poor or absent treatment response and a more rapid disease course. We aim to characterize clinical, laboratory, and electrophysiological features associated with IVIg response in MMN over a follow-up period of up to 20 years.

Thirteen patients fulfilling diagnostic criteria for definite MMN were retrospectively analyzed. Detailed clinical data, comorbidities, IVIg treatment regimens, and serial nerve conduction studies (NCS) were evaluated. Particular attention was given to the extent and distribution of conduction block (CB) and temporal dispersion (TD). IVIg response was defined as ≥ 1-point improvement in MRC strength in at least two muscle groups within 4 weeks after infusion or equivalent functional improvement documented in medical records.

Four of 13 patients (31%) showed poor or absent IVIg response. These patients exhibited a higher number of clinically and electrophysiologically affected motor nerves at disease onset (median 6 vs. 3, p = 0.014) and more nerves with TD (median 2.5 vs. 1, p = 0.025), compared to IVIg responders. CB alone did not reliably distinguish responders from non-responders (median 1 vs. 1, p = 0.74). Anti-GM1 IgM antibodies were detected in 44% of the patients tested, of whom 75% were non-responders. Comorbidities (e.g., diabetes II, malignancy, autoimmune disease) were more frequent among non-responders.

A pattern of early nerve involvement with prominent TD appears to be associated with a more aggressive disease course and poorer IVIg response. Diagnostic evaluation in MMN should not focus solely on focal CB but also systematically assess the number of nerves with TD. Prospective studies with standardized protocols are needed to validate these findings.