Multifocal Motor Neuropathy (MMN): Understanding a Rare Neurological Condition

Multifocal Motor Neuropathy (MMN) is a rare, immune-mediated, purely motor neuropathy with asymmetric, distal-predominant weakness without sensory loss.

Epidemiology

• Prevalence: 1–2 per 100,000.
• Incidence: ~0.6–1 per million/year.
• Male predominance: 2.5:1 to 3:1.
• Onset: Typically 30s–40s.

Clinical Features

• Asymmetric hand/forearm weakness (wrist/finger drop).
• Fasciculations, cramps, myokymia.
• No sensory symptoms, no UMN signs.
• Slowly progressive, stepwise.

Diagnosis

• Nerve conduction: conduction blocks at non-entrapment sites (pathognomonic).
• Anti-GM1 antibodies in ~40–50% (IgM).
• Exclude ALS, Lewis-Sumner, CIDP, vasculitis.

Treatment

• IVIG (first-line): 2 g/kg over 2–5 days; maintenance q4–8 weeks. Response in 80%.
• Subcutaneous IG alternative.
• Rituximab, cyclophosphamide for refractory cases.
• Contraindicated: Corticosteroids, plasma exchange (may worsen).

Early diagnosis and IVIG prevent irreversible axonal loss.