A 35-year-old man presents with progressive asymmetric weakness of the right hand over 6 months, affecting grip strength and finger dexterity. Examination reveals atrophy of the first dorsal interosseous muscle, normal reflexes, and no sensory loss. Nerve conduction studies show a 60% reduction in compound muscle action potential (CMAP) amplitude when comparing proximal versus distal stimulation of the right median nerve at a site away from the wrist. MRI of the brachial plexus shows T2 hyperintensity and contrast enhancement of the affected nerve, marked as **A** in the diagram. Anti-GM1 IgM antibodies are positive. Which of the following is the most appropriate first-line treatment for this patient's condition?

Question

Accepted Answer

B. Intravenous immunoglobulin (IVIG) 2 g/kg induction, then 1 g/kg maintenance every 2–6 weeks. ## Why IVIG is the correct answer

Multifocal motor neuropathy (MMN) is a rare, chronic immune-mediated demyelinating motor neuropathy characterized by asymmetric, distal weakness in individual nerve distributions (radial, median, ulnar territories), conduction block on NCS at non-compression sites, and normal sensory conduction. The MRI finding of T2 hyperintensity and contrast enhancement of the brachial plexus/peripheral nerve (structure **A**) is consistent with demyelinating inflammation in MMN. IVIG is the established first-line treatment with response rates of 70–80%, using an induction dose of 2 g/kg over 2–5 days followed by maintenance therapy of 1 g/kg every 2–6 weeks (EFNS/PNS MMN Guidelines 2024). Early IVIG therapy helps preserve function and slow progression.

## Why each distractor is wrong

- **High-dose corticosteroids**: Corticosteroids are paradoxically ineffective and may worsen MMN, unlike CIDP where they are beneficial. This is a critical board-level distinction and a common pitfall.
- **Plasma exchange (plasmapheresis)**: Like corticosteroids, plasmapheresis is ineffective in MMN and may paradoxically worsen the condition. It is used in CIDP but not in MMN.
- **Oral cyclophosphamide**: While cyclophosphamide is a second-line agent for IVIG-refractory or IVIG-dependent MMN, it is not first-line. IVIG must be tried first given its superior efficacy and safety profile.

**High-Yield:** MMN is treated with IVIG (first-line), NOT corticosteroids or plasmapheresis—this paradoxical ineffectiveness of steroids/plasma exchange is a classic exam distinction from CIDP.

[cite: EFNS/PNS MMN Guidelines 2024; Harrison's Principles of Internal Medicine, 21st ed]

Answer

A. High-dose corticosteroids (methylprednisolone 1 g daily for 5 days)

Answer

C. Oral cyclophosphamide 1–2 mg/kg/day with mesna protection

Answer

D. Plasma exchange (plasmapheresis) twice weekly for 4 weeks

Explanation

Why IVIG is the correct answer

Why each distractor is wrong

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