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MMN vs ALS – Key Distinctions — SCE Neurology MCQ

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ModeratePeripheral Neuropathy & NeuromuscularMMN vs ALS – Key DistinctionsSCE Neurology

A 45-year-old man with known multifocal motor neuropathy (MMN) on regular IVIg presents with asymmetric distal upper limb weakness without sensory loss. NCS show conduction block in motor nerves outside entrapment sites with normal sensory studies. Anti-GM1 IgM antibodies are positive. What distinguishes MMN from ALS clinically?

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Correct answer: E — MMN has purely motor involvement (no UMN signs, no bulbar involvement), conduction block on NCS, positive anti-GM1 IgM antibodies, and crucially responds to IVIg — ALS has combined UMN and LMN signs, no conduction block, and does not respond to immunotherapy

MMN can mimic ALS clinically (progressive asymmetric limb weakness with LMN signs). Key distinguishing features: (1) MMN has NO UMN signs (ALS has both), (2) MMN has NO bulbar involvement (ALS commonly does), (3) MMN shows conduction block on NCS outside entrapment sites (ALS does not), (4) MMN has anti-GM1 IgM in ~50%, (5) MMN responds to IVIg (ALS does not). Misdiagnosis of MMN as ALS would deny the patient effective treatment. A: Important distinguishing features exist. C: Clinical features also distinguish. D: ALS causes progressive weakness. E: MMN has NO sensory loss (pure motor).

Reference: EAN/PNS MMN Guidelines; ABN Neuromuscular Guidelines