Paramyotonia Congenita – SCN4A — SCE Neurology MCQ

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HardPeripheral Neuropathy & NeuromuscularParamyotonia Congenita – SCN4ASCE Neurology

A 40-year-old woman presents with recurrent episodes of muscle stiffness and weakness triggered by cold. Grip myotonia worsens in cold temperatures (paradoxical myotonia). EMG shows myotonic discharges. Genetic testing reveals a mutation in the SCN4A gene. What is the diagnosis?

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Correct answer: A — Paramyotonia congenita

Paramyotonia congenita is an SCN4A (sodium channel) channelopathy characterised by paradoxical myotonia — myotonia that worsens with repeated activity (paradoxical, as in other myotonias it improves with exercise — the 'warm-up' phenomenon). Cold triggers and exacerbates the myotonia. Episodes of weakness may follow prolonged cold exposure. A: DM1 has systemic features and DMPK CTG repeats. B: Myotonia congenita (CLCN1) has warm-up phenomenon. D: HyperKPP has episodic weakness with hyperkalaemia. E: Neuromyotonia has continuous muscle fibre activity from peripheral nerve hyperexcitability.

Reference: EAN Channelopathy Guidelines; ABN Non-Dystrophic Myotonia Guidelines