Single-Fibre EMG – Jitter in Myasthenia Gravis — SCE Neurology MCQ

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ModerateClinical NeurophysiologySingle-Fibre EMG – Jitter in Myasthenia GravisSCE Neurology

A 50-year-old woman with myasthenia gravis undergoes single-fibre EMG. The electromyographer reports increased jitter with blocking in the orbicularis oculi muscle. What does this finding indicate?

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Correct answer: D — Impaired neuromuscular transmission

Single-fibre EMG (SFEMG) measures the variability in time intervals between action potentials of two muscle fibres from the same motor unit ('jitter'). Increased jitter indicates impaired neuromuscular transmission and is the most sensitive electrophysiological test for myasthenia gravis (sensitivity ~95–99%). 'Blocking' occurs when transmission fails completely at one endplate, causing intermittent absence of one potential. SFEMG is particularly useful when RNS and antibodies are negative (seronegative MG). A: Normal jitter excludes NMJ pathology. C: Denervation is detected by conventional EMG. D: Myopathic changes are seen on standard EMG. E: UMN lesions are not detected by SFEMG.

Reference: ABN MG Guidelines (2025); AANEM SFEMG Practice Parameters