In Lambert-Eaton myasthenic syndrome (LEMS), antibodies against P/Q-type voltage-gated calcium channels (VGCC) at the presynaptic terminal cause impaired acetylcholine release. Which neurophysiological finding on repetitive nerve stimulation distinguishes LEMS from myasthenia gravis?

• A Decremental response at low frequencies (3 Hz)
• B Incremental (facilitating) response at high-frequency stimulation (50 Hz) >100% ✓
• C Normal response at all frequencies
• D Single fibre EMG shows increased jitter and blocking

Explanation

LEMS demonstrates a characteristic incremental (facilitating) response on repetitive nerve stimulation at high frequencies (20–50 Hz), with amplitude increasing >100% — because calcium accumulation during repetitive stimulation partially overcomes the calcium channel deficiency. Low-frequency stimulation (3 Hz) shows decremental response in both LEMS and MG. Single-fibre EMG increases jitter in both. The increment >100% on high-frequency stimulation is pathognomonic for LEMS and does not occur in MG.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.