A 60-year-old woman develops proximal limb weakness that worsens throughout the day, ptosis, and diplopia. She also has a 5 cm anterior mediastinal mass on CT. Repetitive nerve stimulation at 3 Hz shows a decremental response of 20%. Which additional finding would specifically suggest Lambert-Eaton myasthenic syndrome (LEMS) rather than myasthenia gravis?
- A Anti-AChR antibodies positive
- B Positive Tensilon (edrophonium) test
- C Incremental response on high-frequency (50 Hz) repetitive nerve stimulation ✓
- D Thymoma on CT chest
Correct answer: C. Incremental response on high-frequency (50 Hz) repetitive nerve stimulation
Explanation
Lambert-Eaton myasthenic syndrome (LEMS) is characterised by an incremental response (facilitation, >100% increase) on high-frequency repetitive nerve stimulation (50 Hz) — the opposite of the decremental response in MG. In LEMS, P/Q-type VGCC antibodies impair presynaptic ACh release; brief exercise temporarily facilitates neuromuscular transmission. Both conditions can have an anterior mediastinal mass (LEMS is associated with small cell lung cancer). Anti-AChR antibodies and edrophonium test are specific to MG, not LEMS.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.