In Lambert-Eaton Myasthenic Syndrome (LEMS), the autoimmune target is the presynaptic voltage-gated calcium channel (VGCC). How does LEMS differ from Myasthenia Gravis (MG) in terms of clinical testing findings?
- A In LEMS, repetitive nerve stimulation (RNS) at high frequency shows a decremental response, same as MG
- B In LEMS, RNS at low frequency shows a decremental response, but high-frequency RNS shows an incremental response ✓
- C In LEMS, single-fiber EMG shows reduced jitter, differentiating from MG which shows increased jitter
- D In LEMS, edrophonium (Tensilon) test is positive, identical to MG
Correct answer: B. In LEMS, RNS at low frequency shows a decremental response, but high-frequency RNS shows an incremental response
Explanation
In LEMS, low-frequency RNS (3 Hz) shows a decremental response (like MG), but high-frequency RNS (50 Hz) or post-exercise facilitation shows a dramatic incremental (facilitatory) response >100%, due to calcium accumulation overcoming presynaptic calcium channel deficiency. This increment distinguishes LEMS from MG. Single-fiber EMG shows increased jitter in both conditions. Edrophonium test is less reliable in LEMS. LEMS is paraneoplastic in ~60% of cases (SCLC).
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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