## Lambert-Eaton Myasthenic Syndrome (LEMS) ### Clinical Presentation The patient presents with the classic triad of LEMS: - Proximal muscle weakness (lower limbs > upper limbs) - Autonomic dysfunction (dry mouth, impotence, constipation) - Areflexia or hyporeflexia ### Electrophysiology **Key Point:** Facilitation with high-frequency stimulation (>10 Hz) is pathognomonic for LEMS. This distinguishes it from myasthenia gravis, which shows decremental response. ### Paraneoplastic Association | Malignancy | Frequency | Mechanism | |---|---|---| | Small cell lung cancer (SCLC) | 50–60% | Anti-VGCC (P/Q-type) antibodies | | Gastric cancer | 5–10% | Anti-VGCC antibodies | | Lymphoma | 3–5% | Anti-VGCC antibodies | | Other lung cancers | <5% | Rare | **High-Yield:** SCLC accounts for >50% of LEMS cases. In a smoker with LEMS, SCLC must be excluded with imaging (chest X-ray, CT). ### Pathophysiology LEMS is caused by autoantibodies against **voltage-gated calcium channels (VGCC)** at the presynaptic terminal. This reduces acetylcholine release, leading to muscle weakness. **Clinical Pearl:** Unlike myasthenia gravis (which worsens with fatigue), LEMS improves with exercise—a key diagnostic clue. ### Management 1. Screen for occult malignancy (especially SCLC) 2. Immunotherapy (IVIg, plasmapheresis) 3. Symptomatic: 3,4-diaminopyridine (K^+^ channel blocker) 4. Treat underlying cancer **Mnemonic:** LEMS = **L**ow acetylcholine release, **E**nhanced by exercise, **M**alignancy (SCLC), **S**eropositive for VGCC antibodies.
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