## Lambert-Eaton Myasthenic Syndrome (LEMS): Presynaptic Pathophysiology **Key Point:** LEMS is a presynaptic autoimmune disorder caused by IgG antibodies against voltage-gated calcium channels (VGCC) on the motor nerve terminal, resulting in impaired acetylcholine release. ### Mechanism of Presynaptic Dysfunction 1. **Antibody-mediated VGCC blockade**: Anti-VGCC antibodies bind to L-type calcium channels on the presynaptic membrane 2. **Reduced calcium influx**: Action potentials fail to trigger adequate Ca²⁺ entry into the nerve terminal 3. **Impaired ACh mobilization**: Calcium-dependent fusion of synaptic vesicles with the presynaptic membrane is reduced 4. **Decreased ACh quanta release**: Fewer acetylcholine molecules are released per action potential 5. **Subthreshold end-plate potentials**: Insufficient depolarization of the muscle membrane → weakness ### Why Repeated Stimulation Improves Strength (Post-Tetanic Potentiation) **High-Yield:** With successive high-frequency action potentials: - Calcium gradually accumulates in the presynaptic terminal (despite VGCC blockade, some Ca²⁺ still enters) - Intracellular [Ca²⁺] rises above baseline - More vesicles are mobilized and released with each stimulus - ACh quanta increase → larger end-plate potentials → muscle contraction improves - This is the **opposite** of myasthenia gravis, where fatigability occurs ```mermaid flowchart TD A[Repeated nerve stimulation]:::action --> B[Successive action potentials]:::action B --> C[Gradual Ca²⁺ accumulation in nerve terminal]:::action C --> D[Intracellular Ca²⁺ concentration rises]:::outcome D --> E[Increased vesicle mobilization & ACh release]:::action E --> F[Larger end-plate potentials]:::outcome F --> G[Muscle strength improves]:::outcome G --> H[Post-tetanic potentiation observed]:::outcome ``` ### Clinical Features Explained | Feature | Explanation | | --- | --- | | Proximal weakness | Presynaptic defect affecting all muscles, but proximal more symptomatic | | Dry mouth, constipation | Autonomic dysfunction from anti-VGCC antibodies affecting autonomic nerve terminals | | Diminished reflexes | Reduced ACh release at neuromuscular junction | | **Incremental response on RNS** | Amplitude increases with successive stimuli due to Ca²⁺ accumulation | | Improvement with activity | Post-tetanic potentiation; opposite of MG | **Clinical Pearl:** LEMS is associated with small-cell lung cancer (SCLC) in ~50% of cases. Always screen for malignancy in LEMS patients. **Mnemonic:** **LEMS = Low calcium entry → Low ACh release → Legs weak (proximal) → Learns to improve with exercise (post-tetanic potentiation)** [cite:Guyton and Hall Textbook of Medical Physiology 14e Ch 8]
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