## Lambert-Eaton Myasthenic Syndrome (LEMS): Presynaptic Pathophysiology **Key Point:** LEMS is a paraneoplastic autoimmune disorder in which IgG antibodies target **voltage-gated calcium channels (VGCC)** on the presynaptic motor nerve terminal, reducing acetylcholine (ACh) release and causing proximal weakness with autonomic dysfunction. ### Mechanism of VGCC Blockade 1. **Anti-VGCC antibody binding** — IgG antibodies bind to L-type calcium channels (Cav1.1) on the presynaptic membrane 2. **Complement-mediated destruction** — MAC formation leads to loss of functional calcium channels 3. **Reduced calcium influx** — Fewer channels available for depolarization-induced Ca²⁺ entry 4. **Impaired ACh release** — Reduced intracellular Ca²⁺ → decreased mobilization and exocytosis of ACh vesicles 5. **Neuromuscular transmission failure** — Insufficient ACh quanta to reliably depolarize the postsynaptic membrane ### The Incremental Response: Why It Occurs **High-Yield:** The **incremental response (>100% increase in CMAP amplitude)** on high-frequency repetitive nerve stimulation (HFS) is the hallmark of LEMS and reflects: - **Calcium accumulation** — Repeated presynaptic action potentials open residual functional VGCC channels - **Increased intracellular Ca²⁺** — Calcium influx accumulates in the presynaptic terminal during HFS - **Enhanced ACh mobilization** — Higher Ca²⁺ concentration drives more vesicles to the release site - **Improved transmission** — More ACh quanta are released with successive stimuli, progressively increasing the CMAP amplitude **Clinical Pearl:** This is the **opposite** of myasthenia gravis (postsynaptic), which shows a **decremental response** because the problem is insufficient receptors, not insufficient neurotransmitter. ### Diagnostic Features of LEMS | Feature | Finding | Significance | |---------|---------|---------------| | **Antibodies** | Anti-VGCC (>85% of LEMS) | Confirms diagnosis; >90% have occult malignancy | | **RNS at low frequency (2–3 Hz)** | Decremental response initially | Similar to MG at baseline | | **RNS at high frequency (20–50 Hz)** | Incremental response (>100%) | Pathognomonic for LEMS | | **Single-fiber EMG** | Increased jitter, blocking | Confirms NMJ dysfunction | | **Autonomic symptoms** | Dry mouth, constipation, impotence | Reflects widespread VGCC dysfunction | | **Malignancy association** | Small cell lung cancer (50–60%) | LEMS is paraneoplastic in most cases | **Mnemonic for LEMS features:** **LAMB** = **L**ow frequency decremental, **A**uto-immune, **M**alignancy-associated (SCLC), **B**etter with activity (high-frequency stimulation improves transmission). ### Contrast: Myasthenia Gravis vs. LEMS ```mermaid flowchart TD A[Neuromuscular Junction Disorder]:::outcome --> B{Site of pathology?}:::decision B -->|Postsynaptic| C[Myasthenia Gravis]:::outcome B -->|Presynaptic| D[Lambert-Eaton]:::outcome C --> E[Anti-AChR antibodies]:::outcome C --> F[Decremental RNS]:::outcome C --> G[Worse with activity]:::outcome D --> H[Anti-VGCC antibodies]:::outcome D --> I[Incremental RNS on HFS]:::outcome D --> J[Better with activity]:::outcome D --> K[Autonomic dysfunction]:::outcome D --> L[Paraneoplastic SCLC]:::outcome ``` ### Why Autonomic Dysfunction Occurs in LEMS VGCC are expressed not only at skeletal NMJ but also in: - **Sympathetic nerve terminals** → dry mouth, impaired sweating - **Parasympathetic terminals** → constipation, urinary retention - **Autonomic ganglia** → erectile dysfunction Antibody-mediated VGCC blockade affects all these sites, causing widespread autonomic symptoms. [cite:Guyton & Hall Textbook of Medical Physiology Ch 9; Harrison Principles of Internal Medicine 21e Ch 428]
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