Neuromuscular Junction Diseases

NMJ Structure & Function - The Synaptic Stage

• Presynaptic Terminal: ACh in vesicles. VGCCs ($Ca^{2+}$) trigger ACh release.
• Synaptic Cleft: Gap with Acetylcholinesterase (AChE) degrading ACh.
• Postsynaptic Membrane (Motor End Plate): Has junctional folds with nicotinic ACh receptors (nAChRs).
• Transmission: AP → $Ca^{2+}$ influx → ACh release → ACh binds nAChRs → $Na^{+}$ influx → End Plate Potential (EPP) → Muscle AP.

⭐ The safety factor of neuromuscular transmission is high, meaning significantly more acetylcholine (ACh) is released than the minimum required to trigger a muscle fiber action potential, ensuring reliable muscle contraction.

Myasthenia Gravis (MG) - Receptor Rebels

• Autoimmune: IgG antibodies attack postsynaptic Acetylcholine Receptors (AChRs).
• Pathophysiology: ↓ functional AChRs → impaired neuromuscular transmission → fatigable muscle weakness.
  • Weakness worsens with activity, improves with rest.
• Key Features:
  • Ocular: Ptosis, diplopia (common initial).
  • Bulbar: Dysphagia, dysarthria.
  • Generalized: Proximal muscle weakness.
• ⭐ > Over 75% of Myasthenia Gravis patients have thymic abnormalities, most commonly thymic hyperplasia (around 65%) or thymoma (around 10-15%).
• Diagnosis:
  • Serology: AChR-Ab (most common), MuSK-Ab.
  • Electrophysiology: Repetitive Nerve Stimulation (RNS) shows >10% decremental response; SFEMG shows ↑ jitter.
  • Ice pack test for ocular symptoms.
• Management:
  • Symptomatic: Acetylcholinesterase inhibitors (e.g., Pyridostigmine).
  • Immunomodulation: Corticosteroids, immunosuppressants.
  • Thymectomy.
  • Crisis: IVIg, plasmapheresis. 📌 My Asthenia Gravis: Muscles Are Getting Weaker.

Lambert-Eaton Syndrome (LEMS) - Calcium's Quiet Blockade

• Pathophysiology: Autoimmune; antibodies target presynaptic P/Q-type voltage-gated $Ca^{2+}$ channels (VGCCs) → ↓ Acetylcholine (ACh) release.
• Clinical Features:
  • Proximal muscle weakness (legs > arms), improves with exercise/repetition ("second wind" phenomenon).
  • Autonomic dysfunction (dry mouth, constipation, erectile dysfunction).
  • Hyporeflexia or areflexia.

⭐ Lambert-Eaton Myasthenic Syndrome (LEMS) is strongly associated with Small Cell Lung Cancer (SCLC), occurring in approximately 50-60% of LEMS patients, often preceding cancer diagnosis.

• Diagnosis:
  • Electromyography (EMG): Low baseline compound muscle action potential (CMAP) amplitude; incremental response (>100% increase) with high-frequency (20-50 Hz) repetitive nerve stimulation or post-exercise.
  • Anti-VGCC antibody detection.
• Treatment:
  • Treat underlying malignancy (especially SCLC).
  • Symptomatic: 3,4-Diaminopyridine (enhances ACh release).
  • Immunosuppression (e.g., prednisone, azathioprine).

Other NMJ Disorders - Toxic & Genetic Twists

• Toxin-Induced:
  • Botulism: Blocks presynaptic ACh release (BoNT). Descending flaccid paralysis.
  • Organophosphates: Irreversible AChE inhibition.

    ⭐ Organophosphate poisoning causes irreversible inhibition of acetylcholinesterase, leading to a cholinergic crisis characterized by 📌 DUMBBELLS (Diarrhea, Urination, Miosis, Bronchospasm/Bradycardia, Emesis, Lacrimation, Lethargy, Salivation/Sweating).

  • LEMS: Antibodies vs presynaptic $Ca^{2+}$ channels; weakness improves with exercise. SCLC link.
• Genetic (CMS):
  • Inherited NMJ protein defects. Infantile myasthenia.

High‑Yield Points - ⚡ Biggest Takeaways

• Myasthenia Gravis: Postsynaptic AChR antibodies. Fatigable weakness (ptosis, diplopia). Thymoma association. Edrophonium test positive.
• LEMS: Presynaptic VGCC antibodies. Often paraneoplastic (SCLC). Proximal weakness improves with exercise.
• Botulism: Toxin blocks ACh release (presynaptic). Descending flaccid paralysis. Avoid honey in infants.
• Organophosphates: Irreversible AChE inhibition. Cholinergic crisis (DUMBBELLS). Treat with atropine + pralidoxime.
• RNS: Decremental response in MG, incremental response in LEMS_._