## Myasthenia Gravis: Pathophysiology and Management ### Autoimmune Destruction at the NMJ **Key Point:** In myasthenia gravis (MG), autoantibodies bind to nicotinic acetylcholine receptors (nAChR) and cause: 1. **Complement-mediated lysis** of the postsynaptic membrane. 2. **Cross-linking and internalization** of receptors → accelerated degradation. 3. **Blockade** of ACh binding sites. Result: 70–80% reduction in functional nAChR density. ### Anticholinesterase Therapy (Correct) **High-Yield:** Anticholinesterase inhibitors (e.g., pyridostigmine, neostigmine) block acetylcholinesterase, prolonging ACh residence time in the synaptic cleft. This increases the probability of ACh binding to the remaining functional receptors and partially restores neuromuscular transmission. **Clinical Pearl:** Pyridostigmine is a **reversible** anticholinesterase with a duration of 3–4 hours, making it suitable for chronic dosing. Edrophonium is a short-acting anticholinesterase historically used for diagnostic testing (Tensilon test), though it is now rarely available. ### Edrophonium as a Diagnostic Agent (Correct) **Mnemonic:** **TENSILON TEST** — Transient Edrophonium-induced Neuromuscular Strength Improvement in MG Lesion Observation Noted. Edrophonium produces rapid, transient improvement in muscle strength (within 30 seconds) in MG patients because it acutely increases ACh availability. However, this test is now largely replaced by serological testing (anti-nAChR antibodies) and single-fiber electromyography (SFEMG). ### Immunosuppressive Therapy: The Trap **Warning:** Mycophenolate mofetil is **NOT** a first-line immunosuppressive agent in MG. It is a **second-line** agent used in patients who do not respond adequately to corticosteroids or azathioprine. | Agent | Role in MG | Mechanism | |-------|-----------|----------| | Corticosteroids (prednisone) | **First-line** immunosuppression | Broad T-cell and B-cell suppression | | Azathioprine | **First-line** steroid-sparing agent | Purine analog; inhibits lymphocyte proliferation | | Mycophenolate mofetil | **Second-line** | Selective B-cell and T-cell inhibitor | | Cyclosporine | **Second-line** | Calcineurin inhibitor | | Rituximab | **Emerging** | Anti-CD20 B-cell depletion | **High-Yield:** The standard immunosuppressive approach in MG is: 1. **Acute:** Corticosteroids (prednisone) + anticholinesterase. 2. **Maintenance:** Azathioprine or mycophenolate (as steroid-sparing agents). 3. **Refractory cases:** Add rituximab or other B-cell-targeting agents. **Key Point:** Mycophenolate is effective in MG but is reserved for patients with inadequate response to or intolerance of first-line agents, not as initial immunosuppression. [cite:Harrison 21e Ch 378]
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.