A 58-year-old woman from Delhi presents with progressive ptosis and diplopia for 3 weeks. She reports worsening of symptoms toward the end of the day and improvement after rest. On examination, she has bilateral ptosis, ophthalmoplegia affecting extraocular movements, and mild proximal muscle weakness in the lower limbs. Repetitive nerve stimulation shows a decremental response. Serology is positive for anti-acetylcholine receptor (AChR) antibodies. Which of the following best explains the pathophysiology of her muscle weakness?

Explanation

## Pathophysiology of Myasthenia Gravis (MG) ### Mechanism of Muscle Weakness **Key Point:** In seropositive myasthenia gravis (anti-AChR antibodies present), autoimmune destruction of acetylcholine receptors at the neuromuscular junction is the primary pathophysiological mechanism. ### Cellular Events at the NMJ 1. **Antibody-mediated destruction**: IgG antibodies bind to AChR at the motor end plate 2. **Complement activation**: Classical complement cascade is activated, leading to membrane attack complex (MAC) formation 3. **Receptor loss**: Direct lysis of the postsynaptic membrane and receptor internalization and degradation 4. **Reduced receptor density**: AChR numbers decrease by 70–90% compared to normal 5. **Impaired neuromuscular transmission**: Fewer receptors available to bind acetylcholine (ACh) released from the presynaptic terminal 6. **Decreased miniature end-plate potentials (mEPPs)**: Individual ACh quanta produce smaller depolarizations 7. **Failure of neuromuscular transmission**: Insufficient depolarization to reach threshold, especially with repetitive stimulation (explaining decremental response on RNS) ### Clinical-Pathophysiological Correlation | Feature | Mechanism | |---------|----------| | Ptosis and ophthalmoplegia | Extraocular muscles have fewer AChR and are most vulnerable | | Fatigue with repetition | Depletion of ACh stores + reduced receptor availability | | Improvement with rest | Resynthesis and mobilization of ACh | | Decremental RNS response | Progressive failure of neuromuscular transmission with repetitive stimulation | | Anti-AChR seropositivity | Diagnostic of autoimmune NMJ disease | **High-Yield:** The decremental response on repetitive nerve stimulation is the functional consequence of reduced AChR density — each successive stimulus encounters fewer available receptors, leading to progressively smaller compound muscle action potentials (CMAPs). ### Distinction from Presynaptic Disorders In Lambert-Eaton myasthenic syndrome (LEMS), antibodies target presynaptic voltage-gated calcium channels (VGCC), reducing ACh release *per se*, not receptor availability. RNS shows an *incremental* response (opposite of MG). **Clinical Pearl:** The presence of anti-AChR antibodies in a patient with classic ocular and generalized weakness makes this diagnosis definitive and distinguishes it from seronegative MG (which may involve anti-MuSK antibodies or other mechanisms).