A 68-year-old woman from rural Maharashtra presents with progressive weakness of eye muscles and difficulty swallowing for 3 weeks. She reports that symptoms worsen towards evening and improve after rest. Examination reveals ptosis bilaterally and weak eye movements. Repetitive nerve stimulation shows a decremental response. Anti-acetylcholine receptor (AChR) antibodies are positive. Which of the following best explains the pathophysiology of her neuromuscular junction dysfunction?

Explanation

## Pathophysiology of Myasthenia Gravis (MG) ### Mechanism of AChR Antibody-Mediated Dysfunction **Key Point:** Myasthenia gravis is an autoimmune disorder where IgG antibodies against nicotinic acetylcholine receptors (AChR) cause neuromuscular junction failure through three mechanisms: 1. **Complement-dependent destruction** — antibodies bind to AChR, activate the complement cascade (C1q → C3b deposition), and cause lysis of the postsynaptic membrane 2. **Receptor cross-linking and internalization** — antibodies cross-link adjacent AChRs, leading to accelerated endocytosis and reduced receptor density 3. **Direct blockade** — antibodies sterically block acetylcholine binding (though less significant than complement lysis) ### Clinical Correlation **Clinical Pearl:** The decremental response on repetitive nerve stimulation occurs because: - Reduced number of functional AChRs at the neuromuscular junction - Each action potential releases a fixed amount of acetylcholine - With fewer receptors, successive stimuli fail to reach threshold for muscle action potential - This produces the characteristic **decremental response** (amplitude decreases with successive stimuli) **High-Yield:** Anti-AChR antibodies are present in ~85% of generalized MG and ~50% of ocular MG. Seropositive patients have more severe disease and better response to immunosuppression. ### Why Repetitive Stimulation Shows Decrement ```mermaid flowchart TD A[Motor nerve action potential]:::action --> B[ACh release from presynaptic terminal] B --> C[Reduced AChR density due to antibodies] C --> D[Fewer receptors available for binding] D --> E[EPP amplitude decreases with successive stimuli] E --> F[Eventually fails to reach threshold]:::urgent F --> G[Decremental response on RNS]:::outcome ``` **Mnemonic: CRIB** — **C**omplement activation, **R**eceptor internalization, **I**mmunoglobulin cross-linking, **B**lockade of binding site [cite:Harrison 21e Ch 385]