## Diagnosis: Myasthenia Gravis This patient presents with the classic triad of myasthenia gravis (MG): **ocular symptoms (ptosis, diplopia), bulbar weakness (difficulty chewing), and fatigability (worsening toward evening)**. The positive edrophonium test confirms the diagnosis. ## Pathophysiology of Muscle Weakness **Key Point:** In myasthenia gravis, IgG autoantibodies bind to nicotinic acetylcholine receptors (AChR) at the neuromuscular junction, causing: 1. **Receptor blockade** — antibodies physically prevent acetylcholine binding 2. **Complement-mediated destruction** — C1q activation leads to membrane attack complex formation and loss of postsynaptic membrane 3. **Cross-linking and internalization** — antibodies cause receptor clustering and endocytosis, reducing the number of functional receptors **Result:** Reduced safety factor for neuromuscular transmission → muscle weakness and fatigue. ## Why Edrophonium Test is Positive Edrophonium is an acetylcholinesterase inhibitor that increases acetylcholine concentration in the synaptic cleft. By increasing available ACh, it temporarily overcomes the receptor deficit, improving muscle strength transiently — a hallmark diagnostic finding in MG. **Clinical Pearl:** The **fatigability pattern** (weakness worsening with use, improving with rest) distinguishes MG from other neuromuscular disorders. This occurs because the already-reduced safety factor deteriorates further as acetylcholine is depleted during sustained activity. **High-Yield:** About 85% of generalized MG and 50% of ocular MG have detectable anti-AChR antibodies. Seronegative MG (~15%) may have antibodies against muscle-specific kinase (MuSK) or LRP4.
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