## Clinical Presentation Analysis **Key Point:** This patient has classic myasthenia gravis (MG) with negative AChR antibodies, defining seronegative MG. ### Diagnostic Features Present | Feature | Finding | Significance | |---------|---------|---------------| | Symptom pattern | Ptosis, diplopia, fatigable weakness | Pathognomonic for MG | | Temporal pattern | Worse in evening, improves with rest | Hallmark of MG | | Edrophonium test | Positive | Confirms neuromuscular junction disorder | | AChR antibodies | Negative | Seronegative MG (10–15% of generalized MG) | | CT chest | Normal | Rules out thymoma | **High-Yield:** Seronegative MG accounts for ~15% of generalized MG cases. These patients may have antibodies against muscle-specific kinase (MuSK) or LRP4, or be truly seronegative. The diagnosis rests on clinical features + positive edrophonium/neostigmine test + electrophysiology (repetitive nerve stimulation showing decremental response). **Clinical Pearl:** Ocular symptoms (ptosis, diplopia) are the initial presentation in ~50% of MG patients. Approximately 80% of ocular-onset cases eventually progress to generalized MG within 2–3 years, but the presence of a positive edrophonium test and negative AChR antibodies in a seronegative patient still confirms MG diagnosis at this stage. ### Why Seronegative MG? The combination of: 1. Typical MG symptoms (fatigable weakness, ptosis, diplopia) 2. Positive edrophonium test (diagnostic for neuromuscular junction dysfunction) 3. Negative AChR antibodies (excludes seropositive MG) 4. Normal CT chest (excludes thymoma-associated MG) ...defines **seronegative MG**, likely with MuSK or LRP4 antibodies or truly seronegative. **Mnemonic: FATIGUE** — Fatigable weakness, Antibodies (may be negative), Temporal variation (worse evening), Improved by anticholinesterases, Gaze-dependent ptosis, Unaffected by rest in other conditions, Edrophonium positive, Extraocular involvement
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