A 28-year-old man with confirmed seronegative myasthenia gravis (anti-AChR and anti-MuSK antibodies both negative) is evaluated for disease severity. Which finding best distinguishes seronegative MG from seropositive MG in terms of clinical and immunological characteristics?

## Seronegative vs. Seropositive Myasthenia Gravis **Key Point:** Seronegative MG (double-seronegative, ~10% of MG cases) typically presents with **milder, predominantly ocular symptoms** that generalize more slowly compared to seropositive MG, which often shows earlier bulbar and systemic involvement. ### Clinical and Immunological Comparison | Feature | Seropositive MG (Anti-AChR+) | Seronegative MG (Double-negative) | |---------|------------------------------|-----------------------------------| | **Frequency** | 80–85% of MG | 10–15% of MG | | **Typical presentation** | Ocular + generalized early | Purely ocular or slow generalization | | **Bulbar/respiratory involvement** | Common (40–50%) | Rare (< 10%) | | **Severity** | Often moderate to severe | Usually mild to moderate | | **Thymic pathology** | Thymoma (10–15%), hyperplasia (60%) | Thymoma rare (< 5%) | | **Antibody target** | Nicotinic AChR | MuSK, LRP4, or unknown | | **Response to immunotherapy** | Excellent | Variable; slower response | | **Prognosis** | Well-defined; responds to standard therapy | More heterogeneous | **Clinical Pearl:** A patient presenting with **pure ocular MG that remains ocular for years** should raise suspicion for seronegative disease. These patients have a more benign course and lower risk of myasthenic crisis. **High-Yield:** Seronegative MG = **"Mild and ocular"** phenotype; seropositive MG = **"Systemic and severe"** phenotype. ### Why Other Options Are Misleading - **Thymic pathology:** Thymoma/hyperplasia is more common in **seropositive** MG (70%), not seronegative. Seronegative patients rarely have thymoma. - **Edrophonium response:** Both seropositive and seronegative MG show positive response to acetylcholinesterase inhibitors; this is not discriminatory. - **Earlier onset and severe bulbar involvement:** This is characteristic of **seropositive** MG, not seronegative. Seronegative MG typically has later onset and milder symptoms. **Mnemonic:** **SERO-negative = Ocular, Slow, Serene** (mild course); **SERO-positive = Systemic, Severe, Surgery-often-needed** (thymectomy).