A 35-year-old woman presents with progressive ptosis and diplopia over 3 months. Clinical examination reveals fatigable weakness of extraocular muscles and levator palpebrae superioris. Antibody testing is positive for anti-acetylcholine receptor (AChR) antibodies. What is the most common associated malignancy in myasthenia gravis?

Explanation

## Most Common Malignancy in Myasthenia Gravis **Key Point:** Thymoma is the most common associated malignancy in myasthenia gravis, occurring in 10–15% of MG patients. Conversely, 30–50% of thymoma patients develop MG. ### Epidemiology of MG and Malignancy | Association | Frequency | Clinical Significance | | --- | --- | --- | | Thymoma in MG | 10–15% | Most common; requires imaging in all MG patients | | MG in thymoma | 30–50% | High prevalence; screen all thymoma patients | | Lung cancer in MG | <2% | Not specifically associated | | Gastric cancer in MG | Rare | No established link | | Breast cancer in MG | Rare | No established link | ### Pathophysiology **High-Yield:** The thymus gland is central to MG pathogenesis: - Thymic hyperplasia occurs in ~65% of seronegative and seropositive MG patients - Thymomas contain myoid cells that express AChR epitopes - Autoimmune response against thymoma cells cross-reacts with neuromuscular junction AChR - Thymectomy improves symptoms in 80–90% of patients with thymoma-associated MG ### Clinical Implications **Clinical Pearl:** All patients with newly diagnosed MG require thoracic imaging (CT chest with contrast) to exclude thymoma, regardless of age or serology status. Early detection and thymectomy significantly improve prognosis. **Warning:** Do not assume absence of thymoma based on negative serology or young age. Seronegative MG still has thymoma association in ~5% of cases. ### Thymoma-Associated MG Features - Earlier onset (typically 40–60 years) - More severe generalized weakness - Higher antibody titers - Better response to thymectomy than non-thymomatous MG