A 52-year-old woman presents with ptosis that worsens during the day, diplopia, and fatigable limb weakness. Single-fiber EMG shows increased jitter and blocking. Anti-AChR antibody is positive. Which paraneoplastic association must be ruled out in this patient?

• A Thymoma (20–25% of MG patients have thymoma) ✓
• B Small cell lung carcinoma (anti-Hu antibodies)
• C Breast carcinoma (anti-amphiphysin antibodies)
• D Ovarian teratoma (anti-NMDA receptor antibodies)

Explanation

Myasthenia gravis (MG) has a strong association with thymic abnormalities: thymoma is present in 10–15% of MG patients, and thymic hyperplasia in 60–70%. Conversely, 30–35% of thymoma patients develop MG. All patients with MG should undergo CT chest to screen for thymoma. Thymectomy is indicated for thymomatous MG (regardless of severity) and for generalized MG in patients 18–50 years without thymoma (MGTX trial, 2016 NEJM, showed thymectomy improved outcomes). Anti-Hu is associated with paraneoplastic sensory neuropathy/encephalitis from SCLC. Anti-amphiphysin with stiff-person syndrome; anti-NMDA with autoimmune encephalitis (not MG).

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.