A 24-year-old woman presents with diplopia, ptosis worsening through the day, and weakness that improves with rest. Anti-AChR antibody is positive. CT chest shows a 3.5 cm anterior mediastinal mass. What is the most important immediate diagnostic test and management decision?
- A Tension test (edrophonium) to confirm diagnosis
- B Pulmonary function tests to assess for respiratory compromise, and plan thymectomy ✓
- C MRI brain to rule out brainstem lesion
- D Start plasma exchange as first-line long-term therapy
Correct answer: B. Pulmonary function tests to assess for respiratory compromise, and plan thymectomy
Explanation
In AChR antibody-positive MG with an anterior mediastinal mass (thymoma), thymectomy is indicated. Before planning surgery, respiratory function must be assessed because myasthenic crisis can be precipitated perioperatively; FVC < 2 L indicates high respiratory risk requiring optimisation. The MGTX trial (2016) confirmed benefit of thymectomy even in non-thymomatous MG. Edrophonium test is rarely used now given antibody tests. Plasma exchange is a short-term bridge (pre-surgery or crisis), not long-term therapy.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.