We report on a 55-year-old woman with MG who was admitted for myasthenic crisis requiring treatment with intravenous immunoglobulin and steroids. The workup suggested acetylcholine-receptor-positive MG and a computed tomography scan found a heterogenous mass arising from or immediately adjacent to the inferior left thyroid lobe extending into the upper mediastinum measuring 40 × 33 × 28 mm and causing right tracheal deviation. This was initially confused for a mass of thyroid or parathyroid origins, however, parathyroid hormone and thyroid-stimulating hormone levels were normal. This multi-lobulated mass was trans-cervically resected from below the left lobe of the thyroid but distinct from it. Histopathological examination revealed a type A thymoma with transcapsular invasion and atypical cells classified as a Masaoka stage II thymoma.
During a 3-month follow-up visit, she showed a remarkable improvement in myasthenic symptoms, including dysphagia and dysarthria, with only residual ocular symptoms. Therefore, the prednisone dose was lowered with no need for non-steroidal immunotherapies. The patient was also scheduled to receive radiation therapy to the mediastinum for the malignant thymoma.