Ectopic cervical thymoma (ECT) is an extremely rare tumor, especially associated with myasthenia gravis (MG). Various locations of ectopic thymoma have been reported, such as the neck, carotid triangle, chest wall, pleura, lung, and heart. Per guidelines, thymectomy is recommended in most MG patients with thymoma, which usually shows an excellent prognosis.

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.  

The diagnosis of ECT is challenging due to unusual locations, rare incidence, and inconclusive results of fine needle aspiration cytology (FNAC) that can lead to initial misinterpretation of a thymoma for thyroid or parathyroid lesions, as in our case. A comprehensive search and a timely diagnosis of thymomas for surgical resection are recommended.