A Case of Microscopic Thymoma and Myasthenia Gravis: Key Points for Clinical Features and Surgical Management

Microscopic thymoma (MT) is defined as solitary or multifocal small thymic epithelial nests <1 mm in diameter, occurring in approximately 15% of patients with myasthenia gravis (MG) and sometimes associated with thymic cysts. Its preoperative diagnosis is challenging due to its small size, and computed tomography (CT) scans are unable to detect it. MTs are considered incipient or precursor lesions of thymoma, detectable only via microscopic examination. We report a case of MT identified pathologically in the thymic tissue surrounding a thymic cyst in a 52-year-old female hospitalized for drooping eyelids and muscle weakness. A mediastinal CT scan revealed an enlarged thymic gland with an associated cyst in the anterior mediastinum. Due to symptomatic disease, we performed a right video-assisted thoracoscopic thymectomy. No macroscopic tumor was observed, but histopathological analysis revealed an acquired multilocular thymic cyst with multiple thymic epithelial islands (MTs) in the surrounding thymic tissue. All previous cases reported, except one, were symptomatic for MG and none were diagnosed before surgery. The morphological pattern of MT differs from conventional thymoma, and the latest WHO classification categorizes it as thymic nodular hyperplasia. To maximize the detection of MT nests, complete resection and thorough histopathological examination of the thymic gland and surrounding tissue are essential, even in the absence of macroscopic lesions. Here, we propose key points for the evaluation and management of MT.