Myasthenia Gravis Following Excision of a Large Thymoma

Abstract

A 63-year-old gentleman presented with a history of fever and weight loss for 6 months and nonproductive cough for 3 months. There was no history of hypertension, heart disease, TB, or asthma. On examination, he had decreased breath sound on right mid zone. His neurological and musculoskeletal examination was normal and he did not have any lymphadenopathy. An initial chest radiograph showed an anterior mediastinal mass extending to the right of the ascending aorta. A contrast computed tomographic (CT) scan showed a lobulated 15×12cm heterogeneous soft tissue mass in the anterior mediastinum. There were no areas of the chest wall or pericardial invasion or lymphadenopathy (Fig. 1). Whole-body PET-CT showed FDG avid, heterogeneously enhancing soft tissue mass lesion measuring ~10.1 × 7.1 × 11.8 cm in the anterior mediastinum, displacing and compressing anterior segment of the right upper lobe and medial segment of the right middle lobe. No other FDG avid lesion in the body was seen. The patient had undergone a CT-guided biopsy which revealed thymoma. IHC on tumor cells were marked by broad spectrum CK. CD20 was not expressed. Ki-67 of the tumor cell population inclusive of lymphocytes/epithelial cells was 60%, opined lymphocyte-rich thymoma. The patient was submitted to median thoracotomy to remove a large mass occupying the upper and anterior mediastinum in position beside the pericardium from just right to the superior vena cava and right atrium, extending laterally and distally in the direction of the diaphragm (Fig. 2). The tumor was well encapsulated and no significant lymphadenopathy was seen. Post-op. histopathology revealed single lobulated soft tissue mass measures 13.5 × 9.5 × 5.5 cm with attached fibro-fatty tissue on the surface. Microscopic examination revealed lobulated tumor with fibrous bands separating nodules composed of sheets, fascicles, and whorls of plump to spindle-shaped cells having bland, vesicular nuclei, inconspicuous nucleoli, and moderate to abundant cytoplasm along with a patchy, dense infiltrate of small lymphoid cells. An occasional mitotic figure is seen. No necrosis or capsular invasion is seen. Features were compatible with a mixed thymoma. Post-operatively, the patient developed respiratory difficulty with collapse of the right lower lobe for which he was put on a ventilator, and also bronchoscopy and broncho-alveolar lavage was done. Electroneuromyography (ENMG) was normal, repetitive nerve stimulation (RNS) test revealed neuromuscular junction dysfunction, his serum antiacetylcholinestrase receptor antibody level was 8.26 nmol/L (normal range being <0.3 nmol/L). Based on radiological, biochemical, and pathological findings, a diagnosis of thymoma (World Health Organization classification type AB and Masaoka classification stage I) associated with myasthenia gravis was made. Post-operatively, pyridostigmine 60 mg 3 times a day and prednisolone 40 mg once daily were started and the patient was discharged in a satisfactory condition.