Seeing Double: a Case of a Massive Type B2 Thymoma Presenting with Pseudo-Internuclear Ophthalmoplegia (P9-13.004)

Abstract

<h3>Objective:</h3> NA <h3>Background:</h3> Thymomas are rare, malignant epithelial cell tumors usually diagnosed in the 5th decade. Median tumor size is approximately 5 cm. An estimated 20 – 25% present as paraneoplastic myasthenia gravis (MG) with positive acetylcholine receptor antibodies (AChR Ab). A rarely described presentation of MG is pseudo-internuclear ophthalmoplegia (p-INO). This presents a diagnostic conundrum as INO is discretely localizable to the brainstem and more commonly associated with cerebrovascular disease or demyelination. <h3>Design/Methods:</h3> NA <h3>Results:</h3> A 27-year-old man presented with a three-month history of binocular horizontal diplopia, night sweats, weight loss and generalized weakness. The rapid progression of weakness occurred almost immediately following the COVID-19 vaccine. Examination demonstrated bilateral INO, worrisome for central nervous system inflammatory demyelination. An MRI of brain and cervical spine showed no evidence of demyelination, but the thoracic MRI showed a large soft tissue mass in the right hemithorax. A chest CT demonstrated a massive multilobulated mass encompassing almost the entirety of the right thoracic cavity. Given the mass in conjunction with the neurological symptoms, AchR Ab was obtained demonstrating positivity (51.2 nmol/L) for MG. Biopsy revealed thymoma type B2 and positron emission tomography showed metastatic disease, stage IVa. The patient was treated with four cycles of cisplatin, cyclophosphamide, and doxorubicin with prednisone, as well as a thymus gland resection. He was additionally started on pyridostigmine 60mg every 4 hours which improved his p-INO and generalized weakness. <h3>Conclusions:</h3> This case represents a rare presentation of myasthenia gravis in a young patient with p-INO. The incidental finding of a large thymoma (measuring ~30cm) discovered on thoracic spine imaging for work-up of demyelinating disease prompts alternative diagnostic consideration when presented with MRI-negative INO. It also evokes the question, should there be more emphasis on expediting CT chest imaging to evaluate for thymoma in patients with antibody positive MG. <b>Disclosure:</b> Dr. Hart has nothing to disclose. Maja Ostojic has nothing to disclose. Dr. Farrell has nothing to disclose. Dr. Esch has received personal compensation in the range of $5,000-$9,999 for serving on a Speakers Bureau for Biogen.