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Fewer than 50 cases of thymoma-associated panencephalitis (TAPE) have been reported. The presence of GABAB-R autoantibodies is rarely reported with thymomas. We present a case of a 45-year-old woman who initially sought psychiatric care for depression and was diagnosed with autoimmune encephalitis with GABAB-R autoantibodies in the setting of thymoma.

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A 45-year-old woman with pre-diabetes and PCOS presented to her psychiatrist with six months of fatigue, apathy, worsening cognition, and 50 lb. weight loss. Examination revealed global bradykinesia, cogwheeling in upper extremities, hyperreflexia with sustained right ankle clonus, and positive Hoffman’s sign bilaterally. She was admitted to Neurology for expedited work up due to these symptoms. 

MRI of the brain showed T2/FLAIR hyperintensities of the left mesial temporal lobe, insula, and right posterior temporal lobe. Full body CT revealed a 10cm mediastinal mass compressing the SVC and right heart. Biopsy revealed thymoma, WHO type B1. A serum paraneoplastic panel was positive for calcium channel P/Q-type autoantibodies. Serum encephalitis panel revealed positivity for AMPA-R,  GABAB-R, GAD-65, and muscle Ach-R autoantibodies. A CSF encephalitis panel revealed GABAB-R and GAD-65 autoantibodies.  

During treatment she became dyspneic and hypertensive. Given concern for mass effect on the right heart with bradyphrenia, thymectomy was performed. Pathology revealed WHO type B2 and B3 combined thymoma. Ultimately, she was treated with IVIG, steroids, and plasma exchange. Repeat brain MRI showed resolution of the T2/FLAIR hyperintensities. Upon follow-up, her exam is near baseline with slightly increased tone in the left hand and leg. 

This case revealed novelties in the presentation of TAPE. The GABAB-R autoantibody is most associated with small cell lung cancer and has a rare association with thymoma. This patient’s clinical presentation also underscores the need to maintain a high degree of suspicion for autoimmune etiologies with new onset of psychiatric symptoms.