A 55-year-old woman with a myxoid chondrosarcoma of the right hand, previously treated with chemotherapy that received one dose of pembrolizumab, with an excellent tumor response. One month later she developed progressive memory impairment and new onset of severe hyperglycemia (glucose > 700 mg/dL) and profound hypothyroidism (TSH of 90 mcIU/mL), attributed to pembrolizumab. She was treated with hormone replacement for autoimmune diabetes and hypothyroidism. Shortly after, she had her first generalized convulsive seizure. Initial MRI brain was unremarkable. Formal neurological evaluation two weeks later was concerning for fluctuating cognitive impairment and staring spells, for which she was admitted. EEG demonstrated focal status epilepticus of the left posterior quadrant. She required multiple agents to control her refractory seizures. MRI brain showed FLAIR hyperintensities in the bilateral hippocampi and cerebral hemispheres. CSF: WBC 3 (76% lymph), normal protein and glucose, negative infectious workup, 7 CSF specific oligoclonal bands, and positive anti-Hu (1:8 CSF and 1:400 serum) and positive serum anti-GAD-65 antibodies (>1:4800). Anti-TPO antibodies were 103 IU/ml.

She received 1 gram of intravenous solumedrol for 5 days. Due to partial response, she received five sessions of plasma exchange. Follow up MRI brain showed worsening FLAIR hyperintensities in the bilateral hippocampi, therefore, she was started on IVIG. Patient’s mental status continued to improve, and she was discharged to acute rehabilitation on a slow prednisone taper.