A 41-year-old female recently diagnosed with triple negative breast cancer on paclitaxel, carboplatin, and pembrolizumab presented with acute onset of confusion and expressive aphasia. A week earlier she was discharged on Keppra 500 mg BID for generalized tonic clonic seizure.  

Her examination and vital signs were within normal limits except for stuttering speech and difficulty finding words. Her repetition and comprehension were intact. Labs were within normal limits with no leukocytosis and normal electrolytes. 

She was admitted with broad differentials of autoimmune encephalitis vs pembrolizumab toxicity vs paraneoplastic syndrome. Her pembrolizumab and other immunologic agents were discontinued. MRI revealed no intracranial abnormality or pathologic enhancement. Lumbar puncture showed normal CSF cell count, culture with negative infectious and paraneoplastic panels.  

Initial EEG reports left temporal polymorphic delta slowing with abundant left temporal sharp waves and she was started on 5 days intravenous Methylprednisolone at 1000 mg/day. She regained spontaneous speech with good eye contact. Repeat EEG showed no sharp waves, and she was discharged home on 80 mg/day Prednisone.