Interesting Manifestation of Autoimmune Encephalitis on FDG PET Scan

We are presenting a 22-year-old man with intractable seizures. Autoimmune epilepsy, vasculitis, and paraneoplastic disorder were among initial differential diagnoses. His initial laboratory tests and toxicology screen were unremarkable. His initial brain MRI demonstrated generalized cortical atrophy. Features such as progressive encephalopathy, neuropsychiatric symptoms, personality change, and autonomic dysfunction were in favor of autoimmune encephalitis. Autoantibody evaluations including anti-NMDA receptor (NR1) IgG were negative in both serum and CSF samples. FDG PET scan demonstrated intense FDG uptake in the basal ganglia, more prominent in the caudate nuclei and putamina, which is one of the known autoimmune encephalitis imaging features.