Progressive Supranuclear Palsy With Post-traumatic Frontal Lobe Damage Mimicking Anti-IgLON5 Antibody Disease.

Phenotypic overlap between autoimmune encephalitis and movement disorders may rarely occur. We present a patient with progressive gait disturbance, vertical supranuclear gaze palsy, cognitive decline, and daytime somnolence, as well as extremity chorea. He was initially suspected of having autoimmune encephalitis. Antibody testing was negative in serum and cerebrospinal fluid. A fall leading to subdural hemorrhage and bilateral post-traumatic frontal lobe changes likely led to his bilateral upper and lower extremity involuntary movements. His mental status improved after lumbar puncture. His condition further improved on treatment with carbidopa levodopa 25/100 mg three times a day.  While the presentation of this case was suspicious for autoimmune encephalitis, the patient was diagnosed with progressive supranuclear palsy and traumatic encephalopathy with involuntary movements due to frontal lobe damage.