An 18-month-old female with a history of eczema and unvaccinated status presented with a 1-week history of developmental regression that profoundly advanced during admission to include depressed level of consciousness, generalized hypotonia, loss of purposeful movements, choreiform and dystonic movements, tonic seizures, and infantile spasms. MRI brain and spine and cerebral spinal fluid indices were normal. EEG initially showed several brief tonic seizures, abundant epileptiform discharges and photoparoxysmal response at low frequencies that raised concern for possible lysosomal storage disease.

She was started on Keppra with resolution of her seizures. Repeat EEG on day 18 of hospitalization however showed worsening pattern with modified hypsarrhythmia and she was initiated on high-dose oral prednisolone for infantile spasms. She underwent extensive neurometabolic and genetic workup which were largely unremarkable. Anti-NMDAR antibody returned positive on day 32 of hospitalization. She was initiated on methylprednisolone and plasma exchange, followed by intravenous immunoglobulin, with dramatic improvement. She was discharged on day 68 of hospitalization, with plan to continue intravenous immunoglobulin monthly.

Our case emphasizes the significance of suspecting anti-NMDAR encephalitis as a cause of progressive neurological deterioration in young children.