Acute onset of unilateral movements?

This previously healthy neurotypical 3-year-old boy presented for frequent stereotyped episodes of left facial and arm twitching with maintained awareness. An electroencephalogram (EEG) and brain magnetic resonance imaging were normal. However, due to a high concern for focal motor seizures, he was discharged on daily levetiracetam. Six days later, he was readmitted for increased irritability and continued left hemibody movements while awake and asleep. Continuous EEG captured the episodes of concern without an EEG correlate. The evaluation for toxic, metabolic, and infectious conditions, including cerebrospinal fluid studies, was unremarkable. His agitation was concerning for an adverse effect of levetiracetam, so he was switched to lacosamide. The evaluation was broadened to include autoimmune disorders. His agitation did not improve, and then he developed low-grade fevers with continued irritability. His left hemibody movements evolved to orofacial dyskinesia and choreoathetoid movements (Video S1). He was started on intravenous immunoglobulins and methylprednisolone for probable autoimmune etiology. He was later found to have antibodies to the NMDA receptor in the cerebrospinal fluid (titer 1:5), confirming the diagnosis of anti-N-methyl-d-aspartate (NMDA)-receptor encephalitis, consistent with his behavioral change and movement disorder. This boy illustrates the clinical presentation of anti-NMDA encephalitis in a child presenting with stereotypical hemibody movements unresponsive to antiseizure medication. His findings highlight the importance of a comprehensive evaluation and early treatment of potential autoimmune causes of acute behavioral and motor changes without signs of infection or previous seizures. Devan J. Peterson: Conceptualization; investigation; supervision; visualization; writing— original draft; writing—review and editing. Olivia S. Yale: Conceptualization; investigation; writing—original draft; writing—review and editing. Janetta L. Arellano: Conceptualization; investigation; supervision; writing—original draft; writing—review and editing. The authors declare no conflicts of interest. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.