Introduction: Anti-NMDA receptor (NMDAr) encephalitis is an autoimmune disease that presents with psychiatric symptoms, epileptic seizures and movement disorders. Early diagnosis is essential for efficacy of immunotherapy; however, anti-NMDAr encephalitis is frequently mistaken for other autoimmune or infectious diseases.

Case description: A five-year-old boy presented with abnormal movements, epileptic seizures and progressive encephalopathy. The magnetic resonance imaging (MRI) and cerebrospinal fluid (CSF) were normal. Antiepileptic drugs were started and after seven months of the beginning of the symptoms he was seen in our service. He was unresponsive and with severe choreoathethosis. The MRI showed FLAIR signal hyperintensity bilaterally in insular cortex and CSF was normal. Electroencephalogram showed disorganized activity with electrographic seizures in right parasagital area. Anti-NMDAr antibodies were positive in CSF and serum. Investigation for associated tumors was negative. A course of methylprednisolone was attempted with no improvement. The child was treated with intravenous immunoglobulin, however in the fourth day he developed mesenteric thrombosis and underwent a large intestinal resection. He remained in intensive care unit for months and presented with several complications. After two months of immunotherapy, when he was clinically stable, the choreoathethosis improved and he was significantly more active and communicative.

Conclusion: Despite severe neurological features, anti-NMDAr encephalitis is a potentially reversible disease. Delay in diagnosis is frequent due to misleading presentation and may cause inferior response to immunotherapy. Our case illustrates some of the problems in the diagnosis and follow-up of patients with anti-NMDAr encephalitis, including rarely described adverse effects of immunotherapy.