Anti-N-methyl-d-aspartate receptor encephalitis presenting as lower-limb Monoparesis and epilepsia partialis continua in early childhood: A case report

Background Anti- N -methyl- d -aspartate receptor (NMDAR) encephalitis is a major form of autoimmune encephalitis in children. Patients typically develop multiple neuropsychiatric symptoms within the first month of disease onset. Early diagnosis is important because timely immunotherapy improves patient outcomes. Case presentation We report a 21-month-old boy who presented with right lower-limb twitching and difficulty standing without behavioral changes or altered consciousness, which represented an atypical manifestation of anti-NMDAR encephalitis. The patient was admitted 26 days after symptom onset. At presentation, no overt neuropsychiatric symptoms were observed. The interictal electroencephalogram (EEG) showed frequent central sharp waves with left predominance. Twitching of the right lower limb was observed during both wakefulness and sleep without an ictal EEG correlate, which is consistent with epilepsia partialis continua. The twitching resolved spontaneously before treatment, whereas right lower-limb paresis persisted. Brain magnetic resonance imaging demonstrated subtle left hemispheric asymmetry characterized by mild prominence of the left central sulcus. 18 F-fluorodeoxyglucose positron emission tomography revealed decreased uptake in the left superior frontal gyrus. As only limited clinical manifestations were present, the patient did not meet the existing diagnostic criteria for anti-NMDAR encephalitis before antibody confirmation. Treatment with levetiracetam and intravenous immunoglobulin was initiated before antibody confirmation. Motor function gradually improved thereafter, and independent ambulation was eventually regained. Subsequently, a cerebrospinal fluid cell-based assay detected anti-NMDAR antibodies and confirmed the diagnosis. Conclusion This case highlights that anti-NMDAR encephalitis may present with isolated focal neurological symptoms in early childhood and should be considered even in the absence of typical neuropsychiatric features.