Abnormal theta-band rhythm: EEG abnormality as potential biomarkers for disease severity in pediatric anti-NMDAR encephalitis

Anti-N-methyl-D-aspartate receptor (NMDAR) encephalitis in children often requires early immunosuppressive therapy before antibody detection. While various electroencephalogram (EEG) patterns, including extreme delta brushes (EDBs), have been reported in adults, pediatric EEG characteristics remain understudied. This study aims to assist clinicians in identifying severe cases early, potentially improving treatment outcomes through prompt intervention. This retrospective case series examined EEG features influenced by disease severity in children with anti-NMDAR encephalitis. We evaluated six children (1–13 years old; four females, two males) treated at Tokyo Metropolitan Neurological Hospital from January 2007 to January 2023. The severity of autoimmune encephalitis in our patients was assessed using the Clinical Assessment Scale in Autoimmune Encephalitis (CASE). The literature proposes a severity classification for the CASE score, wherein scores of 0–8 points are categorized as mild, 9–18 points as moderate, and 19–27 points as severe. In our patients, CASE scores ranged from 4 to 25 (median:19). We reviewed acute-phase EEG recordings, including 13 long-term videos and 58 conventional recordings. None of the patients maintained a normal posterior-dominant rhythm, and only one exhibited EDBs. Notably, three patients with higher CASE scores (≥15) displayed abnormal theta-band rhythm during non-REM sleep and prolonged EEG recovery times. Our findings suggest that abnormal theta-band rhythms may serve as a potential acute-phase EEG biomarker for severe anti-NMDAR encephalitis in children.