The Clinical Significance of Abnormal Electroencephalography (EEG) Patterns in Patients with Neuropsychiatric Disorders Due to Anti-NMDA Receptor Encephalitis: A Comparative Study

<b>Background:</b> Anti-NMDA receptor encephalitis is an autoimmune disease characterized by severe neuropsychiatric disturbances, often misdiagnosed as a primary psychiatric disorder. Early diagnosis is crucial, as delayed immunotherapy is associated with worse outcomes. Electroencephalography (EEG) is a widely available tool for detecting abnormalities that may aid in early detection of cases that should undergo a thorough approach. Although EEG has high sensitivity, its specificity remains a challenge. <b>Methods:</b> This case-control study was carried out in the National Institute of Neurology and Neurosurgery of Mexico and included 241 patients with acute or subacute neuropsychiatric disturbances, raising the suspicion of autoimmune encephalitis and leading to the determination of NMDA receptor antibodies in the cerebrospinal fluid (CSF). EEG patterns were analyzed to determine the frequency of abnormal findings and their diagnostic value. <b>Results:</b> 140 patients were diagnosed as having definite anti-NMDA receptor encephalitis, whereas 101 had a negative determination of NMDA receptor antibodies. Psychosis was very frequent in both groups. However, severe cognitive dysfunction and catatonia were significantly more frequent in anti-NMDA receptor encephalitis patients. EEG abnormalities were significantly more frequent in patients with anti-NMDA receptor encephalitis patients (87.2% vs. 61.2%, <i>p</i> < 0.001). Diffuse slowing (75.7% vs. 46.6%, <i>p</i> < 0.001) and the extreme delta brush pattern (8.8% vs. 0%, OR = 20.6, <i>p</i> = 0.002) were significantly associated with anti-NMDA receptor encephalitis. Logistic regression analysis confirmed that an abnormal EEG remained strongly associated with anti-NMDA receptor encephalitis after adjusting for confounders. <b>Conclusions:</b> EEG abnormalities, particularly diffuse slowing and the extreme delta brush pattern, provide important diagnostic clues in patients with a clinical suspicion of anti-NMDA receptor encephalitis. While EEG has high sensitivity, its specificity is enhanced by recognizing distinct patterns. These findings support the integration of EEG into diagnostic algorithms to guide early detection and management of autoimmune encephalitis.