Diagnostic Yield and Prognostic Factors in Autoimmune Encephalitis: A Prospective Observational Study

Background: Autoimmune encephalitis (AE) has emerged as a leading cause of potentially reversible encephalopathy, often mimicking infectious, metabolic, and psychiatric conditions. With the discovery of neuronal autoantibodies, diagnosis has improved substantially, but diagnostic yield varies across studies. Prognostic factors influencing outcomes remain inadequately defined, especially in prospective cohorts. Objective: To assess the diagnostic yield of antibody testing, neuroimaging, and electrophysiology in patients with AE and to identify prognostic factors associated with functional outcomes. Methods: We conducted a prospective observational study of 150 patients fulfilling consensus diagnostic criteria for possible, probable, or definite AE at St Stephens Hospital Delhi a tertiary care center between march 2021 and february 2023.Clinical, biochemical, radiological, and electrophysiological data were collected. Autoantibody testing was performed in both serum and cerebrospinal fluid (CSF). Modified Rankin Scale (mRS) at discharge and 3 months was used to define outcomes, with favorable outcome defined as mRS ≤2. Logistic regression identified predictors of outcome. Results: Among 150 patients (mean age 42.6 ± 16.3 years; 58% female), antibody positivity was detected in 93 (62%), with higher yield in CSF compared to serum (56% vs 41%). MRI abnormalities were found in 84 patients (56%), predominantly mesial temporal hyperintensities. EEG showed abnormalities in 102 patients (68%), with extreme delta brush observed in 11 cases (7%). At 3 months, 96 patients (64%) had favorable outcomes. Independent predictors of poor prognosis included refractory seizures (OR 3.8, 95% CI 1.9–7.6), ICU admission (OR 4.2, 95% CI 2.1–8.4), abnormal MRI findings (OR 2.7, 95% CI 1.4–5.0), and treatment delay >14 days (OR 3.1, 95% CI 1.6–6.0). Early immunotherapy (≤7 days from onset) and younger age were associated with favorable outcomes. Conclusions: In this large prospective cohort, CSF antibody testing demonstrated the highest diagnostic yield. Early initiation of immunotherapy significantly improved outcomes, while refractory seizures, ICU admission, abnormal MRI findings, and delayed treatment predicted poor prognosis. These findings underscore the importance of early diagnosis and treatment in AE