18F-FDG PET/CT Metabolic Patterns in Clinically Probable Seronegative Autoimmune Encephalitis with Normal MRI Findings: A Retrospective Observational Study.

Seronegative autoimmune encephalitis (AIE) poses a significant diagnostic challenge because of the absence of identifiable neuronal antibodies and frequently normal structural neuroimaging. 18F-FDG PET/CT has emerged as a sensitive functional imaging modality for detecting metabolic abnormalities in such patients. We conducted this study to evaluate the metabolic patterns observed on 18F-FDG PET/CT in patients with clinically suspected seronegative AIE and to assess the diagnostic utility of 18F-FDG PET/CT when MRI findings are normal. Methods: This retrospective observational study included 12 patients with clinically suspected AIE who tested negative for known neuronal antibodies and underwent 18F-FDG PET/CT. Clinical presentation, time to imaging, MRI findings, and PET metabolic patterns were analyzed. Results: Among the 12 patients evaluated (mean age, 23.08 y), seizures (83%), fever (42%), and behavioral changes (25%) were the most common presenting symptoms. Brain MRI findings were normal for all patients. 18F-FDG PET/CT revealed abnormal cerebral metabolism in all patients, with predominant basal ganglia hypermetabolism and characteristic anterior-to-posterior cortical metabolic gradient, found in 92% and 33% of patients, respectively. Medial temporal lobe hypermetabolism was observed in 25% of patients. The mean SUVmax in the basal ganglia was 12.28 (median, 18.72). Conclusion: 18F-FDG PET/CT demonstrated consistent and characteristic metabolic abnormalities among patients with seronegative AIE, even in the presence of normal MRI findings. However, these findings are not disease-specific and should be interpreted in conjunction with clinical and other diagnostic parameters. 18F-FDG PET/CT may serve as a valuable adjunctive diagnostic tool in clinically suspected seronegative AIE.