Aseptic Encephalitis in an Immunocompetent Young Adult: A Case Report.

Encephalitis is a major diagnostic challenge, especially when clinical findings, imaging studies, and cerebrospinal fluid (CSF) analysis suggest parenchymal encephalitis, but no specific etiologic agent or detectable autoimmunity can be identified. We present the case of a 27-year-old man with subacute neuropsychiatric symptoms following an infectious prodromal stage. Neuroimaging studies revealed frontotemporal involvement, and oligoclonal bands (OCBs) were detected in the CSF, without suggesting an infectious or autoimmune etiology. The monophasic clinical course, the absence of relapses, the negative antibody test, and the clinical stability over time supported the diagnosis of aseptic encephalitis. This case report discusses the magnetic resonance imaging findings, highlighting the role of OCBs as a nonspecific marker of immune activation, and the utility of the 2016 Graus diagnostic criteria to rule out probable seronegative autoimmune encephalitis. This case underscores the importance of interpreting immunological findings within a clinical context and applying established diagnostic criteria for different types of encephalitis to prevent misdiagnosis and minimize the risks associated with unnecessary immunosuppressive treatment.