Brain 18F-fluorodeoxyglucose positron emission tomography/computed tomography in COVID 19-associated autoimmune encephalitis: a case report.

Coronavirus disease 2019 (COVID-19)-associated encephalitis is a rare but severe neurological complication. Conventional diagnostic tools, such as magnetic resonance imaging (MRI) and cerebrospinal fluid (CSF) studies, may yield normal results despite significant functional brain impairment. Brain positron emission tomography/computed tomography (PET/CT) with 18F-fluorodeoxyglucose (18F-FDG) has emerged as a valuable adjunctive imaging modality in such cases. We report the case of a 54-year-old woman with COVID-19 who presented with anosmia, ageusia, and altered behavior. Neurological examination revealed cognitive deficits, including bradyphrenia and mixed aphasia. Routine MRI and CSF analyses were unremarkable. However, brain 18F-FDG PET/CT revealed cortical hypometabolism in frontal and temporal lobes and cerebellar changes, consistent with immune-mediated encephalitis. The patient received immunosuppressive therapy with corticosteroids and intravenous immunoglobulin, leading to gradual clinical improvement and complete recovery at 3 months. This case highlights the diagnostic challenge of COVID-19-associated encephalitis when routine studies are inconclusive. PET/CT functional imaging can detect metabolic brain changes not apparent on structural imaging, supporting immune-mediated pathogenesis. Early recognition and immunotherapy may improve neurological outcomes. To conclude, brain 18F-FDG PET/CT is a useful diagnostic adjunct in COVID-19-related encephalitis with normal MRI and negative CSF, aiding timely diagnosis and treatment monitoring.