Acute Sulcal FLAIR Hyperintensity in Severe Tick-Borne Encephalitis: A Potential Prognostic Marker

(1) Background: To report two cases of severe tick-borne encephalitis (TBE) in elderly patients presenting with a previously undescribed subarachnoid T2/FLAIR hyperintensity on repeated MRI examinations, which may serve as an early imaging biomarker of disease severity. (2) Methods: Two unvaccinated 82-year-old patients (one male and one female) presented with acute encephalitis and required intensive care. Serial brain MRI, EEG, CSF analysis, and neurophysiological assessments were performed. (3) Results: Both patients showed rapid progressive neurological deterioration in the context of TBE, confirmed by elevated serum and CSF IgM and IgG titers. Early follow-up MRI revealed striking sulcal hyperintensities on T2/FLAIR sequences, interpreted as protein-rich subarachnoid inflammatory changes. These changes paralleled clinical worsening and resolved on follow-up imaging. The male patient developed meningoencephalomyeloradiculitis, remained comatose, and died from respiratory failure (the brain and spinal cord were examined postmortem). The female patient had meningoencephaloradiculitis with severe dysphagia and was discharged with a modified Rankin Scale score of four. Both patients demonstrated epileptiform EEG activity. The CSF analysis revealed markedly elevated total protein, lactate, tau protein, and CXCL13, as evidence of blood-brain barrier disruption and inflammatory neurodegeneration. (4) Conclusions: We describe acute subarachnoid T2/FLAIR hyperintensity in TBE as an imaging feature that may correlate with severe systemic inflammation and a poor prognosis. This radiological finding could serve as a potential early prognostic marker in TBE.