Case Report: Structured MRI assessment of posterior thalamic infarction in a distribution compatible with posterior choroidal artery territory presenting as Déjerine-Roussy syndrome in an adolescent: differentiating arterial ischemia from venous thrombosis and thalamic neoplasm

Background: Isolated thalamic lesions with post-contrast T1 hyperintensity in children may mimic arterial infarction, cerebral venous thrombosis, inflammatory disease, or neoplasm. Structured interpretation of standard magnetic resonance imaging (MRI) sequences, particularly diffusion-weighted imaging/apparent diffusion coefficient (DWI/ADC) correlation and interval follow-up, can help distinguish these entities. Posterior thalamic infarction in a distribution compatible with posterior choroidal artery territory is rare in pediatric patients. Case Presentation: A previously healthy 15-year-old female developed headache and blurred vision on the day of tetanus-diphtheria (Td) vaccination, followed by progressive right hemibody numbness, impaired hand dexterity, and severe cold allodynia over 10 days. Examination localized to the left thalamus, consistent with Déjerine-Roussy syndrome. MRI on 2 June 2024, day 10 after symptom onset, showed DWI hyperintensity with subtle corresponding ADC reduction in the left posterior thalamic/pulvinar lesion, supporting early-subacute infarction. T2/fluid-attenuated inversion recovery (FLAIR) distribution was compatible with posterior choroidal artery territory involvement. Susceptibility-weighted imaging (SWI) showed no hemorrhage or venous-congestion pattern. Post-contrast MPRAGE demonstrated patchy T1 hyperintensity interpreted as probable enhancement, but true enhancement could not be definitively confirmed because corresponding pre-contrast T1-weighted images were unavailable. Etiologic workup was unrevealing, and the event was classified as cryptogenic pediatric arterial ischemic stroke (AIS). Follow-up MRI showed reduced post-contrast T1 hyperintensity and encephalomalacic evolution. Discussion: The unilateral territorial pattern, DWI/ADC abnormality, lack of hemorrhagic venous features, and follow-up maturation favored arterial infarction over venous thrombosis or neoplasm. Available post-contrast images showed no obvious filling defect of the major dural venous sinuses or principal deep venous structures, although dedicated magnetic resonance venography/computed tomography venography (MRV/CTV) was not performed. Corresponding pre-contrast T1 images were unavailable, limiting definitive enhancement assessment. The temporal association with Td vaccination is reported for pharmacovigilance; causality cannot be established from a single case. Conclusion: This case illustrates a rare adolescent posterior thalamic infarct presenting as Déjerine-Roussy syndrome and highlights the value of DWI/ADC and follow-up MRI in evaluating thalamic lesions with post-contrast T1 hyperintensity.