Bilateral Thalamic Diffuse Midline Glioma, H3K27M-Altered, in a 3-Year-Old Boy.

Diffuse midline glioma, H3 K27-altered, is an aggressive pediatric brain tumor that can mimic non-neoplastic conditions when located outside the brainstem. We describe a 3-year-old boy who presented with progressive ocular misalignment and left-sided weakness without systemic symptoms. Magnetic resonance imaging demonstrated symmetric bilateral thalamic T2/FLAIR hyperintense lesions without contrast enhancement or diffusion restriction, initially raising suspicion for inflammatory or metabolic disease. Cerebrospinal fluid and metabolic investigations were unremarkable. Due to diagnostic uncertainty, surgical resection of the dominant lesion was performed. Histopathology revealed an infiltrative high-grade astrocytic tumor with loss of H3K27me3 and positive H3K27M immunostaining, confirming diffuse midline glioma, H3 K27-altered (WHO grade IV). Postoperatively, the patient developed transient contralateral weakness but remained neurologically stable at 3-month follow-up and was referred for radiochemotherapy. This case highlights that bilateral thalamic lesions in children may represent diffuse midline glioma even when imaging lacks typical malignant features. Early biopsy should be considered in unexplained bilateral thalamic lesions to prevent diagnostic delay and initiate appropriate oncologic management.