Case Report: Giant acute hemorrhagic colloid cyst in the septum pellucidum-foramen of Monro region with obstructive hydrocephalus in an 8-month-old infant.

Colloid cysts are uncommon benign neuroepithelial cystic lesions that typically arise in the anterior third ventricle near the foramen of Monro. Pediatric colloid cysts are rare, and giant hemorrhagic lesions centered in the septum pellucidum-foramen of Monro region have only rarely been reported in infancy. We report the case of an 8-month-old boy who presented with a 1-day history of sudden bulging of the anterior fontanelle, recurrent vomiting, and decreased responsiveness. Cranial magnetic resonance imaging revealed a giant cystic mass extending from the septum pellucidum-foramen of Monro region to the anterior third ventricle, measuring approximately 38 mm × 23 mm × 35 mm. The lesion showed mildly hyperintense signal on T1-weighted and FLAIR sequences, with a focal fluid-fluid level and a T2-hypointense clot-like nodule. Supratentorial ventricular dilatation and periventricular interstitial edema were also observed, indicating acute obstructive hydrocephalus. The patient underwent emergency microsurgical decompression, evacuation of intracystic fluid and hematoma, sampling of wall-like tissue, septostomy, and external ventricular drainage. Intraoperatively, dark-brown intracystic fluid mixed with abundant blood clots was identified. Histopathological examination showed wall-like fibrovascular tissue with edema, hemorrhage, necrosis, and focal calcification. After pediatric intensive care support, anti-infective therapy, and symptomatic management, the patient recovered well. Follow-up MRI at 3 months after surgery showed no definite evidence of residual or recurrent lesion. This case suggests that infant who cannot verbalize headache, bulging of the anterior fontanelle, postprandial vomiting, and decreased responsiveness should be regarded as important warning signs of acute intracranial hypertension. Early radiological recognition, timely relief of cerebrospinal fluid pathway obstruction, and standardized perioperative intensive care are critical for improving clinical outcomes.