The management of obstructive hydrocephalus associated to posterior cranial fossa tumor remains a controversial issue. In this context, the role of endoscopic third ventriculostomy (ETV) performed before tumor removal (the so-called preoperative ETV) is debated and its success rate is reported as being largely variable in the literature. We describe two children who experienced the closure of a preoperatively performed ETV after the surgical resection of their posterior fossa tumor. In both cases tumor removal was performed in the prone position. Radiological investigations and intraoperative evidence documented that the failure of the ETV was due to a blood clot which directly occluded the stoma. We speculate that the prone position favored the migration of blood, through the aqueduct, thereafter, clotting on the most declivous part of the third ventricle once the patient assumed the upright position, with the consequent occlusion of the stoma. Based on a gravitational hypothesis, we named this mechanism the "snow globe effect."
Keywords: endoscopic third ventriculostomy; endoscopy; hydrocephalus; posterior fossa tumor; preoperative.