Childhood hydrocephalus secondary to posterior fossa tumor is both an intra- and extraaxial process.

OBJECTIVE Ventricular dilation secondary to tumor obstruction of the posterior fossa CSF outflow in childhood is an intraaxial process. However, third ventriculostomy or complete tumor removal often fails to reduce the pressure in some children, and in others there is a delayed reduction in intracranial pressure; this is termed the adaptation period. The cause of this adaptation period has not been studied. Venous sinus compression is highly correlated with other forms of childhood hydrocephalus, and this study seeks to follow the changes that occur in sinus cross-sectional area after surgery. METHODS Twelve children with posterior fossa tumors underwent MRI examination that included standard T2-weighted and 3D contrast-enhanced images obtained preoperatively, in the immediate postoperative period, and after several months. The volumes of the lateral and third ventricles and the minimum cross-sectional area of the transverse and sigmoid sinuses were measured. Patients were categorized by 1) shunt status (those who required a shunt vs those who did not) and 2) by age (those younger than 3 years vs those older than 3 years at diagnosis). RESULTS There was a significant reduction in ventricular volume in both the immediate and secondary follow-up periods for all patients. There was preoperative venous sinus compression in all groups, which did not change significantly in the immediate postoperative period but did improve in the secondary follow-up period. The younger children had larger ventricles and smaller sinuses before surgery compared with the older children. CONCLUSIONS In children with obstructed hydrocephalus caused by tumor, there is secondary compression of venous outflow, indicating both an intra- and extraaxial process. The expansion of the sinuses following decompression of the posterior fossa is delayed and may correlate with the adaptation period. Younger children have greater sinus compression than older ones.