 Neurosurgical treatment of progressive PHVD involves surgical placement of a ventricular access device VAD for the removal of cerebrospinal fluid CSF. Though at present clear criteria do not exist for CSF removal, an estimation of usual neurosurgical practice and that used in previous trials standard-threshold is intervention at Levenes EGA-adjusted 97th percentile for ventricular enlargement 4 mm or greater 2 standard deviations 97th percentile or a diagonal width enlargement of the frontal horn 10 mm. For the purpose of this study, low-threshold intervention will be defined as ventricular enlargement 97th percentile with either a frontal diagonal width 7-10 mm or a thalamo-occipital diameter 24 mm. 