Specific Imaging Findings

Periventricular white matter halo, most commonly around the lateral ventricles, shows low density on CT, increased diffusion and high T2 signal, which is best seen as hyperintensity on FLAIR images. Periventricular edema is invariably associated with ventriculomegaly and signs of high ventricular pressure: dilated temporal horns; rounded frontal horns; bowed, thinned and stretched corpus callosum; ballooned third ventricle; sulcal effacement. There is no contrast enhancement. The periventricular rim has indistinct borders on CT, and smooth, well-delineated margins on FLAIR images. It commonly contours the entire lateral ventricles, extending along the temporal horns. Maximum halo thickness is generally around the frontal horns and the trigones. On sagittal T2-weighted imaging dilated hyperintense perivenular subependymal spaces can be seen, radiating outward with finger-like appearance. In long-standing hydrocephalus, the periventricular halo can show ill-defined margins and white matter thinning due to gliotic scarring. With newly diagnosed acute hydrocephalus, the main distinction is between obstructive and communicating hydrocephalus. If no obvious obstructing masses or subarachnoid hemorrhage are evident to explain the hydrocephalus, cisternographic MR techniques (volumetric highresolution heavily T2WI), or CT cisternography, can be used to detect thin webs obstructing the CSF outflow.

Pertinent Clinical Information

Obstructive and non-obstructive conditions leading to imbalance between the production and the resorption of CSF, resulting in accumulation of CSF, cause increased intraventricular CSF pressure. Depending on the rate of the ventricular pressure changes, presentations range from signs and symptoms of frank intracranial hypertension (papilledema, headache, worse in the recumbent position and early in the morning, nausea and projecting vomiting, lethargy) to more subtle gait disturbances, headache, ophthalmoplegia, and cognitive decline.