Differences in the Calculated Transvenous Pressure Drop between Chronic Hydrocephalus and Idiopathic Intracranial Hypertension

BACKGROUND AND PURPOSE: Chronic hydrocephalus is associated with dilated ventricles despite a normal intracranial pressure. In idiopathic intracranial hypertension, the ventricles are normal despite an elevated intracranial pressure. This apparent paradox has largely remained unexplained. It is suggested that a pressure difference between the superficial and deep venous territories of the brain could account for the variation between the 2 diseases. The purpose of this paper is to investigate the cause of this pressure difference.

MATERIALS AND METHODS: Using MR phase-contrast imaging, we calculated the hydraulic diameters of the sagittal and straight sinuses in 21 patients with hydrocephalus, 20 patients with idiopathic intracranial hypertension, and 20 age-matched controls. The outflow resistance of each sinus was estimated using the Poiseuille equation. The outflow pressure was estimated using the flow data. A smaller subset of the patients with hydrocephalus had these studies repeated after successful shunt insertion.

RESULTS: In hydrocephalus, the sagittal sinuses were 21% smaller than those in controls (P < .001); the straight sinuses were not significantly different. In idiopathic intracranial hypertension, both sinuses were not significantly different from those of controls. The pressure drop from the sagittal sinus to the end of the straight sinus was elevated by 1.2 mm Hg in hydrocephalus (P = .001) but not significantly different from that in controls in idiopathic intracranial hypertension. Shunt insertion dilated the sagittal sinuses in hydrocephalus, leaving them 18% larger than normal and eliminating the transvenous pressure change.

CONCLUSIONS: There is a transvenous pressure difference in hydrocephalus that is absent in idiopathic intracranial hypertension. This difference is eliminated by shunt insertion. The findings may have a bearing on ventricular dilation.