RESUMO
BACKGROUND: The lack of absorption of CSF at the vertex in chronic hydrocephalus has been ascribed to an elevation in the arachnoid granulation outflow resistance (Rout). The CSF infusion studies measuring Rout are dependent on venous sinus pressure but little is known about the changes in pressure which occur throughout life or with the development of hydrocephalus. METHODS: Twenty patients with chronic hydrocephalus underwent MR venography and MR flow quantification techniques. The venous outflow pressure was estimated from the sinus blood flow and the cross-sectional area of the transverse sinuses. Adult controls as well as a normal young cohort were selected to estimate the change in sinus pressure which occurs throughout life and following the development of hydrocephalus. Significance was tested with a Student's t-test. RESULTS: The size of the transverse sinuses was unchanged from the 1(st) to the 5(th) decade of life, indicating a stable outflow resistance. However, the blood flow was reduced by 42%, indicating a likely similar reduction in pressure gradient across the sinuses. The sinuses of hydrocephalus patients were 38% smaller than matched controls, indicating a 2.5 times increase in resistance. Despite the 24% reduction in blood flow, a significant increase in sinus pressure is suggested. CONCLUSIONS: The size of the venous sinuses normally does not change over the age range investigated but sinus pressure is reduced proportional to an age-related blood flow reduction. Hydrocephalus is associated with much smaller sinuses than normal and an elevation in venous pressure may explain the lack of CSF absorption into the arachnoid granulations in chronic hydrocephalus.
RESUMO
OBJECT: Both idiopathic intracranial hypertension (IIH) in adults and idiopathic hydrocephalus in children have been shown to involve elevations in venous pressure that resolve once the cerebrospinal fluid pressure is reduced. It has been assumed that the venous pressure elevations in both conditions are not hemodynamically significant, but measurement of venous collateral flow in IIH has shown these pressure elevations to be of consequence. The authors used the same methodology to see if the venous pressure elevations noted in childhood hydrocephalus are important. METHODS: Fourteen patients with idiopathic childhood hydrocephalus underwent magnetic resonance imaging with flow quantification. The degree of ventricular enlargement, total blood inflow, and superior sagittal/straight sinus outflow was measured. The degree of collateral venous flow was calculated for each venous territory. The findings were compared with findings in 14 age-matched controls. RESULTS: In children with hydrocephalus the cerebral blood inflow was normal, but the superior sagittal sinus (SSS) and straight sinus outflows were reduced by 27% and 38%, respectively, compared with measurements in controls (p = 0.03 and 0.002). These findings suggest that approximately 150 ml of blood per minute was returning via collateral channels from that portion of the brain drained by the SSS, and 60 ml/minute was returning from collaterals in the deep venous territory. CONCLUSIONS: Similarly to patients with IIH, children with hydrocephalus show a significant elevation in collateral venous flow, indicating that the same venous pathophysiological process may be operating in both conditions. Whether or not the ventricles dilate may depend on the differences in brain compliance between adults and children.
