Cerebrospinal fluid absorption block at the vertex in chronic hydrocephalus: obstructed arachnoid granulations or elevated venous pressure?

Abstract

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.

Keywords: Cerebral blood flow; Chronic hydrocephalus; MR venography; Normal pressure hydrocephalus; Sagittal sinus pressure.

Figure 1

MRI imaging of a chronic hydrocephalus patient before and after shunt failure. A A T2 axial image of a 32 year-old male with chronic hydrocephalus and a left occipital shunt currently functioning. B A T2 axial image following removal of the shunt showing enlargement of the ventricles. C The MR venogram with functioning shunt appears normal, the thin arrow indicates the sagittal sinus and the thick the dominant transverse sinus. D The MR venogram with the shunt removed shows the sagittal and transverse sinuses to be smaller than previously. This is most easily seen at the level of the arrows. E A T2 axial image of the sagittal sinus 2 cm above the torcular taken at the level of the thin arrows in C and D) showing the cross-sectional area of the lumen to be 43 mm². F The follow-up T2 axial image at the same level as 1e but with shunt removed shows the lumen to be 21 mm². G A sagittal reconstruction of the mid portion of the right transverse sinus (at approximately the level of the thick arrows above) taken from the MRV raw data shows the sinus to be 40 mm² in area. H The same reconstruction as 1 g following shunt removal shows the sinus area to be 20 mm².