Vascular compliance in normal pressure hydrocephalus

Abstract

Background and purpose: Normal pressure hydrocephalus (NPH) is considered to be a combination of altered CSF resorption and a reversible form of cerebral ischemia. The hypothesis tested in this study was that a reduction in venous compliance in the territory drained by the superior sagittal sinus (SSS) is associated with NPH and cerebral ischemia.

Methods: This prospective study involved 27 patients without evidence of hydrocephalus. This group was subdivided into those with normal MR findings and those with evidence of ischemia or atrophy. Ten patients with NPH then underwent MR flow quantification studies of the cerebral vessels. Five of these patients had the same studies performed after CSF drainage. Vascular compliance was measured in the SSS and straight sinus territory by use of MR flow quantification with net systolic pulse volume (NSPV) and arteriovenous delay (AVD) as markers.

Results: Vascular compliance of patients with ischemia or atrophy was significantly higher than that of healthy subjects (mean NSPV in the SSS, 417 microL and 274 microL, respectively). Patients with NPH showed lower compliance than that of the healthy subjects in the SSS (mean NSPV, 212 microL and 274 microL, respectively; mean AVD, 42 ms and 89 ms, respectively). After intervention, the NPH group showed compliance approximating the group with ischemia/atrophy.

Conclusion: Vascular compliance is significantly different in the brains of healthy subjects as compared with that in patients with ischemia/atrophy or NPH.

fig 1.

A, Scout image shows site of flow quantification acquisition. B, Axial scout image in the same plane as the flow study shows the carotid artery (long arrow), the straight sinus (short thin arrow), and the SSS (short thick arrow). C, Axial phase image in the same plane as B.

fig 1.

fig 2. Arterial and venous flow in the SSS territory in a healthy patient. fig 3. Arterial and venous flow in the SSS territory in a patient with atrophy

fig 4.

A, Blood flow in SSS territory before tap test shows very similar arterial and venous flows with minimal delay. B, Blood flow in SSS territory immediately after removal of 30 mL of CSF. Note the arterial curve now peaks earlier, is higher, and is thinner. The venous curve peaks later, is lower, and is wider. C, Net blood flow before and after tap test. The divergence of the curves after intervention has led to a larger overall pulsation in systole (positive half of curve).

fig 5.

A, The pathophysiology of atrophy. Straight black arrows indicate the magnitude of the pulsations; white arrow near the SSS, volume of CSF leaving the system through the incisura of the tentorium; curved black arrows between the capillary beds, bulk fluid flow; SSS, superior sagittal sinus; ST, straight sinus; V, ventricle. B, The pathophysiology of NPH. Straight black arrows denote vessel pulsation magnitudes; curved black arrows, interstitial fluid bulk flow; white arrow, CSF flow; SSS, superior sagittal sinus; ST, straight sinus; V, ventricle.