Cerebral arteries in hypertension: structural and hemodynamic aspects.

Chronic hypertension is associated with structural alterations in the cerebrovascular bed, with increased vessel wall thickness and media/lumen ratio over a large range of arterial sizes. In the relaxed cortical surface arteries of spontaneously hypertensive rats and in some segments of the basal arteries, the enlarged media encroaches on the lumen. The altered vessel geometry increases the steepness of the resistance curve and enables the blood vessels to withstand higher intraluminal pressure. On the other hand, the risk for ischemic complications is increased because of an elevated minimal resistance. Pathological, degenerative changes occur when the compensatory mechanisms are insufficient to protect the microvessels from the pressure increases. Small intraparenchymatous cerebral arteries develop hyaline degeneration, fibrinoid necrosis, and microaneurysms with the appearance of hemorrhages and lacunae. Hypertension predisposes for atherosclerosis in the cervicocranial and intracranial basal arteries, increasing the risk for large hemispheric infarcts and transitory ischemic attacks. Stenosis/occlusion of the arteries and embolization from an atheromatous plaque might have a larger impact on the hypertensive than the normotensive brain because of less efficient collateral circulation related to the altered vessel geometry.