An important cause of the neurological dysfunction in AD is ischemia. This is caused by cerebral amyloid angiopathy (CAA), which is found in about 90% of AD cases, and cerebral atherosclerosis and small vessel disease, which is found in the majority of patients. Middle age hypertension is also a risk for AD. Soluble Aβ is a potent vasoconstrictor of cerebral vessels. Amyloid deposition on the vascular wall results in loss of smooth muscle and narrowing of the lumen. With the loss of their smooth muscle, cerebral arterioles lose their ability to constrict and dilate in response to regional brain activation. Narrowing of capillaries decreases cerebral perfusion and impairs the blood brain barrier function. Occluded capillaries form the seed of SPs. Thus, the brain capillary network is diminished and does not regenerate because of senescence. Other consequences of CAA are lobar hemorrhage, cerebral infarcts, and leukoencephlopathy. These changes are superimposed on cerebral atherosclerosis and its complications.