A review on blood pressure and ischaemic white matter lesions.

Hypertension or high blood pressure is constantly reported to be the main risk factors for ischaemic white matter lesions (WMLs). These lesions show the histopathological picture of diffuse demyelination and moderate loss of axons in subcortical structures. The main hypothesis regarding the association between high blood pressure and ischaemic WMLs is that long-standing hypertension causes lipohyalinosis of the media and thickening of the vessel walls with narrowing of the lumen of the small perforating arteries and arterioles which nourish the deep white matter. Episodes of hypotension may then lead to hypoperfusion and hypoxia-ischaemia in the white matter. In line with this, low blood pressure has also been reported to be a risk factor for WMLs. However, also other pathogenetic mechanisms may be involved. Hypertension may cause disturbances in the blood-brain barrier, which may cause lesions in the white matter by cerebral oedema, by activation of astrocytes or by destructive enzymes or other poisons which pass through the damaged vessel walls. The renin-angiotensin system is an example of a system that may be involved in the pathogenesis of both hypertension and arteriosclerosis. Its effector peptide angiotensin II has several blood-pressure-increasing effects, such as direct vasoconstriction and activation of the sympathetic nervous system. It also promotes hyperplasia and hypertrophy in vascular smooth muscle cells. Recently an association between hypertension and Alzheimer's disease has been reported. It is not clear whether this may be the reason for the common occurrence of WMLs in cases of late-onset Alzheimer's disease, as Alzheimer's disease may also cause lesions in the cerebral microvasculature. The association between WMLs and hypertension may thus be mediated through several different pathogenetic pathways.