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Vascular dementia is diagnosed when there is a reduction in blood flow to the brain for any one of a number of reasons. The reduction in blood flow can be because of a reduction of blood flow in the small capillaries in the brain, causing what is called ‘micro-infarcts’, or tiny areas of the brain where blood (and therefore oxygen and nutrients) is no longer delivered. This can be because of small clots (e.g., due to atrial fibrillation, or micro-emboli from a plaque in an artery feeding the brain), infections (which causes closing of the small capillary blood vessels), and auto-immune disease (which cause inflammation of the small vessels).
Vascular dementia can also be due to narrowing of the larger arteries supplying the brain (e.g., the carotid artery). A large number of people undergoing coronary artery bypass surgery (up to 50% in some studies) develop micro-emboli when on the heart bypass machine, resulting in a one-time damage to the brain (stable dementia).