Many people suffer from hardening of the arteries, including those of the brain, as they grow older. This means that the wall of the blood vessels thickens and narrows, causing a slowing down of the blood flow. It is, however, not this narrowing that causes the dementia, but a complete blockage causing the death of brain cells. Many people live with narrowed arteries for twenty or thirty years without any effect on their intellect; it is only a sudden, complete blockage that causes a stroke and an accumulation of these blockages that leads to MID. In many cases disease in the cardiovascular system elsewhere — outside the head — has resulted in small particles or clots travelling along the arterial system to a small artery in the brain that they then block, depriving that part of the brain of its blood supply with the resulting death of cells. This therefore is the sudden blocking off of small arteries referred to above whereas arteriosclerosis, when it occurs, blocks off arteries very slowly and may take years to cause significant narrowing of a blood vessel. It is not therefore always arteriosclerosis in the brain itself that causes the problems, but changes in the blood vessels elsewhere in the body, especially those between the heart and the top of the neck. Further causes of small strokes are discussed later.
Infarcts in the brain look similar, irrespective of the way in which they are caused. When they have been present for a long time there is often a small hole as the body’s natural processes remove the dead tissue. This hole usually fills up with fluid. Where this hasn’t happened there is usually a patch of softened material. Examining the brain with the naked eye can usually reveal all of these changes. Looking down the microscope will confirm that the microscopic changes that one expects to be associated with a stroke are also present.
If enough tissue is destroyed, there will be a reduction in brain size and weight with enlargement of the ventricles inside the brain and wasting of the ridges on the brain’s surface.
As one might expect, the severity of the dementia parallels the number of areas of the brain that have been destroyed. It has been suggested that dementia only occurs after a certain amount of brain tissue has died and this threshold has been estimated to be approximately 50 ml — the equivalent of ten teaspoonfuls. In many cases, however, we come across people who have multiple infarct dementia, but in whom there is less than 50 ml of dead brain tissue. Whether or not a person develops dementia seems to depend not only upon the total amount of brain tissue that has been lost, but also upon the site of the strokes. Some brain structures are more important than others in this respect.
Smoking
It is now well established that people who smoke cigarettes are more likely to have a stroke than non-smokers and it has even been suggested that about a third of strokes or similar episodes may be caused by cigarettes. There is, however, very little specific evidence to link smoking with dementia as opposed to paralysis of the limbs or face. Nevertheless because there is no doubt about the effects of smoking on the blood vessels, it makes sense to cut down or preferably to cease smoking altogether if there is any evidence of even a single stroke, whether this is affecting mental function or the limbs.
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