Types of vascular dementia
There are several different types of vascular dementia. They differ in the cause of the damage and the part of the brain that is affected.
The different types of vascular dementia have some symptoms in common and some symptoms that differ. Their symptoms tend to progress in different ways.
A stroke happens when the blood supply to a part of the brain is suddenly cut off. In most strokes, a blood vessel in the brain becomes narrowed and is blocked by a clot. The clot may have formed in the brain, or it may have formed in the heart (if someone has heart disease) and been carried to the brain. Strokes vary in how severe they are, depending on where the blocked vessel is and whether the interruption to the blood supply is permanent or temporary.
A major stroke occurs when the blood flow in a large vessel in the brain is suddenly and permanently cut off. Most often this happens when the vessel is blocked by a clot. Much less often it is because the vessel bursts and bleeds into the brain. This sudden interruption in the blood supply starves the brain of oxygen and leads to the death of a large volume of brain tissue.
Not everyone who has a stroke will develop vascular dementia, but about 20 per cent of people who have a stroke do develop this post-stroke dementia within the following six months. A person who has a stroke is then at increased risk of having further strokes. If this happens, the risk of developing dementia is higher.
Single-infarct and multi-infarct dementia
These types of vascular dementia are caused by one or more smaller strokes. These happen when a large or medium-sized blood vessel is blocked by a clot. The stroke may be so small that the person doesn't notice any symptoms. Alternatively, the symptoms may only be temporary - lasting perhaps a few minutes - because the blockage clears itself. (If symptoms last for less than 24 hours this is known as a 'mini-stroke' or transient ischaemic attack (TIA). A TIA may mistakenly be dismissed as a 'funny turn'.)
If the blood supply is interrupted for more than a few minutes, the stroke will lead to the death of a small area of tissue in the brain. This area is known as an infarct. Sometimes just one infarct forms in an important part of the brain and this causes dementia (known as single-infarct dementia). Much more often, a series of small strokes over a period of weeks or months lead to a number of infarcts spread around the brain. Dementia in this case (known as multi-infarct dementia) is caused by the total damage from all the infarcts together.
Subcortical vascular dementia is caused by diseases of the very small blood vessels that lie deep in the brain. These small vessels develop thick walls and become stiff and twisted, meaning that blood flow through them is reduced.
Small vessel disease often damages the bundles of nerve fibres that carry signals around the brain, known as white matter. It can also cause small infarcts near the base of the brain.
Small vessel disease develops much deeper in the brain than the damage caused by many strokes. This means many of the symptoms of subcortical vascular dementia are different from those of stroke-related dementia.
Subcortical dementia is thought to be the most common type of vascular dementia.
Mixed dementia (vascular dementia and Alzheimer's disease)
At least 10 per cent of people with dementia are diagnosed with mixed dementia. This generally means that both Alzheimer's disease and vascular disease are thought to have caused the dementia. The symptoms of mixed dementia may be similar to those of either Alzheimer's disease or vascular dementia, or they may be a combination of the two.