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The Journal of Quality Research in Dementia, Issue 6 (lay summary)

Preventing dementia following a stroke

Dr A Peter Passmore

Reader, Public Health Medicine & Primary Care
Room 223, Whitla Medical Building, 97 Lisburn Road, Belfast, BT9 7BL.Telephone 028 9097 2158, email
p.passmore@qub.ac.uk

Vascular dementia is caused by problems with the supply of oxygen to the brain. Having a stroke more than doubles the risk of developing dementia and around one in three people who have a stroke develop dementia, however our understanding of how to prevent dementia following a stroke remains limited.

A stroke is an interruption in the supply of blood to a particular part of the brain. There are two main causes of stroke: 
a) a blood vessel within the brain becomes clogged or blocked (ischemic stroke, about 83% of strokes)
b) a blood vessel ruptures, causing blood to leak into the brain (hemorrhagic stroke)
When the blood supply is interrupted in this way brain cells are deprived of oxygen, leaving them permanently damaged.

The effects of a stroke vary from person to person and depend on how much of the brain is damaged and where in their brain the stroke happened. A single stroke can be enough to cause dementia if it affects a particular part of the brain. This type of vascular dementia is known as single-infarct dementia ('infarct' means a damaged area).

Vascular dementia is more commonly caused by a series of small strokes. These can be so tiny that the person might not notice any symptoms or the symptoms may be only temporary. This is called multi-infarct dementia.
Vascular dementia progresses in a similar way to Alzheimer's disease, but progression is often 'stepped' rather than gradual, declining suddenly as the person has a new stroke.

People who have a mini-stroke, which are also known as transient ischaemic attacks or TIA, are at much higher risk of having a stroke. Other risk factors for stroke include age, high blood pressure, high cholesterol levels, diabetes, smoking, having an irregular heartbeat (atrial fibrillation) or problems with blood clotting.

Treating a stroke

If people can get to an ambulance or hospital soon after having a stroke they might be eligible for a clotbusting drug. This can restore the normal blood flow thus reducing the amount of brain damage and chances of developing vascular dementia. However as this treatment is not suitable for people who have had a hemorrhagic stroke and also must be given within a certain timeframe following the first signs of a stroke (usually three hours) it is not suitable for everyone.

Following a stroke patients should be admitted to a stroke unit where their needs can be assessed by a multidisciplinary team. The team should consider rehabilitation and preventative measures as appropriate. For cerebral haemorrhage the main intervention is very good control of blood pressure levels. For other types of stroke treatment can include anticoagulants to improve blood flow and prevent clotting, good control of blood pressure and cholesterol levels and possibly a surgical intervention to remove the inner lining of the carotid artery if it has become thickened or damaged to the point where it is preventing blood flow to the brain (carotid endarterectomy).

Cognition and dementia after stroke

Research shows that having a stroke doubles the risk of developing dementia and that 30% of people who have a stroke then develop dementia.

Many studies have shown that keeping your heart and arteries in good condition also helps to prevent dementia. Frustratingly we don't yet have a clear understanding of the exact mechanism that links heart and circulation problems with dementia. We're stuck with the common sense assumption that having a good blood supply to the brain helps to keep it as healthy as possible.

The same common sense link suggests that when someone has had a stroke, the underlying reasons for the stroke (such as high blood pressure, or high cholesterol levels) will also have had an effect on the structure and function of the brain. Having a poor supply of blood to the brain over time means that, even before a stroke occurs, it's likely that the brain has been damaged, particularly in the deeper parts of the brain where the blood vessels are smaller (this is known as white matter change). It's also possible that the plaques and tangles that are linked to Alzheimer's disease have begun to develop.

Following a stroke people can develop vascular dementia or what is known as mixed dementia, which is a combination of vascular dementia and Alzheimer's disease. It's also possible for people to develop Alzheimer's disease on its own, with no sign of vascular dementia.

It's usual to wait for at least three months after a stroke before judging whether someone's mental skills have been affected by the stroke. Before this point they might simply be confused or experiencing delirium as a temporary after effect of the stroke. Delirium is different to dementia because it develops very quickly and is usually temporary. Having delirium means that you cannot concentrate, think clearly and can be unaware of what is going on around you. Often people with delirium can see or hear things that are not really there, but seem very real to them (hallucinations).

Following a stroke it's quite common for people to experience changes in what is known as their 'executive function'. This is a term used to describe a set of mental processes that helps us connect past experience with present action. We use executive function when we perform such activities as planning, organizing, strategizing and paying attention to and remembering details. Changes in executive function can easily be confused with depression, which is also common after a stroke.

A change in executive function is not a definite sign of dementia. Even at three months after a stroke, distinct changes in mental skills might remain stable or even improve. However some changes in executive function can mean that the chances of developing dementia are increased.

Prevention of dementia after stroke

There is a lack of research and therefore a lack of understanding about how to prevent damage to memory and mental skills (known as cognitive impairment) and dementia following a stroke. In terms of research, this area is often tacked onto studies into stroke and physical disability rather than studied for its own sake.

To date research has shown that the following are risk factors linked to the development of dementia after a stroke:

Age
Low education level
Dependency before stroke
Diabetes mellitus
Atrial fibrillation
Myocardial infarction
Epileptic seizures
Sepsis
Cardiac arrhythmias
Congestive cardiac failure
Silent cerebral infarcts
Global and temporal lobe atrophy
White matter changes
Stroke severity
Cause
Location
Recurrence of stroke

A large international study, known as the PROGRESS study [1,2], investigated the benefits of managing blood pressure levels on the health of over 6,000 people in the three and a half years following a stroke. Patients who were given extra treatment to manage their blood pressure levels had a reduced risk of recurrent stroke and a significantly reduced development of severe cognitive impairment and dementia.

Another study, the PRoFESS study [3], looked at antiplatelet intervention and blood pressure lowering treatments at an earlier stage after a stroke. Antiplatelet drugs interact with platelets, a type of blood cell, to block them from gathering into harmful clots. The PRoFESS study was a double-blind, placebo-controlled clinical trial performed at 695 sites in 35 countries, with 20,332 patients. The initial results of this enormous study do not show any reductions in loss of mental skills (cognitive outcomes) or levels of dementia following stroke due to these interventions. However the study will be investigating cognition in more detail, so future results might reveal helpful information, although it's possible that the study itself is over too short a time period to provide useful evidence.

There is no clear evidence about the effects that treatments given following stroke have on mental skills or the development of dementia. In particular studies are urgently needed to investigate the effects of statins on the development of dementia following stroke. These cholesterol-lowering drugs are now routinely given following ischemic stroke. They are known to reduce the likelihood of a second or subsequent stroke and it seems logical that, as they affect cardiovascular health, they would have an impact on vascular dementia, however this research has yet to be carried out.

Similarly, as it is now routine to encourage and support people who have had a stroke to adopt a healthier lifestyle, it would be helpful to know what impact these changes have on peoples' chances of developing dementia.

There is a clear need to improve understanding of which treatments can help to prevent dementia and other cognitive impairments following a stroke.

References

[1] PROGRESS Collaborative Group, (20010 Randomised trial of a perindopril-based blood-pressure-lowering regimen among 6105 individuals with previous stroke or transient ischaemic attack. Lancet.  358, 1033-1041

[2] Tzourio C, Anderson C, Chapman N et al. PROGRESS Collaborative Group,(2003) Effects of blood pressure lowering with Perindopril and indapamide therapy on dementia and cognitive decline in patients with cerebrovascular disease. Arch Int Med.163,1069-1078

[3}Rationale, design and baseline data of a randomized, double-blind, controlled trial comparing two antithrombotic regimens (a fixed-dose combination of extended-release dipyridamole plus ASA with clopidogrel) and telmisartan versus placebo in patients with strokes: the Prevention Regimen for Effectively Avoiding Second Strokes Trial (PRoFESS). Cerebrovasc Dis. 2007;23,368-80.