What Is A Stroke?
A stroke can be defined as a lack of blood flow to the brain resulting in a loss of brain function. Typically a stroke is only classified as such when sensory deficit persists for greater than 24 hours. If the symptoms resolve completely within 24 hours this is referred to as a TIA or transient ischaemic attack. Such an event which resolves within 24 hours may also be called a ‘mini-stroke’, a term which has recently gained in popularity recently.
The sensory deficit associated with stroke can range from mild with complete resolution through to severe or fatal depending upon the region of the brain which loses blood supply. Restoration of function can also range from complete to none, making stroke the leading cause of disability worldwide.
Types of Stroke
Strokes can be divided into two broad categories depending upon the cause of the interrupted blood flow to the brain. Approximately 20% of strokes are haemorrhagic strokes while the remaining 80% are classified as ischaemic strokes.
A haemorrhagic stroke is the result of the rupture or ‘haemorrhage’ of one of the major blood vessels supplying the brain. This leads to blood accumulating in the sub-arachnoid space (defined as a sub-arachnoid haemorrhage or SAH) or accumulation of blood in the intra-cerebral space (defined as an intra-cerebral haemorrhage or ICH).
Ischaemic strokes are due to a lack of blood supply through small perforating arteries in the brain which supply localised areas of brain tissue. Interruption of this blood supply leads to small, localised regions of cellular death due to a lack of oxygen (ischaemia). This lack of blood flow through small perforating arteries of the brain can have a number of causes which allows a further classification of ischaemic stroke into several distinct subtypes.
The first of these subtypes is called small vessel disease (SVD) and is a consequence of fatty deposits building up on the blood vessel wall, eventually causing a blockage of the small perforating blood vessels and resulting ischaemia in the downstream brain tissue. This fatty build up, known as atherosclerosis, occurs in all vessels throughout the body primarily as a consequence of a high fat diet. These deposits, or plaques, can also form in large vessels such as the carotid artery in the neck or the coronary artery supplying blood from the heart. If such a plaque ruptures the lipid core can cause blood to clot around the ruptured plaque, leading to possible blockage of the vessel. Alternatively fragments of the clot can break off and enter the blood stream. Such circulating clot fragments, called emboli, can then get lodged in the smaller vessels of the brain and lead to localised ischaemia. The second and third subtypes of ischaemic stroke therefore are defined as large vessel disease (LVD), where ischaemia is a consequence of large vessel occlusion or emboli, and cardioembolic stroke, where ischaemia results from an emboli of cardiac (heart) origin. The final subtype of ischaemic stroke is due to other defined but rare causes such as cervical artery dissection.
Prevalence of Stroke
Stroke is the third biggest cause of death in the developed world after heart disease and cancer, causing an estimated 10% of all deaths worldwide. It is also the biggest cause of disability. An individuals lifetime risk of developing a stroke is approximately 1 in 6. This can be affected by a range of risk factors which increase an individuals chance of suffering from a stroke, and while some of these are fixed others can be modified to decrease risk.
Risk Factors For Stroke
Risk factors for stroke can be divided into two broad groups – those that can be modified and those which are fixed. Fixed risk factors include age, male gender, genetics and family history. Modifiable risk factors include diet, high blood pressure, exercise, drinking and smoking. By eating a sensible diet and not smoking or drinking excessively an individuals chance of suffering a stroke in later life can be considerably reduced.
Genetics and Stroke
An individuals genetic makeup is fixed from birth. It has long been known however that genetics may contribute up to 50% of an individuals risk of developing a stroke in future. If it were possible to test for genes that increase an individuals risk when they were very young however then other environmental risk factors such as diet and exercise could be monitored much more closely in at risk individuals as an ‘early warning mechanism’. Such lifestyle modification would greatly benefit such genetically susceptible individuals, allowing treatment of conditions such as atherosclerosis much more aggressively to avoid additional risk of future stroke.
Genetics plays a role in almost every aspect of human biology including disease susceptibility. An individuals response to their environment is determined by their genes. The genetics of stroke is complex, with many possible mechanisms by which biological processes such as blood clotting, plaque formation in blood vessels and breakdown of circulating emboli, could lead to increased risk of stroke. The ISGC is an international collaboration of stroke physicians and scientists who have agreed to pool resources and expertise in an effort to unravel the genetic basis of stroke. This will be a first step to developing both more effective treatments and allowing earlier intervention for stroke treatment as well as potentially identifying new targets for drug treatment.
This website has been developed to highlight the role of the ISGC and its progress in understanding the genetic basis of stroke. If you would like further information on stroke or aspects of stroke treatment and patient care that are not covered as part of this website then please see our links page for related websites containing further information on a range of issues relating to stroke.