With an aging population, strokes assume greater importance, as it not only causes major permanent disabilities but it can potentially create social burdens within a family framework as it will tie down the caregivers. Understanding the mechanisms for strokes and the causative factors can help us take proactive measures to prevent strokes.
Mechanisms and causes of stroke
Identifying the main mechanisms of stroke can help one to take preventive measures. Mechanisms of strokes can be divided into 3 broad categories.
Firstly, narrowing of brain arteries (ischaemic strokes) is an important cause of stroke and dementia. Ischaemic strokes can be due to blockage of large or small arteriesor “debris” from elsewhere in the arterial circulation that have travelled to the brain artery (mainly fromclots arising from the heart chambers orcholesterol plaques from aorta or neck arterial walls).
The main causes for this type of stroke are :- 1) Poorly controlled diabetes mellitus, high blood pressure and elevated bad cholesterol predispose to plaque build-up in the arterial wall; 2) Procedures such as insertion of tubes or devices into the aorta or neck arteries, or surgeries such as heart artery bypass graft surgery or valve surgery are responsible for plaque“debris” from other parts of the arterial circulation; 3) Blood clot originating from the heart and travelling to the brain results mainly from the presence of an abnormal heart rhythm called atrial fibrillation.
Secondly, strokes can also occur as a result of bleeding into the brain. The main causes for this type of stroke include uncontrolled high blood pressure, rupture of a brain arterial aneurysm, excessive blood thinning medication or inflammation of brain arteries.
Thirdly, although arterial narrowing may not be enough to occlude a major artery,it may decrease the blood flow to such a degree that areas of the brain which are at the border zone of 2 different arterial blood suppliesmay not get sufficient blood flow resulting in a stroke.
Stroke can also occur when the plaque extends across the opening of a side branch of the major artery compromising its blood flow. Hence, minor narrowing of the brain arteries of less than 50% may also be important.
Current guidelines for high blood pressure advise against lowering the blood pressure excessively as it can reduce blood flow to these borderline threatened areas of the brain and precipitate a stroke.
Incidence of brain arterial narrowing
Studies have identified brain arterial narrowing in 45% to 62% of patients with ischaemic stroke. One French autopsy study showed the presence of plaques and significant narrowing in 62% and 43% of stroke patients, respectively. While autopsy studies have shown the presence of plaque in the brain arteries as early as the first decade of life, significant disease was extremely rare up to the fourth decade. Generally,brain arterial disease develops more slowly as compared to other arteries; as much as up to 20 years later.
The most rapid development of intracranial disease is in the sixth and seventh decades. In Asians, the middle cerebral arteries which supply blood to the parts of the brain controlling the power and sensation are most commonly affected.
Arteries in the brain are different from arteries outside the brain in that there is less elastic tissue and there is more muscle tissue. Brain arteries of all ages have elevated antioxidant enzyme activity compared to other arteries.
This difference may protect the brain arteries by making it more resistant to cholesterol deposition and less responsive to chemical and neural stimuli. Microscopic vessels (vasa vasorum) in the walls of the major arteries provide nutrition and remove waste. The presence of vasa vasorum is associated with pathological age-related changes which cause progressive damage to the brain arteries.
The absence of vasa vasorum in the smaller brain arteries means that these arteries are less vulnerable to these age related changes and may explain the later onset of disease in the brain arteries.
Studies have shown that in contrast to the major arteries elsewhere, cholesterol deposits are rare inbrain arteries of patients <15 years of age. In addition, the plaques developed mainly asfibrous plaques with fewer fatty deposits and less complex lesions.
Complex plaques which contain calcium deposits or tear were present after the fifth decade and were present mainly in the initial segments of the neck arteries and rarely in the brain arteries.
Studies of the brain arteries of human foetuses from mothers with high cholesterol show a similar pattern with less cholesterol deposits in the brain arteries compared to other arteries, demonstrating this protective effect in brain arteries.
Disease of the brain arteries occurs earlier and more extensively in Asians as compared to Caucasian populations. High blood pressure, diabetes mellitus and high cholesterol are important risk factors for narrowing of the brain arteries.
Brain arteries are structurally different from other arteries and they are less vulnerable to age related changes by having higher antioxidant enzyme activity. They are also more resistant to cholesterol deposition and less responsive to chemical and neural stimuli.
The presence of high blood pressure, diabetes mellitus and hyperlipidaemia increases the stiffness of major arteries such as the aorta in later life and reduces anti-oxidant activity in arteries. This increased arterial stiffness increases the pulse pressure (increasing difference between the upper systolic blood pressure value and the lower diastolic blood pressure value).
This increased pulse pressure, commonly seen in the elderly, together with the reduction in antioxidant activity increases the stress in the brain arteries and accelerating its degeneration.
Unlike the plaques in other arteries, the plaques in brain arteries are low in cholesterol content, are rarely complex and are generally stable.
Unlike heart arteries and other arteries, brain arteries are more resistant to the effects of cholesterol and hence degeneration is seen much later in life. As such it is important that you enjoy the benefits of this safe window period of the first few decades of your life, and avoid factors “outside the brain” that can cause stroke.
The following is a simple check list : – 1) Avoid diagnostic procedures that are invasive which involve the insertion of tubes or devices into or near the neck arteries unless absolutely necessary. Hence, instead of undergoing invasive angiograms (X-rays of vessels) of the heart, neck or brain arteries performed by the insertion of plastic tubes into your arteries, you should always consider having a non-invasive computed tomography (CT) scan of the arteries instead as it is safer and less costly; 2) Surgeries that are associated with stroke such as heart bypass surgery or surgery of neck arteries must be carefully weighed against other alternatives as these procedures carry a real risk of major stroke; 3) If you are 60 years and above and have palpitations, see your doctor to make sure that you do not have atrial fibrillation; 4) If you are losing weight, having fast heart beat and feeling warm, check for the presence of excessive thyroid hormones as it can precipitate the onset of atrial fibrillation and thereby predispose to a stroke; 5) do not exercise vigorously if your high blood pressure is not well controlled as the pressure can rise dangerously above 200 mm Hg during exercise; 6) if you are 60 years and above, do not over treat your high blood pressure as having an excessively low blood pressure is associated with an increased risk of stroke; 7) if you are on blood thinning medications such as warfarin, ask your doctors for advice on the food and drugs that can interact with the medication and increase the risk of bleeding; 8) Optimal control of your medical conditions such as high blood pressure, diabetes mellitus and high cholesterol levels are especially important in the sixth decade where degeneration of brain arteries are most rapid. Always remember that it is always possible to prevent stroke – you just need to make the right