Читать книгу A Statin Nation - Dr Malcolm Kendrick - Страница 12

The other major disastrous event often caused by atherosclerosis is a stroke. As with heart attacks, there are variations on a theme. You will be glad to know there are only two basic types of stroke, but they do have a number of different causes.

The commonest type is an ischaemic stroke, ischaemia meaning lack of oxygen supply to a part of the body. This stroke is normally a two-step process. First, atherosclerotic plaques build-up in the carotid arteries in the neck, and then a blood clot forms over the plaque. Then, and this is where a stroke is different to an MI, the clot breaks off and travels up into the brain where it jams as the artery narrows. In turn this blocks the blood supply to an area of the brain, causing a cerebral infarct. Somewhat strangely, this is called a stroke and hardly ever a cerebral infarction (CI). You can also get infarcts deeper in the brain. These lacunar infarcts are usually smaller.

DIAGRAM 6

Another common cause of infarcts in the brain is AF. Here the upper chambers of the heart are fibrillating, i.e. not contracting in a controlled manner, somewhat like VF. However, because the contraction of the atria is much less critical to blood flow, people can live for many years with AF. In fact, there may not even be any symptoms.

However, AF is still a major health problem because, if the atria are fibrillating, the blood does not flow through smoothly and can form whirls and eddies, which makes it much more likely for clots to form. These can then break off, travel into the ventricles and then head for the heart. And because the carotid arteries are the first major arteries to branch out of the aorta, any blood clots that form in AF are likely to head up these arteries and into the brain, where they get stuck, causing a stroke.

Having said this, though, the clots that form in AF can travel anywhere in the body. Kidneys, arms, legs. Winston Churchill had AF, and because of this had multiple small strokes. I was certain I read somewhere that he lost a thumb due to a blood clot blocking the artery at the base of the thumb, but I am now not sure this is true. How he made it to age ninety is beyond the understanding of his GP. 4

The other, major form of stroke is the bleeding, haemorrhagic kind. A blood vessel in the brain bursts and blood rushes into the brain tissue, destroying parts of it. Haemorrhagic strokes tend to be more severe than ischaemic strokes, and they tend to be triggered by high blood pressure, though not always.

This type of stroke is often secondary to a thinned and ballooned out area of the artery, an aneurysm. This weakened area is more likely to pop under pressure. If you have aneurysms in the arterial system at the base of the brain (the circle of Willis), they cause a subarachnoid haemorrhage. This is not quite the same as a stoke because the bleeding happens outside the brain, but it puts great pressure on the brain, forcing it down the spinal column and can cause severe damage – and death.

DIAGRAM 7

Finally, many strokes are defined as cryptogenic, which is a fancy way of saying, ‘We don’t know what caused it.’ Doctors don’t like saying that in any area of medicine, so pseudoscientific terms have been developed to stop them admitting ‘we simply haven’t a clue’. Hence:

 Cryptogenic stroke = stroke of unknown cause

 Idiopathic pulmonary fibrosis = progressive lung damage of unknown cause

 Essential hypertension = high blood pressure of unknown cause

Anyway, back to strokes. When I was a fresh-faced young doctor, there was no effective treatment for strokes and the attitude of most GPs was pretty laissez-faire. Strokes were something that happened to the elderly, who should probably just lie in bed and see what happens. Yes, this was much the same as the original six weeks of bed rest for an MI.

Things have certainly changed. We are now supposed to call strokes ‘brain attacks’ to emphasise how serious they are, and how quickly we should act, as with a heart attack. Although, to be honest, I do not think I have ever heard anyone call a stroke a brain attack – but I am sure it will start happening.

Nowadays, if someone is suspected of having a stroke, they are rushed to hospital at high speed and then … Well, there is a delay. Because the correct treatment for cerebral infarction would most likely kill anyone having a cerebral haemorrhage. This is because the correct treatment for an infarction is tissue plasminogen activator (tPA). It is a clot buster.

If it given early enough – within about six hours – it can blow apart the clot that caused the stroke and will significantly reduce the damage caused. However, if you give tPA to someone having a haemorrhagic stroke it will, instead, blow apart any clot that has formed to stop further bleeding into the brain, with drastic consequences.

Unfortunately, there is no way of knowing from the clinical signs if someone is having a cerebral infarct or a cerebral bleed. The only way to find out is with a brain scan, which means that you must try and get people suffering a stroke into a scanner as quickly as possible. If it is ischaemic, they get tPA. If it is a haemorrhage, they do not – they must not. About 80 per cent of strokes are ischaemic.

What I find fascinating is that if you have an MI, and the heart stops beating, irreversible brain damage occurs in about four to five minutes. If you have a stroke, and the blood stops flowing through a part of the brain, you can protect the rest of the brain if you give tPA within six hours. Maybe someone can explain this to me.

One drastic way to reduce the risk of ischaemic strokes is to look for large plaques in the carotid arteries and remove them surgically. You may have been offered a carotid artery scan as part of a health screen, which seem to be becoming increasingly popular. If you have greater than a certain amount of blockage, then a surgeon can open the artery and hook out most of the plaque. Sometimes they may put a stent in to keep the artery open. This can also be done after a stroke to stop another one happening in the future.

If you have AF, the treatment of choice is to take an anticoagulant to stop blood clots forming in the atria. The most commonly used anticoagulant in this case is warfarin (coumadin in the US). This is an extremely effective treatment and reduces the risk of stroke considerably. In a major study, warfarin reduced the risk of ischaemic stroke from 7.4 to 2.3 per cent per year.5 For every hundred people that represents five fewer strokes per year, or fifty fewer over ten years. And that, my friends, is as good as any ‘preventative’ medicine ever gets. Of course, there are associated risks, such as an increased risk of bleeding, etc. But overall, in AF, I would advise warfarin asap.

In the last few years, newer, life-long anticoagulants have replaced warfarin. They are no more effective than warfarin, but they do not need to be monitored all the time – as with warfarin. You, your friends or relatives may have an (International Normalised Ratio) INR test every few weeks or so to check the dose of warfarin is correct. This is not needed with the newer drugs; however the newer drugs are about 80 times as expensive as warfarin. Kerchingggg!

At this point I think I have covered as much as you need to know about the underlying process of CVD. Essentially, it is a disease of the blood vessels themselves, consisting of thickened areas called atherosclerotic plaques. The final event is, usually, a blood clot forming on top of an already existing plaque.

The next questions are what causes the disease, and how to prevent it. However, before that, I think there is an equally pressing need to look in more detail at the factors currently considered to be the most important cause. Fat(s) and cholesterol in your diet, causing a raised blood cholesterol level, leading to atherosclerosis/atherosclerotic plaques.