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Thu 17 Jan 2008 04:00 AM

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Keeping the arteries clear

Stroke prevention efforts should focus on reducing primary risk factors.

Mobile screening measures stroke risk with ultrasound scans of the carotid arteries in the neck. Companies offering such services typically promise that the test can give patients ‘peace of mind', but ultrasound carotid screening remains controversial. Groups like the American Stroke Association don't endorse mass screening. Some professional organisations are more receptive, but would restrict screening to older people with several risk factors for stroke.

Most strokes are not related to carotid stenosis

There are several questions to consider: Is carotid ultrasound the best way to screen for stroke? If the test finds something, what's next? And if it doesn't, is the patient in the clear?

Often unheralded

There are two major types of stroke: ischemic and hemorrhagic. Ischemic strokes, which account for over 80% of strokes, occur when a clot cuts off the blood supply to the brain. Hemorrhagic strokes are caused by a blood vessel bursting open. Transient ischemic attacks (TIAs), sometimes called ministrokes, are similar to ischemic strokes, although by definition the obstruction is temporary.

The mortality rate from strokes has been declining for decades, but the incidence - the number of people having them each year - may be going up, and stroke is a leading cause of disability. Part of the argument for screening is that strokes often seem to strike people out of the blue. Over 70% are ‘first events', although some people may miss early warning signs or fail to recognise the symptoms of a TIA or mild stroke.

Clogged supply lines

You have two common carotid arteries, one running up each side of the front of your neck. If you check your pulse in your neck, you're feeling blood coursing through the carotid arteries. Near the jaw, the common carotids split into the external carotid artery, which supplies the face and other parts of the head, and the internal carotid artery, which supplies the brain.

The common and internal carotids are the chief blood supply lines to the cerebral cortex - the thinking part of the brain. Between 5% and 10% of Americans ages 65 and older have carotid arteries that have been significantly narrowed by plaque, a buildup of fat and cellular debris that's similar to the plaque in coronary arteries that leads to heart attacks.

Plaque in the carotid artery can mean big trouble. It may rupture so that a blood clot forms, blocking the artery and blood flow to the brain. The blood clots may break off and block arteries further ‘downstream'. There's also a chance that the plaque will break up, shedding pieces of fatty gunk that go on to plug up smaller vessels in the brain.

But these are just possibilities. In fact, in many people, the atherosclerotic narrowing doesn't cause symptoms, let alone stroke. So a ‘clogged artery' sounds scary - and seems worth finding with a screening ultrasound - but it's more complicated than that. Many studies have been done, and the results vary with how long people were followed, the definition of clogged - stenosis is the medical term - and a number of other factors.

Overall, however, the results suggest that were you to identify 100 people who had significant carotid stenosis and no history of stroke, stroke symptoms, or TIAs, and if they weren't treated in any way, then one or two would have a stroke in each subsequent year. That isn't zero risk, certainly, but it's pretty small.
Here's another way to think about it: most strokes are not related to carotid stenosis. One study found that less than 10% of first-time stroke patients have carotid atherosclerosis. So screening for clogged carotids lets a lot of people at risk for stroke fall through the cracks.

Cleaning out the carotid

For years, carotid endarterectomy (pronounced end-ar-ter-EK-toe-me) has been the main way of treating carotid stenosis, and about 100,000 of these operations are performed each year in the United States. A surgeon makes a small incision in the neck and carotid and, in effect, reopens the blocked artery by scooping out the atherosclerotic plaque.

Rather than screen for carotid stenosis, it would be more effective to focus on conditions that cause or contribute to the stenosis

The procedure has been dogged by concerns that it may cause the problem it's meant to prevent by setting loose the bits of plaque and tiny blood clots that are capable of causing a stroke during the procedure, or perhaps contributing to one later.

It's been shown, though, that getting a carotid endarterectomy does, in fact, reduce the risk of future stroke for people with severe carotid stenosis and a history of symptoms (which can include a TIA). It's a hard call if the stenosis is less severe.

And it's an even harder one for people with carotid stenosis but no history of symptoms or TIAs - the very people that a carotid screening program might identify. Large studies in highly specialised hospitals have shown that the benefits may outweigh the risks, but those calculations are based on low complication rates, and the rates are likely to be higher in most hospitals.

The American Stroke Association came out in favour of carotid endarterectomy for asymptomatic stenosis in 2007, but limited the recommendation to highly selected patients. The guidelines also say that only surgeons with low morbidity and mortality rates should perform the procedure. Carotid angioplasty is an alternative to carotid endarterectomy for some patients. Like coronary angioplasty, the procedure involves propping open a narrowed artery (in this case the carotid) with a stent - essentially, a little wire tube. Some doctors have high hopes for carotid angioplasty, but it's hard to tell from study results so far whether it's an improvement over carotid endarterectomy.

Carotid narrowing doesn't need to be treated with a scalpel or a stent. Aspirin will reduce the risk of blood clots forming. For people who also have high blood pressure, any medication that ratchets it down will bring down stroke risk, too.

Studies are showing that for people in high-risk groups - those who have high blood pressure, or a history of heart problems - statins (Lipitor, Zocor, other brands) cut stroke risk, perhaps by stabilising atherosclerotic plaque instead of by lowering ‘bad' LDL cholesterol levels. The stroke association's guidelines recommend that anyone with known carotid narrowing should take a statin.
Stick to the basics

Ultrasound scans are already being used to screen for cardiovascular disease. But does ultrasound scanning of the carotids for stroke make sense?

When we contacted Dr. Farzaneh Sorond, a stroke specialist at Harvard-affiliated Brigham and Women's Hospital in Boston, she ticked off the drawbacks:

• Ultrasound scans may find carotid stenosis, but among asymptomatic individuals, the benefits of interventions like carotid endarterectomy are not clear.

• Results vary by operator. Depending on the quality of the scanners, some tests might lead to unnecessary procedures or, at the very least, follow-up tests.

• Even if a scan has indicated that your carotid arteries are ‘clean,' that doesn't mean a patient is safe from stroke. There are lots of other risk factors.

"The use of carotid ultrasound for stroke screening is more about financial gains for companies than health benefits for patients," says Dr. Sorond.

Dr. Sorond concedes that carotid scans might identify some cases of treatable stenosis before symptoms start, but the chances are slim - too slim to endorse mass screening. In the future, researchers may identify a subset of asymptomatic people who should be scanned, but there's no agreement on who they are or how they should be identified.

Rather than screen for carotid stenosis, it would be more effective, as Dr. Sorond points out, to focus on conditions that cause or contribute to the stenosis: diabetes, high cholesterol, hypertension, obesity. The next step is getting those risk factors under control. Often that means a combination of medication - particularly blood pressure-lowering medications and now, increasingly, statins - and all those familiar lifestyle changes, like getting more exercise, that are easier said than done. It's conservative advice, but sometimes conservative is the way to go.

Ultrasound scans of the carotid arteriesA transducer generates sound waves to create an image of the carotid arteries in the neck. Atherosclerotic plaque often forms where the common carotid branches into external and internal carotid arteries.

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