17th January 2020
I thought I should write a blog on coronary artery calcification (CAC), as it has become the latest hot topic. CAC scans, and CAC scoring are now increasingly popular, and the results are worrying lots of people who wonder what they mean, and exactly how worried they should be. I get many e-mails on this issue from people who have been scared witless, or another word ending in***tless, by having a high CAC score.
What is coronary artery calcification
Coronary artery calcification (CAC) is the deposition of calcium in artery walls. It represents the final stages in the life cycle of some/many/most atherosclerotic plaques. At the risk of oversimplification, it is generally accepted that atherosclerotic plaques go through four stages
Forget small and bigger. The important ones are vulnerable and calcified. What is a vulnerable plaque? It is a plaque that reaches a certain size (undefined) containing an almost liquid core, with a thin cap. If this thin cap ruptures, it exposes the liquid core to the bloodstream triggering a major blood clot than can fully block a coronary artery and cause a myocardial infarction.
This is generally described as plaque rupture. If it happens in an artery in the neck, a carotid artery, the clot will normally not be big enough to block the artery. But it can break off and head up into the brain, causing a stroke.
Which means that it is these ‘vulnerable’ plaques that are dangerous, and these are often not calcified, and therefore cannot be seen on a CAC scan.
Over time, assuming the vulnerable plaques do not rupture and kill you, some of them (all of them?) shrink down in size, become more solid and start to calcify. At which point they are less likely to rupture and may be considered relatively benign.
Calcification of areas of damage in the body is not restricted to atherosclerotic plaques. Almost any damaged area in the body, that is not perfectly repaired, is likely to calcify to some extent or another. Scars tend to be white, and the white is calcium.
At the extreme end of calcification is a condition called myositis ossificans, whereby almost any damage ends up becoming bone. With damaged muscle turning into bone. This does not end well.
Anyway, assuming you have plaques developing and growing in your arteries, they will in time calcify. Or at least some of them will. Are some people genetically more likely to get calcification than others? Almost certainly.
Some things are known to increase the rate of calcification. Statins, for example. Here from the Cleveland clinic:
- Patients with coronary artery disease (CAD) who are treated with statins experience an increase in coronary calcification, an effect that is independent of plaque progression or regression.
- Paradoxically, high-intensity statin therapy is associated with the largest increases in coronary calcification despite promoting atheroma regression 1
With statins the plaques get smaller and the calcium load gets bigger.
Another drug that whacks up the rate of calcification is warfarin (often called coumadin in the US).
‘The vitamin K antagonist, warfarin, is the most commonly prescribed oral anticoagulant. Use of warfarin is associated with an increase in systemic calcification, including in the coronary and peripheral vasculature. This increase in vascular calcification is due to inhibition of the enzyme matrix gamma-carboxyglutamate Gla protein (MGP). MGP is a vitamin K-dependent protein that ordinarily prevents systemic calcification by scavenging calcium phosphate in the tissues.’ 2
High intensity exercise also stimulates CAC.
‘Emerging evidence from epidemiological studies and observations in cohorts of endurance athletes suggest that potentially adverse cardiovascular manifestations may occur following high-volume and/or high-intensity long-term exercise training, which may attenuate the health benefits of a physically active lifestyle. Accelerated coronary artery calcification, exercise-induced cardiac biomarker release, myocardial fibrosis, atrial fibrillation, and even higher risk of sudden cardiac death have been reported in athletes.’ 3
An interesting mix, I think.
- Statins increase calcification
- Warfarin increases calcification
- Intense exercise increases calcification
Yet, all three reduce the risk of dying of cardiovascular disease. Yes, even statins – a bit.
But, let’s turn this around for a second. If you have no calcification in your arteries, you have a greatly reduced risk of dying of cardiovascular disease. Which means that calcification can be both good, and bad? Yes, you are right, this area is not straightforward at all.
Even if you look at non-calcified atherosclerosis, or pre-calcified atherosclerosis, the picture is complex.
For many years I have studied the Masai villagers, on and off. They are fascinating because, amongst Masai males, the diet almost entirely consists of cholesterol and saturated fat – or at least it did. Nowadays, I believe it is more McDonalds and Subway.
Despite their previous super-high saturated fat and cholesterol diet, their cholesterol levels were the lowest of any population studied. However, they developed atherosclerosis at around the same rate as any Western male of the same age.
Added to this, and just to make things even more complicated, there were no recorded cases of any male Masai villagers dying of CVD. Which made me think, at one time, that atherosclerosis and death from CVD must be unrelated phenomenon.
You think not? Here, for example, is a study on the Masai from 1971 by George Mann (who helped to set up the Framingham Study and then became a trenchant critic of the cholesterol hypothesis).
Atherosclerosis in the Masai
‘Do the Masai not develop atherosclerosis or do they have it but remain immune to occlusive disease because of some other protective circumstances? The question was answered with autopsy material collected over a five-year period. The Masai do have atherosclerosis but they are almost immune to occlusive disease.’ 4
Now, if we bring these facts together, what do they tell us. At the risk of running the thinking too fast, these facts tell us that atherosclerosis, calcified or not, is necessary for someone to die from CVD. However, it is not sufficient, by itself, to cause death from occlusive disease.
In epidemiology this is the well-recognised concept of ‘necessary but not sufficient’. It actually applies to many/most diseases. For example, you cannot get TB, or die of TB, without infection with the tuberculous bacillus. However, you can be exposed to the bacillus and not have TB.
Why, because your immune system fought it off. Which means that the tuberculous bacillus is necessary but not sufficient, to cause infection and death from TB. As a slight aside, one sign of TB is calcified nodes in the lungs. Which can mean that you have active TB. Alternatively, it can mean that you had active TB, which has now been cleared out, leaving only calcification.
Turning back to atherosclerosis, and using the Masai as one example, it is clear that you can get atherosclerosis, and calcified atherosclerosis, and not die of CVD, or even have an increased risk of CVD. Why, because other factors are required to kill you. Which is why it can be said that atherosclerosis is necessary, but not sufficient, to cause heart attacks and strokes.
To put this another way, you are exceedingly unlikely to die from an acute blockage to an artery without any atherosclerosis [or at least this is vanishingly rare], but just having atherosclerosis is not sufficient to cause heart attacks and strokes.
Which means that for example, if your atherosclerosis is (only) caused by intense exercise you are at no significant increased risk of dying CVD. In this case your calcified atherosclerosis is not sufficient to cause CVD.
‘A new study of mostly middle-aged men in JAMA Cardiology found the most avid exercisers—averaging eight hours per week of vigorous exercise—did indeed show greater levels of coronary artery calcium (CAC). Nevertheless, they were less prone to dying over the average follow-up period of 10.4 years compared to men who exercised less, suggesting they can safely continue their workout regimens.’ 5
However, if your CAC score has gone through the roof because of say: diabetes, smoking, steroid use, air pollution, heavy metal toxicity, high Lp(a), lack of various nutrients etc. then you are at great risk of dying of CVD, and you need to do something about it.
At one time atherosclerosis was defined as either athero…sclerosis, or arterio…sclerosis, in acknowledgement that there seem to be two distinct and different type of …sclerosis in your arteries. This concept seems to have fallen by the wayside.
This may be a mistake. Some years ago, the AHA tried to define all the different types of lesion* that could be found in arteries. The report was so big, that it got split in two 6.7. Then it got ever bigger, and then they gave the project up. The reports are long, and mind splittingly boring. One of them was a ‘twenty cups of coffee’ read. Followed by three Red Bulls.
(*lesion = abnormal thing)
What I learned, I think, in the moments when I was still conscious, was that atherosclerotic plaques are most certainly not all the same. Which lead me to think that we should attempt to bring back arteriosclerosis as a concept.
By which I mean the idea that some plaques develop, primarily, in response to biomechanical stress – such as is caused by physical exercise. On the other hand, some plaques develop in response to factors that independently damage the endothelium – such as a high blood sugar level, or smoking. With the addition of high clotting factors.
Whilst all plaques are now called atherosclerotic plaques, they do not all look the same, and they probably do not act the same. The arteriosclerotic lesions are thinner and more fibrous, they have no real lipid core and are very unlikely to rupture. They are, still, sometimes called fibroatheroma.
On the other hand atherosclerotic lesions are thicker, have a lipid core, more likely to narrow the artery and are also more likely to rupture, causing an occlusive blockage – leading to a stroke and/or heart attack.
Which is why the Masai (the most heavily exercising population on the planet – at the time) had …sclerosis yet remained ‘almost immune to occlusive disease’. Which is also why people who exercise intensely can develop …sclerosis and calcification but are not at an increased risk of dying of CVD.
However, both arterio and athero… sclerosis can calcify. So, they (probably) look much the same on the CAC san.
Moving on, again.
Sensitivity and specificity
Getting back to the CAC test, and what it means. The next issue is one that plagues all screening tests. Namely, what is the sensitivity, and what is the specificity? Something I always get the wrong way around in my head, then I must go back and look it up, to get it clear again.
To explain. A perfect screening test is one that is 100% sensitive and 100% specific. No test has ever achieved that, and I doubt any test ever will.
Sensitivity means, how good is the test at picking up that someone with the disease is identified as having the disease. Specificity means, how good is the test at making sure that people who do not have the disease are accurately told that they do not.
If we look at breast cancer, the first sign of breast cancer can often be that a woman feels a lump in her breast. However, many things that are not breast cancer, can cause a lump in the breast. Let us say 50% of palpable lumps are not breast cancer. If this is true, then the specificity of manual examination of the breast, in detecting breast cancer, would be 50%.
What of mammography? While it is clearly much better than manual palpation (from a sensitivity point of view) many cancers that cannot be felt, can still be seen on a scan, but it is actually worse from a specificity point of view.
This is because many/most ‘abnormal’ things seen on a mammogram will turn out to be benign. Sensitivity and specificity are often inversely related.
Some things sit in an intermediate area. In breast cancer screening a lot of women are told they may have breast cancer, but what has been detected is an abnormality called ductal carcinoma in situ (DCIS). This is something that may, or far more likely may not, progress to become a significant breast cancer. Should it be treated, or not?
The specificity problem is a problem for almost all screening tests. You have managed to find something abnormal on your test. Is it really abnormal? Does it need treatment? Would it have been better not to have found this ‘abnormality’ at all.
This is not a simple argument. Although it is usually presented in the most black and white terms if you question the breast cancer screening programmes. ‘Do you want women to die of breast cancer?’ Is a statement I have often heard from the pro-screening side. How does one answer this? ‘Well, of course I do. I see it as my role, as a doctor, to ensure that as many women as possible die from breast cancer.’
The real debate, of course, is far more complex and nuanced. Do the harms of finding benign abnormalities (with all the anxiety, further investigations, possible mastectomies etc. that this causes) outweigh the benefits of finding breast cancer at an early stage? Currently, the answer seems to be … yes.
If you want a far more detailed review of this area, you could buy the book ‘Mammography Screening’ by Peter Gøtzsche.
‘If Peter Gøtzsche did not exist, there would be a need to invent him … It may still take time for the limitations and harms of screening to be properly acknowledged and for women to be enabled to make adequately informed decisions. When this happens, it will almost entirely due to the intellectual rigour and determination of Peter Gøtzsche.’ Iona Health President RCGP (Royal College of General Practitioners)
Screening and scanning always seems a fantastic idea. Pick up a disease early, then you can treat it, even cure it. Presented in this, the simplest form, who could argue against it?
But it is not simple, in medicine very few things are. Breast cancer screening – in fact most cancer screening programmes – are far from black and white. You can argue for them, you can argue against them.
And, at present, cancer screening programmes are much better than CAC screening, for many other reasons. I will only deal with the most important one. Which is that… We don’t know what to do about the finding!
If you find a small, early stage, not yet spread anywhere, breast cancer you can remove it. It is gone, never to return. But what are you going to do with calcified plaques? You certainly can’t remove them. You do not know if they are going to rupture. They probably won’t. If you manage to stop the calcification getting worse, are you doing any good. Who knows? What caused them in the first place?
It is not even the calcified plaque that is the problem. The calcified plaque is only really a marker for earlier stage vulnerable plaques. If these start to calcify, this is probably a good thing, but whilst calcification is going on, the CAC score will be getting worse – while your risk of suffering a myocardial infarction is falling.
Sensitivity and specificity, false positives – and CAC scans (Pandora’s box)
My first general comment here is that you should never start screening and scanning until you are extremely certain, based on strong evidence, that you understand the natural history of the disease you are screening for.
Also, that you fully understand what the results of your test mean. And that you have an effective treatment for any abnormality you find.
These criteria are all missing with CAC scans.
Yes, a negative scan – no calcium detected – has reassurance value. If you have no calcium in your arteries, you almost certainly do not have any type of …sclerosis in your arteries. So, your risk of CVD is low.
However, positive scans, like positive mammograms, will include a very high number of false positives. Then what? You have been told you have significant calcification in your arteries. But it is ‘good’ calcification, or ‘bad’ calcification. Are you at increased risk, or not? This, no-one can tell you, for sure.
Equally, if you have calcification in your arteries, what are you going to do about it? Take statins… that makes it worse. Do more exercise…. that makes it worse. If you don’t know why you have calcification in the first place, it becomes impossible to take steps to do anything about it.
This is somewhat analogous to having a genetic test to discover if you have Huntington’s Chorea – if one of your parents had it. Do you want to find out that you have a disease – which will kill you – that you can do absolutely nothing about?
In a similar way should you have a test for Alzheimer’s, to find out if you are going to get the disease. Do you really want to know that you are going to have a terrible and devastating disease, and that there is nothing that can be done to prevent it?
In fact, CAC scans meet most of my criteria for ‘a bloody awful test that should not be done.’ It may or may not mean anything, there is no clear guidance as to what you can do about it if it is positive, and it spreads fear and anxiety in many, many, people. I should know, my inbox is stuffed with e-mails from people terrified by their CAC score.
My first recommendation is that, if you have not had a CAC scan, do not have one.
My second recommendation is that, if you have had a CAC scan, and it shows no calcification, good. Do not have another one.
If, however, you have had a CAC scan and it shows significant calcification. What then? What then indeed? You may want to read this paper: ‘Non-invasive vulnerable plaque imaging: how do we know that treatment works?’
‘Atherosclerosis is an inflammatory disorder that can evolve into an acute clinical event by plaque development, rupture, and thrombosis. Plaque vulnerability represents the susceptibility of a plaque to rupture and to result in an acute cardiovascular event. Nevertheless, plaque vulnerability is not an established medical diagnosis, but rather an evolving concept that has gained attention to improve risk prediction. The availability of high-resolution imaging modalities has significantly facilitated the possibility of performing in vivo regression studies and documenting serial changes in plaque stability. This review summarizes the currently available non-invasive methods to identify vulnerable plaques and to evaluate the effects of the current cardiovascular treatments on plaque evolution.’ 8
It will, at least, give you some idea of the other forms of investigation that are available.
Or, you might want to read this one: ‘New methods to image unstable atherosclerotic plaques.’
‘Atherosclerotic plaque rupture is the primary mechanism responsible for myocardial infarction and stroke, the top two killers worldwide. Despite being potentially fatal, the ubiquitous prevalence of atherosclerosis amongst the middle aged and elderly renders individual events relatively rare. This makes the accurate prediction of MI and stroke challenging. Advances in imaging techniques now allow detailed assessments of plaque morphology and disease activity.
Both CT and MR can identify certain unstable plaque characteristics thought to be associated with an increased risk of rupture and events. PET imaging allows the activity of distinct pathological processes associated with atherosclerosis to be measured, differentiating patients with inactive and active disease states. Hybrid integration of PET with CT or MR now allows for an accurate assessment of not only plaque burden and morphology but plaque biology too.
In this review, we discuss how these advanced imaging techniques hold promise in redefining our understanding of stable and unstable coronary artery disease beyond symptomatic status, and how they may refine patient risk-prediction and the rationing of expensive novel therapies.’ 9
The key words in that abstract are ‘hold promise.’
My final recommendation is that we should NOT be doing CAC scans, until it can be proved in a well conducted clinical trial, that we can do something positive and beneficial about the findings.
Yes, a ‘negative’ CAC is reassuring. This, however, must be set aside against the psychological damage caused by a ‘positive’ CAC scan. At present we are playing a form of psychological Russian Roulette. Half the population walks away reassured, half the population reels away, scared witless.
Also, often puzzled and disappointed. I have lost count of the number of people who have written to me saying that they: don’t smoke, exercise regularly, are not overweight, have low cholesterol levels, do not have high blood pressure, do not have high blood sugar levels, etc. etc. yet they have a terrifyingly high CAC score. What should they do?
Well, what can they do?
I don’t know. Because I don’t know what the test means. Not for sure. Not enough to provide any advice that I can be certain is right. Some boxes are better left unopened, however tempting it may be to peek inside.
Just because you can do something does not mean that you should.