The Plaque Hypothesis: Focus on Vulnerable Lesions to Cut Events

December 15, 2022

A new strategy for the management of atherosclerotic plaque as a source of major adverse cardiac events is needed with the focus shifting from the flow-limiting coronary artery luminal lesions to the overall atherosclerotic burden, be it obstructive or nonobstructive, according to a new review article.

The article, by Peter H. Stone, MD, and Peter Libby, MD, Brigham and Women's Hospital, and William E. Boden, MD, Boston University School of Medicine, Boston, Massachusetts, was published online in JAMA Cardiology on December 14.  

The review explored new data from vascular biology, atherosclerosis imaging, natural history outcome studies, and large-scale clinical trials that support what the authors refer to as "The Plaque Hypothesis" — the idea that major adverse cardiac events such as myocardial infarction (MI) and cardiac death are triggered by destabilization of vulnerable plaque, which may be obstructive or nonobstructive.

"We need to consider embracing a new management strategy that directs our diagnostic and management focus to evaluate the entire length of the atheromatous coronary artery and broaden the target of our therapeutic intervention to include all regions of the plaque (both flow-limiting and non–flow-limiting), even those that are distant from the presumed ischemia-producing obstruction," the authors concluded.

Stone explained to | Medscape Cardiology that for several decades, the medical community has focused on plaques causing severe obstruction of coronary arteries as being responsible for major adverse cardiac events. This approach — known as the Ischemia Hypothesis — has been the accepted strategy for many years, with all guidelines advising the identification of the stenoses that cause the most obstruction for treatment with stenting.

However, the authors pointed out that a number of studies have now suggested that, while these severe obstructive stenoses cause angina, they do not seem to be responsible for the hard events of MI, acute coronary syndrome (ACS), and cardiac death. 

Several studies including the COURAGE trial and BARI-2D, and the recent ISCHEMIA trial have all failed to show a reduction in these hard endpoints by intervening on these severe obstructive lesions, Stone noted. 

"We present evidence for a new approach — that it is the composition and vascular biology of the atherosclerotic plaques that cause MI, ACS, and cardiac death, rather than simply how obstructive they are," he said.  

Stone pointed out that plaque seen on a coronary angiogram only looks at the lumen of the artery, but plaque is primarily based in the wall of the artery, and if that plaque is inflamed it can easily be the culprit responsible for adverse events even without encroaching into the lumen.

"Our paper describes many factors which can cause plaques to destabilize and cause an ACS. These include anatomical, biochemical, and biomechanical features that together cause plaque rupture or erosion and precipitate a clinical event. It is not sufficient to just look for obstructive plaques on a coronary angiogram," he said. "We are barking up the wrong tree. We need to look for inflamed plaque in the whole wall of the coronary arteries."

The authors described different factors that identify a plaque at high risk of destabilization. These include a large area of vulnerable plaque, a thin capped atheroma, a severe inflamed core, macrocalcifications, a large plaque burden, and a physical profile that would encourage a thrombus to become trapped.  

"Atherosclerotic plaques are very heterogeneous and complex structures and it is not just the mountain peaks but also the lower foothills that can precipitate a flow-limiting obstruction," Stone noted. 

"The slope of the mountain is probably very important in the ability for a thrombus to form. If the slope is gradual there isn't a problem. But if the slope is jagged with sharp edges this can cause a thrombus to become trapped. We need to look at the entirety of plaque and all its risk features to identify the culprit areas that could cause MI or cardiac death. These are typically not the obstructive plaques we have all been fixated on for many years," he added. 

"We need to focus on plaque heterogeneity. Once plaque is old and just made up of scar tissue which is not inflamed it does not cause much [of] a problem — we can probably just leave it alone. Some of these obstructive plaques may cause some angina but many do not cause major cardiac events unless they have other high-risk features," he said. 

"Cardiac events are still caused by obstruction of blood flow but that can be an abrupt process where a thrombus attaches itself to an area of destabilized plaque. These areas of plaque were not necessarily obstructing to start with. We believe that this is the explanation behind the observation that 50% of all people who have an MI (half of which are fatal) do not have symptoms beforehand," Stone commented. 

Because these areas of destabilized plaque do not cause symptoms, he believes that vast populations of people with established cardiovascular risk factors should undergo screening. "At the moment we wait for people to experience chest pain or to have an MI — that is far too little too late." 

To identify these areas of high-risk plaques, imaging techniques looking inside the artery wall are needed such as intravascular ultrasound. However, this is an invasive procedure, and the noninvasive coronary CT angiography also gives a good picture, so it is probably the best way to begin as a wider screening modality, with more invasive screening methods then used in those found to be at risk, Stone suggested.  

Plaques that are identified as likely to destabilize can be treated with PCI and stenting.  

While systemic therapies are useful, those currently available are not sufficient, Stone noted. For example, there are still high levels of major cardiac events in patients treated with the PCSK9 inhibitors, which bring about very large reductions in LDL cholesterol. "These therapies are beneficial, but they are not enough on their own. So, these areas of unstable plaque would need to be treated with stenting or something similar. We believe that the intervention of stenting is good but at present it is targeted at the wrong areas," he stated.  

"Clearly what we've been doing — stenting only obstructive lesions — does not reduce hard clinical events. Imaging methods have improved so much in recent years that we can now identify high-risk areas of plaque. This whole field of studying the vulnerable plaque has been ongoing for many years, but it is only recently that imaging methods have become good enough to identify plaques at risk. This field is now coming of age," he added.

The next steps are to start identifying these plaques in larger populations, more accurately characterizing those at the highest risk, and then performing randomized trials of pre-emptive intervention in those believed to be at highest risk, and follow up for clinical events, Stone explained.  

Advances in detecting unstable plaque may also permit early evaluation of novel therapeutics and gauge the intensity of lifestyle and disease-modifying pharmacotherapy, the authors suggested.

This work was supported in part by the National Heart, Lung, and Blood Institute, the American Heart Association, the RRM Charitable Fund, the Simard Fund, and the Schaubert Family. Libby is an unpaid consultant to or involved in clinical trials with Amgen, AstraZeneca, Baim Institute, Beren Therapeutics, Esperion Therapeutics, Genentech, Kancera, Kowa Pharmaceuticals, MedImmune, Merck, Norvo Nordisk, Novartis, Pfizer, and Sanofi-Regeneron; and is a member of the scientific advisory board for Amgen, Caristo Diagnostics, Cartesian Therapeutics, CSL Behring, DalCor Pharmaceuticals, Dewpoint Therapeutics, Elucid Bioimaging, Kancera, Kowa Pharmaceuticals, Olatec Therapeutics, MedImmune, Moderna, Novartis, PlaqueTec, TenSixteen Bio, Soley Thereapeutics, and XBiotech, Inc.

JAMA Cardiol. Published online December 14, 2022. Full text.

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