Identification of vulnerable and dangerous carotid astherosclerotic plaques

Atherosclerotic plaques are a potential source of cardiovascular events. There is growing awareness that stenotic severity alone has limited value in predicting the complication rates and various cellular processes related to inflammation and proteolytic activity are probably more importantly associated with plaque instability and rupture.

Moreover, stroke is one of the most frequent manifestations of atherosclerosis and the third leading cause of death after ischaemic heart disease and cancer. Approximately 80% of all strokes are ischaemic and 30 - 50 % of these are caused by carotid atherosclerotic plaques (1). However, not all plaques become symptomatic and will lead to stroke. Conventionally, advanced atherosclerotic lesions are estimated to be more dangerous and internal carotid artery stenosis is considered to represent a higher risk of cerebral ischaemia when it exceeds 70 % narrowing of the lumen section.

However, as the overwhelming majority of patients with severe carotid stenosis remain asymptomatic for years and even indefinitely, it is self - evident that the degree of stenosis is not the only factor that represents a risk of cerebral ischaemia. It was shown that besides the degree of stenosis, the structure of the atherosclerotic plaque or pathophysiological features within the plaque which increase its instability may play a key role in predicting stroke risk (2). Therefore, the concept of the vulnerable or unstable plaque which has a predisposition to breaking and fissuring, and hence for embolisation to the brain or sudden occlusion of the arterial lumen, with the consequent congruent neurological symptoms was accepted (3).

It is generally accepted that invasive treatment of carotid atherosclerosis should be performed in high grade and symptomatic stenosis, but invasive interventions at a lower degree of stenosis are still a matter of debate. The rapidly developing molecular and functional imaging strategies allow identification of pathophysiological processes in carotid artery stenosis which were shown to be importantly associated with plaque vulnerability, and in the future these techniques could probably help to identify the most vulnerable patients who need vascular intervention. Important pathophysiological processes associated with plaque vulnerability are inflammation, lipid accumulation, proteolytic activity, apoptosis and angiogenesis (4). 

Methods for identification of vulnerable plaques

For identification of subjects who are at risk of cardiovascular atherosclerotic complications different scores, models and techniques are used. Most of them are non - specific and based on the statistical probability that a population of subjects with proven risk factors of atherosclerosis or circulating – blood markers is at risk of developing atherosclerosis.
However, there is an emerging interest in medical science to find strategies that allow identification of individuals who are at the highest risk of cardiovascular complications.  As it has been recognised that atherosclerosis is a chronic inflammatory disease and that its intensity is probably related to the complication rate, there is growing interest in detection of inflammation of the vessel wall in vivo.

A - Pathohistological examinations


Using post-mortem pathohistological investigation it was first shown that atherosclerosis is an inflammatory process. This technique represents the gold standard for identification of the inflammatory process. However, it is not useful for in-vivo testing and because of its invasiveness it is not acceptable as a screening tool.


B - Circulating inflammatory markers


In recent decades a close interrelationship between some circulating markers and risk factors or manifested atherosclerotic cardiovascular disease was shown. However, these markers are not specific and there is no definite answer as to whether they represent the cause or the consequence of the atherosclerotic inflammatory process.
Probably the most specific indicators of inflammation of the vessel wall are increased levels of some interleukins such as interleukin – 6 and interleukin – 8. The drawback of these tests is that the markers are determined in blood samples from peripheral veins therefore the source of the markers is not known, and also information on the location of the inflammatory atherosclerotic process is lacking.

C - Imaging of atherosclerotic plaques


Imaging of the arterial wall provides insight into the structure of the vessel wall, including pathomorphological deterioration and presentation of atherosclerotic plaques. The new functional imaging strategies allow identification of molecular and pathophysiological processes in carotid artery stenosis.

Traditional imaging methods of carotid artery disease include angiography, duplex ultrasound and computer tomography angiography (CTA). These techniques mainly focus on anatomic features of atherosclerotic lesions; however some techniques are also able to detect the morphological characteristics of plaque vulnerability such as a large lipid core, a thin fibrous cap and plaque ulceration.

Duplex ultrasonography and CTA are used to determine the degree of stenosis in carotid artery disease. With regard to plaque morphology, some studies comparing imaging results to histopathological findings as gold standard and found a relatively low sensitivity (45 %) and a somewhat higher specificity (75 %) of imaging techniques (5). Computer-assisted duplex ultrasound image analysis using grey-scale median (GSM) showed similar correlations between GSM values and histopathological findings, varying from 46 % to 75 % (6, 7). CTA has also showed an ability to identify plaque ulceration, calcification and the size of the lipid cores that were correlated with histology (8).

High-resolution magnetic resonance imaging (MRI) was shown to be capable of differentiating between the morphological characteristics of symptomatic and asymptomatic carotid plaques. In the study of U-King-Im and co-workers it was shown that symptomatic patients are more likely to have a thin fibrous cap, intra-plaque haemorrhage, a large lipid core and other characteristics which indicate plaque instability (9). In another study high resolution MRI was able to detect intra-plaque haemorrhage and other characteristics associated with an increased risk of thromboembolic events (10).

All these techniques provide only limited information on morphological plaque features and therefore new techniques have been developed to image biological and pathological processes at the molecular level. The most widely studied imaging methods were those which identified inflammatory processes in carotid artery plaques and the leading technique in this field is positron emission tomography (PET). Radionuclide tracers for macrophage metabolism and inflammatory markers have been developed to image inflammatory activity. The most frequently used radionuclide tracer is a glucose analogue (fluorodeoxyglucose - FDG), which accumulates in the glucose - using cells in proportion to their metabolic activity (11). According to preliminary observations, it was suggested that PDG-PET identifies only those plaques that are most actively inflamed and at the highest risk of rupture (12). Using FDG-PET Rudd first showed that unstable plaques accumulate more FDG (13). It was also indicated that the intensity of the FDG-PET signal correlates with histological macrophage markers in human atherosclerotic plaques (14). In a pilot study of patients with TIA, radionuclide uptake was related to patients presenting symptoms (15).

Recently other radionuclide tracers have also been used in imaging of carotid plaques like 99mtechnetium, interleukin – 2. Other promising radionuclide tracers used to image inflammation in atherosclerosis are 125I - monocyte chemotactic protein – 1 and amino malonic acid monoclonal antibody.

In addition to radionuclide tracers a variety of magnetic nanoparticles have been developed to detect inflammation in atherosclerotic plaques (4).
In spite of the promising results of these techniques of functional imaging, some basic requirements like quantification of radionuclide accumulation, specificity and reproducibility are still unanswered.

D - Imaging the activity of proteolytic enzymes in atherosclerotic plaques


As it has been suggested that the release of proteolytic enzymes such as matrix metalloproteinases (MMPs) and cathepsin cystein proteases (CCPs) are responsible for plaque destabilisation, non-invasive visualisation and quantification of proteolytic enzymes activity is of great potential in the risk assessment of carotid artery stenosis. Several studies have shown that the proteolytic activity of an atherosclerotic plaque is related to its instability (16). Radiolabelled molecules designed to specifically target proteolytic enzymes have been developed for imaging using PET or single photon emission computer tomography (SPECT). It was also shown that SPECT may be an important tool not only in the estimation of the proteolytic activity of plaques, but also for monitoring the effect of therapy strategies like statins (17).

Another new technique to image proteolytic activity in atherosclerotic plaque is near infrared fluorescence (NIRF). This technique is based on the near infrared spectra of light and provides fluorescent images for detection of the enzymatic activity of MMPs and CCPs. The first NIRF application for characterisation of human carotid artery plaques was reported recently (18).


Table 1. Morphological imaging : techniques and values.

Table 2 - Functional imaging : Techniques and values.

 
Abbreviations: CTA – computer tomography angiography; MRI – magnetic resonance imaging; PET – positron emission tomography; SPECT – single photon emission tomography; NIRF – near infrared fluorescence; GSM – gray scale median

Conclusion:

In conclusion, the detection of atherosclerosis in its early phases, following its development and identifying non-stable lesions which are prone to cause cardiovascular events, is of outmost importance for prevention of cardiovascular events. Using imaging techniques it is possible to identify deterioration of the vessel wall in atherogenesis.

Traditional imaging techniques focused primarily on the anatomical features of atherosclerotic plaques. Current new imaging procedures provide the opportunity to follow functional cellular processes and to identify vulnerable atherosclerotic lesions, particularly carotid plaques.

It is also expected that these techniques may be useful for evaluation of the individual therapeutic strategy and to identify subjects in whom vascular interventions represent the greatest benefit. Although new approaches are promising, no large clinical trials have been carried out so far and problems related to reproducibility and safety are not completely solved. Therefore, additional research is needed before implementing these new imaging techniques in everyday clinical practice.