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Assoc. Prof Iana Simova
Intima-media thickness (IMT) is a marker of subclinical atherosclerosis (asymptomatic organ damage) and should be evaluated in every asymptomatic adult or hypertensive patient at moderate risk for cardiovascular disease. Intima-media thickness values of more than 0.9 mm (ESC) or over the 75th percentile (ASE) should be considered abnormal. Carotid artery ultrasound scan is the method of choice and results are reliable provided certain standards are followed.
Atherosclerosis most often develops gradually and slowly, starting from childhood and proceeding into adulthood with varying velocity and susceptibility to complications. The first structural change that can be detected in atherosclerosis is an increase in IMT. Intima-media thickness is an important atherosclerotic risk marker. However, this increase is not synonymous with subclinical atherosclerosis, but is related to it. Indeed, increase in IMT is also the result of nonatherosclerotic processes such as smooth muscle cell hyperplasia and fibrocellular hypertrophy leading to medial hypertrophy and compensatory arterial remodeling. Therefore This process may be an adaptive response to changes in flow, wall tension, or lumen diameter. The uniform thickening progresses in straight arterial segments as the patient ages and all known vascular risk factors accelerate this process. Therefore IMT is an important atherosclerotic risk marker but cannot be accepted as a risk factor and should not be subjected to treatment (1).
Screening for multisite artery diseases is important in asymptomatic adults at moderate cardiovascular risk, as well as in hypertensive patients (2, 3). The clinician searches for evidence of asymptomatic organ damage, which can further determine cardiovascular risk and lead to reclassification of intermediate risk patients into low or high risk categories (4-6).
Who: Subjects with a 10-year risk of fatal cardiovascular disease between 1% and 5%, , i.e. those at moderate cardiovascular risk. A large proportion of asymptomatic middle-aged adults belong to this category.
What: For further cardiovascular risk assessment, these patients should be considered for IMT measurement and/or screening for atherosclerotic plaques by carotid artery ultrasound (class of indication IIa, level of evidence B).
Who: Hypertensive individuals at moderate risk
What: IMT measurement is advised in a search for target organ damage; asymptomatic vascular damage could be detected with ultrasound scanning of carotid arteries searching for vascular hypertrophy or asymptomatic atherosclerosis. Damage is defined as the presence of IMT >0.9 mm or plaque. The other markers of asymptomatic vascular (target organ) damage are: pulse pressure ≥ 60 mmHg, carotid-femoral pulse wave velocity > 10 m/s and ankle-brachial index < 0.9.
Why: There is evidence that asymptomatic target organ damage predicts cardiovascular death independently of SCORE.Level of evidence: Class of indication IIa, level of evidence B.
Who: Patients at intermediate risk: 6-20% 10-year risk of myocardial infarction or coronary heart disease death, without established coronary artery disease or its equivalents, those with a family history of premature cardiovascular disease in a first-degree relative, individuals younger than 60 years old with severe abnormalities in a single risk factor who otherwise would not be candidates for pharmacotherapy and women younger than 60 years old with at least two risk factors (7).
What: Carotid ultrasound scanning useful for refining cardiovascular disease risk assessment
Vascular ultrasound (IMT measurement) is not recommended for routine measurement in clinical practice for risk assessment for a first atherosclerotic cardiovascular disease event (8). Serial studies of IMT to assess progression or regression in individual patients are not recommended (9).
Examination of the carotid wall gives every clinician an opportunity to evaluate subclinical alterations in wall structure that precede and predict future cardiovascular clinical events. B-mode ultrasonography is a noninvasive, safe, easily performed, reproducible, sensitive, relatively inexpensive and widely available method for detection of early stages of atherosclerosis and is accepted as one of the best methods for evaluation of arterial wall structure. IMT is defined as a double-line pattern visualised by echo 2D on both walls of the common carotid artery (CCA) in a longitudinal view. Two parallel lines (leading edges of two anatomical boundaries) form it: lumen-intima and media-adventitia interfaces – Figure 1.
Figure 1. Intima-media thickness (IMT) definition – IMT is measured as the distance between lumen-intima (yellow line) and media-adventitia (pink line) interfaces.
Figure 2. Proper location for IMT measurement
One of the main problems in interpreting IMT results from clinical trials is the differences in measurement methodology. These discrepancies can refer to either one or more of these parameters: the precise definition of the investigated carotid segment, the use of mean or maximal IMT, the measurement of near and far wall or only far wall IMT, the insonation at a single or different angles, employing manual tracking or an automated software, including carotid plaques or not and uni- or bilateral measurements (10-12).To avoid this problem standards for IMT measurement have been developed.
Type of equipment
What to include in observation
How to conduct observation
How to measure
Normal IMT values and reference ranges are age- and sex-dependent – there is a significant steady increase in IMT with advancing age in all carotid segments (16-18) and significantly higher IMT values in men than in women – Table 1.
Table 1. Normal IMT values – median (P50), 25th and 75th percentile (P) IMT values for men and women at different age categories, separately for right (A) and left (B) CCA (1)
Which IMT values should be considered as abnormal, however, is a controversial topic. The relationship of IMT with cardiovascular risk is continuous and dichotomising this parameter (i.e. determining a threshold IMT value) would be incorrect. Nevertheless it should be noted that in the latest ESH/ESC hypertension guidelines (2013) carotid IMT > 0.9 mm has been re-confirmed as a marker of asymptomatic organ damage, although it has been proven that in middle-aged and elderly patients the threshold values indicating high cardiovascular risk are higher (19,20). The American Society of Echography (ASE) Task force recommends that IMT ≥ 75th percentile is considered high and indicative of increased cardiovascular risk. Values from the 25th to the 75th percentile are considered average and indicative of unchanged cardiovascular risk. Values ≤ 25th percentile are considered low and indicate lower than the expected cardiovascular risk. There are also more conservative cut-off suggestions: IMT values ≥ age-adjusted 97.5th percentile to be defined as abnormal (and predictive of increased vascular risk). The reason for that is that in a large cross-sectional study the association of CCA-IMT with vascular risk has been found to be present only for values falling in the highest quintile of the population values.
Intima-media thickness is accepted as a marker of subclinical atherosclerosis and IMT screening can help the clinician to reclassify a substantial proportion of intermediate cardiovascular risk patients into a lower or higher risk category. In order to implement IMT screening in our daily practice, however, we should be aware of the standards of measurement, as they are described here.
Iana Simova, MD, PhD, Department of Noninvasive Cardiovascular Imaging and Functional Diagnostics, National Cardiology Hospital, Sofia, BulgariaAuthor’s disclosures : None declared
Other resources:E-journal article on subclinical atherosclerosis and the use of IMT as a screening tool for cardiovascular disease (2011)A Spanish experience of carotid risk reclassification in according with carotid intima-media thickness (2009)