What is the SYNTAX Score and How Should We Use It?

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In recent years, studies of multivessel coronary angioplasty randomized patients between surgery and intervention. The fairness of this randomization assignment has become a subject of great interest, since it may influence outcomes. One of the most prominent recent studies, the SYNTAX trial, compared multivessel percutaneous coronary intervention (PCI), including patients with left main narrowings, to coronary artery bypass surgery. The results of this randomized study demonstrated that patients who had SYNTAX scores >34 appeared to do much better with bypass surgery than those with lower SYNTAX scores, in whom PCI was just as good for major adverse cardiac events, with lower stroke rates.

What is the SYNTAX score?

The SYNTAX score is an angiographic grading tool to determine the complexity of coronary artery disease. The SYNTAX score was developed by the group at the Thoraxcenter, Erasmus Medical Center in The Netherlands, headed by senior investigator Prof. Patrick Serruys. The SYNTAX score was derived from preexisting classifications, which included the American Heart Association (AHA) classification of coronary artery tree segments modified for the ARTS study, the Leaman score, the American College of Cardiology (ACC)/AHA lesion classification system, the total occlusion classification system, the Duke and International Classification for Patient Safety (ICPS) classification system for bifurcation lesions, and a consensus opinion from among the world’s experts. The SYNTAX angiographic grading tool was published in 2005.1 The SYNTAX score is the sum of the points assigned to each individual lesion identified in the coronary tree with >50% diameter narrowing in vessels >1.5mm diameter. The coronary tree is divided into 16 segments according to the AHA classification (Figure 1). Each segment is given a score of 1 or 2 based on the presence of disease and this score is then weighted based on a chart, with values ranging from 3.5 for the proximal left anterior descending artery (LAD) to 5.0 for left main, and 0.5 for smaller branches. The branches <1.5 mm in diameter, despite having severe lesions, are not included in the SYNTAX score. The percent diameter stenosis is not a consideration in the SYNTAX score, only the presence of a stenosis from 50–99% diameter, <50% diameter narrowing or the total occlusion. A multiplication factor of 2 is used for non-occlusive lesions and 5 is used for occlusive lesions, reflecting the difficulty of PCI.

Further characterization of the lesions adds points. For example, a total occlusion duration >3 months, a blunt stump, a bridging collateral image, the first segment visible beyond the total occlusion, and a side branch >1.5 diameter all receive one point. For trifurcations, one diseased segment gets three points, two diseased segments get four points, three diseased segments get five points, and four disease segments get six points. For bifurcation lesions, one point is given for types A, B, and C; two points are given for types D, E, F, and G; and one point is given for an angulation >70 degrees (Figure 2). Additionally, an aorto-ostial lesion is worth one point, severe tortuosity of vessel is worth two points, lesion length greater than 20 mm is worth one point, heavy calcification is worth 2 points, thrombus is worth 1 point, and diffuse disease or small vessel is at 1 point per segment involvement. For multiple lesions less than three reference vessel diameters apart, these are scored as a single lesion. However, at greater distance than three vessel diameters, these are considered separate lesions. The types of bifurcations are shown in Figure 2. Segments in which bifurcations are evaluated are those involving the proximal LAD and left main, the mid LAD, the proximal circumflex, mid circumflex, and crux of the right coronary artery. With regard to trifurcation lesions, these also are additive in number of segments involved. The SYNTAX score algorithm then sums each of these features for a total SYNTAX score. Table 1 summarizes the SYNTAX grade categories. A computer algorithm is then queried and a summed value is produced.

How were the SYNTAX scores validated?

The SYNTAX score was used in a series of patients undergoing three-vessel PCI, such as the ARTS II trial.2 The variables were then associated with outcome events in the PCI studies. Low SYNTAX scores are <18, intermediate SYNTAX scores range from 18-27 and high SYNTAX scores are >27. High scores are associated with increasing cardiac mortality, major adverse cardiac events, and a specific, predefined combination of end points. The SYNTAX angiographic grading system was used alone to identify potential risk for revascularization. When comparing all clinical and angiographic factors, it turns out that the SYNTAX score, in addition to age, gender, smoking, diabetes and acute coronary syndromes, is one of the highest predictors of cardiac mortality and major adverse cardiac events in patients undergoing multivessel and, specifically, unprotected left main PCI. A SYNTAX score of >34 also identifies a subgroup with a particularly high risk of cardiac death independent of age, gender, acute coronary syndrome, ejection fraction, Euro score and degree of revascularization.

How do we use the SYNTAX scores?

The SYNTAX score is a useful differentiator for the outcome of patients undergoing three-vessel PCI. Examples of the types of SYNTAX score are provided below on figures from the original paper (Figure 3). The patients with the highest scores have the highest risk and the lowest scores, the lowest risk. The SYNTAX scores can be divided into three tertiles. The high scores indicate complex conditions and represent greatest risks to patients undergoing PCI. High scores have the worst prognosis for revascularization with PCI compared to coronary artery bypass graft surgery (CABG). Equivalent or superior outcomes for percutaneous intervention were noted in comparison to coronary artery bypass graft surgery for patients in the lowest 2 tertiles (Figure 4). The SYNTAX angiographic tool thus provides the first evidence-based approach to employing optimal revascularization strategies for patients with complex coronary artery disease.

In comparison to the simpler AHA/ ACC scoring system, the SYNTAX score shows similar performance, with the exception of a better ability to discriminate patients at risk of major adverse events. The SYNTAX score, however, does not account for prior stenting and thus its use in patients who have had prior PCI must be cautionary. Likewise, coronary artery bypass graft surgery is a confounding variable when calculating the SYNTAX score. The best discriminating feature of the SYNTAX score was between the lowest and highest tertiles of grading. When the algorithm can be managed simply from a PDA, the usefulness of this score is likely to extend itself into everyday clinical practice when considering multivessel coronary intervention, especially those involving unprotected left main stenting.