Tuesday, October 15, 2013

CT calcium scoring: who and when?

There are a couple of good articles on this topic in a recent issue of CCJM (here and here).

CT calcium scoring has been widely misunderstood by both health professionals and the public for a number of reasons. My own confusion concerned the pathophysiologic rationale. After all we were taught that calcification of atherosclerotic plaques was characteristic of advanced coronary artery disease, right? Moreover it was all too often the young, soft (and presumably non-calcified) plaques, those little blisters on the endothelial lining, that ruptured and caused catastrophic events. How then could calcium imaging be an early detection tool let alone one to assess preclinical risk? Those questions bugged me for a while and had me digging in Big Robbins, Braunwald's Heart Disease and the Path Guy for better answers. I wanted to know exactly when in the pathogenic sequence of atherosclerosis calcification occurred.

It turns out my assumptions were simplistic. Although atherosclerosis may be well underway when calcification occurs, calcium incorporation is not confined to the late stages of the process. Evidently calcium is incorporated before significant stenosis is seen even though considerable plaque maturation may have occurred. This is due in part to the phenomenon of outward remodeling. Moreover, plaques that rupture may be mature plaques but whose fibrous caps have been eroded by inflammatory mediators.

That said, non-calcified plaques can still rupture, making the use of calcium scoring in symptomatic patients problematic, one of the discussion points in the first of the two CCJM articles. Those authors agree with the prevailing view which is that scoring is useful mainly for the screening of asymptomatic patients and has little role in symptom evaluation. Cardiac calcium scoring for symptomatic patients gets little mention in U.S. Guidelines. An older guideline focused on cardiac CT in general gives it no better than a IIb recommendation. The current unstable angina/NSTEMI guidelines, which include evaluation of undifferentiated patients with chest pain, do not recommend it at all.

The second CCJM paper, a companion editorial, takes a more positive view of this controversial use of calcium scoring:

Taken together, these data suggest that the absence of coronary calcification in people at low to intermediate risk indicates a very low likelihood of significant stenotic coronary artery disease and foretells an excellent prognosis.
These data have already been incorporated into the British National Institute for Health and Clinical Excellence (NICE) guidelines, in which calcification scoring is an integral part of the management algorithm in patients with chest pain who are at low risk.

I was initially surprised that the NICE guidelines would embrace it. It's not so surprising on further consideration given that it is cheaper than invasive or noninvasive angiography or stress imaging.

For the more widely accepted indication of asymptomatic screening it is important to keep in mind that the test is considered optional at best. That is, no clinical scenario is given a class I recommendation. Its main advantage in asymptomatic patients is that it will reclassify a fair number of asymptomatic patients who are clinically determined to be at intermediate risk.

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