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Updated December 08, 2014.
An uncommon but dreaded complication following the use of stents to treat blockages in the coronary arteries is stent thrombosis.
Stent thrombosis is the sudden, often complete occlusion of a coronary artery at the site of the stent, caused by a thrombus ( blood clot). Stent thrombosis often results in sudden death, or in a large myocardial infarction (heart attack) that produces significant cardiac damage.
Most people who experience this catastrophe have no warning symptoms (such as angina) prior to the acute event.
How Often Does Stent Thrombosis Occur?
Stent thrombosis most commonly occurs early - within days to a few weeks after the stent procedure. However, it can occur at any time, even several years later. Overall, the risk of stent thrombosis is estimated to be between 2 - 3% after three years. Most of these events occur during the first year; after the first year, it is estimated that one out of approximately 200 - 300 stent patients each year will experience this problem. But stent thrombosis not uncommonly causes sudden death outside of the hospital, so it is likely that not all cases of stent thrombosis are accurately identified - and the incidence actually may be somewhat higher than is reported.
What Causes Stent Thrombosis?
The causes of stent thrombosis are not completely understood. It appears that the metal components of the stent can trigger blood clotting - this is probably why most stent thromboses occur early on, before the growth of new tissue has a chance to fully cover the metal struts.
Late stent thrombosis (events occurring more than a year after stent placement) may be related to the failure of tissue growth to completely cover the metal struts. Because drug-eluting stents (DES) are designed to inhibit tissue growth (in order to inhibit stent restenosis), it has been speculated that late thrombosis may be more common with DES than with bare metal stents (BMS). Some (but not all) studies support this speculation. The apparent excess of late stent thrombosis with DES seems to be diminished with recent-generation DES.
Late stent thrombosis may also be related to a recurrence of new atherosclerotic plaques inside the stents. Some evidence suggests that this “neoatherosclerosis” may occur more readily with DES than with BMS.
Other factors that seem to be associated with a higher risk of stent thrombosis include poor stent placement technique, diabetes, significant kidney disease, small coronary arteries, cardiomyopathy, cocaine usage, and smoking.
Can Stent Thrombosis Be Prevented?
The risk of stent thrombosis can be greatly diminished, though not entirely eliminated, by taking two anti-platelet blood thinners to reduce blood clotting: aspirin and one of the P2Y12 receptor blockers. The P2Y12 blockers used for this purpose are clopidogrel (Plavix), prasugrel (Effient), and ticagrelor (Brilinta). Taking aspirin plus one of the P2Y12 blockers is referred to as “dual-anti-platelet therapy,” or DAPT.
It is critical for everyone receiving a stent to take DAPT - especially in the early weeks and months, when the threat of thrombosis is higher. Current recommendations are for stent recipients to take DAPT for at least one year. If a patient has a high risk of bleeding, or is likely to require surgery within a year, DAPT should still be used (whenever possible) for at least six months. For patients in whom taking long-term DAPT is going to pose a problem, most cardiologists will consider using a BMS instead of a DES, because of the possibility that the long-term risk of thrombosis is higher with DES.
New Information on DAPT
The recommendations regarding long-term DAPT are now being reevaluated. In November, 2014 data was presented from the long-awaited DAPT study, which enrolled and followed 10,000 patients receiving coronary artery stents. The results of this study suggest that continuing DAPT for up to 30 months after stent placement results in a reduced incidence of late stent thrombosis. Further, this study suggests that discontinuing DAPT - even several years after a stent has been placed - is associated with a “spike” in the risk of stent thrombosis. Based on this study is seems likely that formal recommendations on the optimal duration of DAPT therapy stent placement will be revisited, and that much longer-term usage of DAPT will become the norm.
Because DAPT is itself not a risk-free therapy - it can cause serious bleeding, and its use poses a real dilemma if surgery becomes necessary - the need for long-term DAPT has to be taken into account any time stent therapy is being considered.
The Bottom Line
The use of modern stents has greatly reduced the risk of restenosis. However, the risk of catastrophic stent thrombosis, while uncommon, remains a serious issue after stent therapy. The long-term use of DAPT greatly lowers the risk of stent thrombosis, but carries its own substantial risks.
Sources:
Mauri L, Kereiakes DJ, Yeh RW, et al. Twelve or 30 months of dual antiplatelet therapy after drug-eluting stents. N Engl J Med 2014; DOI:10.1056NEJMoa1409312.
Lüscher TF, Steffel J, Eberli FR, et al. Drug-eluting stent and coronary thrombosis: biological mechanisms and clinical implications. Circulation 2007; 115:1051.
Iakovou I, Schmidt T, Bonizzoni E, et al. Incidence, predictors, and outcome of thrombosis after successful implantation of drug-eluting stents. JAMA 2005; 293:2126.
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