Unstable angina occurs in approximately 6 out of every 10,000 people. Coronary artery disease due to atherosclerosis is by far the most common cause of unstable angina. A coronary artery spasm is a rare cause of angina. Initially, narrowing (in the form of atheroschlerotic plaques) of the coronary arteries may be severe enough to cause stable angina with exertion or stress. This narrowing, however, sometimes causes no symptoms at all. For reasons that are not well understood, the plaques reach a point at which they rupture. Under these circumstances, a plug formed by platelets and blood clots may form, causing an already narrowed blood vessel to become highly susceptible to becoming completely blocked. This unstable situation may progress to complete blockage of the vessel, followed by heart attack. Once this type of lesion exists in a coronary artery, and whether the event leads to a heart attack or not, the chest pain produced by the extreme narrowing of the coronary artery may become unstable. Unlike stable angina, in which chest pain can be induced by exercise or stress, chest pain in unstable angina may occur at rest and without any precipitating factors. Pain may increase in severity, frequency, or duration at low levels of activity or for no identifiable reason. This type of angina may also occur soon after a heart attack. Risk factors for unstable angina are similar to those for stable angina and coronary artery disease, including: Occasionally, sudden overwhelming stress can precipitate an episode of angina.
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