UNSTABLE ANGINA PECTORIS: PATHOLOGIC ASPECTS
Authors: Falk E A 
Published: August 2018

Unstable angina pectoris represents a common and important manifestation of acute ischemic heart disease encompassing the broad spectrum of clinical syndromes between stable effort angina and acute myocardial infarction. This group of patientsisfar from uniform concerning underlying pathogenetic mechanisms and prognosis, but generally the risk of infarction or deathis increased during the unstable period. Most patients are presenting with new or worsening effort angina or angina at rest,and especially patients with rest anginaassociated with transient ECG changes seem to constitute a high risk subgroup. Transient reductions in coronary blood flow,rather than increases in myocardial oxygen demand, seem to play the major role in rest angina, indicating an underlying 'dynamic' coronary stenosis.Furthermore, unstable angina seems to beagood clinicalmarker for actively progressing coronary-artery disease.

Pathologically, a rapidly evolving coronary-artery lesion represented by a disrupted atherosclerotic plaque with variable degree of plaque hemorrhage and luminalthrombosis usually is present in patientscoming to autopsy after a period of rest angina. The thrombus at the rupture site may be mural and limited (just sealing therupture) or occlusive depending on the degree of preexisting atherosclerotic stenosis. An occlusive thrombus is seldom seen over ruptured plaques causing less tha15% stenosis (histologic area stenosis), but is found with increasing frequency when stenosis severety increases beyond 15%.Most occlusive thrombi have a layered structure with thrombus material of differing age indicating an episodic growth by repeated mural deposits. Aggregated platelets usually can be identified in the mostrecent part of the thrombus, while older parts are more homogeneous due to fibrin infiltration/stabilization. Additionally,microemboli and microinfarcts are frequently found in the myocardium downstream tocoronary thrombi. So, the period of unstable angina preceding a fatal heart attackseems to be characterized by an ongoing thrombotic process in a major coronary artery where recurrent mural thrombus formation alternates with intermittent thrombus fragmentation and peripheral embolization. Such a dynamic thrombosis (with or without a concomitant focal vasospastic phenomenon) at the site of an unstable (ruptured) atherosclerotic lesion obviously may lead to the other clearly thrombus-related acute ischemic events: myocardial infarction or sudden death.

Clinical studies using coronary angiography and coronary angioscopy during the acute phase of unstable angina have revealed a high frequency of ulcerated (unstable) atherothrombotic lesion in arteries responsible for the acute ischemia. Furthermore, episodic platelet activation (usually associated with chest pain) has recently been demonstrated in patients with unstable angina.

The mechanism underlying pain/ischemia(predominantly spasm?) and the rapid plaque progression (plaque hemorr.hage/luminal thrombosis?) during unstable angina maydiffer. Accordingly, therapy directed against a possible spasm (nitrates, calcium antagonists) usually relieves pain effectively without having any documented effect on infarction/survival, while antithr-ombotic therapy (aspirin, heparin) clearlyimproves the prognosis without apparent antianginal effect. Therefore, with the objective not only of relieving pain but also of improving the prognosis, more attention should be paid to the potentially fatal thrombotic process that apparently isgoing on in a major coronary artery of many patients with unstable angina.