The metamorphosis of myocardial infarction following coronary recanalization.

The "metamorphosis" of acute myocardial infarction (AMI) in terms of pathological features and complications in the reperfusion era is herein discussed. Typically, the AMI following coronary artery recanalization is characterized by frequent subendocardial location, since a prompt coronary lumen recanalization is able to prevent the transmural progression of myocardial necrosis from the endocardium towards the epicardium. Transmural AMI may develop when recanalization occurs late (>6 hours) or is not effective (persistent coronary occlusion). Moreover, reperfused AMI frequently appears reddish because of interstitial haemorrhage, which is thought to be caused by vascular cell damage with leakage of blood out of the injured vessels. Hemorrhage occurs always within the area of necrosis and it is significantly related to the infarct size and to the coronary occlusion time. At histology, typical features of reperfused AMI consist of contraction band necrosis and interstitial hemorrhage. Moreover, a more pronounced inflammatory cellular response is visible within the necrotic area when comparing reperfused with non-reperfused AMI. Reperfusion after prolonged coronary occlusion is also associated with secondary impairment of microcirculatory flow ("no-reflow" phenomenon), that is due to endothelial swelling, luminal obstruction and external compression but may also be aggravated by distal embolization. Finally, the reperfused AMI with its typical subendocardial, non-transmural location, is characterized by a lower incidence of expansive remodelling and related complications, in terms of cardiogenic shock, myocardial rupture, aneurysm and pseudoaneurysm formation and thromboembolism. Moreover, pericardial involvement is a rare occurrence. Unfavorable mechanical consequences of intramyocardial haemorrhage could consist in increased myocardial stiffness, propensity to wall rupture and delayed healing process. However, prospective in vivo large-scale studies in patients with reperfused AMI are needed to assess the prognostic value of hemorrhagic AMI in terms of morbidity and mortality. The knowledge and correct interpretation of these findings at post-mortem by the general and forensic pathologists is of great importance, to provide useful information to the clinicians.