The Histopathologic Evolution of Myocardial Infarction

The histologic criteria for determining the age of a myocardial infarct were established by Mallory et al in 1939. The present study was undertaken to reexamine more quantitatively the evolution of myocardial infarction and to determine the prevalence of two recently described pathologic findings: wavy fibers and myocytolysis. The medical records of 1,155 patients dying of myocardial infarction from 1964 to 1973 were reviewed, and 192 cases in which the age of the infarct could be determined accurately were studied. As in the study by Mallory et al, acute inflammatory changes predominated during the first week, chronic inflammatory changes peaked during the second week, and proliferation of connective tissue dominated during the third week. In 35 percent (7/20) of the myocardial infarctions that were 22 to 35 days old, healing was not complete. Ninety-four percent (47/50) of the myocardial infarctions that were less than 24 hours old had “wavy fibers,” but only one-fifth of all myocardial infarctions had focal myocytolysis. Waviness appears to be due to ischemic injury of fibers and may be the earliest light-microscopic change in evolving myocardial infarction, while myocytolysis does not appear to be related to ischemia per se. Although the therapy for acute myocardial infarction has changed markedly since 1939, this has not altered the pathologic evolution of the myocardial lesion.