Pathology of Acute Myocardial Infarction 🧫💔 | Gross & Microscopic Changes | USMLE Step 1 Pathology

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Pathology of Acute Myocardial Infarction 🧫💔 | Gross & Microscopic Changes | USMLE Step 1 Pathology

Acute myocardial infarction (MI) refers to irreversible myocardial cell death due to sustained ischemia and typically involves necrosis of more than 1 cm of myocardium. The designation “acute” applies to infarcts less than 3–5 days old, when neutrophilic infiltration dominates the histologic picture. MIs can be classified as non-reperfused (permanent coronary occlusion) or reperfused, where blood flow is restored after irreversible damage has already occurred. The gross morphological changes follow a predictable timeline: by 18–24 hours, the myocardium appears pale; 1–3 days, pallor with hyperemia develops; 3–7 days, a yellow soft lesion with a hyperemic border is seen; 10–21 days, the infarct becomes maximally yellow; and by 6 weeks, it transforms into a firm white fibrotic scar. Microscopic changes also progress sequentially: in 1–3 hours, wavy myocardial fibers appear; 4–12 hours, you see coagulative necrosis, loss of striations, contraction bands, edema, and early neutrophilic infiltration. At 18–24 hours, pyknotic nuclei and marginal contraction bands are evident. By 1–3 days, there is abundant PMN infiltration, nuclear dissolution (karyolysis), and complete loss of striations. Between 3–7 days, macrophages and mononuclear cells infiltrate, initiating the fibrovascular repair process. From 10–21 days, granulation tissue becomes prominent, and by 6 weeks, a dense fibrotic scar replaces the infarcted area. This video provides a detailed and visual breakdown of the gross and microscopic evolution of acute MI, helping you master time-based pathology questions in USMLE Step 1 and correlate clinical outcomes with histological stages.

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