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Myocardial Infarction - Introduction (II) - Gross Pathology and Microscopic Pathology

Gross pathology:

Upon examination an MI appear grossly as pale yellow areas in the myocardium
• If reperfusion has occurred, the infarcted areas may appear red
• May be either subendocardial, transmural or multifocal
• In the first 6 to 12 hours, usually no grossly detectable changes unless using tetrazolium incubation
• After 18 to 24 hours, there may be either myocardial pallor or mottling
• In 2 to 3 days, the infarcted zone begins to appear yellow as polymorphonuclear leukocytes infiltrate the tissue, the pallor increases as more polymorphonuclear leukocytes continue to infiltrate the infarcted myocardium
• At 7 days, distinct gelatinous early scar with red borders and depression on cut surface is present
• At 14 days, gelatinous change transitions to early white scar
• By 7 to 8 weeks, cicatrization may be complete
• Complications with structural changes after myocardial infarction include rupture of papillary muscle , ventricular rupture , ventricular aneurysm or pseudoaneurysm

Myocardial necrosis in an acute MI appear grossly as pale yellow areas in the myocardium
• If reperfusion has occurred, the infarcted areas may appear red
• May be either subendocardial, transmural or multifocal
• In the first 6 to 12 hours, usually no grossly detectable changes unless using tetrazolium incubation
• After 18 to 24 hours, there may be either myocardial pallor or mottling
• In 2 to 3 days, the infarcted zone begins to appear yellow as polymorphonuclear leukocytes infiltrate the tissue, the pallor increases as more polymorphonuclear leukocytes continue to infiltrate the infarcted myocardium
• At 7 days, distinct gelatinous early scar with red borders and depression on cut surface is present
• At 14 days, gelatinous change transitions to early white scar
• By 7 to 8 weeks, cicatrization may be complete
• Complications with structural changes after myocardial infarction include rupture of papillary muscle, ventricular rupture, ventricular aneurysm or pseudoaneurysm


Microscopic examination:

Hypereosinophilia of myocyte sarcoplasm (myofibers, myocytes), nuclear pyknosis and karyolysis
Coagulation necrosis - hypereosinophilia with blurring or loss of the striated pattern of the myocyte sarcoplasm
Colliquative myocytolysis (hydropic change of myocytes) in subendocardial location
Contraction band necrosis (which may be part of reperfusion injury including interstitial hemorrhage) is frequently present
Wavy and thinned myocytes (fibers) can also be seen, however wavy myocytes without thinning should not be interpreted as infarcted myocardium
Inflammatory response starts at around 4 hours with margination and progresses as shown in the sequential histopathologic changes table
If reperfusion occurs, contraction band necrosis is prominent with interstitial hemorrhages

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