What are the histologic microscopic findings of myocardial infarction?

Histologic Microscopic Findings of Myocardial Infarction

Myocardial infarction is defined pathologically as myocardial cell death characterized by coagulation necrosis and/or contraction band necrosis, evolving primarily through oncosis and to a lesser degree through apoptosis. 1

Fundamental Pathologic Features

Cell death patterns require careful histological analysis by an experienced observer to distinguish the specific type of necrosis present. 1

Types of Necrosis

• Coagulation necrosis: The predominant pattern in transmural infarction, appearing as yellowish-brown tissue in the center of infarcted foci 2
• Contraction band necrosis: Particularly prominent when reperfusion occurs, producing myocytes with contraction bands and large quantities of extravasated erythrocytes 1
• Coagulative myocytolysis: Characteristic of subendocardial infarction throughout the entire focus 2

Temporal Evolution of Microscopic Findings

Very Early Changes (<6 hours)

• Stretching and waviness of myocardial fibers, especially at the border of the infarcted area, develops within less than 1 hour and perhaps within minutes after circulation fails 3
• This waviness results from rhythmical pull by normal myocardium against the paralyzed infarcted area and outward bulging during systole 3
• Minimal or no polymorphonuclear leukocytes may be seen if the interval between onset and death is brief (e.g., <6 hours) 1
• Wavy fiber and colliquative myocytolysis appear as non-specific ischemic lesions in peripheral and subendocardial areas 2

Acute Phase (6 hours to 7 days)

• Presence of polymorphonuclear leukocytes is the defining histologic feature of acute myocardial infarction 1
• Complete necrosis of all myocardial cells at risk requires at least 2-4 hours or longer, depending on collateral circulation, persistent or intermittent coronary occlusion, and individual oxygen demand 1
• It takes several hours (approximately 6 hours) before myocardial necrosis can be identified by standard microscopic post-mortem examination 1

Healing Phase (7 to 28 days)

• Presence of mononuclear cells and fibroblasts with absence of polymorphonuclear leukocytes characterizes healing infarction 1
• Removal of necrotic sarcoplasm by mononuclear phagocytes creates a reticular-appearing tissue 4
• Collagen fibers are deposited within areas of previous sarcolemmal sheaths 4

Healed Phase (≥29 days)

• Scar tissue without cellular infiltration manifests as the healed infarction 1
• The entire process leading to a healed infarction usually takes at least 5-6 weeks 1

Microscopic Patterns by Infarct Type

Transmural Infarction

• Yellowish-brown coagulation necrosis in the center of the infarcted focus 2
• Coagulative myocytolysis at the marginal zone 2
• Associated with obstructive thrombus formation in 79% of cases 2

Subendocardial Infarction

• Coagulative myocytolysis throughout the entire focus is the characteristic pattern 2
• One to two layers of preserved myofibers adjacent to the endocardium 4
• Vacuolar change in deeper fibers 4
• Focal areas of coagulation necrosis of variable size in myocardium external to fibers with vacuolar change 4
• Coagulation necrosis usually not associated with neutrophil infiltration 4
• Associated with plaque hemorrhage obstruction in 27% of cases and frequently with multivessel disease 2

Special Considerations with Reperfusion

Reperfusion significantly alters the microscopic appearance of the necrotic zone. 1

• Produces myocytes with prominent contraction bands 1
• Large quantities of extravasated erythrocytes (hemorrhage) within the infarct zone 1
• This pattern differs from non-reperfused infarction and must be recognized to avoid misinterpretation 1

Critical Diagnostic Pitfalls

Timing Discordance

The clinical and electrocardiographic timing may not correspond exactly with pathological timing. 1, 5

• ECG may show evolving ST-T changes and biomarkers may remain elevated when pathologically the infarction is already in the healing phase 1, 5

Sudden Death Cases

• Patients dying before pathological changes develop (within minutes to hours) present diagnostic challenges 1
• Waviness of myocardial fibers may be the only detectable microscopic finding in very early sudden death 3
• This pattern can establish the diagnosis even after considerable autolysis 3

Grading of Ischemic Severity

Ischemic lesions can be graded in descending order of severity: coagulation necrosis > coagulative myocytolysis > wavy fiber > colliquative myocytolysis 2