What are the microscopic findings of myocardial infarction (MI)?

Microscopic Findings of Myocardial Infarction

Myocardial infarction is characterized pathologically by coagulation necrosis and/or contraction band necrosis, with the presence of polymorphonuclear leukocytes defining acute MI histologically. 1

Temporal Evolution of Microscopic Changes

The microscopic appearance of MI evolves in a predictable sequence that depends critically on the time elapsed since ischemic injury:

Hyperacute Phase (<6 hours)

• Minimal or no polymorphonuclear leukocytes are present if death occurs within 6 hours of onset, making early histologic diagnosis challenging 1
• Waviness of myocardial fibers may be the only detectable microscopic finding in very early sudden death cases 1
• Cell death is not immediate after ischemia onset—it takes as little as 20 minutes to develop in some models, but standard microscopic examination cannot identify necrosis until approximately 6 hours have elapsed 2

Acute Phase (6 hours to 7 days)

• The defining histologic feature is the presence of polymorphonuclear leukocytes (neutrophils) infiltrating the necrotic myocardium 1, 3
• Cell death manifests as coagulation necrosis and/or contraction band necrosis, evolving primarily through oncosis and to a lesser degree through apoptosis 2, 1
• Complete necrosis of all at-risk myocardial cells requires at least 2-4 hours or longer, depending on collateral circulation, intermittent versus persistent occlusion, and individual oxygen demand 2, 1

Healing Phase (7 to 28 days)

• Mononuclear cells (macrophages and lymphocytes) and fibroblasts replace polymorphonuclear leukocytes as the dominant cellular infiltrate 1, 3
• The absence of polymorphonuclear leukocytes with presence of mononuclear cells is the key distinguishing feature of healing infarction 1

Healed Phase (≥29 days)

• Scar tissue without cellular infiltration characterizes the healed infarction 1, 3
• The entire process from acute injury to complete healing typically requires at least 5-6 weeks 2, 1

Special Microscopic Patterns

Reperfusion-Modified Infarction

• Reperfusion significantly alters the microscopic appearance, producing myocytes with prominent contraction bands and large quantities of extravasated erythrocytes 1
• This pattern differs markedly from non-reperfused infarction and must be recognized to avoid misinterpretation 1

Critical Diagnostic Pitfalls

Clinicopathologic Timing Discordance

• The clinical and electrocardiographic timing may not correspond exactly with pathological timing 1, 3
• ECG may show evolving ST-T changes and biomarkers may remain elevated when the infarction is pathologically already in the healing phase 1, 3
• This discordance occurs because biomarkers reflect ongoing myocyte death and membrane disruption, while histology reflects the tissue's inflammatory and reparative response 2

Very Early Death

• Patients dying within minutes to hours of symptom onset present the greatest diagnostic challenge, as pathological changes may not yet be detectable 1
• Careful analysis of histologic sections by an experienced observer is essential to distinguish true necrosis from artifact 2