What are the autopsy features of myocardial infarction (heart attack)?

Autopsy Features of Myocardial Infarction

Myocardial infarction can be definitively identified at autopsy through specific pathological features including polymorphonuclear leukocyte infiltration, myocardial necrosis, and eventually scar tissue formation, with distinct temporal stages that correlate with the time elapsed since the ischemic event. 1

Temporal Classification of Myocardial Infarction at Autopsy

Myocardial infarction can be classified pathologically into distinct temporal phases:

1. Evolving Phase (≤6 hours):

   • Minimal or no polymorphonuclear leukocytes may be visible if death occurs within 6 hours 1
   • Stretching and waviness of myocardial fibers, especially at the border of the infarcted area 2
   • Myocardial edema detectable on post-mortem MRI within 3 hours of onset 3

2. Acute Phase (6 hours-7 days):

   • Characterized by presence of polymorphonuclear leukocytes 1, 4
   • Coagulation necrosis becomes visible
   • Increased T2-weighted signal intensity on post-mortem MRI 5

3. Healing Phase (7-28 days):

   • Presence of mononuclear cells and fibroblasts
   • Absence of polymorphonuclear leukocytes 1

4. Healed Phase (≥29 days):

   • Manifested as scar tissue without cellular infiltration
   • Complete healing process typically takes 5-6 weeks 1

Macroscopic Findings

• Early infarcts (first few hours): May appear pale or show no visible changes
• 12-24 hours: Pallor and softening of the affected myocardium
• 1-3 days: Yellow-tan discoloration with hyperemic borders
• 3-7 days: Well-demarcated yellow-tan areas with soft, friable centers

• 7 days: Progressive firming of infarct borders

• Healed infarcts: White, firm scar tissue replacing myocardium

Microscopic Findings

• Early changes (≤6 hours):

  • Stretching and waviness of myocardial fibers 2
  • Edema (detectable on post-mortem MRI) 3
  • Enzyme loss (detectable after 5-6 hours using histochemical methods) 6

• Established changes (6 hours-7 days):

  • Coagulation necrosis of myocytes
  • Contraction band necrosis (especially in reperfused infarcts)
  • Polymorphonuclear leukocyte infiltration 1, 4
  • Loss of cross-striations and nuclear pyknosis

• Late changes (>7 days):

  • Granulation tissue formation
  • Mononuclear cell infiltration
  • Fibroblast proliferation
  • Progressive collagen deposition 1

Special Considerations

Reperfusion Effects

Reperfusion alters the macroscopic and microscopic appearance by producing:

• Myocytes with contraction bands
• Large quantities of extravasated erythrocytes
• More rapid inflammatory response 1

Sudden Cardiac Death

In cases of sudden death where pathological changes haven't had time to develop:

• Fresh thrombus in coronary arteries may be the only finding
• Evidence of plaque rupture, erosion, or thrombosis should be carefully sought
• These cases should be classified as Type 3 myocardial infarction when there are clinical features of ischemia or fresh coronary thrombus at autopsy 1

Advanced Diagnostic Methods

• Post-mortem MRI:

  • Can detect myocardial edema within 3 hours after onset of ischemia 3
  • Elevated T2-weighted signal intensity compared to normal myocardium (2.5 vs. 1.9 when normalized to skeletal muscle) 5
  • Central zone of intermediate signal intensity bordered by a rim of increased signal intensity 3

• Histochemical techniques:

  • Creatine phosphokinase and non-specific dehydrogenase methods become positive only 5-6 hours after infarction 6

Pitfalls in Autopsy Diagnosis

1. Very early infarcts (<6 hours) may show minimal histological changes and can be missed with routine examination
2. Autolysis can obscure histological features, though waviness of myocardial fibers may still be detectable 2
3. Distinguishing different types of MI requires correlation with coronary findings and clinical history 7
4. Post-mortem artifacts can mimic infarction changes
5. Clinical and pathological timing discrepancies - ECG changes and biomarker elevations may persist when pathologically the infarction is already in the healing phase 1

The pathological identification of myocardial infarction should be made with careful attention to these temporal changes, and when possible, correlated with clinical history and coronary artery findings to determine the specific type of myocardial infarction according to the universal classification system 1.