Myocardial infarction

Myocardial infarction, abbreviated MI, is death of cardiac muscle due to a compromised blood supply. In the context of pathology, they are occasionally seen at autopsy.

Myocardial infarction
Diagnosis in short
Contraction band necrosis - a finding of MI. H&E stain.
LM	wavy fibers, myocyte necrosis with loss of cross striations, contraction bands, edema, hemorrhage, neutrophilic infiltrate, pyknosis of nuclei, loss of nuclei (karyolysis), loss of striations, macrophage and mononuclear infiltration, fibrovascular response, fibrovascular response, granulation tissue, myocardial fibrosis (old)
LM DDx	cardiac amyloidosis for old infarct, myocarditis
Gross	myocardial pallor, hyperemia (redness), yellow lesion with hyperemic border, white (fibrosis)
Site	heart
Associated Dx	coronary artery atherosclerosis+/-plaque rupture, +/-thrombosis, cocaine use
Signs	abnormal EKG - esp. ST elevation
Symptoms	classic findings: retrosternal chest pain +/- with radiation down the arms, nausea & vomiting, diaphoresis
Blood work	elevated troponin, CK-MB
Prognosis	dependent on extent
Clin. DDx	varies by presentation - usu. aortic dissection, acute pneumonia, pulmonary embolism, GI pathology,

General

Clinical

• Usually diagnosed clinically - with blood work (troponin, CK-MB) or EKG.
• MI may be precipitated by cocaine use... and further exacerbated by treatment with a beta-blocker.
• Acute myocardial infarction (abbreviated AMI) = MI < 6 hours old.
  • Usually no PMN infiltrate.

Classic symptoms:

• Retrosternal chest pain +/- with radiation down the arms.
• Nausea & vomiting.
• Diaphoresis.

Enzymatic tests:

• CK: peaks at day 1, resolves after 2-3 days.
• AST: peaks close to day 2, resolves after 4-5 days.
• LDH: peaks day 2, resolves after ~6 days.

Complications

Complications of MI:

• Contractile dysfunction.
• Cardiac arrhythmia.
• Aneurysm formation, e.g. left ventricular aneurysm.
• Ventricular rupture:
  • Ventricular free wall rupture.
  • Ventricular septal rupture.
• Fibrinous pericarditis.
  • Dressler's syndrome AKA postmyocardial infarction syndrome; pericarditis post-myocardial infarction +/- pericardial effusion (clinically tamponade).
• Mural thrombosis.
• Extension of MI.

Gross

Sequence:

• 18-24 hours - myocardial pallor.
• 1-3 days - pallor, moderate hyperemia (redness due to congestion with blood).
• 3-7 days - yellow lesion with hyperemic border.
• 10-21 days - maximally yellow.
• 6 weeks - white (fibrosis).

Microscopic

Sequence:

• 1-3 hours - Wavy (myocardial) fibers
• 4-12 hours - Coagulative necrosis & loss of cross striations, contraction bands, edema, hemorrhage, PMN infiltrate.
• 18-24 hours - Coagulative necrosis, pyknosis of nuclei, and marginal contraction bands.
• 1-3 days - Loss of nuclei (karyolysis), loss of striations, abundant PMNs.
• 3-7 days - Macrophage and mononuclear infiltration, fibrovascular response.
• 10-21 days - Fibrovascular response, prominent granulation tissue.
• 6 weeks - Fibrosis.

Images:

• MI (upmc.edu).

Contraction band necrosis

General:

• Mediated by catecholamines.
• Thought to arise in reperfusion from hypercontraction.

Microscopic:

• Thick intensely eosinophilic staining bands (on H&E) ~ typically 4-5 micrometres wide
  • Span the short axis of myocyte.
  • Can be thought of bunched-up striae.

Notes:

• Better seen with special stains (Masson or Gomori trichrome).

Images

• 

  CBN - high mag. (WC)

• 

  CBN - very high mag. (WC)

See also

• Heart.