Is unstable angina the same as a myocardial infarction (MI)?

Unstable Angina vs. Myocardial Infarction: Key Differences

No, unstable angina is not the same as myocardial infarction (MI), though they are both part of the acute coronary syndrome spectrum with important distinctions in pathophysiology, diagnosis, and prognosis.

Fundamental Differences

Unstable angina and myocardial infarction differ primarily in:

1. Cardiac Biomarkers:

   • Unstable angina: Normal cardiac biomarkers (troponin, CK-MB)
   • Myocardial infarction: Elevated cardiac biomarkers indicating myocardial necrosis 1

2. Pathophysiology:

   • Unstable angina: Typically involves non-occlusive thrombus formation on disrupted atherosclerotic plaque with preserved myocardial tissue
   • Myocardial infarction: Involves more extensive coronary occlusion leading to myocardial cell death 1, 2

3. ECG Findings:

   • Unstable angina: May show transient ST depression or T-wave changes
   • NSTEMI: ST depression or T-wave inversion with positive biomarkers
   • STEMI: ST-segment elevation with positive biomarkers 3

Acute Coronary Syndrome Classification

The American College of Cardiology and American Heart Association define the acute coronary syndrome spectrum as follows:

• Unstable Angina (UA):

  • Rest angina, new-onset severe angina, or increasing angina
  • Normal cardiac biomarkers
  • Often transient ECG changes during symptoms 1

• Non-ST Elevation MI (NSTEMI):

  • Similar presentation to unstable angina
  • Elevated cardiac biomarkers
  • ST depression or T-wave inversion on ECG 1

• ST Elevation MI (STEMI):

  • Prolonged chest pain
  • Elevated cardiac biomarkers
  • ST-segment elevation on ECG 1

Clinical Implications

The distinction between unstable angina and MI has important implications:

1. Prognosis:

   • Patients with MI (both STEMI and NSTEMI) generally have worse outcomes than those with unstable angina due to myocardial necrosis 4

2. Treatment Approach:

   • Unstable angina: Typically managed with antiplatelet therapy, anticoagulation, and risk stratification
   • NSTEMI: Similar to unstable angina but may require more aggressive intervention
   • STEMI: Requires immediate reperfusion therapy 5

3. Risk Stratification:

   • Patients with unstable angina who have high-risk features (persistent pain, dynamic ECG changes) should be managed similarly to NSTEMI 3

Common Pathophysiological Mechanism

Despite their differences, unstable angina and MI share a common pathophysiological basis:

• Both typically result from coronary plaque disruption or erosion with subsequent thrombus formation
• The extent of thrombus formation and coronary occlusion determines whether myocardial necrosis occurs 1, 2
• Unstable angina typically involves more platelet-rich thrombus, while MI involves more fibrin-rich thrombus 2

Evolving Terminology

Recent guidelines are moving toward a more comprehensive classification system:

• Acute Myocardial Ischemic Syndromes (AMIS) for unstable angina and MI
• Non-Acute Myocardial Ischemic Syndromes (NAMIS) for stable angina 1

This reflects the understanding that these conditions represent a spectrum of ischemic heart disease with varying presentations and outcomes.

In summary, while unstable angina and myocardial infarction share pathophysiological mechanisms and clinical features, the key distinction is the presence of myocardial necrosis (detected by cardiac biomarkers) in MI that is absent in unstable angina.