What is the difference between Non-ST-Elevation Myocardial Infarction (NSTEMI) and unstable angina in terms of diagnosis and management?

Difference Between NSTEMI and Unstable Angina

The primary difference between NSTEMI and unstable angina is the presence of myocardial necrosis in NSTEMI, detected by elevated cardiac troponin levels, while unstable angina shows no evidence of myocardial damage despite similar clinical presentations. 1

Diagnostic Differences

Definition and Pathophysiology

• Unstable angina is defined as myocardial ischemia at rest or minimal exertion without evidence of acute cardiomyocyte injury/necrosis 1
• NSTEMI is characterized by myocardial ischemia with evidence of myocardial necrosis, demonstrated by elevated cardiac biomarkers (primarily troponin) 1
• Both conditions typically result from disrupted or eroded atherosclerotic plaque with subsequent thrombus formation, but in NSTEMI the thrombus causes sufficient ischemia to result in myocardial damage 1

Biomarker Evaluation

• The critical diagnostic difference is the presence of elevated cardiac biomarkers:
  • NSTEMI: Positive cardiac troponin (above the 99th percentile of normal population) 1
  • Unstable angina: Negative cardiac troponin (below the 99th percentile) 1
• With high-sensitivity troponin assays, the diagnosis of unstable angina has decreased while NSTEMI diagnoses have increased (approximately 20% relative increase) 1
• Serial troponin measurements (0h/1h, 0h/2h, or 0h/3h protocols) are recommended to differentiate between these conditions 1

ECG Findings

• Both conditions typically present with ST-segment depression, T-wave inversion, or no ECG abnormalities 1
• Neither shows persistent ST-segment elevation (which would indicate STEMI) 1
• ECG changes may be transient or absent in unstable angina 1

Management Differences

Risk Stratification

• NSTEMI patients have a substantially higher risk of death compared to unstable angina patients 1
• GRACE risk score models should be considered for estimating prognosis in both conditions 1
• Risk assessment guides the timing and aggressiveness of intervention 1

Antithrombotic Therapy

• Both conditions require antiplatelet therapy with aspirin 2
• P2Y12 inhibitors (clopidogrel, ticagrelor, prasugrel):
  • NSTEMI patients derive greater benefit from more potent antiplatelet therapy 1
  • Prasugrel should be considered in preference to ticagrelor for NSTE-ACS patients proceeding to PCI 1
  • Unstable angina patients appear to derive less benefit from intensified antiplatelet therapy 1
• Anticoagulation with heparin (unfractionated or low-molecular-weight) is recommended for both conditions 1, 3

Invasive Strategy Timing

• NSTEMI patients benefit more from an early invasive strategy (within 72 hours) 1
• Unstable angina patients appear to derive less benefit from an invasive strategy within 72 hours 1
• Coronary angiography is advised for all high-risk patients regardless of diagnosis 1
• MDCT coronary angiography may be considered as an alternative to invasive angiography when there is low-to-intermediate likelihood of CAD and troponin/ECG are inconclusive 1

Monitoring Requirements

• NSTEMI patients require more intensive cardiac monitoring:
  • Low-risk NSTEMI: Rhythm monitoring up to 24 hours or until PCI 1
  • High-risk NSTEMI: Rhythm monitoring for >24 hours 1
• Monitoring requirements for unstable angina may be less stringent due to lower risk of arrhythmias 1

Clinical Outcomes and Prognosis

• Mortality is significantly higher in NSTEMI compared to unstable angina 1, 4
• The 6-month mortality for NSTE-ACS has decreased from 17.2% to 6.3% over recent decades due to improved management strategies 1
• Even patients with unstable angina have a non-negligible cardiovascular risk compared to controls without ACS 4
• Long-term management for secondary prevention is essential for both conditions, including smoking cessation, statin therapy, blood pressure control, and ACE inhibitors 3

Common Pitfalls and Caveats

• The distinction between unstable angina and NSTEMI may not be apparent at initial presentation, requiring serial troponin measurements 1
• Minor troponin elevations are common in patients diagnosed with unstable angina, making the distinction from NSTEMI challenging 4
• The introduction of high-sensitivity troponin assays has led to reclassification of many cases previously diagnosed as unstable angina to NSTEMI 1
• Type 2 MI (secondary to oxygen supply-demand mismatch rather than plaque rupture) can complicate the diagnosis and requires consideration of underlying causes like anemia, hypoxemia, tachyarrhythmias, or hypertension 1
• D-dimer may be elevated in both conditions but is not specific for coronary disease and should not be used for diagnosis 5