Silent myocardial ischemia or infarction, or silent myocardial infarction (MI), is characterized by a reduction in blood flow to the heart that doesn't manifest clinically with chest pain/discomfort or other symptoms of angina, dyspnea, nausea, or diaphoresis. Silent MI is commonly undetected in patients who have chronic symptoms of coronary disease, according to an article in the European Heart Journal Supplements (EHJS).1 Indeed, it is estimated that 70% to 80% of transient ischemic episodes aren't symptomatic.2
The first step in managing patients with silent ischemia is identification, and diagnostic ECG has a clear role to play, as ST-segment changes provide objective evidence it occurs. These subclinical events account for most episodes of myocardial ischemia, and they can be detected on an exercise stress test, 24-hour Holter monitoring, or bedside ECG, as well as various types of noninvasive imaging. In recent years, physiology assessments during invasive coronary angiography, like fractional flow reserve and instantaneous wave-free ratio, provide additional ways to uncover subclinical ischemia.1
From there, physicians can decide on the most appropriate course of action, whether that's optimization of medical therapy and management of cardio vascular (CV) risk factors or either percutaneous or surgical revascularization.
Types of Silent Ischemia and Risk Factors
Researchers don't have a firm grasp on how common silent ischemia is at the population level, but it's estimated that up to half of patients with ischemic heart disease have subclinical episodes and that these silent events are about 20 times more common than symptomatic ones.1
Silent ischemia is broken down into three types:
- Type 1 (least common): Occurs in asymptomatic patients with chronic ischemic heart disease but no anginal symptoms.
- Type 2: Occurs in patients with a prior documented MI.
- Type 3 (most common): Occurs in patients with stable, unstable, or vasospastic (or Prinzmetal) angina.
Silent ischemia is particularly prevalent among patients with diabetes or obstructive sleep apnea, among geriatric patients who have just undergone surgery, and among critically ill patients in the ICU who are admitted for noncardiac reasons. It's also been shown to be more common in men than in women, although women are more likely to have acute coronary syndromes without clinical symptoms and less likely to present with significant changes on ECG.3
Where Diagnostic ECG Fits into the Care Pathway
The ECG—whether it's performed in the hospital or doctor's office, as part of exercise stress testing or with ambulatory monitors—is a key tool at clinicians' disposal to uncover silent myocardial ischemia. The Texas Heart Institute notes that characteristic ST-segment changes and disturbances in heart rhythm, such as ventricular tachycardia and ventricular fibrillation, can be seen when utilizing ECG.4 Diagnostic ECG can quickly and accurately determine whether silent ischemia is present. Despite similarities in clinical presentation, ECG is capable of differentiating between ischemic and non-ischemic changes such as between STEMI and pericarditis or myocarditis.
Early diagnosis and triage can help prevent further adverse outcomes tied to delays in patient care. In fact, research has shown even silent ischemia is associated with worse clinical outcomes over the long term, both in patients with no history of coronary disease and in those with established heart disease, according to the EHJS authors.1
The authors point to a study in the Journal of the American College of Cardiology that followed men with no prior history of coronary disease for an average of 10 years.5 Those with exercise-induced silent ischemia—defined as ST-segment depression on ECG during and after maximal symptom-limited exercise test—were associated with greater risks of any acute coronary event and of coronary heart disease mortality. Additionally, this observed correlation was stronger in men who had high cholesterol, hypertension, or a history of smoking. The same research group subsequently reported that asymptomatic ST-segment depression during exercise testing also increased the risk of sudden cardiac death over a median follow-up of 18 years.6
More recently, another study linked silent MI defined by ECG to the development of heart failure over a median follow-up of 13 years, with a stronger association in individuals younger than 53.7
How to Lower Risk and Improve Long-term Outcomes
Technological advances can ease the detection of silent myocardial ischemia. However, physicians still face challenges when deciding what to do in response to finding an absence of symptoms. "Furthermore, one of the dilemmas, currently with no clear answer, is whether screening should be performed in asymptomatic patients at high risk for myocardial ischemia, but especially when and how to treat them."1
Authors of the EHJS study highlight this difficulty by pointing to the results of the ISCHEMIA trial, which included "a good number" of patients with silent ischemic episodes.8 The trial, which enrolled patients with stable ischemic heart disease and a moderate degree of ischemia, failed to show that a routine invasive strategy that included angiography followed by as-needed revascularization resulted in better outcomes compared with optimal medical therapy alone.
Overall, however, there is limited data evaluating how effective coronary revascularization is for the treatment of silent ischemia.2
In general, the initial approach for treating patients with silent myocardial ischemia should be similar to the general approach used for heart disease and angina.9 That means quitting tobacco; managing traditional risk factors like diabetes, hypertension, and dyslipidemia; exercising regularly; maintaining a healthy weight; eating well; reducing stress; checking in with a doctor regularly; and screening ECGs at annual check-ups. Some drug therapies might include beta-blockers, calcium channel blockers, nitrates, statins, aspirin, and ranolazine, with oxygen treatment used in some cases.
Still, the EHJS authors noted it's not clear how much these efforts improve long-term outcomes in patients with asymptomatic ischemia, who have an unknown prognosis in the modern era.1 "In light of current and recent evidence, the treatment of silent myocardial ischemia must be personalized based on the patient's clinical characteristics, symptoms, the degree of ischemia, and the extent of coronary artery disease," they conclude.
Resources:
1. Indolfi C, Polimeni A, Mongiardo A, et al. Old unsolved problems: when and how to treat silent ischaemia. European Heart Journal Supplements. November 2020;22(Suppl L):L82-L85. https://academic.oup.com/eurheartjsupp/article/22/Supplement_L/L82/5989607
2. Gul Z, Makaryus AN. 2023 Jan 19. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan–. https://www.ncbi.nlm.nih.gov/books/NBK536915/
3. Aziz F. Coronary artery disease in women: an unsolved dilemma. Journal of Clinical Medicine Research. April 2014;6(2):86-90. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3935528/
4. The Texas Heart Institute. Silent ischemia. TexasHeart.org. https://www.texasheart.org/heart-health/heart-information-center/topics/silent-ischemia/. Accessed June 30, 2023.
5. Laukkanen JA, Kurl S, Lakka TA, et al. Exercise-induced silent myocardial ischemia and coronary morbidity and mortality in middle-aged men. Journal of the American College of Cardiology. July 2001;38(1):72-79. https://www.sciencedirect.com/science/article/pii/S0735109701013110
6. Laukkanen JA, Mäkikallio TH, Rauramaa R, et al. Asymptomatic ST-segment depression during exercise testing and the risk of sudden cardiac death in middle-aged men: a population-based follow-up study. European Heart Journal. March 2009;30(5):558-565. https://academic.oup.com/eurheartj/article/30/5/558/586995
7. Qureshi WT, Zhang ZM, Chang PP, et al. Silent myocardial infarction and long-term risk of heart failure: the ARIC study. Journal of the American College of Cardiology. January 2018;71(1):1-8. https://www.jacc.org/doi/10.1016/j.jacc.2017.10.071
8. Maron DJ, Hochman JS, Reynolds HR, et al. Initial invasive or conservative strategy for stable coronary disease. New England Journal of Medicine. April 2020;382:1395-1407. https://www.nejm.org/doi/full/10.1056/nejmoa1915922
9. Beaumont. Silent ischemia – myocardial ischemia without angina. Beaumont.org. https://www.beaumont.org/conditions/silent-ischemia. Accessed June 30, 2023.
