In 1912, Herrick published an article describing the "clinical features of obstruction of the coronary arteries" in JAMA. There, he describes the entity of unrecognized myocardial infarction. His title still suggests that the pathophysiology in ischemia is the same in patients with silent MI, where poor perfusion to the cardiomyocytes leads to tissue hypoxia and infarction. The difference must be in how this abnormality is perceived and acted upon by patients. Autonomic neuropathy of afferent nerves has been a proposed mechanism, especially in diabetic patients. In 1977, a study looking at 5 patients with silent myocardial infarction and diabetes, and found pathologic changes in the autonomic nerves supplying the myocardium. These changes were consistent with diabetic neuropathy. These changes were not seen in diabetics and non-diabetics in patients with painful ACL presentations. "Gating" phenomenon has been described as another potential mechanism, where muting of afferent pain signals in the dorsal horns by additional sensory input (ex. dypnea) may overwhelm pain input and dampen down the pain perception. Additional factors have focused on supratentorial interpretation of pain, which can be influenced by other medical conditions such as depression. There was a theory that endorphins may play a role in suppressing pain in these patients, however studies involving the administration of naloxone in the setting of silent ischemia in exercise testing had no influence.Silent MI is likely an under-recognized condition. Data from the Framinham Study suggested approximately 30% of all MI's are silent. This statistic was slightly more common in women. This was also seen in an additional study, where ~20% of patients had silent myocardial infarctions. Q waves on ECG are the most common way to identify this. Considering these ECG changes can resolve in some patients over time, its possible we are missing a portion of silent MI's as a result. Not to mention those that die of sudden cardiac death.
Many studies have found age and hypertension to be associated with silent MI. However, just given these factors are associated in general with coronary artery disease may confound these findings. In a study from Iceland, the risk of silent MI increased by 10% per year of age in those with myocardial infarction. Dementia and cognitive impairment are some of the possible explanations for this. As previously stated, diabetes is a risk factor for silent MI, interestingly diabetics tend to report less pain in confirmed MI as well. A study in JAMA reviewed over 400,000 patients presenting myocardial infarction and found that silent MI was common, and was associated with differences in treatment, where they were less likely to receive aspirin or thrombolysis/PCI. Diabetes was more common in this group as well. The Cardiovascular Health Study found women to be 45% more likely to have unrecognized infarction compared to men, although it was not identified as an independent risk factor. Public perceptions regarding the baseline risk for women to have cardiovascular disease may play a role in the under-identification of MI.
You would think that silent disease would represent milder disease and improved survival, however this may lead to a lack of appropriate therapy. Long term follow up of patients with silent MI show little differences in mortality. The Cardiovascular Health Study showed a 21% vs 25% mortality at seven years in unrecognized and recognized MI respectively. The Honolulu Heart Study actually found an increase in mortality associated with unrecognized MI.
Patients presenting without chest pain but additional concerning features for ischemia need to be investigated. These individuals carry a prognosis similar to recognized myocardial infarction and shouldn't be treated differently. See below for a review article.
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