Some myocardial infarctions (MIs) are minimally symptomatic or asymptomatic; consequently, the acute event often goes undetected. These unrecognized MIs might affect future morbidity and mortality. Sheifer and associates performed a MEDLINE search to review relevant publications from 1966 to the present that discussed either “silent” or “unrecognized” MIs to clarify the pathophysiology of these events.

Some authors hypothesize that unrecognized MIs occur when myocardial ischemia does not cause symptomatic discomfort. Others suggest that symptoms of an MI occur, but the patients may fail to recognize that these symptoms are indicative of an MI. Either of these scenarios results in a “defective anginal warning system” and a subsequent lack of recognition of the acute event. Potential causes of decreased perception of MI include inadequate stimulation of cardiac sensory receptors; afferent neuron dysfunction that blocks pain perception (as occurs in patients with autonomic neuropathy); gating mechanisms that hide the pain of an MI when it is combined with other sensory stimuli such as dyspnea; and neuropsychiatric factors that blunt pain sensation at the supratentorial level. Psychosocial factors, including stoicism and denial, higher pain thresholds, or depression, may also diminish recognition of symptoms of an acute MI.

Results of studies reviewed by Sheifer and colleagues found that unrecognized MI may represent 22 to 35 percent of all MIs. In the Framingham study, for example, approximately one half of the affected patients reported no symptoms, while the rest of the patients had nonspecific symptoms that were not associated with an MI.

Many studies suggest that women have a higher risk for unrecognized MI; however, many clinicians and patients incorrectly believe that coronary events are less common in women, which decreases the likelihood of an accurate recognition of cardiac symptoms. This situation is compounded by the higher frequency of atypical symptoms occurring with cardiac events in women.

Three other risk factors that appear to be associated with unrecognized MI include the absence of angina, the absence of congestive heart failure, and a lower forced expiratory flow in one second. These risk factors may be related to diagnosis bias; for example, patients with lung disease may have excess respiratory signals that effectively block the perception of pain. Hypertension, increasing age, and diabetes mellitus have also been considered risk factors for unrecognized MI; however, this supposition has not been confirmed by actual direct comparison studies. The prognosis of patients with unrecognized MI does not differ significantly from that of patients with diagnosed MI.

The authors conclude that unrecognized MI accounts for at least 20 percent of all MIs, and the prognosis for both recognized and unrecognized events is poor. Further studies are needed to clarify the risk factors for unrecognized events. Regular screening electrocardiography may be useful in high-risk patients, such as elderly patients with multiple coronary risk factors. In stable patients, noninvasive testing for residual myocardial ischemia and left ventricular function, with stress testing, echocardiography, and nuclear myocardial stress perfusion imaging, may help predict outcomes and encourage treatment as well as aggressive risk-factor modification. Treatment with accepted postmyocardial therapies, including aspirin, a beta blocker, an angiotensin-converting enzyme inhibitor, and a lipid-lowering medication, is appropriate, unless contraindicated.