Normally, the brain and heart function in seamless harmony. The heart supplies the brain with nutrients and oxygen, while the brain regulates cardiac function. It is a long-standing fact that cardiovascular disease is a major source of neurological dysfunction. In addition, however, it has become recently apparent that besides playing a major role in basic cardiac homeostatis, the brain directly influences certain cardiovascular disease states. Among these influences are profound effects on the incidence of, and outcome from, coronary artery disease, congestive heart failure, and cardiac arrhythmias.
The influences of the heart on the brain
There is a strong association between heart disease and subsequent neurological dysfunction. Focal neurological syndromes due to cardiac diseases most often arise from emboli, typically associated with arrhythmias, valvular heart disease, myocardial infarction, as well as rarer situations, such as intracardiac tumours. Cardiac surgery, particularly coronary artery bypass grafting and valve replacements, may be associated with showers of athero-, air, and thromboemboli at the times of aortic manipulation.
In addition to embolic injury, other cardiac causes of neurological dysfunction include cardiac arrhythmias or cardiac arrest, which may result in anoxic injury to the brain. Finally, congestive heart failure may lead to a number of subtle neurological findings.
Cerebral embolic strokes of cardiac origin
The percentage of strokes that are cardioembolic is at least 20%, and may be as high as 50%. The major underlying cardiac conditions leading to cardioembolic strokes include atrial fibrillation, myocardial infarction, valvular heart disease, infectious endocarditis, and non-bacterial thrombotic endocarditis. Less common conditions which may be associated with cardioembolic strokes include patent foramen ovale, intracardiac tumours, and calcifications of the mitral or aortic valves/annulus.
Atrial fibrillation is a major risk factor for stroke, with a stroke rate of approximately 5%/year (Nademanee & Kosar, 1998). However, the risk varies greatly. For those under 65, without risk factors, the risk is about 1%/year. For others with increasing age, systemic arterial hypertension, diabetes, valvular heart disease, a dilated left atrium, thyrotoxic atrial fibrillation, mitral annular calcification, previous TIA/stroke, or poor ventricular function the risk can be as high as 10–12%/year (Nademanee & Kosar, 1998). The actual incidence of embolic infarction is undoubtedly higher than these percentages, as evidenced by the increased numbers of ‘silent’ brain infarctions on brain imaging studies in patients with arrhythmias (Ezekowitz et al., 1995).