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DISEASES OF THE MYOCARDIUM AND ENDOCARDIUM
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Terminology of Myocardial and Endocardial Diseases
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The myocardium is the middle layer of the heart wall and is predominantly composed of cardiac muscle. A spectrum of acquired, developmental, and idiopathic disorders causes acute or chronic forms of cardiac disease with myocardial involvement. Inflammatory disease of the myocardium, termed myocarditis, includes infectious and autoimmune disorders. In contradistinction to the adult age group, myocardial ischemia is rare in children. Causes of myocardial ischemia in children include vasculitis (e.g., Kawasaki disease), thromboembolism, cardiac surgery, and coronary artery anomalies. Imaging techniques that evaluate myocardial perfusion and viability include stress echocardiography, coronary angiography, single-photon emission computed tomography (SPECT) scintigraphy, positron emission tomography, and perfusion MRI.1
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The term cardiomyopathy is nonspecific in that it applies to any disorder that involves the cardiac muscle. There are various classification schemes for cardiomyopathy. Primary, or idiopathic, cardiomyopathy refers to heart muscle disease of unknown cause or related to a genetic abnormality (e.g., arrhythmogenic right ventricular dysplasia), whereas the secondary form has a known underlying cause (e.g., infective myocarditis, chronic hypertension, or vasculitis) or is associated with a systemic disorder (e.g., muscular dystrophy). Some practitioners reserve the term cardiomyopathy for those conditions in which there is a structural or functional abnormality of the myocardium that is not associated with coronary artery disease, hypertension, valvar heart disease, congenital heart disease, or pulmonary vascular disease.
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Cardiomyopathy can also be divided into acute and chronic forms. The World Health Organization classifies chronic cardiomyopathies into dilated, hypertrophic, and restrictive categories. Dilated cardiomyopathy is the most common variety; this is characterized by impaired systolic function and poor ventricular contractility. Dilated cardiomyopathy is most often caused by infective myocarditis or exposure to myocardial toxins. Ventricular wall thickening in the less common hypertrophic form of cardiomyopathy can cause systolic outflow obstruction or impaired diastolic filling. Hypertrophic cardiomyopathy frequently has a genetic basis. Restrictive cardiomyopathy is the least-common variety. Grossly impaired ventricular filling causes poor diastolic function. Restrictive cardiomyopathy can be idiopathic or caused by disorders that lead to cellular infiltration of the myocardium, such as glycogen storage disease.
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The endocardium is the endothelial lining membrane of the heart and the underlying connective tissue bed. Because the cardiac valves are predominantly endocardial structures, valvar dysfunction is a common manifestation of endocardial diseases. Endocardial involvement is an important component of many diseases that also affect the myocardium. The most common acquired endocardial disease is endocarditis, which is inflammation of the endocardium. Endocarditis is most often caused by infection, but noninfectious varieties such as rheumatic fever are also common (Table 10-1).
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