It is typical to see dilatation of all cardiac chambers in dogs with dilated cardiomyopathy (DCM), although left atrial and ventricular enlargement may predominate. The ventricular wall thickness appears decreased compared with the lumen size. Papillary muscles often look flattened and atrophic; endocardial thickening may be noted. The atrioventricular (AV) valves generally have only mild to moderate, if any, degenerative changes. Histopathologic findings include scattered areas of myocardial necrosis, degeneration and fibrosis, especially in the left ventricle.

The presence of narrowed (attenuated) myocardial cells with a wavy appearance is reported to be a common finding. Such abnormal cells have also been noted in some Newfoundlands thought to be predisposed to DCM but without any clinical or echocardiographic evidence of the disease. Inflammatory cell infiltrates and myocardial hypertrophy are inconsistent features.

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