Mac Callum's plaques is a condition featuring the thickening of left atrium's wall and its endocardial wall above the mitral valve due to fibrosis.
It is one complication of chronic rheumatic heart disease. Other complications of chronic rheumatic heart disease are valvular effect (stenosis, insufficiency or can be both), valvular leaflets become thickened by fibrosis, frequent valvular calcification, cordae tendinae become thickened, shortened and fused.
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Mac Callum's plaques Wikipedia(Text) CC BY-SA