A small papillary frond-like vegetation that is found on the closing edge of valve cusps and leaflets. It is typically seen in the central closing edge of the aortic valve cusps. The excrescence resembles a fine thread-like structure. Microscopically, they have a fibrous core, and a surface of endothelial cells. They are thought to arise from localized trauma to the valve from repetitive closure, with organization of the resulting fibrin and platelet vegetation. They are related to the much less common, and larger, organized vegetation called papillary fibroelastoma. The latter lesions, for many years, were considered to be benign tumors or hamartomas. Structurally, however, they appear to be due to non-bacterial vegetation that undergoes replacement of the fibrin and platelets by fibrous and elastic connective tissue. Clinically, they are rarely significant. However, if a larger lesion, particularly on a stalk, prolapses into a coronary ostium, it may lead to sudden death or myocardial infarction. In contrast, the Lambl's excrescence is too small, and is only an incidental finding at autopsy.
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