Hemorrhagic acid mucopolysaccharide ground substance containing myxoma cells (see higher magnification)

Myxoma cells, as seen here, are stellate or globular. Whether these cells are neoplastic is controversial.

The myxoma is the most common cardiac tumor in adults. Myxomas are classified as benign based on pathologic characteristics and their failure to metastasize. However, as with other pathologically benign cardiac tumors their clinical behavior can be malignant. Myxomas are usually sporadic but familial cases with an autosomal dominant inheritance pattern have been reported. Carney syndrome or complex is a familial multiple neoplasia syndrome. 7% of cardiac myxomas are associated with this syndrome. Myxomas grow from the endocardium as sessile or pedunculated polypoid masses, usually from the atrial septum into the left atrial chamber. The tumors are of variable size; the larger ones can obstruct flow through the mitral valve, causing intermittent syncope. Pounding of valve leaflets by the tumor can cause damage. Grossly, the tumors are gelatinous and friable. Fragmentation leads to embolism. They are often hemorrhagic. Histologically, characteristic cells are stellate or globular myxoma cells. Also present are smooth muscle cells and endothelial cells embedded in abundant mucopolysaccharide matrix. The surface of the tumor is covered with endothelium. Identification of structures is difficult with suggestion of gland formation and poorly-identified vessels in some cases. The tumor is thought to originate from primitive multipotential mesenchymal cells.