Narrowing of the opening between the left ATRIUM and left VENTRICLE of the HEART as a result of rigidity of, and adhesion between, the cusps of the MITRAL VALVE. Now rare, it is due, almost invariably, to RHEUMATIC FEVER. The atrium has to work harder to force blood through the narrowed channel. The effects are similar to those of MITRAL INCOMPETENCE. Shortness of breath and palpitations and irregular beating (fibrillation) of the atrium are common consequences in adults. Drug treatment with DIGOXIN and DIURETICS helps, but surgery to dilate or replace the faulty valve may be necessary.