The commonest cause of mitral stenosis (MS) is rheumatic fever. Congenital MS occurs in a very small group and other causes are extremely rare. Hence if MS is documented, especially in the developing countries, it is most often of rheumatic etiology. Mitral stenosis is considered severe if the valve area is less than 1 sq cm in an adult. An area more than 1.5 sq cm is considered mild and between 1.5 and 1 sq cm as moderate. Normal mitral valve has an area of around 5 sq cm and mitral stenosis is said exist when the valve area is less than 2 sq cm.
As the mitral valve becomes narrow, the pressure in the left atrium and consequently that in the pulmonary veins and capillaries rise. When the pulmonary capillary pressure reaches above 25 mm Hg, there is a chance for transudation of fluid into pulmonary alveoli. This is known as pulmonary edema and causes severe breathlessness. To begin with this occurs only during exercise while later on when the severity of mitral stenosis increases, this can occur at rest as well. Usually this occurs when the mitral valve area has gone below 1 sq cm. Hence a valve area less than 1 sq cm constitutes critical mitral stenosis. When the pulmonary capillary pressure is chronically elevated, it leads to reactive pulmonary arterial hypertension and right ventricular failure as a consequence.
The modern treatment of mitral stenosis is balloon mitral valvotomy if the valve is pliable and non-calcified. If the valve is calcified, attempts of balloon valvotomy may sometimes result in severe mitral regurgitation due to leaflet tear. This mandates urgent valve replacement. Hence elective mitral valve replacement is often resorted to in calcific mitral stenosis.