The mitral valve is one of four valves that regulate blood flow through the heart. More specifically, the mitral valve controls blood flow from the left upper chamber of your heart (left atrium) to the left lower chamber (left ventricle). Mitral valve stenosis is a narrowing of the mitral valve.
When the mitral valve is narrowed, blood flow to the heart’s left lower chamber (left ventricle) is limited. The blood is trapped in the left upper chamber (left atrium), putting pressure on the blood vessels that bring blood from the lungs to the heart. When a valve is narrow, it doesn’t open as much as it should, so the blood pressure backs up within the heart and causes the blood vessels in the lungs to have too much pressure. Over time, this may cause the heart's upper left and right chambers to enlarge.
In most cases, mitral valve stenosis is present from birth (due to a congenital heart defect) and, in some cases, can progress over time. Less commonly, mitral valve stenosis can develop after birth due to an infection (rheumatic fever).