Mitral valve stenosis (MVS) is a type of heart valve disease defined by the narrowing or stiffening of the mitral valve opening in the heart. This condition restricts the proper flow of oxygenated blood from the upper left chamber to the lower left chamber. The resulting obstruction places a significant burden on the heart, ultimately affecting the body’s overall circulation.
The Mechanism of Mitral Valve Stenosis
The mitral valve is positioned between the heart’s left atrium and left ventricle, functioning as a one-way door. A healthy valve typically has an opening area ranging between 4 and 6 square centimeters.
When the valve becomes stenotic, its orifice shrinks, often falling below 2.5 square centimeters before symptoms appear. This reduced opening forces the heart to maintain blood flow against resistance, creating an abnormal pressure gradient across the valve. The left atrium must generate higher pressure to push blood through the narrowed opening into the left ventricle.
This sustained high pressure causes the left atrium to enlarge, and the pressure is transmitted backward into the veins of the lungs. The resulting fluid backup, known as pulmonary congestion, leads to symptoms like shortness of breath. Over time, the consistently elevated pressure can also lead to changes in the pulmonary blood vessels, potentially causing high blood pressure in the lungs.
Primary Cause: Rheumatic Fever
The overwhelming majority of mitral valve stenosis cases worldwide are the long-term consequence of acute rheumatic fever. This inflammatory condition is a delayed, non-infectious complication following an untreated or inadequately treated infection by Group A beta-hemolytic streptococcus (GABHS), the bacteria responsible for strep throat or scarlet fever. Rheumatic fever develops two to six weeks after the initial strep infection in susceptible individuals.
The body’s immune system creates antibodies to fight the streptococcal bacteria, but a phenomenon called molecular mimicry causes these antibodies to mistakenly attack the body’s own tissues, including the heart valves. This autoimmune attack on the heart is termed rheumatic carditis, which is the initial stage of Rheumatic Heart Disease (RHD). The mitral valve is affected in 65 to 70 percent of RHD cases, making it the most vulnerable valve.
The acute inflammatory damage leads to chronic, progressive scarring and deformation of the valve structure. Over the next decade or two, the valve leaflets thicken, stiffen, and their edges fuse together in a process called commissural fusion. This gradual scarring and calcification shrinks the valve opening, often leading to severe MVS 10 to 20 years after the initial childhood infection. Due to this long latency period, MVS is frequently diagnosed in adults in their 30s, 40s, or 50s.
Other Contributing Causes
While rheumatic fever is the dominant cause, several less common conditions can also lead to MVS.
- Congenital MVS: This rare form is present from birth due to a structural flaw. It is usually detected in infants or children, and its severity varies widely.
- Mitral Annular Calcification (MAC): This is the age-related buildup of calcium deposits around the valve ring. This degenerative process, common in older adults, stiffens the valve structures, restricting leaflet movement and causing stenosis.
- Radiation Damage: Therapeutic radiation delivered to the chest for cancers (such as Hodgkin’s lymphoma or breast cancer) can induce chronic inflammation and calcification of the valve tissue. This damage typically manifests as MVS decades later, often 10 to 30 years after treatment.
- Systemic Autoimmune Disorders: Extremely uncommon causes include systemic autoimmune disorders, such as lupus or rheumatoid arthritis. These conditions can cause inflammation that leads to thickening and scarring of the mitral valve leaflets.