Aortic valve stenosis
Causes and risk factors
A number of conditions contribute towards aortic stenosis. Three conditions that are known to cause aortic stenosis are:
1. Calcification of a bicuspid valve
Bicuspid aortic valve is the most common cause of aortic stenosis in patients under age 65. Normal aortic valves have 3 thin leaflets called cusps. About 2% of people are born with aortic valves that have only 2 cusps (bicuspid valves). Although bicuspid valves usually do not impede blood flow when the patients are young, they do not open as widely as normal valves with 3 cusps. The turbulent blood flow causes excessive wear and tear leading to calcification, scarring, and reduced mobility of the valve leaflets over time. About 10% of bicuspid valves become significantly narrowed, resulting in the symptoms and heart problems of aortic stenosis.
2. Senile calcific aortic stenosis
The most common cause of aortic stenosis in patients 65 years and over is called "senile calcific aortic stenosis." With aging, protein collagen of the valve leaflets is destroyed, and calcium is deposited on the leaflets. Once valve leaflet mobility is reduced by calcification, turbulence across the valve increases, causing scarring, thickening, and stenosis of the valve. Why this aging process progresses to cause significant aortic stenosis in some patients but not in others is not known.
3. Rheumatic fever
Rheumatic fever rarely causes isolated aortic stenosis. Rheumatic fever is a condition resulting from untreated infection by group A streptococcal bacteria. Damage to valve leaflets from rheumatic fever causes increased turbulence across the valve and more damage. The narrowing from rheumatic fever occurs from the fusion of the commissures of the valve leaflets. Rheumatic aortic stenosis usually occurs with some degree of aortic regurgitation. Under normal circumstances, the aortic valve closes to prevent blood in the aorta from flowing back into the left ventricle. In aortic regurgitation, the diseased valve allows leakage of blood back into the left ventricle as the ventricular muscles relax after pumping. These patients also have some degree of rheumatic damage to the mitral valve.
- Subvalvular aortic stenosis
- Hypertrophic obstructive cardiomyopathy
Valvular aortic stenosis results in chronic left ventricular pressure overloading. At any stage of life, however, the natural history of aortic stenosis largely reflects the functional integrity of the mitral valve. As long as adequate mitral valve function is maintained, the pulmonary bed is protected from the systolic pressure overloading imposed by aortic stenosis. In contrast to mitral valve disease where the pulmonary circuit is directly involved, compensatory concentric left ventricular hypertrophy allows the pressure overloaded ventricle to maintain stroke volume with modest increases in diastolic pressure, and patients can remain asymptomatic for many years.Eventually, left ventricular hypertrophy results in either diastolic dysfunction with the onset of congestive symptoms or myocardial oxygen needs in excess of supply with the onset of angina.
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