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Aortic stenosis is obstruction of the blood flowing out of the left ventricle of the heart and into the aorta for circulation to the rest of the body. It is estimated that slightly less than three percent of structural defects of the heart diagnosed in the fetus are due to aortic stenosis.
The most common form of aortic stenosis is obstruction of the aortic valve and is known as aortic valvar stenosis. The normal aortic valve consists of three thin and pliable valve leaflets that spread apart easily to allow blood to flow into the aorta, but fetuses with valvar stenosis typically have only two leaflets, known as a bicuspid aortic valve. These leaflets may be thickened and less pliable than normal, so the valve does not open as freely and the left ventricle of the heart must work harder to eject blood into the aorta. This additional workload causes the muscle of the left ventricle to become enlarged (hypertrophied). Other causes of aortic stenosis include muscular obstruction below the aortic valve and narrowing of the aorta immediately above the valve.
Aortic stenosis is detected by an echocardiogram of the fetal heart. In cases of moderate to severe aortic stenosis, the left ventricle may look normal or only slightly hypertrophied. The aortic valve may appear abnormal and turbulence may be seen near the opening of the valve. Doppler ultrasound can confirm aortic stenosis.
If aortic stenosis is critical, the left ventricle may appear to be dilated and contracting poorly and the mitral valve may also be involved. The aortic valve may be thickened and domed when the ventricles are pumping blood. Blood flow in the aortic arch may be reversed, which further confirms the critical nature of the aortic valve obstruction. Critical aortic stenosis can progress to hypoplastic left heart syndrome (HLHS).
The echocardiogram may appear normal in early pregnancy and aortic stenosis may develop later in the pregnancy. In addition, mild aortic stenosis with normal growth of the left ventricle during the mid trimester of pregnancy can progress to more severe aortic stenosis by the time of birth.
Valvular aortic stenosis is not usually associated with malformations outside the heart, although it can be part of several syndromes. These include Turner’s syndrome, Shone’s complex, and more rarely William’s syndrome.
The management of aortic stenosis depends on how severe it is and how old the fetus is at the time of diagnosis. Generally, fetuses with severe aortic stenosis tolerate the condition prior to birth, without significant problems with heart function. Once the diagnosis is made however, serial echocardiogram studies can be done to measure the growth of the left heart structures and evaluate if the fetus is likely to develop HLHS. If so, prenatal intervention may be considered. If the studies show that the fetus has normal or near normal ejection fraction, the chances are increased for a normal, 2 ventricle circulation after the child is born. The results of the echocardiogram studies and the options for prenatal and postnatal treatment will be reviewed with the parents.
In pregnancies that continue without prenatal intervention, the baby should be delivered in a tertiary care center. Prenatal diagnosis and follow-up should help improve the condition of the fetus and the outcome of postnatal treatment with either balloon or surgical repair of the aortic valve soon after birth.
Survival of fetuses diagnosed with critical aortic stenosis that has progressed to HLHS has increased in recent years, partly due to due to improvement in prenatal detection of less severe forms of the condition and partly due to improvement in treatment.
In an attempt to improve left ventricular function and growth, prenatal balloon aortic valvuloplasty has been performed at some centers to widen the stenotic aortic valve and help the fetal heart develop more normally.
This procedure is generally done under maternal anesthesia. Ultrasound images guide placement of the needle. Anesthesia is also administered to the fetus and the needle is then inserted through the fetal chest wall and into the fetal heart. A thin tube (catheter) with a special balloon attached is inserted and the balloon is inflated to expand the narrowed valve so blood can flow through it more easily. The tube and the needle are then removed. Success of the procedure depends on many factors, including position of the fetus within the womb. Early studies have shown that the valve was expanded in at least half of fetuses undergoing the procedure. Some mothers experienced bleeding after the procedure and needed to have an emergency caesarean section. The long-term risks and benefits of the procedure remain unknown.
As most fetuses with aortic stenosis with remain stable in the mother’s womb, fetal balloon aortic valvuloplasty has usually been limited to fetuses with poor prognosis, who are likely to progress to hypoplastic left heart syndrome. Typical findings in such fetuses include:
Serial fetal echocardiography should be performed every two-four weeks, both in mothers who undergo a fetal therapeutic procedure and those who do not. These echocardiograms will assess and monitor cardiac function and growth of the structures of the heart to compare the results of fetuses that undergo therapy and those that do not.
Click for larger image.
1.Normal heart in systole (during ventricular ejection) showing aortic valve opening completely.
2.Aortic stenosis with thickened and fused valve leaflets preventing full opening during systole.
3.Fusion of a commissure in a malformed, bicuspid aortic valve.
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