Aortic stenosis is the congenital obstruction of the left ventricular outflow tract of the heart. Stenosis can occur at, above, or below the aortic valve (Becker and Anderson, 1981). Subvalvular aortic stenosis can be either fixed or dynamic. Fixed aortic stenosis is due to the presence of a discrete membranous diaphragm or a diffuse fibromuscular ring below the valve. Dynamic subvalvular aortic stenosis demonstrates a constantly changing pressure gradient across the valve and is most commonly due to muscular thickening of the septum. This form of subvalvular aortic stenosis is often called “asymmetric septal hypertrophy” (ASH), “idiopathic hypertrophic subaortic stenosis” (IHSS), or “hypertrophic obstructive cardiomyopathy” (HOCM). A transient form of dynamic subvalvular aortic stenosis has also been described, secondary to fetal hyperglycemia (Gutgesell et al., 1976).

Valvular aortic stenosis occurs secondary to abnormalities of the cusps of the aortic valve. Congenital unicuspid or bicuspid aortic valves may be stenotic at birth or they may become stenotic later in adult life. Other causes of valvular aortic stenosis include dysplastic or thickened cusps and fusion of the commissures that separate the cusps.

Supravalvular aortic stenosis can occur secondary to a localized narrowing of the ascending aorta, the presence of a membrane just superior to the origin of the coronary arteries, or a diffuse narrowing of the aortic arch and great arteries. Because the obstruction occurs above the origin of the coronary arteries at the sinuses of Valsalva, the coronary arteries are also exposed to the elevated left ventricular pressures.

With all types of congenital aortic stenosis, if the stenosis becomes severe, secondary endocardial fibroelastosis can occur, leading to thickening of the endocardium and subsequent mitral insufficiency or cardiomyopathy. Aortic stenosis diagnosed early in gestation may evolve over time into hypoplastic left heart syndrome (Sharland et al., 1991).

The overall incidence of congenital heart disease is 4 in 1000 to 10 in 1000 livebirths (Hoffman, 1990). Aortic stenosis accounts for 3% to 6% of all congenital cardiovascular malformations, although it makes up 1% to 3% of all cardiac lesions in newborns presenting with significant cardiac defects (Rowe et al., 1981; Kitchener et al., 1993). It occurs up to four times more commonly in males than in females, although this sex predominance may be less marked in newborn populations. Overall, aortic stenosis has an incidence of 3.5 in 10,000 livebirths, while congenital bicuspid aortic valve may be as common as 1 in 100 livebirths.

Prenatal diagnosis of aortic stenosis is unreliable. Because supravalvular and subvalvular forms of aortic stenosis are usually not clinically apparent during the newborn period, prenatal diagnosis of these conditions is rarely successful. Prenatal sonographic features suspicious for aortic stenosis include enlargement or hypoplasia of the left ventricle. However, it is important to realize that left ventricular size can also be normal despite the presence of significant aortic stenosis. The right ventricle is also usually dilated with critical aortic stenosis, as blood flow is redistributed to the right ventricle and through the ductus arteriosus. In addition, critical aortic stenosis is usually associated with poststenotic aortic root dilation.

Incomplete opening of the aortic valve and increased aortic turbulence on Doppler echocardiography are both sonographic signs consistent with aortic stenosis during the newborn period (Figures 50-1 and 50-2), but prenatal visualization of these signs is extremely difficult because of the small size of the fetal aortic root. Prenatal diagnosis of a significant pressure drop (more than 50 mm Hg) across the valve is also suggestive of aortic stenosis (Jouk and Rambaud, 1991). Prenatal visualization of abnormal thickening of the interventricular septum in aortic stenosis has been documented (Stewart et al., 1986). In severe cases of congenital aortic stenosis, antenatal Doppler studies may demonstrate significant mitral regurgitation. Aortic stenosis may also be the underlying cause of some cases of hydrops fetalis or intrauterine growth restriction, and therefore aortic stenosis should be considered during the sonographic assessment of these conditions.

The main alternatives that must be considered in the prenatal diagnosis of aortic stenosis include primary cardiomyopathy (see Chapter 57) and hypoplastic left heart (see Chapter 48) (Huhta et al., 1987). In addition, coarctation of the aorta must be considered, but the prenatal diagnosis of this condition is also very limited (see Chapter 51). Unlike aortic stenosis, primary cardiomyopathy is usually associated with a normal aortic valve and no evidence of poststenotic dilation. Such primary cardiomyopathies may be secondary to endocardial fibroelastosis, viral or bacterial myocarditis, or some glycogen storage diseases.

Hypoplastic left heart syndrome is usually associated with both mitral-and aortic-valve atresia. In addition, the hypoplastic left ventricle may be globular in shape, rather than ellipsoid, which can be demonstrated on a four-chamber cardiac view by the left ventricle failing to reach the apex of the heart. In some cases, differentiation of aortic stenosis from hypoplastic left heart syndrome can be difficult because the enlarged right ventricle seen with aortic stenosis may make the left ventricle appear small or even hypoplastic. Other features that aid in differentiating aortic stenosis from hypoplastic left heart syndrome include failure of growth in left ventricular, aortic, or mitral dimensions on serial examinations, as well as severe restriction of interatrial shunting in cases of hypoplastic left heart syndrome (McCaffrey and Sherman, 1997). Of note, aortic stenosis and hypoplastic left heart syndrome are now considered part of a clinical spectrum with many cases of early fetal aortic stenosis evolving as gestation advances into the classic hypoplastic left heart syndrome. Therefore, attempts to differentiate between these two clinical conditions in utero may be purely academic (Hornberger et al., 1995).

The antenatal natural history of congenital aortic stenosis can be quite varied, with almost all cases of subvalvular and supravalvular aortic stenosis resulting in no fetal compromise, while critical valvular aortic stenosis can lead to intrauterine growth restriction, hydrops fetalis, and severe hemodynamic compromise during the early newborn period. The presence of hydrops with a structural cardiac malformation is usually considered an ominous finding.

Left ventricular pressure overload may lead to ventricular enlargement, relative coronary hypoperfusion, subendocardial ischemia, and consequently, significant impairment of cardiac function. This can result in severe metabolic acidosis, leading to death in a significant number of cases, either before or soon after birth (Sharland et al., 1991). Sustained elevated left ventricular pressure secondary to outflow tract obstruction can also lead to the intrauterine development of endocardial fibroelastosis and subsequent cardiomyopathy, which also increases the mortality rate for congenital aortic stenosis. In cases of aortic stenosis diagnosed prenatally, left ventricular volume and aortic root dimensions tend to fall off from the normal percentiles as gestation progresses, and this information may be useful in predicting the appropriate form of repair during the newborn period (Simpson and Sharland, 1997).

In one series of 30 cases of prenatally diagnosed left ventricular dysfunction, an initial appearance of aortic stenosis evolved over time into complete hypoplastic left heart syndrome in five cases (Sharland et al., 1991). There may be a spectrum of diseases in the fetus, involving primary left ventricular endocardial fibroelastosis, critical aortic stenosis, and hypoplastic left heart syndrome. Therefore, if screening takes place early enough, an initial clinical presentation of aortic stenosis may evolve over time into hypoplastic left heart syndrome (Sharland et al., 1991). For this reason, it is important to evaluate the appearance of the fetal heart whenever obstetric ultrasound examinations are performed in gestation.