● AORTIC STENOSIS
Definition, Spectrum of Disease, and Incidence
Aortic stenosis is defined as narrowing at the level of the aortic valve leading to an obstruction of left ventricular outflow (Fig. 21.1). The stenosis is classified as valvular, subvalvular, or supravalvular according to the anatomic location of the obstruction in relation to the aortic valve. Valvular aortic stenosis is the most common type diagnosed prenatally; the other two are rarely encountered on prenatal ultrasound, especially in an isolated form.
In valvular aortic stenosis, the valve leaflets are dysplastic and are either tricuspid with fused commissures, bicuspid, unicuspid, or noncommissural. The spectrum of aortic stenosis varies from an isolated mild lesion, mild aortic stenosis (Fig. 21.1), to a severe lesion, critical aortic stenosis, which may lead to a secondary dysfunction of the left ventricle with signs of endocardial fibroelastosis (see Fig. 22.2). The latter condition will be discussed together with the hypoplastic left heart syndrome in Chapter 22. Table 21.1 lists differentiating characteristics of mild versus critical aortic stenosis.
Aortic stenosis occurs in 3% to 6% of structural heart defects and is more common in boys, with a ratio of 3:1 to 5:1 (1–3). Isolated mild aortic stenosis is rare in fetal series (4). Bicuspid aortic valve (BAV) is a very common condition and is discussed later in this chapter. The prenatal diagnosis of BAV is uncommon (5).
Mild aortic stenosis
Critical aortic stenosis
Systolic aortic flow
LV wall echogenicity
Short diastole and mitral regurgitation
Aortic isthmus flow
Partially reverse flow
Left-to-right shunt when mitral regurgitation
LV, left ventricle.
Mild aortic stenosis is difficult to detect prenatally due to a normal four-chamber anatomy in the majority of cases (Fig. 21.2A). The typical left ventricular myocardial hypertrophy found postnatally is absent or occasionally noted in late gestation. The five-chamber view may, however, show a poststenotic dilation of the ascending aorta (Figs. 21.2B and 21.3). This dilation can also be prominent and reach the transverse aortic arch and thus be seen in the three-vessel-trachea view. At the level of the aortic valve, thickened valve leaflets, doming of the cusps, and lack of complete valve opening during systole may be observed (Figs. 21.2B and 21.3). A cross section at the level of the aortic valve (short axis of right ventricle) may show the number of cusps and the presence of any thickening of the commissures (5) (Fig. 21.4).
The detection of mild aortic stenosis is mainly achieved by the routine use of color Doppler with the detection of turbulent flow across the aortic valve (Figs. 21.5 to 21.7). The three-vessel-trachea view on color Doppler shows often a turbulent antegrade flow through a normal-size or slightly dilated transverse aortic arch (Fig. 21.7). Spectral Doppler reveals high peak systolic velocities of greater than 150 cm/s (Figs. 21.8 and 21.9). In a fetus with a normal left ventricular contractility, the higher the peak velocity the more severe is the stenosis. The shape of the Doppler envelope may show in these cases a slow acceleration to the peak velocity (Figs. 21.8 and 21.9).
The diagnosis of mild aortic stenosis has been reported in early gestation (6). The authors have also diagnosed in few fetuses mild aortic stenosis by demonstrating elevated peak systolic velocities across the aortic valves (>100 cm/s) as compared to normal velocities across the pulmonary valves (30 cm/s) (Fig. 21.10). Poststenotic dilation of the aortic root was already noted in one of the cases (Fig. 21.10). In general, suspicion of aortic stenosis in early gestation is achieved either due to high velocities on the routine use of color Doppler or in the visualization of a dilated ascending aorta.
Tomographic ultrasound imaging of a three-dimensional (3D) volume can demonstrate various features of aortic stenosis by displaying multiple planes at different anatomic levels. Application of spatiotemporal image correlation (STIC) allows for evaluation of ventricular anatomy and contractility and can be used to monitor progress of disease. Surface-rendering mode combined with STIC can be used to obtain an en face plane of the stenotic aortic valve (Fig. 21.11). This is better achieved in the third trimester, but shadowing from the ribs can be a limiting factor. A similar view can be provided by using the biplane approach (see Chapter 15), visualizing the dysplastic valve in the left ventricular long-axis view and getting the corresponding 90 degrees orthogonal plane with an en face view of the valve (Fig. 21.12). Rendering of the ventricular volume using inversion mode can also be of help in differentiating ventricular contractility between the right and left ventricles. Turbulent color Doppler flow is best demonstrated with tomographic imaging and also in glass-body mode (Fig. 21.13).