Aortic stenosis
The earliest description of a stenotic aorta as the leading cause of death of a young man has been attributed to Lazarus Riviére in 1659, who found an enlarged LV and round masses occluding the aortic orificium during necropsy46. A prenatal diagnosis of aortic stenosis is relatively rare, occurring in less than 10% of newborns requiring prostaglandin therapy and postnatal intervention47. The frequency has been estimated to be 0.2 to 0.5 in 1.000 live births, with a clear male predominance (4:1).
Aortic stenosis (AS) markedly varies in its clinical severity and anatomical complexity, underscoring the need for a thorough cardiac survey during targeted ultrasound to estimate whether a bi- or univentricular circulation should be pursued47. Unfortunately, a large proportion of fetuses might present with rather normal LV dimensions (e.g., a normal four-chamber view) in the midtrimester, often precluding a prenatal diagnosis at an earlier gestational age. According to the affected segment, obstructions of the left ventricular outflow tract can occur at three anatomic levels: valvular and (less frequently in fetal life) supravalvular or subvalvular. In AS, the gross valvular dysmorphology encompasses a diminished commissural separation leading to an increased rigidity of the valve tissue. Morphologically, there is fibrous continuity between the leaflets of the aortic and mitral valves48. Owing to this spatial proximity, AS is frequently accompanied by mitral valve disease that worsens ventricular dysfunction. In general, fetal cardiac adaptive changes to an increased hemodynamically significant pressure load to the developing myocardium are characterized by myocyte hyperplasia, LV dilation, endocardial fibroelastosis, myocardial fibrosis, and early systolic and diastolic dysfunction49.
In moderate and less severe AS, fetal cross-sectional echocardiography reveals impaired mobility of cusp tissue, an alteration in the phasic movement of the aortic valve with reduced lateral and increased superior excursions of valve echoes, and an increase in the internal aortic root dimension beyond the level of the valve annulus50. However, proper imaging of the valve remains challenging as a number of different planes are needed to display the valve (long axis view; axial view of the valve annulus). The LV in these cases may appear normal or only mildly hypertrophied, and an increased blood flow during color Doppler interrogation can confirm the diagnosis of aortic stenosis. In contrast, in critical AS, the left ventricle is markedly dilated and hypocontractile with a typical echogenic inner appearance (endocardial fibroelastosis). Color Doppler mapping fails to demonstrate forward flow across the valve due to deteriorated ventricular dysfunction.
The case depicted in figure 2 shows the anatomic sequelae related to valvular aortic stenosis. Most obvious is the enlarged LV with bulging of the IVS to the right in the 4CV, 5CV and LVOT views caused by an increase in pressure, as discussed above. The mitral valve leaflets are also thickened. It has been shown that congenital valvular AS is frequently a progressive disorder. Axt-Fiedner et al. reported on a case of AS diagnosed in the first trimester with a normal four-chamber view but increased blood flow velocity across the aortic valve. This fetus showed subsequent progression to hypoplastic left heart syndrome51. In a recent large retrospective multicenter study, the natural history of AS has been further elucidated. Based on z-scores of the aortic and mitral valve and LV dimensions, the risk of emerging HLH has been assessed to estimate the proportion of ideal fetal candidates for fetal valvuloplasty52. The authors demonstrated significantly smaller aortic and mitral valve diameters and reduced growth velocities of the mitral valve in fetuses who ended up with a univentricular circulation.