Aortic stenosis
The earliest description of a stenotic aorta as the leading cause of
death of a young man has been attributed to Lazarus RiviƩre 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.