Hypoplastic left heart
This term describes an anomaly characterized by a variable degree of an
underdeveloped, non-apex-forming left ventricle and stenosis/atresia of
the mitral and aortic valves, consequently failing to support systemic
circulation. This congenital cardiac lesion results in a univentricular
circulatory system dependent on right ventricle performance and an open
ductus arteriosus for perfusion. It is among the most common heart
defects, with an estimated incidence of 0.16 to 0.36 per 1000 live
births, accounting for 4.8% to 9% of CHDs56. The
first description dates back to 1851 when von Bardeleben in 1851
described a severely ill newborn who died 27 weeks after birth from
progressive heart failure. Postmortem examination revealed extremely
diminished left heart structures and an atretic (obliterated) ascending
aorta57,58. Interestingly, from a semantic
perspective, HLH syndrome has been discussed to be a misnomer as it
constitutes a heterogeneous, i.e., causally nonspecific, developmental
field defect59. The most commonly held view regarding
the development of HLH is postulated to be altered biomechanical stimuli
(e.g., decreased ventricular filling resulting in reduced myocardial
strains) during a critical period of ventricular development with
advancing gestation60. Nonetheless, it has been
hypothesized that the left ventricular hypoplasia might be caused by a
primary defect in ventricular development. A recent study on postmortem
HLH specimens suggested that the lesions seen represent acquired disease
occurring following closure of the embryonic interventricular
communication rather than sequelae of abnormal
embryogenesis61. Morphologically, classic HLH
comprises hearts with mitral and aortic valve atresia with an intact
ventricular septum. On the other hand, there are variants of HLH
characterized by different degrees of left heart obstruction with mitral
atresia, a ventricular septal defect and a patent aortic valve or those
with aortic valve atresia56,62. Recently, Crucean and
colleagues described three distinct subgroups relying on the
morphological appearance: (I) a slit-like LV (24%); (II) a miniaturized
LV (6%); and (III) a thickened LV with endocardial fibroelastosis (EFE;
70%). Interestingly, only the slit-like ventricle grouping was found to
map to the current nomenclature: the combination of mitral atresia with
aortic atresia. Cases of a miniaturized LV all had normally formed,
although small, aortic and mitral valves, and the last and predominating
group was found to have a range of aortic valve malformations associated
with thickened left ventricular walls despite being described as either
atresia or stenosis62,63.
In a typical hypoplastic left heart condition, prenatal ultrasound shows
a normally located right ventricle with a small hypocontractile LV often
associated with hypoplasia of the ascending aorta. In contrast to CoA,
the apex is formed by the RV, and there is markedly diminished LV
function in order to support systemic circulation in HLH (see figure 4).
As stated above, the shape and size of the LV can considerably vary
(e.g., small, round, slit-like, dilated or even normal). In almost all
cases, the aortic valve is atretic, but a common feature of HLH is an
obstruction of both the LV inflow and outflow tracts in specific
combinations of mitral valve (MV) atresia/aortic atresia (MA/AA), MV
stenosis/aortic stenosis or MV stenosis/aortic atresia. The latter
conditions account for nearly half of all HLH cases61.
The left ventricular outflow is scarcely demonstrated in the 5CV/LVOT
views during echocardiographic examination, as exemplified in figures 3
& 4. These figures further demonstrate a marked size discrepancy of the
left atrium being smaller than the right atrium, with the flap of the
foramen ovale excursing toward the RA as a consequence of the obstructed
inflow. Notably, HLH may be accompanied by different degrees of foramen
ovale restriction (up to 22%), potentially leading to chronic left
atrial hypertension, which is transmitted onto pulmonary venous
structures, resulting in pulmonary hypertension and remodeling. Jadcak
et al. stated that an early appearance and a longer presence of FO
obstruction rather than the extent of obstruction are associated with an
increase in short-term mortality 64. Therefore, an
additional thorough assessment of pulmonary venous (PV) Doppler (PVD)
with a forward/reverse velocity-time integral (VTI) ratio of
<3 has been found to be the strongest predictor of postnatal
hemodynamic stability and the need for fetal
intervention65. Despite a trend toward improved
preoperative morbidity in neonates with a prenatal diagnosis of HLH, a
recent systematic review revealed no significant impact on preoperative
or postoperative Stage I mortality66. According to
recent data from Richard et al., frequent serial fetal echocardiograms
may not necessarily be needed to predict the need for an unexpected,
emergent procedure67. However, this inevitably
underscores the need for an exact delineation of the phenotypic features
of the disease since hypoplasia of the left ventricle can be found in a
number of other cardiac lesions (including an unbalanced AV canal, a
double outlet right ventricle or a common arterial trunk). Moreover,
borderline cases of HLH might be left undetected in many early to
midgestation fetal exams and often tend to progress to HLH until birth.