Hypoplastic left heart syndrome (HLHS) represents a spectrum of abnormalities in which there is underdevelopment of the left-sided cardiac structures, such as the left ventricle, the mitral valve, and aortic valve, such that the systemic circulation cannot be adequately supported (Rychik, 2005). Classic HLHS involves both aortic valve atresia, and either atresia or stenosis of the mitral valve (Simpson, 2000).

The spectrum of malformations can include congenital hypoplasia of the left ventricular wall, atresia of the aortic and/or mitral valves, and coarctation or hypoplasia of the aortic arch. Critical aortic stenosis can evolve into HLHS (see Chapter 50), and unbalanced atrioventricular canal defects (see Chapter 45) in which the left ventricle is quite underdeveloped and can also behave similar to HLHS (Simpson, 2000). Each of the components of HLHS may occur with varying degrees of severity, ranging from aortic stenosis with a small left ventricle to complete aortic and mitral atresia with a slit-like left ventricular remnant. Hypoplastic left ventricle and mitral atresia may also occur without aortic atresia, but such an anomaly is rare (Kiel, 1990). The cause of HLHS is unknown, but it may be due to abnormal intracardiac streaming during weeks 5 to 8 of embryonic life (Harh et al., 1973). HLHS may also evolve during prenatal life from isolated severe aortic stenosis (see Chapter 50), which may result in decreased right-to-left shunting between the atria, and subsequent hypoplasia of the left ventricle (Sharland et al., 1991).

Postnatally, left-to-right shunting occurs in the newborn, returning oxygenated blood from the pulmonary veins through a patent foramen ovale into the right atrium. The right ventricle provides both pulmonary and systemic circulations, the latter through a patent ductus arteriosus with retrograde flow to the aortic arch and coronary vessels. The mitral valve is hypoplastic; the tricuspid valve is often large and regurgitant. The aortic outflow tract may end blindly below the coronary arteries, and the aortic valve and arch may be hypoplastic. Associated cardiac anomalies are common with HLHS. Other associated cardiac anomalies include ventricular septal defect, aortic-arch interruption, and transposition of the great vessels (Kiel, 1990). Central nervous system abnormalities have also been described in association with HLHS, including microcephaly, holoprosencephaly, and agenesis of the corpus callosum (Sanders et al., 1996).

HLHS accounts for up to 9% of all cases of congenital heart disease, with an incidence of 1 to 2 per 10,000 livebirths (Kiel, 1990; Hoffman and Kaplan, 2002). Twice as many males as females may be affected.

HLHS is generally easy to detect prenatally by means of the standard four-chamber cardiac view. This should demonstrate an inequality in ventricle size, with the left ventricular cavity often appearing as a small remnant to the left of the right ventricle (Figure 48-1) (Silverman et al., 1984). The left ventricular wallmay be hypocontractile or immobile and may also appear echogenic (Kluckow et al., 1993). The apex of the left ventricle will usually endmore proximally than the right ventricular apex, and the ventricular cavity may be in a globular shape rather than in the normal elliptical shape. The right ventricular cavity is often enlarged, and the left atrium is usually small, with left-to-right bowing of the interatrial septum (Sanders et al., 1996). The aortic outflow tract may be atretic, with hypoplasia of the ascending aorta. However, the aortic arch and descending aorta should be visible because of retrograde filling through the ductus arteriosus. Doppler echocardiography may also be helpful in demonstrating a lack of antegrade flow into the left ventricle, abnormal flow through the foramen ovale, and retrograde flow in the ascending aorta (Figure 48-2) (Blake et al., 1991).

Even though these findings should be detectable at 18 to 20weeks of gestation, it is possible that isolated aortic stenosis (see Chapter 50) may evolve over time into HLHS, so that the typical features of HLHS may not become visible until the late second trimester or the third trimester (Sharland et al., 1991). The detection rate of prenatal ultrasonography for HLHS is unclear in the general population, with only 28% of all cases of HLHS being detected prenatally in one series (Montana et al., 1996). However, as sonographer experience has improved over the past decades, it is likely that a much higher prenatal detection rate is now possible. In another population-based study from Australia, 66 of 78 cases (85%) of HLHS were correctly diagnosed antenatally (Chew et al., 2007). Additionally, the positive predictive value of prenatal ultrasonography for HLHS is high, with up to 95% of cases of prenatally diagnosed HLHS confirmed on postnatal examination (Chang et al., 1991).

Other conditions that should be included in the differential diagnosis of HLHS include left-sided cardiac masses, aortic stenosis, and univentricular heart (Sanders et al., 1996). Large cardiac masses, such as rhabdomyomas (see Chapter 58), may completely obliterate the left ventricular cavity, thus mimicking the sonographic appearance of HLHS. However, normal ventricular dimensions will be present, and Doppler echocardiography should demonstrate normal flow through the mitral and aortic valves. Severe aortic stenosis may be difficult to differentiate from HLHS because complete obliteration of the left ventricle may occur as the stenosis worsens (see Chapter 50). It is possible that aortic stenosis in early fetal life may progress to hypoplastic left ventricle as gestation advances (Sharland et al., 1991; Wilkins-Haug et al., 2005). Differentiation should be possible based on Doppler echocardiography demonstrating antegrade flow in the ascending aorta with aortic stenosis, as compared with retrograde flow in HLHS. Univentricular heart is a condition in which the entire atrioventricular junction is connected to a single ventricle. There is little agreement on the precise definition of univentricular heart, which makes prenatal differentiation difficult.

HLHS may evolve in utero from critical aortic stenosis (Sharland et al., 1991; Kluckow et al., 1993; Wilkins-Haug et al., 2005). The precise antenatal natural history of HLHS is therefore very variable. This malformation has several degrees of severity, ranging from critical aortic stenosis with normal left ventricular size to complete atresia of the aortic and mitral valves with near absence of the left ventricle. Previously, because of the lack of adequate pediatric surgical intervention, the vast majority of cases diagnosed prior to 24 weeks of gestation resulted in termination of pregnancy. In a series of 77 cases of HLHS diagnosed prior to 24 weeks, 72 resulted in termination of the pregnancy (Allan et al., 1991). However, as pediatric surgical management options have improved over the last 20 years and as the potential for in utero intervention has appeared, it is likely that the voluntary pregnancy termination rate may be significantly less in contemporary practice.

Expectant management of prenatally diagnosed HLHS is associated with a poor prognosis. In one series of 20 cases of prenatally diagnosed HLHS, 9 pregnancies were terminated (Blake et al., 1991). Four of the 11 expectantly managed pregnancies resulted in intrauterine fetal death, 7 resulted in livebirths, and 5 of these 7 infants died within 1 week of birth. Intrauterine congestive heart failure may occur with HLHS because of right ventricular overload. In another series, nonimmune hydrops developed in 4 of 20 cases of prenatally diagnosed HLHS. Only one of these cases resulted in a live born infant, following administration of digoxin to the mother (Blake et al., 1991).