**2. The morphologic Spectrum of hypoplastic left heart syndrome**

Dr. Maurice Lev first described the hypoplastic left heart as hypoplasia of the aortic tract in 1952 [6]. However, the term HLHS was initially used by Noonan and Nadas in 1958 [7]. HLHS is a diagnosis that incorporates a spectrum of left ventricular inlet, ventricular and outlet obstructions that may include: aortic atresia or stenosis; mitral stenosis, atresia or agenesis (**Figure 1**); hypoplasia of the ascending aorta that may extend to the entire aortic arch; a non-apex forming left ventricle with variable degrees of left ventricular hypoplasia; commonly, a discrete coarctation of the aorta; and an intact ventricular septum in most patients [6–8].

A normal left ventricle has a mitral valve and gives rise to the aorta. Furthermore, the apical portion of a left ventricular septal surface shows finer trabeculations than a morphologic right ventricle, and the subarterial septal surface is smooth (**Figure 2a**). Hearts with HLHS show variability in the size of the left ventricle (**Figure 2b**–**d**), and the types of mitral valve malformations (**Figure 3b**–**e**) and aortic valve obstruction (**Figure 4b** and **c**) along with the varying degrees of hypoplasia of the ascending aorta and aortic arch (**Figure 5a** and **b**) with or without coarctation.

In classical HLHS, the left ventricle has an intact ventricular septum. Ventricular size may vary from a mild degree of hypoplasia to being morphologically absent with no discernible inlet or outlet. The hypoplastic ventricle is commonly muscle bound with a thick myocardial wall causing the hypoplastic left ventricle to bulge on the epicardial surface. The hypoplastic left ventricle almost universally shows endocardial fibroelastosis that, at times, can be quite prominent (**Figure 2b**–**d**). Additionally, the left atrium will demonstrate varying degrees of hypoplasia.

A normal mitral valve consists of an anterior and posterior leaflet with tension apparatus attached to papillary muscles (**Figure 3a**), and the anterior leaflet is in fibrous continuity with the aortic valve. In the setting of HLHS, mitral valve can vary from well-formed but miniature to dysplastic and stenotic to imperforate or congenitally absent (**Figure 3b**–**e**).

The normal aortic valve consists of three semilunar cusps (**Figure 4a**) and the aorta consists of the ascending aorta, aortic arch, isthmus, and descending aorta. Aortic valves in hearts with HLHS can vary from having three cusps to bicuspid and are dysplastic and stenotic or atretic. Hearts with HLHS can have ascending aortas that may uncommonly be mildly hypoplastic (**Figure 5a**), but most of the time they vary from moderately to extremely hypoplastic with an almost thread-like appearance (**Figure 5b**). The hypoplastic ascending aorta serves as a conduit to perfuse both coronary arteries in a retrograde manner. In addition, the aortic arch may be hypoplastic. Coarctation of the aorta - a shelf-like, circumferential, paraductal lesion - is common in the setting of HLHS and may be mild to severe. Furthermore, HLHS is ductal-dependent condition, and the arterial duct is usually widely patent and large (**Figure 5b**). The right heart is enlarged and shows right ventricular hypertrophy. A patent oval fossa or atrial septal defect (**Figure 5c**) is present. Sometimes, the flap valve of the oval fossa can show aneurysmal dilatation bulging into the right atrium.

*Hypoplastic Left Heart Syndrome DOI: http://dx.doi.org/10.5772/intechopen.104723*

#### **Figure 1.**

*Illustration of 4 types of HLHS and Norwood procedure with modified BTT shunt (e) and Sano shunt (f). AA: aortic atresia, AS: aortic stenosis, BTT: Blalock-Taussig-Thomas shunt, MA: mitral atresia, MS: Mitral stenosis (graphic illustration by Dr. Raymond Morales).*

Hearts with premature closure of the oval fossa at an early gestational age result in malformations such as mitral atresia or stenosis and/or aortic atresia or stenosis, variable degrees of hypoplasia of the left ventricle, and endocardial fibroelastosis. The severity of the impact on the heart depends on the gestational timing of the closure or restriction of the oval fossa [9]. **Figure 6a**–**c** are images of a heart with premature closure of the oval fossa. Rarely, in patients born with premature closure of the oval

#### **Figure 2.**

*Variations in the degree of left ventricular hypoplasia in hearts with HLHS. (a) Septal view of a normal left ventricle, (b) hypoplastic left ventricle with a stenotic and dysplastic aortic valve and endocardial fibroelastosis, and (c) three-chamber view showing the muscle bound, non-apex forming hypoplastic left ventricle with endocardial fibroelastosis, mitral valve stenosis and aortic valvar atresia, (d) diminutive and atretic (no inlet or outlet) left ventricular chamber in a heart with aortic and mitral atresia, and the muscle bound left ventricular segment of the heart bulges on the epicardial surface, and the position of the left ventricle is outlined by the anterior and posterior descending coronary arteries.*

fossa and inflow obstruction such as mitral atresia and in cases of typical HLHS with inflow obstruction, an escape channel for the pulmonary venous return may be present [10]. This escape channel may be a levoatriocardinal vein that allows pulmonary blood to egress from the left atrium or a pulmonary vein and reaches the right atrium via the left innominate/superior caval veins (**Figure 6d**) [11]. Rarely the escape of pulmonary venous blood from the left atrium may be accomplished by partial or total anomalous pulmonary venous return [12].

A hypoplastic left ventricle with aortic stenosis and an intact ventricular septum (**Figure 7a**) is not classified as an HLHS. Furthermore, isolated cardiac malformations such as a right-dominant unbalanced atrioventricular septal defect (AVSD) (**Figure 7b**) or a heart with double outlet right ventricle (DORV) with its ventricular septal defect and mitral valve stenosis (**Figure 7c** and **d**), both of which have interventricular communications and a hypoplastic left ventricle are arguably not categorized as having HLHS because
