The term pulmonary stenosis refers to narrowing of the right ventricular outflow tract; pulmonary atresia implies complete occlusion of the right ventricular outflow tract. Pulmonary atresia, when associated with an intact ventricular septum, is also considered as a hypoplastic right ventricle and is described in detail in Chapter 47. Pulmonary atresia with coexistent ventricular septal defect is generally considered part of the spectrum of tetralogy of Fallot and is described in detail in Chapter 52. Pulmonary stenosis usually results from fusion of the three cusps of the pulmonary valve. Other causes of pulmonary stenosis include narrowing of the infundibular portion of the right ventricular outflow tract, hypoplasia of the pulmonary artery, or pulmonary valve dysplasia, in which the valve leaflets are thickened and immobile. Pulmonary artery hypoplasia and supravalvar pulmonary stenosis may occur in association with Williams syndrome or with congenital rubella or toxoplasmosis (Gutgesell, 1990; Rhodes et al., 2008). Supravalvar pulmonary valve stenosis may also occur in association with Noonan syndrome, which has a phenotype similar to Turner syndrome, but a normal karyotype (Mendez and Opitz, 1985; Rhodes et al., 2008). Approximately 60% of cases of Noonan syndrome will have a dysplastic pulmonary valve (Bashore, 2007).

Pulmonary stenosis may lead to right ventricular hypertrophy, a decrease in right ventricular chamber size, and poststenotic dilation of the pulmonary artery (Romero et al., 1988). Poststenotic dilation of the pulmonary artery is rarely present in utero and usually takes several months of neonatal life to develop (Gutgesell, 1990). In cases of critical pulmonary stenosis, hypoplasia of the right ventricular cavity may occur together with hypertrophy of the ventricular wall and dilation of the right atrium. In such cases, neonates usually have an interatrial communication through either a patent foramen ovale or a secundum atrial septal defect. Other associated cardiac defects that may be seen with pulmonary stenosis include ventricular septal defect, total anomalous pulmonary venous return, and aortic stenosis (Romero et al., 1988).

Congenital pulmonary stenosis may be subdivided into three anatomical types: (1) a dome-shaped pulmonary valve with a narrow opening but preservation of mobile valve leaflets, (2) a dysplastic thickened pulmonary valve with a narrow outflow tract, and (3) a unicuspid or bicuspid pulmonary valve (Bashore, 2007). Differentiation of these types by prenatal sonography is quite limited.

Pulmonary stenosis is a relatively common congenital cardiac defect, and in many mild forms the diagnosis may not be made until late in childhood. The incidence of congenital pulmonary stenosis is approximately 1 in 1500 livebirths, while pulmonary atresia occurs in approximately 1 in 10,000 livebirths (Hoffman and Kaplan, 2002). Pulmonary stenosis may account for 5% to 10% of all pediatric cardiology patients at tertiary care centers (Gutgesell, 1990).

Prenatal sonographic features of congenital pulmonary stenosis include decreased size of the right ventricular chamber, thickening of the right ventricular walls, right atrial enlargement, increased diameter of the pulmonary artery, thickening of the pulmonary valve, and incomplete valvular opening (Figure 49-1). However, several of these features may be extremely difficult, if not impossible, to demonstrate reliably with two-dimensional echocardiography in the fetus. Duplex and color flow Doppler sonography may be used in the prenatal diagnosis of pulmonary stenosis to demonstrate poststenotic turbulence of flow in the pulmonary artery as well as significant tricuspid regurgitation. The demonstration of reversed flow across the ductus arteriosus is also suggestive of severe pulmonary stenosis or atresia and in one study was found in all seven fetuses with this prenatal diagnosis (Mielke et al., 1997).

The ability of routine prenatal sonography to detect pulmonary stenosis or atresia in unselected populations seems limited. In the largest study evaluating the prenatal detection rates for various congenital cardiac malformations, only 9 of 180 cases of pulmonary stenosis or atresia were detected prenatally (Montana et al., 1996). In another series of 11,984 fetuses examined by prenatal sonography, there were 19 cases of pulmonary stenosis or atresia of varying severity, only two of which were detected prenatally (Tegnander et al., 1995). However, if the diagnosis of pulmonary stenosis or atresia is made by targeted prenatal sonography, it appears to be accurate, with all seven cases in one series confirmed on postnatal examination (Mielke et al., 1997). In another series of 12 cases of prenatally diagnosed pulmonary stenosis or atresia, the four-chamber view was abnormal in all cases, and 10 of the 12 cases were confirmed on postnatal examination (Hornberger et al., 1994). Another problem in prenatal diagnosis of pulmonary stenosis is the possibility of late appearance of the typical sonographic features, which may lead to the diagnosis being missed during a second trimester survey of fetal anatomy (Todros et al., 1988).

Differentiation of pulmonary stenosis from pulmonary atresia through the use of prenatal sonography is difficult, with the only reliable feature allowing differentiation being the presence of antegrade pulmonary blood flow in pulmonary stenosis. In cases of pulmonary stenosis without documented antegrade pulmonary flow, it may be impossible to differentiate prenatally between stenosis and atresia (Hornberger et al., 1994). Tetralogy of Fallot should be considered in the differential diagnosis of all cases of pulmonary stenosis or atresia. Hypertrophy of the right ventricular walls is almost always present in both isolated pulmonary stenosis and tetralogy of Fallot. The presence of a ventricular septal defect and an overriding aorta, together with the sonographic features of pulmonary stenosis should be sufficient to lead to the diagnosis of tetralogy of Fallot (see Chapter 52). Pulmonary atresia with intact ventricular septum should be considered as part of hypoplastic right ventricle (see Chapter 47).