Fetal Pulmonary Stenosis

Fetal pulmonary stenosis occurs when there is obstruction to blood flow from the heart into the pulmonary arterial circulation. This obstruction may be valvar, subvalvar, supravalvar, or mixed. In valvar stenosis, 75% are characterized by a doming stenotic pulmonary valve, while 20% have markedly thickened, immobile cusps, characterized by the presence of disorganized myxomatous tissue. A doming type of a stenotic pulmonary valve may be associated with a poststenotic dilatation of the main pulmonary artery, while the dysplastic type of pulmonary valve stenosis does not have. In 5% of valvar stenosis, the pulmonary valve annulus is markedly narrow, resulting in stenosis with a concomitant hypoplasia of the pulmonary valve. The pulmonary cusps may be absent or rudimentary resulting in both stenosis and regurgitation. Absent pulmonary valve is more commonly associated with tetralogy of Fallot, a variant that is characterized by severely dilated pulmonary trunk and branches, due to the concurrent occurrence of valvular stenosis and regurgitation, associated with a malalignment ventricular septal defect, overriding of the aorta and agenesis of ductus arteriosus. In subvalvar pulmonary stenosis, the subpulmonary outflow tract is narrowed by either a muscle or a fibrous tissue, which could be from a malaligned infundibular septum, as seen in tetralogy of Fallot, from concentric right ventricular hypertrophy or from hypertrophic cardiomyopathy. Rarely, a fibrous subpulmonary obstruction may be caused by an anomalous chordal attachment from a tricuspid valve or common atrioventricular valve. Supravalvar pulmonary stenosis may be from a discrete single or multiple narrowing distal to the pulmonary valve at the sinotubular junction or at the bifurcation of the main pulmonary artery or diffuse hypoplasia of the main pulmonary artery or branches. Pulmonary stenosis may occur as an isolated condition or may be associated with other intracardiac anomalies such as tetralogy of Fallot, transposition of great arteries, double outlet right ventricle, tricuspid atresia, and corrected transposition of great vessels. Fetal pulmonary stenosis is often reported in monochorionic diamniotic twin pregnancies complicated with twin-to-twin transfusion syndrome with pulmonary stenosis or even pulmonary atresia in the recipient twin. Mild-to-moderate pulmonary valve stenosis is well tolerated, and term delivery is expected without any neonatal intervention. Moderate-to-severe pulmonary valve stenosis may need postnatal infusion of prostaglandin if there is antenatal evidence of ductal flow reversal and inadequate antegrade flow across the pulmonary valve at birth with resultant profound desaturation. Neonatal intervention in the cardiac catheterization laboratory may be indicated for nonsurgical relief of pulmonary valve obstruction. Certain institutions offer fetal pulmonary valvuloplasty for critical pulmonary stenosis/pulmonary atresia with intact ventricular septum complex in order to avoid a single ventricle repair postnatally or in the presence of fetal hydrops or marked ventricular dysfunction. Selective fetoscopic laser photocoagulation (SLFP) therapy has shown to cause regression of pulmonary stenosis in the recipient twin in twin-to-twin transfusion syndrome.