Aortic Stenosis

Aortic Stenosis is the narrowing of the aortic valve opening caused by the  failure of  valve leaflets to open normally which causes a variable degree of left outflow tract obstruction. Clinical severity ranges from an asymptomatic bicuspid valve in older children and adults to life-threatening, critical obstruction in newborn.

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Congenital aortic stenosis (AS) occurs in 0.2–0.5 per 1,000 live-birth¹. It accounts for 3% of all congenital heart defect (CHD). Approximately 10% of patient presents with neonatal critical AS².

Congenital aortic stenosis is caused by abnormal morphological development of the aortic valve. The most frequent type of congenital aortic stenosis is a bicuspid aortic valve accounting for approximately 90% of AS cases. Valvular abnormalities can be accompanied by supra or subvalvular stenosis.³

Congenital aortic stenosis occurs due to improper development of the aortic valve in the first 8 weeks of fetal development. The embryological process that forms the aortic valve leaflets begins by cavity formation of truncus arteriosus. The aortic valve usually has three leaflets. Incomplete formation of the cavity causes various morphological abnormalities of the aortic valve including  bicuspid valve with or without commissural fusion, tricuspid valve with commissural fusion, mono cuspid valve and myxomatous leaflet, i.e., dysplastic valve. The exact etiology is unknown and is multifactorial. Majority of the time it occurs sporadically, sometimes it can be a part of a genetic syndrome.

The underlying pathophysiology of aortic stenosis is left outflow tract obstruction and increase in afterload to the left ventricle. The degree of obstruction varies from mild, moderate to severe and expresses from an asymptomatic bicuspid valve in older children and adults to life-threatening critical obstruction to hypoplastic left heart syndrome in the newborn. Increase in afterload due to obstruction leads to an increase in the left ventricular myocardial muscle mass to generate extra force to overcome this. In critical aortic stenosis cases, the left ventricular contractility is depressed, and the left ventricle becomes markedly dilated. Even in critical condition, the fetal systemic output is maintained due to interatrial communication (foramen ovale) and ductus arteriosus. The marked increase in the left ventricular end diastolic pressure decreases the blood flow from the left atrium into the left ventricle leading to a gradual reduction of left ventricular cavity volume and evolving hypoplastic left heart syndrome. Degeneration of the endocardium at the same time leads to the development of endocardial fibroelastosis. An obstructive lesion during early development of the heart can lead to a complex cardiac malformation at the end of pregnancy due to alteration of intracardiac blood flow and pressure. In milder cases, the left ventricle can cope with the increased afterload and patient presents with clinical symptoms later in life.^4,5

Associated cardiac malformations are seen in at least 20% of cases. Coexistent cardiac abnormalities include coarctation of aorta and ventricular septal defect and concomitant pulmonary stenosis. Aortic stenosis is not usually associated with other structural malformations unless it is a part of a genetic syndrome/chromosomal abnormality. Common genetic syndrome includes Turner syndrome, Shone’s complex and William’s syndrome⁶.

The risk of recurrence for AS is high mainly if the etiology is a bicuspid aortic valve. Genetic studies have shown that there is a 13%-15% recurrence risk of aortic stenosis in a baby if the mother is affected, a 5% risk if the father is affected, and a 2% risk if one child is affected.⁷

Ultrasound findings depend on the severity of the obstruction. Mild AS is associated with increased pressure of the left outflow tract flow. This can be diagnosed only by color Doppler interrogation which shows acceleration and aliasing of blood flow through the aortic valve. Aortic stenosis can be confirmed by Pulse Wave Doppler, which shows high peak velocities across the aortic valve. The left ventricular function is usually normal in mild cases, and the 4-chamber view can look completely normal which can be easily overlooked prenatally. With progressive obstruction, the left ventricle becomes more globular, dilated and less contractile. The myocardium becomes hypertrophied and progressive dysfunction leads to endocardial fibroelastosis which is identified by marked internal wall hyperechogenicity.

With increasing severity, mitral valve regurgitation is commonly seen. The aortic valve appears thickened and echogenic. In cases of critical aortic stenosis, the systolic function is impaired, and no forward flow is seen across the aortic valve which leads to retrograde filling of the aortic arch and the isthmus from the arterial duct. In advanced cases the left ventricle becomes hypoplastic^8,9.

• A Globular and dilated left ventricle
• Aliasing through the aortic valve
• Retrograde aortic arch flow in critical AS

Milder cases are not easy to diagnose and the advanced disease where the left ventricle is already hypoplastic is difficult to differentiate from the other etiologies of hypoplastic left heart syndrome (HLHS), i.e. aortic atresia/mitral atresia.

Following prenatal detection, a detailed fetal cardiac assessment is indicated. The severity of aortic stenosis needs to be assessed. The most challenging issue after prenatal diagnosis is the risk of progression to HLHS. A normal-sized or slightly dilated left ventricle can progress to HLHS after birth. Careful evaluation of the fetal heart by a pediatric cardiologist is essential to determine the need of in-utero intervention.  Furthermore, extracardiac defects should be carefully excluded during the fetal anatomic survey.

ISUOG guidelines recommend inclusion of colour Doppler for routine prenatal screening in order to improve the detection of milder cases of Aortic stenosis.¹⁰

The prognosis depends upon the severity of obstruction and the left ventricular size and function. If the left ventricular size and components are sufficient to generate a systemic output then interventions for the aortic valve per se, i.e.,  catheter (transluminal aortic valvuloplasty) or surgery is performed. If there is an established hypoplastic left heart circulation with under developed left ventricle, the management option after birth is staged operations leading to Fontan’s circulation, i.e. a single ventricular circulation.

Once diagnosed at the time of routine screening, the patient should be referred to a fetal cardiologist for further detailed evaluation of the heart and counseling. A thorough fetal anatomy investigation is recommended. The overall risk of aneuploidy is low however invasive testing with CGH array can be discussed to rule out any evidence of microdeletion/duplication syndrome.

1. van der Linde D, Konings EE, Slager MA, Birth prevalence of congenital heart disease worldwide: a systematic review and meta-analysis. J Am Coll Cardiol. 2011 Nov 15;58(21):2241-7.

2. Hoffman JI, Kaplan S, Liberthson RR. Prevalence of congenital heart disease. Am Heart J. 2004; 147:425–439.

3. Kitchiner D, Jackson M, Malaiya N, Walsh K, Peart I, Arnold R. Incidence and prognosis of obstruction of the left ventricular outflow tract in Liverpool (1960-91): a study of 313 patients. Br Heart J. 1994;71(6):588.

4. Hornberger LK, Sanders SP, Rein AJ, Spevak PJ, Parness IA, Colan SD. Left heart obstructive lesions and left ventricular growth in the midtrimester fetus. A longitudinal study. Circulation. 1995; 92:1531–1538.

5. Mcelhinney, D. B, Vogel, M, & Benson, C. B. Assessment of left ventricular endocardial fibroelastosis in fetuses with aortic stenosis and evolving hypoplastic left heart syndrome. Am J Cardiol (2010).

6. Wang L, Ming Wang L, Chen W. Bicuspid Aortic Valve: A Review of its Genetics and Clinical Significance. J Heart Valve Dis. 2016 Sep;25(5):568-573.

7. Fesslova V, Brankovic J, Lalatta F, Recurrence of congenital heart disease in cases with familial risk screened prenatally by echocardiography. J Pregnancy. 2011

8. Yamamoto Y(1), Hornberger LK. Progression of outflow tract obstruction in the fetus. Early Hum Dev. 2012 May;88(5):279-85.

9. McCaffrey FM(1), Sherman FS. Prenatal diagnosis of severe aortic stenosis. Pediatr Cardiol. 1997 Jul-Aug;18(4):276-81.

10. International Society of Ultrasound in Obstetrics and Gynecology, Carvalho JS, Allan LD, Chaoui R, Copel JA, DeVore GR, Hecher K, Lee W, Munoz H, Paladini D, Tutschek B, Yagel S. ISUOG Practice Guidelines (updated): sonographic screening examination of the fetal heart. Ultrasound Obstet Gynecol. 2013 Mar;41(3):348-59. doi: 10.1002/uog.12403.

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