Down syndrome (trisomy 21) is a relatively common genetic syndrome, occurring in 1 of 800 to 1000 live births. Congenital heart defects are present in 40 to 60% of infants affected by Down syndrome.
Before birth, the fetal heart shunts blood around the lungs, because the lungs are not functional in utero. A portal between the left and right sides of the heart keeps pressures equalized.
Shortly after birth, several mechanical and physiologic processes occur that close the shunts, route blood to the lungs for oxygenation, increase pressures on the left side of the heart, and lead to the complete separation of the pulmonary and systemic vascular systems.
While congenital heart defects also allow blood to bypass the lungs during fetal life, these abnormalities can cause significant distress when an infant takes its first few breaths. As the lungs expand and circulatory dynamics change, defective hearts often mix oxygenated and unoxygenated blood, and symptoms of hypoxia and/or heart failure may appear.
Specific Heart Defects in Down Syndrome
- Complete atrioventricular canal defect (accounts for 60% of heart defects in Down syndrome infants): The AV canal, or endocardial cushion, is defined by a block of tissue that normally forms the walls between the heart’s atria (top chambers) and ventricles (bottom chambers). This tissue also contributes to the formation of valves between the atria and ventricles. When development of this region is incomplete, blood can backwash freely between the two sides of the heart and leak backward through the valves. Infants are often cyanotic (blue) at birth, and heart failure can occur fairly quickly. Surgical repair is usually performed early in the child’s life (sometimes at 3 to 4 months).
- Ventricular septal defect (32%): The ventricular septum is a wall that normally separates the left and right ventricles. VSDs are openings in this wall. VSDs usually cause murmurs, but a murmur may only become audible several weeks after birth, when left-to-right pressure gradients are fully developed. Some VSDs close spontaneously in infancy; those that cause heart failure may require surgical closure.
- Tetralogy of Fallot (6%): This complex anomaly includes:
- A large VSD: (See above)
- Pulmonary stenosis: Narrowing of the passageway from the right ventricle to the lungs
- Right ventricular hypertrophy: Thickening of the right ventricular muscle (caused by backpressure from pulmonary stenosis)
- Overriding aorta: The aorta normally exits from the left ventricle as it carries blood to the body. In tetralogy, the aorta is hooked to both ventricles.
- Atrial septal defect (1%): An opening in the wall separating the atria, like VSDs, an ASD allows blood to shunt from left to right after birth. Surgical closure is necessary for those that cause large shunts and heart failure.
- Isolated mitral cleft (1%): A defect in the mitral valve that allows blood to backwash across the valve. These require surgical repair if regurgitation is significant.
(Sources: The Merck Manual, 16th Edition:2051-61; Am Fam Physician 1999;59(2):381-91)
Some anatomical heart problems may not announce themselves immediately after birth; they may cause progressive difficulty only in the days and weeks following delivery. And, while many congenital heart defects produce easily detectable signs—such as a murmur—some potentially life-threatening anomalies don’t produce murmurs.
In order to ensure that no serious cardiac abnormality is overlooked, all newborn infants who are suspected of having Down syndrome should have an electrocardiogram and echocardiogram (ultrasound of the heart).
Stephen Allen Christensen - Dr. Steve Christensen's writing has appeared in magazines, professional journals, poetry anthologies, and children's books since 1976.
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