I. Genetics
II. Incidence of Congenital Heart Malformations

GLOSSARY
atrial septal defect a hole in the wall of the heart (septum) that
divides the left and right atrium. ventricular septal defect a hole in the septum, that divides the
left and right ventricles.

• I. GENETICS
  Down syndrome, trisomy 21 is the most common chro¬mosome abnormality and occurs in approximately 1 in every 650 births. It alone accounts for 1 in 20 congenital cardiac malformations. In virtually all individuals with Down syndrome 47 chromosomes are found with an extra copy of chromosome 21 (trisomy 21), although in approximately 3% of individuals it may originate from an extra copy of all or part of the long arm of chromosome 21 translocated to another chromosome. The recurrence risk is approximately 1% after a child with trisomy 21, and the recurrence is higher if one of the parents carries a translocation.
• II. INCIDENCE OF CONGENITAL HEART MALFORMATIONS
  Congenital heart disease occurs in more than 40% of patients with Down syndrome. Malformations include atrioventricular septal defects and approximately 33% of all atrioventricular septal defects are associated with Down syndrome. This congenital defect is referred to as an endocardial cushion defect or atrioventricular canal malformation which results from a defective closure of the endocardial cushions. These are a group of abnor¬malities unified because of a complete absence of normal atrioventricular septal structures. This is a much more complicated lesion than the more common atrial septal defect and is much more difficult to repair surgically. For more information about atrial septal defects see the chapters Atrial Septal Defect and Embryology. To learn more about ventricular septal defects see the chapter Congenital Heart Disease.
• BIBLIOGRAPHY
  Pyeritz, R. E. Genetics and cardiovascular diseases. In Heart Disease, sixth edition. E. Braunwald, D. P. Zipes, and P. Libby, eds. W. B. Saunders, Philadelphia, 2001.