Down Syndrome is a chromosomal defect in which there is an extra chromosome inserted into the DNA other than the normal 46 chromosomes.
Of children born with a cardiac defect, about one in twenty have trisomy 21. This is highly associated with congenital heart disease, particularly  atrioventricular heart defects (AV canal or endocardial cushion defects).  These children tend to have below average intelligence and have other defects associated with this syndrome such as duodenal and anal atresia and Hirschprungs’s disease (enlargement or obstruction of the colon).
Please see my previous posts on “endocardial cushion defects”, or “AV canal”, “embryology” and “morphology”.
AV canal is the most commonly associated defect with Down syndrome. This defect is a failure of the heart to completely form at the crux, or center of the heart. As a result, there are malformed mitral an tricuspid valves with an associated membranous VSD (ventricular septal defect) and an ASD (atrial septal defect).
Where there is one defect (especially left-sided) there may be others. Always rule out other defects such as bicuspid aortic valve, ASD’s, VSD’s, PDA’s, and coarctation of the aorta.
Trisomy 18 (Edward’s Syndrome)
This defect occurs in about one of every 3500 births and is very often fatal to the newborn due to the extensive nature of the defects. Extra cardiac defects include mental retardation, clenched fists, crossed legs, low birth weight and small skull development. Cardiac defects include persistent PDA, VSD or ASD’s.
Trisomy 13 (Patau’s Syndrome)
This defect occurs in about one of every 7000 births and is typically fatal to the neonate. Extra-cardiac defects include cleft palate, brain malformations and polydactyly ( an extra digit, typically the thumb of little finger). Cardiac defects include atrial malformations, VSD’s, ASD.s, and AV canal.
Apert’s Syndrome
This is a genetic disorder resulting from premature closure of the cranial sutures between the bones of the skull. It is characterized by malformations of the skull and facial features and usually involves “webbing” and/or fusion of the bony structures of the hands and feet. Cardiac malformations include ASD’s and VSD’s.
Cri du chat
A chromosomal disorder characterized by a distinctive cat-like cry and abnormalities of the skull and face. Associated cardiac abnormalities include VSD’s, PDA, ASD, AV canal and tetralogy of fallot.
DiGeorge Syndrome
A chromosomal disorder (deletion of chromosome 22),  resulting in the absence of the thymus and parathyroid glands. Presentations include deformed palate, recurrent infections, neuro-muscular disorders, kidney malfunctions and mental deficiencies.  Associated heart defects include  VSD, PDA, interrupted aortic arch, Tetralogy of Fallot and Truncus Arteriosis.