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Prevalence at birth of complete atrioventricular septal defect (CAVSD) is suggested to be 1/5,000 in Europe. Both sexes are equally affected.

Infants typically present within the first year of life with symptoms of congestive heart failure which may include feeding difficulties, excessive sweating, tachycardia, tachypnea, mild wheezing, failure to thrive, and poor peripheral blood perfusion. If untreated, affected individuals develop irreversible pulmonary hypertension which improves signs of congestive heart failure but worsens tolerance to effort and results in cyanosis and Eisenmenger syndrome. Recurrent pulmonary infections are common. Morphologically, the ventricles may be equal or nearly equal in size (balanced) or one of the ventricles may be significantly larger than the other (unbalanced). The balanced form is typically observed in Down syndrome. Unbalanced ventricles are associated with varying degrees of malalignment of the common atrioventricular valve over the hypoplastic ventricle and hypoplasia of the arterial valve above. Tetralogy of Fallot (TOF) is observed in 5-10% of CAVSD.

CAVSD is strongly associated with Down syndrome and heterotaxy. Cilia gene mutations could be involved in isolated AVSD. A small fraction of CAVSD cases have been associated with NR2F2 (15q26.2), GATA4 (8p23.1), GATA6 (18q11.2) and CRELD1 (3p25.3) mutations.

Echocardiography is used for postnatal diagnosis. Typical findings include a left and superior QRS axis in the frontal plane and counterclockwise depolarization. Chest X-ray may show ventricular enlargement and a greater anterior position of the left atrioventricular valve due to the inlet septal defect, which gives a ''goose neck'' appearance to le left ventricular outflow tract.

Differential diagnosis includes other congenital cardiac anomalies that result in early heart failure including partial atrioventricular canal, atrial septal defects and large ventricular septal defects.

CAVSD can be detected with fetal echocardiography; the detection rate is approximately 67%.

Atrioventricular septal defects (AVSD) can occur in the offspring of mothers with CAVSD.

Digoxin, diuretics and angiotensin-converting enzymes may be used to treat congestive heart failure prior to surgery. Surgical repair is advised at 3 months of age and is typically performed between 3 and 6 months of age. The three classical techniques are single patch technique, two patch technique (most frequent), and the modified single patch technique. To avoid surgically induced AV block, the position of the AV node and bundle of His must be considered, with the AV node positioned more posteriorly and inferiorly, the non-branching bundle running slightly on the left side of the septal crest with the branching bundle being exposed. Pulmonary artery banding can be proposed before 3 months in very ill patients despite optimal medical therapy. Combined repair of other cardiac anomalies is often performed with desirable results. In case of associated tetralogy of Fallot, complete repair is usually delayed up until 1 year of age. Lifelong follow-up every 2 to 3 years is recommended.

Without surgery, many of the affected individuals will die in infancy. Prognosis after surgical repair is good; however, patients who weigh less the 3 kg or are less than 2.5 years old have a higher risk of mortality during surgery. Pregnancy increases the risk of developing left AVV regurgitation, arrhythmias, deterioration of heart failure, and is not recommended for women with severe pulmonary hypertension.