The AV canal or endocardial cushion is an area of the heart that develops during the first two months of development. This occurs when the superior and inferior portions of the endocardial cushion fuse with the septum primum, which is the primitive atrial septation. In a partial AV canal defect, commonly associated with Down’s syndrome, the mitral valve formation is abnormally inferior leading to improper positioning of the two leaflets that comprise the mitral valve. This abnormal leaflet position results in a widened cleft and inadequate coaptation. Due to the AV canal defect and inferior mitral leaflet positioning, this creates an abnormal communication between the left ventricle and right atrium similar to an ASD. The improper mitral leaflet positioning also applies to the tricuspid valve and its leaflets as well. Thus, in the severe form of the AV canal mitral valve regurgitation, there is also an associated ASD and tricuspid regurgitation. This anomaly presents within the first 2 years of life and has a mortality rate of up to 30%.

These children present with cyanosis, tachypnea, dyspnea, failure to thrive, and eventual volume overloading leading to congestive heart failure. Upon auscultation, one would hear a fixed S2 split. Additionally, there is the pansystolic murmur best heard at the apex radiating to the axilla generally associated with the mitral regurgitation. Also there could be an S3 gallop due to the flow from the right atrium into the left ventricle during early diastole.