The first is the development of the septum primum, a crescent-shaped piece of tissue forming the initial divider between the right and left atria.

As the septum secundum grows, it leaves a small opening called the foramen ovale.

Eventually, the septum primum is nothing more than a small flap that covers the foramen ovale on its left side.

At birth, there is a reversal in the pressure gradient between the atria, resulting in functional closure of the valve of the foramen ovale.

An Atrial septal defect is a relatively common heart malformation that occurs when the interatrial septum fails to develop properly.

In normal physiologic circumstances, the septum primum acts as a one-way valve preventing blood flow as described above; but, if pathologic conditions cause right atrial pressure to exceed left atrial pressure, blood may flow through the foramen ovale from right to left.

Children born with this defect may be asymptomatic, however, over time pulmonary hypertension and the resulting hypertrophy of the right side of the heart will lead to a reversal of this shunt.

On CT scan, it shows homogeneous, dumbbell mass which is non-enhancing at the interatrial septum.