Interatrial septum

The interatrial septum is a thin muscular structure that separates the right and left atria of the heart.

The interatrial septum is not a flat structure but has regions of variable thickness and complex embryological origins:

• Fossa ovalis of right atrium: A central depression on the right atrial side, representing the closed foramen ovale. Its prominent margin, called the limbus of the fossa ovalis, is formed by the lower edge of the septum secundum.

• Valve of the foramen ovale: Seen on the left atrial side as a small fold that corresponds to the septum primum.

The interatrial septum is generally thin in the region of the fossa ovalis but thicker superiorly and inferiorly due to muscular ridge. It does not form the entire wall between the atria; parts of the atrial walls (like the atrioventricular septum) are adjacent to it.

Development:

The interatrial septum develops from two primary structures:

• Septum primum – A thin crescent-shaped membrane that grows downward from the roof of the common atrium toward the endocardial cushions. The initial gap beneath it is the foramen primum, which closes as the septum primum fuses with the cushions.

• Septum secundum – A thicker crescent-shaped fold that develops to the right of the septum primum, leaving an incomplete closure known as the foramen ovale.

The overlapping of these two septa creates a valve-like arrangement (the valve of the foramen ovale) that permits right-to-left shunting of blood during fetal life.

At birth, when the lungs expand and left atrial pressure rises, the septum primum is pressed against the septum secundum, functionally closing the foramen ovale. Over time, this usually fuses completely to form the adult interatrial septum.

Relations

• Right atrium: The fossa ovalis and its limbus are visible.

• Left atrium: The valve of the foramen ovale appears as a subtle ridge.

• Superiorly, it is continuous with the roof of the atria.

• Inferiorly, it blends into the atrioventricular septum.

Variations and Clinical Relevance

• Patent foramen ovale (PFO): In ~25% of adults, incomplete fusion of the septum primum and secundum leaves a small opening which can allow interatrial shunting under certain conditions.

• Atrial septal defects (ASDs):

  • Ostium secundum defects: Due to excessive resorption of septum primum or deficient septum secundum.

  • Ostium primum defects: Failure of septum primum to fuse with endocardial cushions.

  • Sinus venosus defects: Near the entry of the superior vena cava.

• Aneurysm of the fossa ovalis: A redundant and bulging interatrial septum may be observed on echocardiography.