8th March 2024

Atrial Septal Defect

Atrial septal defect (ASD) is the commonest form of acyanotic heart disease in which a pathological communication is formed that allows shunting of blood between the left and right atrium. It constitutes approximately 5-10% of congenital heart diseases and co-exists with other congenital defects in 30-50% of cases. 

A patent foramen ovale (PFO) is a different form of interatrial shunt caused by failure of the foramen ovale to close completely after birth. PFOs are present in 25% of the general population and the majority of adults remain asymptomatic. PFOs may result in paradoxical embolisation of thrombus from the venous circulation into the systemic circulation and the risk increases in the presence of an atrial septal aneurysm. 

ASD can be divided into the following subtypes:

Image Source: Wikimedia Commons (2005). Atrial septal defect, Licensed under a creative commons licence. Available for free at: https://en.wikipedia.org/wiki/Atrial_septal_defect svg (Accessed: 26 January 2024).

Ostium secundum

These constitute 80% of ASDs and are located within, or surrounding, the fossa ovalis.  They form as a result of poor development of the septum primum of varying degrees during embryologic development and have associations with other forms of congenital defects, including mitral valve prolapse, pulmonary stenosis and partial anomalous pulmonary venous drainage. 

Ostium primum

They account for 15-20% of ASDs and are part of the spectrum of atrioventricular septal defects (AVSD). Primum ASD is caused by failure of the septum primum to fuse with the endocardial cushions at the interatrial septal base and are frequently associated with a cleft in the anterior leaflet of the mitral valve, leading to varying degrees of mitral regurgitation. Partial AVSD includes primum ASD, cleft mitral valve, and ventricular septal defect (VSD). Complete AVSD is characterised by a common AV valve with primum ASD and inlet VSD, often seen in Down’s syndrome.

Sinus venosus defect

Sinus venosus defects constitute 5-10% of ASDs and involve a portion of the atrial wall derived from the sinus venosus but there is no true communication between the right and left atria. Transoesophageal echocardiography (TOE) is generally required for assessment in adults, especially when there is unexplained right chamber dilatation. They can be further subdivided into superior and inferior defects. The commonest of the two is a superior sinus venosus, located below the orifice of the superior vena cava (SVC). The right upper lobe and middle pulmonary veins connect to the junction of the SVC and right atrium or on the SVC itself. An inferior sinus venosus results from the breakdown between the right atrial wall and the inferior right-sided pulmonary veins. 

Unroofed coronary sinus

Caused by the absence of the common wall between the coronary sinus and left atrium, this defect accounts for less than 1% of ASDs and is often associated with complete AV septal defect; absence of coronary sinus and a persistent left sided SVC.