perioperative echo

Atrial Septal Defects

Atrial Septal Defect Evaluation

1. Background

  • Atrial Septal Defects account for 6-10% of congenital heart disease.
  • Commonest acyanotic congenital lesion.
  • Accounts for 30-40% of clinically important intra-cardiac shunts in adults.
  • PFOs are present in 20-25% of the adult population.

2. Classification

Septal defects can be classified according to their anatomical location:

  1. Sinus Venosus – SVC type
  2. Sinus Venosus – IVC type
  3. Ostium Primum
  4. Ostium Secundum
  5. Unroofed Coronary Sinus
  6. Patent Foramen Ovale
ASD anatomical location

Anatomical classification of ASDs

3. Embrological Development of IAS

Developmental stages atrial septum

Stages of development of the atrial septum

From the 5th week of gestation, the septum primum begins to develop and grow towards the endocardial cushion. Before the septum primum fuses with the endocardial cushion, fenestrations appear in its cephalic portion. These coalesce to form the ostium secundum.

 In the meantime, to the right of the septum primum, the septum secundum starts to form as an invagination of the atrial wall. The septum secundum stops growing at the end of the seventh week of gestation, leaving a posterior and inferior gap known as the fossa ovalis. A flap valve allows blood to flow from the fetal right atrium to left atrium.

At birth, changes in intracardiac pressures occur. There is a drop in RA pressures, and an increase in LA pressures. This causes the flap valve to be pushed against the septum secundum. Eventually in two thirds of the population, the flap valve fuses with the septum secundum creating a morphologic and physiologic separation of the atria. The lack of fusion between the flap valve and septum secundum results in a probe patent foramen ovale (PFO).

4. Patent Foramen Ovale

A PFO is a separation between the septum primum and septum secundum located in the anterior superior atrial septum. It is not considered a true ASD as there is no deficiency in atrial septal tissue. In many cases, a PFO might be only functionally patent and have a tunnel-like appearance, because the septum primum forms a flap valve. A PFO can also be a circular or elliptical true opening between the two atria.

Schematic representation of PFO. Dark green = Septum secundum. Light green = Septum primum.

Schematic representation of PFO. Dark green = Septum secundum. Light green = Septum primum. Ref: Calvert et al. PFO: Anatomy, outcomes and closure. Nature Reviews Cardiology 2011.

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Definitions:

  1. Patent Foramen Ovale – Presence of right to left shunt demonstrated by agitated saline contrast or doppler, in the abscence of a true atrial septal deficiency.
  2. PFO with left to right flow – refers to when the atrial hemodynamics result in opening the potential communication of the foramen, resulting in left to right shunting of blood demonstrated by Doppler imaging.
  3.  Stretched PFO – When a PFO is stretched open by atrial hemodynamics, thus creating a defect in the septum, it is referred to as a ‘‘stretched’’ PFO. This can result in left to right or right to left shunting of blood flow, depending on the differences in the right and LA pressure.

Anatomical variants of PFO. Calvert et al. Anatomical variants of PFOs. Nature Cardiology Reviews 2011.

5. Ostium Secundum ASD

An ostium secundum ASD most often occurs as the result of a true deficiency of septum primum tissue; it is the most common form of a true ASD. These defects can vary in shape and can be elliptical or round. The video above shows an ostium secundum ASD. These ASDs typically range in size from several millimeters to as large as more than 3 cm in diameter.

Watch the above video by A/Prof Greg Scalia – ASD Anatomy, Echocardiography and Interventions. Structural Heart Disease, Australia.

6. Ostium Primum ASD

The ostium primum defect is within the spectrum of AV septal defects, also known as AV canal defects or endocardial cushion defects. They represent 2-3% of ASDs.
 
The defect is called an ostium primum ASD (see video below) when there is failure of fusion between the free edge of the septum primum and the endocardial cushion. If the defect is associated with abnormal development of the AV valves, then it is called an endocardial cushion defect.

7. Sinus Venosus Defect

Sinus venosus defects are less common than ostium secundum ASDs and are not true ASDs. These defects occur as a result of a partial or complete absence of the sinus venosus septum between the SVC and the right upper pulmonary vein (SVC type) or the right lower and middle pulmonary veins and the RA (inferior vena cava type). Sinus venosus defects cannot be closed by device and typically require baffling of the right pulmonary veins to the LA by way of an ASD patch. The video below shows a SVC sinus venosus defect (Courtesy of Prof Joshua Zimmerman).

8. Other Defects

A. Coronary Sinus Defects. A coronary sinus septal defect or an ‘‘unroofed’’ coronary sinus is one of the more rare forms of atrial communication. In this defect, the wall of the coronary sinus within the LA is deficient or completely absent. See the video by Dr Hani Mahmoud-Elsayed.

B.  Common Atrium. Rarely, all components of the atrial septum, including the septum primum, septum secundum, and AV canal septum are absent, resulting in a common atrium

C. Atrial Septal Aneurysm. An atrial septal aneurysm (ASA) is a redundancy or saccular deformity of the atrial septum and is associated with increased mobility of the atrial septal tissue. ASA is defined as excursion of the septal tissue (typically the fossa ovalis) of greater than 10 mm from the plane of the atrial septum into the RA or LA or a combined total excursion right and left of 15 mm. The prevalence of ASA is 2%–3%. ASA has been associated with the presence of a PFO, as well as an increased size of a PFO, and an increased prevalence of cryptogenic stroke and other embolic events.

D. Eustachian Valve and Chiari Network. The eustachian valve is a remnant of the valve of the IVC that, during fetal life, directs IVC flow across the fossa ovalis.  The eustachian valve extends anterior from the IVC–RA junction. A Chiari network is a remnant of the right valve of the sinus venosus and appears as a filamentous structure in various places in the RA, including near the entry of the IVC and coronary sinus into the RA. A Chiari network is present in 2%–3% of the general population and is associated with the presence of PFO and ASA.

The above video depicts an unroofed coronary sinus. An eustachian valve is seen in the clip below.

References:

  1. Rojas et al. Embryology and Developmental Defects of the Interatrial Septum. AJR 2010.
  2. Webb & Gatzoulis. Atrial Septal Defects in the Adult. Circulation 2006.
  3. Sivestry et al. Guidelines for the Echocardiographic Assessment of Atrial Septal Defect and Patent Foramen Ovale: From the American Society of Echocardiography and Society for Cardiac Angiography and Interventions. JASE 2015.