What is the anatomy of the interatrial (IA) septum?

Anatomy of the Interatrial Septum

The interatrial septum (IAS) is a complex fibromuscular structure that consists of three main components: the venous inflow component, the vestibule surrounding the mitral orifice, and the interatrial septum itself, which has no clear anatomic demarcations between these components. 1

Structural Components of the Interatrial Septum

True Interatrial Septum

• The true interatrial septum represents only about 20% of the entire interatrial septal area, with the fossa ovalis being its central component 2
• The fossa ovalis has mean cranio-caudal and antero-posterior diameters of 12.1±3.6 mm and 14.1±3.6 mm, respectively 2
• The fossa ovalis is typically located approximately 10.1±4.4 mm above the inferior vena cava ostium, 20.7±5.2 mm from the right atrioventricular ring, and 12.6±5.2 mm below the right atrial roof 2

Fossa Ovalis Morphology

• Four types of fossa ovalis anatomy have been observed: smooth (56.3%), patent foramen ovale (24.4%), right-sided septal pouch (11.9%), and net-like formation (7.4%) 2
• The fossa ovalis is lined by smooth endocardium and represents the thinnest part of the interatrial septum 1
• In a normal heart, the fossa ovalis appears as a depression on the right atrial side with a raised muscular rim (limbus) surrounding it 3

Membranous Septum

• The membranous septum is formed between two valve commissures: on the left side, between the right and noncoronary cusps, and on the right side, between the septal and anterior leaflets of the tricuspid valve 1
• The atrioventricular (AV) component of the membranous septum is where the bundle of His penetrates the left ventricle 1
• The AV septum separates the right atrium and the left ventricle with septal tissue composed primarily of left ventricular myocardium, with contributions from right atrial and ventricular myocardium 1

Interatrial Communications and Defects

Patent Foramen Ovale (PFO)

• PFO is a small interatrial communication confined to the region of the fossa ovalis, characterized by no deficiency of septum primum and a normal limbus with no deficiency of the septum secundum 4
• The mean PFO channel length is 10.5±5.2 mm, with tunnel-like PFOs (channel length ≥12 mm) observed in 8.9% of specimens 2
• PFO represents a tunnel-like passageway between the free edge of the overlapping ovale fossa valve and its muscular rim 3

Atrial Septal Defects (ASDs)

• Secundum ASDs are the most common type, resulting from deficiency in the septum primum, excessive resorption of the septum primum, or deficiency in the septum secundum, located in the region of the fossa ovalis 4
• Primum ASDs are a variant of atrioventricular septal defect, characterized by interatrial communication just above the atrioventricular valve 4
• Sinus venosus defects are characterized by a vena cava and/or pulmonary vein(s) that overrides the atrial septum or septum secundum 4
• Unroofed coronary sinus defects are direct communications between the left atrium and the coronary sinus 4

Associated Anatomical Variations

• Atrial septal aneurysm is an abnormally large and redundant septum primum that results in aneurysmal protrusion into one or both atria 4
• Cor triatriatum sinister is a partition that divides the left atrium into a posterior chamber receiving pulmonary veins and an anterior chamber communicating with the left atrial appendage 4
• Hyperlipomatous interatrial septum is increased interatrial septal thickness caused by lipid deposition 4

Imaging of the Interatrial Septum

Echocardiographic Assessment

• Transthoracic echocardiography can adequately display the IAS in most patients, with subxiphoid views being particularly useful in infants 5
• 3D echocardiography provides "en-face" views of the IAS from both right and left atrial perspectives, allowing better visualization of defects 1
• Left atrial strain imaging can help assess the function of the interatrial septum, though the septal region poses technical challenges due to its fibromuscular composition 1

Advanced Imaging Techniques

• Cardiac CT and MRI can provide detailed anatomical assessment of the interatrial septum and are particularly useful for planning interventional procedures 1
• Multi-modality imaging is recommended for comprehensive assessment of the interatrial septum, especially when planning interventional procedures 1

Clinical Implications

Conduction System Relationship

• The interatrial septum plays an important role in both intra- and interatrial electrical conduction 6
• Electropathological changes in the IAS such as discordant activation of the right and left septal layers may facilitate intraseptal re-entry and promote development of atrial tachyarrhythmias 6
• A thicker IAS has been associated with atrial tachyarrhythmias and lower success rates of catheter ablation 6

Interventional Considerations

• Understanding the true interatrial septum is crucial for safe transseptal access to the left atrium during interventional procedures 2
• 3D echocardiography provides reliable sizing of complex defects for device closure, while 2D echocardiography may lead to selecting a smaller-sized device 1
• The "en-face" anatomically oriented view of the IAS creates a common language between interventionists and echocardiologists, making catheter navigation across the septum faster compared to 2D imaging with fluoroscopy 1

Developmental Aspects

• The atrial septum develops from a ridge of mesenchyme in the roof of the undivided atrial cavity 7
• Muscularization of the mesenchymal ridge and vestibular spine reinforces the attachment of the primary muscular septum to the atrioventricular insulating plane 7
• The propensity for malformations of the atrial septum reflects its complicated morphogenesis, involving several myocardial structures and mesenchymal tissues of intracardiac and extracardiac origin 7