Defining Atrial Septal Aneurysm on Echocardiography
An atrial septal aneurysm is defined as an abnormally large and redundant septum primum that protrudes more than 10 mm beyond the plane of the interatrial septum during the cardiac cycle, as measured by echocardiography. 1
Diagnostic Criteria
The standard echocardiographic definition requires two key measurements:
- Excursion threshold: The interatrial septum must demonstrate >10 mm excursion from the centerline (or plane of the atrial septum) during the cardiac cycle 1, 2
- Base diameter: Some definitions also require at least a 15 mm diameter base of the involved interatrial septum 3
The aneurysm typically involves the region of the fossa ovalis and septum primum, appearing as a localized saccular deformity that protrudes into one or both atria 1, 4, 5
Optimal Imaging Approach
Transesophageal echocardiography (TEE) is superior to transthoracic echocardiography (TTE) for diagnosing atrial septal aneurysm:
- TEE can visualize the atrial septum in all patients, while TTE misses the diagnosis in approximately 47% of cases 2
- The aneurysm is best visualized on TTE using the subcostal four-chamber view and parasternal short-axis view at the level of the aortic root 6
Movement Patterns and Classification
The aneurysm demonstrates characteristic dynamic motion during the cardiac cycle:
- Type 1R: Bulging into right atrium only 5
- Type 2L: Bulging into left atrium only 5
- Type 3RL: Major excursion to right atrium, lesser to left 5
- Type 4LR: Major excursion to left atrium, lesser to right 5
- Type 5: Bidirectional and equidistant movement to both atria 5
The aneurysm shows marked variations in contour and size throughout the cardiac cycle, which helps differentiate it from other intra-atrial structures like tumors or large eustachian valves 7
Associated Findings
When diagnosing atrial septal aneurysm, actively look for:
- Interatrial shunting (present in 54% of cases): patent foramen ovale, atrial septal defect, or sinus venosus defect 2
- Valvular regurgitation (present in 74% of cases, particularly mitral and aortic) 3, 5
- Supraventricular arrhythmias (present in 16% of cases) 3, 5
Clinical Pitfall
The aneurysm may be an isolated finding in 32% of patients, but it is frequently associated with other cardiac abnormalities 2. Always perform contrast echocardiography to evaluate for associated patent foramen ovale, as this combination increases embolic risk 2. The presence of actual thrombus attached to the aneurysm is rare (only 1% of cases), so the embolic risk likely relates to associated shunts or arrhythmias rather than direct thrombus formation 2.