Atrial Septal Aneurysm

An atrial septal aneurysm (ASA) is a redundant, hypermobile segment of the interatrial septum, usually at the fossa ovalis, that bulges into the right atrium, left atrium or both. Echocardiographic criteria are a base diameter of at least 15 mm with a total phasic excursion of at least 10 mm from the plane of the septum.

It affects roughly 2% of unselected adults. It coexists with a PFO in over half of cases and is also linked with secundum atrial septal defect and mitral valve prolapse. Causes are a congenitally redundant septum primum or thin fossa ovalis, and acquired remodelling from chronic interatrial pressure differences.

Pathophysiology. Stasis within the aneurysm sac allows in situ thrombus. The mobile septum also widens a coexisting PFO and aims inferior caval flow at it, increasing right-to-left shunting. The abnormal atrial myocardium predisposes to supraventricular arrhythmia.

In patients with a PFO, an aneurysm is an independent marker of recurrent embolic events.

Most patients are asymptomatic and the diagnosis is made on echocardiography done for another reason. Presentations are cryptogenic ischaemic stroke or TIA, systemic embolism, and palpitations from supraventricular tachycardia or atrial fibrillation. Examination is normal.

Red flags.

• Embolic stroke, TIA or peripheral embolism with an aneurysm
• Thrombus visible within the sac
• Atrial fibrillation or flutter
• Positional hypoxaemia suggesting a significant right-to-left shunt

• Transthoracic echocardiography. Subcostal and apical four chamber views show the mobile septum. Measure base width and total excursion.
• Agitated saline contrast. Detects a coexisting PFO. Use the technique and timing criteria for intracardiac versus intrapulmonary shunt: left atrial bubbles within 3 cardiac cycles indicate an intracardiac shunt.
• Transoesophageal echocardiography. More sensitive. It confirms the dimensions, classifies the aneurysm as right bulging, left bulging or bidirectional, and shows PFO, thrombus and left atrial appendage thrombus.
• Cardiac MRI or CT. An alternative when echocardiographic windows are poor.
• Embolic workup. Brain imaging, vascular imaging, and ambulatory rhythm monitoring for at least 4 weeks or an implantable loop recorder.

Differential diagnosis.

• Lipomatous hypertrophy of the interatrial septum (thickened septum over 15 mm sparing the fossa ovalis)
• Atrial myxoma attached to the septum by a stalk
• Redundant Eustachian valve or Chiari network
• Septal thrombus
• Left atrial septal pouch
• Secundum atrial septal defect

• Isolated aneurysm without embolic event or arrhythmia: no treatment.
• Embolic event with aneurysm: aspirin 75 to 100 mg orally once daily, or clopidogrel 75 mg once daily if aspirin is not tolerated, long term (usual maximum 325 mg daily for aspirin). If a PFO is present and the criteria for closure are met (age 18 to 60 years, cryptogenic ischaemic stroke, no alternative cause), closure is considered; the combination of PFO and aneurysm gives the greatest benefit. An aneurysm without a shunt does not warrant closure.
• Thrombus in the sac: warfarin started at 5 mg orally once daily (2 to 3 mg in older adults, liver impairment or interacting drugs), adjusted to an INR of 2.0 to 3.0, with no fixed maximum. Check INR every 2 to 3 days at the start, weekly until stable, then every 4 to 12 weeks. Continue for at least 3 months and repeat echocardiography before stopping.
• Atrial fibrillation or flutter: apixaban 5 mg orally twice daily, reduced to 2.5 mg twice daily when at least two of age 80 years or over, weight 60 kg or under and serum creatinine 133 micromol/L or over are present. Also 2.5 mg twice daily at creatinine clearance 15 to 29 mL/min, and avoided under 15 mL/min. Maximum 10 mg daily. Long term.

Special groups. In pregnancy, warfarin is avoided in the first trimester; use enoxaparin 1 mg/kg subcutaneously every 12 hours with anti-Xa monitoring when anticoagulation is required (1 mg/kg once daily at creatinine clearance under 30 mL/min).

Monitoring. Echocardiography at 3 months for thrombus resolution. Ambulatory ECG with new palpitations. INR every 4 to 12 weeks once stable, to a target of 2.0 to 3.0 while on warfarin.

Referral and escalation. Cardiology and stroke services for any embolic event. Urgent referral for thrombus within the sac or new atrial fibrillation.