Sinus Venosus Atrial Septal Defect

Sinus venosus atrial septal defect is an interatrial communication located at the junction of the superior or inferior vena cava with the right atrium, outside the fossa ovalis. It is almost always accompanied by partial anomalous drainage of the right pulmonary veins into the vena cava or right atrium.

It accounts for about 2 to 3% of atrial septal defects, though some surgical series report up to 10%. The superior type, with the right upper pulmonary vein (often the right middle vein too) draining to the superior vena cava junction, makes up about 90% of cases. The inferior type, with right lower veins draining to the inferior vena cava, is rare.

It is often missed on transthoracic echocardiography because the defect is posterior and lies outside the usual septal views. The usual clue is unexplained right atrial and right ventricular dilatation.

Untreated, the volume load causes right heart enlargement, atrial flutter and fibrillation, sinus node dysfunction, paradoxical embolism and, in a minority, pulmonary arterial hypertension. Symptoms usually emerge in the third to fourth decade.

Pathophysiology.

The vena cava atrial junction is unroofed, so pulmonary venous blood drains to the right atrium directly through the anomalous veins and through the defect. The left-to-right shunt volume loads the right atrium and ventricle.

Abnormal development near the sinus node explains sinus node dysfunction and ectopic atrial rhythm. Because the defect has no septal rim and the veins must be redirected, device occlusion cannot be used.

Pulmonary vascular resistance rises late in a small minority. Once it is fixed and high, shunt closure is harmful.

Many adults are asymptomatic for years. Symptoms are exertional dyspnoea, fatigue, palpitations, reduced exercise tolerance and, less often, right heart failure, stroke from paradoxical embolism or cyanosis from late shunt reversal.

Signs are a wide, fixed split of the second heart sound, a pulmonary flow murmur, a tricuspid flow rumble in larger shunts, a right ventricular heave and an irregular pulse when atrial fibrillation is present.

• ECG: ectopic atrial rhythm or an abnormal (superior) P wave axis, incomplete right bundle branch block, atrial flutter or fibrillation.
• Transthoracic echocardiography: right atrial and right ventricular dilatation with no visible secundum defect, flow from the superior vena cava into the left atrium, and dilated superior vena cava. Request subcostal and suprasternal views and agitated saline (injection from an arm vein shows bubbles in the left atrium).
• Transoesophageal echocardiography, cardiac CT angiography or MRI define the defect, the number and course of anomalous veins, flow quantification (pulmonary to systemic flow ratio, Qp:Qs) and right ventricular volumes. CT or MRI is the key test when transthoracic images are unrevealing.
• Right heart catheterisation when pulmonary hypertension is suspected: pulmonary vascular resistance, shunt fraction and vasoreactivity.
• Baseline blood count, creatinine, NT-proBNP and ambulatory ECG for atrial arrhythmia.

Differential diagnosis.

• Ostium secundum atrial septal defect.
• Partial anomalous pulmonary venous return with an intact septum.
• Scimitar syndrome and unroofed coronary sinus.
• Right ventricular volume overload from tricuspid or pulmonary regurgitation, and arrhythmogenic cardiomyopathy.

Indications for closure. Right ventricular volume overload with a pulmonary to systemic flow ratio of 1.5 or more, with or without symptoms, provided pulmonary vascular resistance is under 3 Wood units. Resistance of 3 to 5 Wood units needs individual decision in a specialist centre, usually after treatment of pulmonary vascular disease and repeat catheterisation. Do not close when resistance is over 5 Wood units without a net left-to-right shunt, or with Eisenmenger physiology.

Procedures.

• Surgery is the usual treatment. A single patch baffle redirects the anomalous veins through the defect into the left atrium when the veins enter low. The Warden procedure (division of the superior vena cava cranial to the highest anomalous vein, anastomosis of the upper end to the right atrial appendage, and a baffle that directs the veins to the left atrium) is used when veins enter high. Operative mortality is under 1% in experienced centres.
• Complications are superior vena cava or pulmonary vein obstruction (up to 5%), sinus node dysfunction and atrial arrhythmia (about 10%), and residual shunt.
• Transcatheter covered stent repair of the superior vena cava, which diverts the veins into the left atrium, is performed in selected adults and requires pre-procedure CT and balloon testing to exclude coronary or pulmonary artery compression.

Medication.

• Aspirin 75 to 100 mg orally once daily for 3 to 6 months after patch repair or covered stent placement (children 3 to 5 mg/kg daily); stop if bleeding or intolerance; no routine renal adjustment.
• Furosemide 20 to 40 mg orally once or twice daily for right heart failure (titrate to weight; electrolytes and creatinine within 1 week of changes).
• Atrial fibrillation or flutter: anticoagulate at a low threshold because paradoxical embolism risk is high. Apixaban 5 mg orally twice daily (2.5 mg twice daily if two of age 80 or over, weight 60 kg or under, creatinine 133 micromol/L or over); creatinine and haemoglobin at least yearly. Rate control with metoprolol succinate 25 to 200 mg once daily (maximum 400 mg daily; reduce in severe hepatic impairment); heart rate and blood pressure at each visit. Catheter ablation of flutter is effective, and is best done at or before surgery.
• Endocarditis prophylaxis is not needed for the unrepaired defect. For 6 months after repair with prosthetic material, and lifelong after previous endocarditis, give amoxicillin 2 g orally 30 to 60 minutes before dental procedures involving gingival manipulation (azithromycin 500 mg or doxycycline 100 mg with penicillin allergy).
• Pulmonary arterial hypertension (when present): specialist treatment with ambrisentan 5 to 10 mg once daily or tadalafil 40 mg once daily, with repeat catheterisation before any decision to close. Both drugs are contraindicated or avoided in pregnancy, and tadalafil is contraindicated with nitrates.

Monitoring. Clinical review, ECG and echocardiography at 1, 6 and 12 months after repair, then every 1 to 2 years. Surveillance imaging of the superior vena cava and pulmonary veins at 6 to 12 months (CT or MRI). Ambulatory ECG yearly.

Referral and escalation. Refer every suspected case to an adult congenital heart disease centre. Urgent referral for stroke or transient ischaemic attack, right heart failure, sustained arrhythmia or suspected pulmonary hypertension.

Special groups.

• Pregnancy: tolerated when there is no pulmonary hypertension, with risk of paradoxical embolism and arrhythmia. Avoid air in intravenous lines, and give enoxaparin 40 mg subcutaneously once daily during immobility and after delivery (20 mg daily when creatinine clearance is under 30 mL/min).
• Children: repair is rarely needed before school age unless there is heart failure.
• Older adults: surgery remains beneficial, but residual atrial fibrillation is common.
• Region specific: late presentation with pulmonary hypertension is frequent where echocardiography is limited, so reserve closure for patients with confirmed resistance criteria.