Sinus Venosus Atrial Septal Defect - Causes, Symptoms, Diagnosis, and Treatment

What Is Sinus Venosus Atrial Septal Defect (SVASD)?

Also known as sinus venosus syndrome, it is an anomalous venous connection between the two atria (intraarterial); this abnormal connection typically happens in the unfolding wall that normally separates the left atrium from the superior vena cava- a large valveless vein that carries deoxygenated from the upper half of the body into the right atrium, or the inferior vena cava- second largest vein that carries deoxygenated blood from the middle and lower part of the body.

There are two types of SVASDs-

1. Superior SVASD- This type of defect occurs when the opening of the superior vena cava overrides the atrial septum and drains deoxygenated blood into the right and left atria.

2. Inferior SVASD- It occurs when the opening of the inferior vena cava overrides the atrial septum and drains into both atria.

Both SVASDs result in the shunting of blood from the left to the right atrium due to high atrial pressure in the former. The affected individual might not have any symptoms during childhood but may become symptomatic during their third decade of life with symptoms like dyspnea (shortness of breath), arrhythmias (abnormal heartbeats), etc.

Diagnosis is difficult due to the anatomical complexity but can be done with the help of transthoracic echocardiography (TTE) and complimentary imaging studies; surgical management is the only treatment option.

What Causes SVASD?

• A typical heart has four chambers- two upper chambers (right and left atrium) separated by the atrial septum and two lower chambers (right and left ventricles) separated by the ventricular septum.

• In adults, the deoxygenated blood (or venous blood) is collected from the entire body (by the superior and inferior vena cava) and drained into the right atrium, which is sent to the lungs for oxygenation through the pulmonary arteries by the right ventricle.

• The oxygenated blood is then pumped back into the left atrium through pulmonary veins, which are then transported to the left ventricle. The left ventricle then distributes the oxygenated blood throughout the entire body with the help of the aorta (the largest artery in the entire body).

• The above mechanism is not possible during fetal life because an unborn baby’s lungs are nonfunctional; they get their oxygen supply from the placenta. The oxygen-rich blood from the placenta enters the fetus through the umbilical cord, which then passes through the fetal liver and reaches the right atrium of the heart.

• The right atrium transfers the oxygen-rich blood to the left atrium, bypassing the right ventricle and the left ventricle through a hole in the atrial septum, known as the foramen ovale.

• When the child is born and begins to breathe, the foramen ovale closes, leaving a depression of various shapes known as fossa ovalis.

• In a few children, the foramen ovale remains patent even after birth leading to atrial septal defects (ASD). There are different types of ASD, and SVASD is one such defect.

• In SVASD, there is anatomical communication between the right and left atrium, but it is outside the confines of the fossa ovalis.

• The exact cause of SVASD is unknown; however, absorption of the sinus venosus (a cardiac chamber upstream of the right atrium that harbors the dominant pacemaker, is known to cause SVASD).

What Are the Symptoms Of SVASD?

In SVASD, blood flows from the left atrium to the right atrium, which then transfers the excess blood into the right ventricle. This leads to overworking of the right ventricle and, by extension, causes increased pulmonary hypertension. As a result, the affected individual will have the following symptoms-

1. Dyspnea.

2. Easy fatiguability.

3. Arrhythmias.

4. Low exercise tolerance.

How Is SVASD Diagnosed?

SVASD has non-specific symptoms, and typical imaging techniques like TTE cannot detect them because of their posterior location. Diagnosis can be made with the help of physical examination and special cardiac imaging.

A physical examination of most ASD patients will reveal a soft systolic ejection murmur over the upper left sternal border. Patients with SVASD will also have a diastolic flow murmur along the systolic one.

The special imaging that will be needed for SVASD patients are-

1. Echocardiography- It helps to identify the left to right shunt and the extent of the right ventricular overload. It also reveals a bald posterior wall suggestive of SVASD suggestive of SVASD.

2. Cardiac MRI (magnetic resonance imaging)- Helps to observe right ventricular hypertrophy along with delineation of individual pulmonary vein connection.

3. Electrocardiography- It can demonstrate right atrial and ventricular hypertrophy along with a displaced sinus node ( an area of specialized cells in the right upper heart chamber that control the heart's rhythm).

How Is SVASD Treated?

• Patients with asymptomatic SVASD do not require any medical care; however, if the patient presents with heart failure, they should be attended by professionals experienced in such defects.

• Surgical repair of the SVASD required cardiopulmonary bypass and aortic cross-clamping. The right atrium is opened, and the defect is identified; a synthetic patch or pericardium (a fluid-filled layer that surrounds the heart) is used to close the defect, followed by redirecting the abnormal pulmonary veins.

• The patch acts as a closure for the ASD and as a baffle to redirect the anomalous pulmonary venous drainage into the left atrium. The postoperative course following the closure of the SVASD is usually benign. Hemodynamic management might need vasoconstrictive infusions, but they are rarely needed. Extubation is accomplished early in the postoperative care, and the patient can be discharged in 4 to 6 days following repair.

• Surgery is contraindicated in patients with severe irreversible pulmonary hypertension and bidirectional shunt. Surgical complications include postoperative narrowing of the superior vena cava, superior vena cava syndrome, and sinus node dysfunction.

• Medical management can be done to manage the associated complications of SVASD, but it is ineffective as a treatment option for SVASD. Some of the commonly used medications are inotropic agents (like Digoxin and Dopamine) and loop diuretics (like Furosemide).

Conclusion:

SVASD is a congenital cardiac anomaly that is caused due to structural abnormalities of the atrial septum. It is asymptomatic till the third or fourth decade of life, which can then spontaneously dissolve or become symptomatic leading to arrhythmias, dyspnea, and syncope (rarely). Diagnosis is made with the help of specialized imaging techniques, which are also used to plan the type of surgical procedure. The prognosis is good in patients with early intervention with very few long-term complications.