Left-sided superior vena cava
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A left-sided superior vena cava (SVC) is the most common congenital venous anomaly in the chest, and in a minority of cases can result in a right-to-left shunt 3,4.
A left-sided SVC is seen in 0.3-0.5% of the normal population and in ~5% of those with congenital heart disease 3. It is only seen in isolation in 10% of cases since the vast majority are accompanied by a normal right-sided SVC, termed SVC duplication.
The vast majority of cases are asymptomatic and the presence of the vessel is only identified incidentally during CT scanning of the chest, or as a result of line placement. In those patients who have a right-to-left shunt as a result of drainage directly into the left atrium (8%), the shunt is usually not large enough to cause cyanosis since it only drains the left upper limb and left side of the head and neck.
A number of associations are recognized, which may result in investigation and identification of the abnormal vessel. They include:
- congenital heart defects are present in 4.4% of patients with left-sided SVC 3
A left-sided SVC forms when the left anterior cardinal vein is not obliterated during normal fetal development. The persistent left-sided SVC passes anterior to the left hilum and lateral to the aortic arch before rejoining the circulatory system. There are a number of possible drainage sites:
- coronary sinus (92%) functionally insignificant since venous return from the head, neck and upper limbs is delivered to the right atrium 3
- left atrium (8%) results in a right-to-left shunt, which is usually not large enough to cause cyanosis or symptoms 3
In the vast majority of cases (82-90%) a normal (but small) right-sided SVC is also present, and a persistent bridging vein (left brachiocephalic vein) is seen in 25-35% of cases 3.
Direct visualization of a left-sided SVC is not possible however its presence can be implied if a catheter or line is in an unexpected left paramediastinal location.
CT, especially with contrast, is able to elegantly demonstrate the anomalous vessel coursing inferiorly to the left of the arch of the aorta and anterior to the left hilum. It is in direct continuation of the confluence of the left internal jugular vein and the left subclavian vein. Communication between the normal right SVC (which is present in 82-90% of cases) may also be seen.
Depending on the timing of the scan and the side of the injection variable amounts of contrast may be seen within the vessel.
CT is also able, especially with the benefit of reformats, to delineate the site of drainage (usually coronary sinus).
The diagnosis may be suspected in cases of left arm injection for a VQ scan, in patients with drainage into the left atrium. In such cases essentially all the radiotracer will appear in the systemic circulation rather than the normal 2% which cross the lung due to intrapulmonary shunts 3.
Transthoracic echocardiography may be enhanced by the use of agitated saline if there is clinical suspicion of a left-sided SVC. Placement of a peripheral intravenous cannula in the left arm, agitation of saline, and subsequent injection should coincide with visualization of the right ventricular outflow tract and coronary sinus in the parasternal long-axis view; the latter (coronary sinus) will fill before the former (right ventricular outflow tract) in the presence of a left-sided SVC 6.
Treatment and prognosis
Except in cases where a large right to left shunt is present, a left-sided SVC has essentially no physiologic impact and is entirely asymptomatic.
Its importance stems from venous procedures, such as line placement or pacemaker implantation where failure to recognize this variant can result in incorrect positioning 4.
When able to be traced on CT there is no real differential. Some if thin slice cuts are not available an anomalous left brachiocephalic vein (ALBCV) may mimic appearances.
An abnormal catheter position on chest x-ray, which runs to the left of the mediastinum has a limited differential (see: differential of left paramediastinal catheter position) 5.
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