Blood finds a way: pictorial review of thoracic collateral vessels
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In the healthy patient, blood returns to the heart via classic venous pathways. Obstruction of any one of these pathways will result in blood flow finding new collateral pathways to return to the heart. Although significant anatomic variation exists and multiple collateral vessels are often present in the same patient, it is a general rule that the collateral pathways formed are a function of the site of venous blockage. Therefore, knowledge of typical collateral vessel systems can provide insight in localizing venous obstruction and characterizing its severity and chronicity. In addition, knowledge of collateral anatomy can be essential in interventional procedural and/or surgical planning, especially when placing catheters in patients with venous blockage. In this pictorial review, we provide a systematic approach to understanding collateral pathways in patients with venous obstruction in the upper body.
KeywordsVeins Collaterals Superior vena cava Superior vena cava syndrome Thrombosis Obstruction
Inferior vena cava
Superior vena cava
Venous obstruction occurs secondary to mass effect, stenosis, and/or thrombosis.
No matter the site of obstruction, blood always finds a way back to the heart via collaterals.
The pattern of collateral vessels that develop is a function of the site of obstruction.
Axillary and subclavian venous obstruction form collaterals to the ipsilateral shoulder and neck.
SVC collaterals form as a function of the obstruction’s position relative to the azygos vein.
The azygos vein is an otherwise unassuming vein which drains the intercostal system into the lower SVC. However, the importance of the azygos vein in relation to the site of obstruction in these thoracic collateral systems cannot be overstressed. Conceptually, it is important to note that the azygos system can potentially connect to the entire venous supply of the body. Blockages in one part of the thorax tend to divert at least some of the flow into the azygos system where it is subsequently rerouted to bypass the blockage. Any blockage above the azygos vein may be rerouted into the SVC through the azygos system. However, when the SVC is blocked at the level of the azygos, blood may only enter the heart through the inferior vena cava (IVC).
Overview of collateral imaging
Characterization of collateral vessels is not a common primary indication for imaging, and collaterals are often first identified incidentally or after a complication has occurred. CT angiography is the most commonly used imaging modality to evaluate collateral pathways. A venous phase of imaging can be obtained by performing the diagnostic portion of the scan at 60–90 s post a standard weight-based dose of intravascular contrast administration. Benefits of this imaging modality include its superior spatial resolution with the ability to view images using multiplanar reformats and its ability to image during different phases of contrast enhancement (venous versus arterial), especially as collateral vessels can be difficult to appreciate in the absence of contrast. It is important to note the side of contrast injection may affect the ability to visualize the obstruction, as the collateral flow may only be visualized if injected on the ipsilateral side. Use of diluted contrast during injection can reduce streak artifact during initial phase imaging which may otherwise limit visualization of an area of obstruction or collateral pathways. Drawbacks to this imaging modality include its radiation exposure, its need for intravenous contrast, and its limited enhancement of the veins.
Digital subtraction angiography in interventional radiology remains the gold standard for characterization of collateral pathways with venograms indicated to confirm suspected venous blockages and provide real-time information on collateral filling. The main drawback to this modality is that it does not provide a detailed look at what is happening extravascularly, leading to an incomplete picture in the evaluation of the blockage.
Finally, venous Doppler ultrasonography is an inexpensive readily available technique that is well tolerated by patients and has no radiation or contrast exposure which can assist in evaluation of venous pathways. Venous ultrasonography can be limited by unreachable anatomy, poor acoustic windows, patient body habitus, and skill of the sonographer [6, 7].
Axillary and subclavian obstruction
In the past, axillary and subclavian obstructions were relatively rare. However, the incidence of such obstructions has increased proportionately with the increasing frequency of indwelling subclavian catheter use . Axillary and subclavian obstruction results in collaterals to the shoulder and neck.
Superior vena cava obstruction
SVC obstruction presents a substantial physiological challenge as it involves the final pathway for venous return of the upper body. Three patterns of collateral vessel formation are observed in SVC obstruction, all resulting from the location of the blockage relative to the azygos vein [9, 10]. As detailed in the anatomy section, the azygos vein delivers venous return to the inferior SVC and is the only major vein to feed into the SVC apart from the right and left brachiocephalic veins. As the azygos system functionally connects in some way to the entire venous system, its importance in bypassing SVC obstruction is not surprising.
Blockages above the azygos
Blockages at the level of the azygos
Blockages below the azygos
Collateral placement of lines
Venous obstruction may result from mass effect, stenosis, or thrombosis. No matter the cause, blood always finds a way around blockages to return to the heart. The path blood takes around blockages is predictable depending on the location and size of the obstruction. Thus, by studying collateral vessels, we can gain insight into an obstruction’s location and severity. Axillary and subclavian obstructions form shoulder and neck collaterals that can bypass the blockage. Collaterals secondary to SVC obstruction form in a pattern dependent on the location of the obstruction relative to the azygos vein. It is the role of the radiologist to apply knowledge of these collateral pathways to assist in diagnosis and intervention in patients with venous obstruction.
We would like to thank Nadezhda Kiriyak, Sarah Klingenberger, and Gwen Mack for their assistance with the illustrations and figures.
All authors contributed to the collection of the cases and the development/content of the manuscript. All authors read and approved the final manuscript.
The authors state that this work has not received any funding.
Ethics approval and consent to participate
Consent for publication
The authors declare that they have no competing interests.
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