Friday, October 12, 2012

The Retroportal Artery

In a flow study before a hepatic embolization procedure, identifying collaterals that could take the drug-eluting beads or radiospheres off target is the highest priority.  Some vessels such as the gastroduodenal artery are usually fairly straightforward to find.  Others, such as the cystic artery or right gastric artery (see the post on 10/9/2012) are intermittently seen... and then there is the possibility of a number of even smaller arteries to form uncommon off-target collateral flow.

One artery that fits this description is the retroportal artery.  This "small but constant branch of the SMA" runs dorsal to the portal vein (retroportal) and pancreatic head. It anastomoses with the retroduodenal artery and supplies branches to the common bile duct... and it also anastomoses with the right hepatic artery. Its course has been described as similar to that of a replaced right hepatic artery, but it clearly anasomoses with the right hepatic artery rather than supplying a liver territory.

The first description of the retroportal artery was a paper by Northover and Terblanche in 1978, which claimed that in nearly 50% of specimens studies (9/20), the origin of the retroportal artery was the SMA, and that the others mostly arose from the celiac artery (9/20).

They further divided the retroportal artery into two types based on resin cast models -- a type I and type II.

Type I was considered a retroportal artery that quickly anastomosed with the retroduodenal artery at the lower part of the CBD... this kind is unlikely to be a hemodynamically signifcant factor in hepatic flow.

In type II, the retroportal artery passed upwards behind the CBD and anastomosed with the right hepatic artery... the configuration discussed earlier.

In the schematic on the left, the portal vein and pancreas are viewed from behind.  The retroportal artery connects with the retroduodenal artery early, a Type 1, as discussed above.

The retroportal artery not often discussed, except as a player in flow dynamics between the hepatic and gastroduodenal artery (and potentially between the celiac and SMA systems), when it can become signficantly important. With occlusion of the GDA, flow through the retroportal can be reversed back into the mesenteric system.

Hepatofugal flow is an obvious problem for delivery of radiospheres. In the example below, even though the GDA was occluded, the retroportal artery could not be, and the planned radioembolization therapy had to be abandoned.

In this flow study for a radioembolization procedure, the right hepatic artery (green arrow), gastroduodenal artery (light blue arrow), and gastroepiploic artery (dark blue arrow) are identified.

After coiling the GDA, a small retroportal artery appears (the left hepatic is also more visible).  The origin of the retroportal artery is not entirely clear... it arises at the region of the bifucation of the anterior and posterior segments of the right hepatic lobe. It then appears to extend to the gastroduodenal artery.

In a recent case report, it was suggested that temporary occlusion of the common hepatic artery with a balloon could be useful in situations when there was hemodynamically significant hepatofugal flow through a retroportal artery, and the retroportal artery could not be coiled.  If there is robust flow through the GDA, the retroportal flow direction would be reversed as long as the balloon was in place.

Another more subtle problem with the retroportal artery (or any collateral flow to the hepatic artery) is the potential for dilutional effects.  In one case series, the authors discussed suboptimal right hepatic infusion chemotherapy due through what they felt was dilution from unexpected hemodynamically-significant inflow from a retroportal artery. In their situation, the retroportal artery was embolized, and normal concentrations of the chemotherapy resumed.

1. "Gastrointenstial Angiography" Reuter SR, Redman HC, Cho KJ. 3rd ed. (1986).
2. Northover JM, Terblanche J. "A new look at the arterial supply of the bile duct in man and its surgical implications." Br. J. Surg. Vol. 66 (1979) 379-384
3. Mahvash A, Zaer N, Shaw C, et al. "Temporary Balloon Occlusion of the Common Hepatic Artery for Administration of Y90 Resin Microspheres in a Patient with Patent Hepatoenteric Collaterals" Journal of Vascular and Interventional Radiology 2012;23(2):277-280.
4. Yamagami T, Kato T, Tanaka O, et al. "Influence of hepatopetal flow of the retroportal artery on efficiency of repeated hepatic arterial infusion chemotherapy."  JVIR. (Oct 2005) 16:10  pp 1391-1395