Ultrasonography of portosystemic shunting in dogs : Doppler studies before, during and after surgery

Abstract

Portosystemic shunting occurs when anomalous veins allow the portal blood to enter the systemic veins directly without first flowing through the hepatic sinusoids. Portosystemic shunting can occur via acquired portosystemic collaterals or via congenital portosystemic shunts and may result in clinical signs of hepatic encephalopathy due to high blood ammonia levels.

It is necessary to differentiate congenital portosystemic shunts non-invasively from other conditions that cause hyperammonemia, since congenital portosystemic shunts require surgical treatment while the other conditions do not. Portography, ultrasonography, scintigraphy, CT and MRI have been used to diagnose portal vein disorders. Although Doppler ultrasonography is the only technique that does not require the use of anesthesia, and can provide detailed anatomic and hemodynamic information about the abdominal vasculature, its accuracy in detecting congenital portosystemic shunts was reported to be insufficient. In the present studies, gray scale ultrasonography with color Doppler mode appeared to be an accurate method to: diagnose a congenital portosystemic shunt, specify its type, distinguish it from acquired portosystemic collaterals and rule it out.

The definitive therapy for congenital portosystemic shunts would ideally be complete occlusion of the shunt. However, in most dogs, only partial shunt ligation can be performed because attenuating the shunt vessel results in post-ligation portal hypertension. Acute portal hypertension can usually be successfully avoided, though development of chronic portal hypertension remains a frequent complication. Regardless of the technique used for shunt attenuation and for assessing post-ligation portal hypertension, the clinical outcome remains unpredictable. This is largely because there is no method currently available to determine the optimal degree of shunt narrowing. As a consequence of an exaggerated shunt closure, dogs may develop postoperative complications related to acute or chronic portal hypertension. To help avoid this problem, intraoperative ultrasonography greatly increases the likelihood of determining the optimal shunt diameter. Congenital extrahepatic portosystemic shunts do not have to, and actually should not, be completely occluded. Once the flow becomes hepatopetal in the entire portal vein and in the shunt adjacent to the portal vein, further narrowing of the shunt is unnecessary.

Complete occlusion of a congenital extrahepatic portosystemic shunt has been suggested to result in a better clinical outcome compared to partial ligation. Several surgeons propose that a second surgery should be performed in every dog whose shunt has partially been attenuated in order to occlude their shunt completely. We have shown that dogs that underwent a partial ligation of a congenital portosystemic shunt do not have to be routinely re-operated on. Color Doppler ultrasonography performed at least one month after surgical ligation of a shunt can reveal whether a persisting hyperammonemia is the result is of a functional shunt, acquired portosystemic collaterals, or both. Dogs that have developed acquired portosystemic collaterals and dogs, whose extrahepatic portosystemic shunt was partially ligated and the flow direction in the entire portal vein and in the shunt adjacent to the portal vein became hepatopetal should not be re-operated on.