Portosystemic Shunts: Page 2 of 3

Diagnosis

Diagnosing portosystemic shunts can sometimes be challenging, but your vet will be equipped with a direct plan to get to the bottom of your pet’s problems in the most logical way possible. A bile acid blood test is usually the first step your vet will use to determine liver dysfunction. This test evaluates the capacity of the liver to function properly by indirectly evaluating portal blood flow. Most dogs or cats with liver shunts have an extreme elevation in these test values, indicating that the liver is not functioning as it should be. This test, however, doesn’t completely rule out portosystemic shunts because levels may vary from only slightly elevated to normal.

A second test, called the protein C assay, may be recommended by your veterinarian to differentiate a shunt from microvascular dysplasia if the bile acid blood test comes back with elevated results. If the protein C assay suggests the presence of a shunt, then surgery will likely be recommended. If the protein C assay is normal in a patient with markedly elevated bile acids, then microvascular dysplasia is most likely present and surgery likely will not be recommended.

Abdominal ultrasound will often be advised, if for no other reason than to evaluate the urinary bladder for the presence of stones. Dogs and cats with shunts may develop stones that may be missed on routine x-rays. If stones are present and surgery for the shunt is indicated, the bladder stones will be removed at the same time.

Further diagnostic tests include portal scintigraphy, or liver scan, and venous portogram, which is a specialized x-ray study. Scintigraphy has the advantage of being non-invasive and not requiring anesthesia. The disadvantage is that it does not give a precise location of the shunt. If a shunt is suspected based on the results of the scintigraphy, but cannot be found at the time of surgery, a venous portogram is needed. The venous portogram requires anesthesia and open abdominal surgery to place a catheter in a portal vein. A contrast material (x-ray dye) is injected and radiographs taken to trace the course of the dye.

A shunt may be associated with a rectal vein instead of the portal vein, and thus not show on a portogram or be found at surgery.

Treatment

There are both medical and surgical options when treating patients with liver shunts.

If you choose surgery for your pet, medical therapy may help to stabilize your pet prior to surgery. Medical therapy can also be used for long-term management, where surgery is not performed, or if surgery does not completely resolve the shunting vessel. Many patients can be managed on appropriate medical therapy with a good quality of life. Medical therapy is tailored based on the individual patient's condition and response to treatment. Common therapies to treat portosystemic shunts include diet and antibiotic administration.

A diet low in animal-based protein will reduce the nitrogen content of the diet, easing the demands placed on your pet’s liver, as nitrogen is one of the toxins that the liver breaks down. Furthermore, you can supplement your pet’s diet with lactulose (a sugar) to reduce the absorption of compounds rich in nitrogen and cause them to be lost in the stool. The dose is adjusted to produce soft but formed stools.

Antibiotic therapy reduces the number of certain bacteria in the gut and treats any bacteria that are absorbed into the blood stream. Debilitated dogs and cats may also require fluid therapy, enemas, and other treatments prior to more definitive treatment.

Appropriate treatment, either medial or surgical, is determined on a case-by-case basis. Medical management is often helpful in controlling your pet’s symptoms, however surgical correction is often a more definitive treatment.

The purpose of surgery for a portosystemic shunt is to correct the vascular malformation. This means closing down the shunting vessel as far as possible to stop the bypass of blood around the liver. Surgical correction can be accomplished using ligatures (suture), constriction devices (Ameroid constrictor), or cellophane banding. The surgeon will choose the appropriate surgical procedure to perform on your pet depending on the type of abnormal vessel(s). Regardless, the surgeon will explore your pet’s abdomen to find and identify the shunt(s).

Surgery is required to treat single portosystemic shunts. Single extrahepatic shunts are treated by either tying a piece of suture material around the vessel (suture ligation) or by placing a device or piece of sterile cellophane that slowly closes the vessel off.

Suture ligation is the traditional method and is still performed by many surgeons today. A suture is placed around the shunting vessel and temporarily tightened to look for signs of portal hypertension, which is caused by an unacceptable increase in the pressure in the veins draining the portal system. Unacceptable rises in portal blood pressure can result in life-threatening complications after surgery. If no signs of portal hypertension are present, the shunt can be completely ligated. Patients with total ligation seem to do better long term, however, total ligation is not always possible.

If signs of portal hypertension are observed, the shunt is partially ligated (closed-off) so that the flow of blood through the shunt is reduced but some flow remains to prevent portal hypertension. There is controversy over how to proceed if the surgeon feels that complete ligation is risky. Some surgeons re-operate one month later to ligate the shunt further, while others choose to wait to see if scar tissue will form and further constrict the shunt with time. Scintigraphy or a portogram can be repeated after surgery, and if shunting is still present, most veterinarians choose to perform further ligation of the troublesome vein.

Another option for the surgeon is an ameroid constrictor. This is a stainless steel ring lined with a material that absorbs fluid, swells and thus slowly compresses and constricts the vessel. The total constriction (ligation) of the shunt takes 30 to 40 days and the patient may need to be treated medically during this time. Ameroid constrictors are designed to reduce the chances of severe portal hypertension, because they allow the liver to slowly adapt to the increased blood flow.