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Types of Shunts:
Shunts may be congenital (found at birth) or acquired (developing after birth). Dogs with acquired shunts usually have cirrhosis, or "hardening" of the liver, secondary to severe liver disease. These dogs develop multiple shunting blood vessels to relieve high blood pressure in the liver. There is no effective surgical treatment for these patients, short of a liver transplantation.
Congenital shunts are usually single blood vessels that are present at birth. In large breed dogs, they are found inside of the liver ("intrahepatic") and may be a result of improper or incomplete closure of the fetal shunt. Surgical treatment of these shunts is possible, but difficult, because of the location of the abnormal blood vessel.
Small and toy breed dogs usually have "extrahepatic" shunts: the blood vessel is located outside of the liver. These shunts are easier to find and treat and therefore the outcome of surgery is better.
Clinical Signs of Portosystemic Shunts
Clinical signs are often seen at a young age and may include poor growth, behavioral changes (circling, disorientation, unresponsiveness, staring into space, head pressing, blind staggers), seizures, and quiet demeanor. Many of the clinical signs may be confused with puppy hypoglycemia (low blood sugar). Other less common signs include diarrhea, vomiting, and excessive drinking or urinating.
In many animals the signs are seen 1-3 hours after eating meat or puppy chow. Proteins in the food are broken down by intestinal bacteria to ammonia and other toxins which are absorbed and, instead of being filtered by the liver, are allowed to reach the brain. The depression and signs are often temporary; once the proteins are emptied from the colon, the signs usually abate. Some animals may not show clinical signs until they are anesthetized to be castrated or spayed. These animals may take days to recover from anesthesia, depending on what drugs were used. Other animals show no signs until they are older, when they develop bladder and kidney problems from excreting toxins and forming urine crystals and stones.
Diagnosis
To diagnose a shunt we may need to rule out toxicity, hydrocephalus ("water on the brain"), and low blood sugar in puppies. We look for abnormalities on bloodwork that indicate poor liver function, such as low protein, albumin, and blood urea nitrogen, which are chemicals produced by the liver. X-rays of the abdomen may show a small liver, indicating that it is has not developed properly. Urine sediment may contain ammonium biurate crystals, which look like starfish or spiky balls.
We also run special diagnostic tests. Blood ammonia concentration can be measured; this test will diagnose liver disease in 90% of affected animals. It is more accurate (95 to 100%) if an ammonia challenge is done, where the puppy is given ammonia by enema or by mouth and the blood is tested to see if the liver clears the ammonia. Blood for ammonia concentration measurements must be kept chilled and must be analyzed within 30 minutes after it was drawn.
Even more accurate are bile acid concentrations. A blood sample is taken after a 12 hour fast, and then the puppy is fed a normal meal. Two hours later another blood sample is taken. Bile acid concentrations are high in most types of liver disease, including shunts. Bile acid concentrations are altered by hemolysis (breakage of the blood cells) and lipemia (fat in the blood) but are minimally affected by temperature and storage and can be sent through the mail. These tests tell us that liver disease is present but do not verify the presence of a shunt. Bile acids levels that are normal after feeding in a 6 or 8 week old puppy indicate the puppy probably does not have a shunt, as long as there is no hemolysis of the blood sample; therefore this test can be a good screening tool for breeders.
To be 100% sure that a shunt is present, we either need to use ultrasound (which is more useful in large dogs), a contrast study with x-rays ("portogram"), nuclear scans ("scintigraphy"), or surgery to find the shunt. Scintigraphy and ultrasound are not invasive and are therefore the safest tests, with scintigraphy being the most accurate of the two.
Portosystemic shunts must be differentiated from hepatic microvascular dysplasia, a disease that has the same clinical signs, liver biopsy results, and blood changes. This disease is seen in Yorkies, Cairn terriers, and other small breeds and must be ruled out by portogram or scintigraphy.
Medical Treatment
Dogs with shunts that are seizuring or comatose should be given intravenous fluids with dextrose (sugar), intravenous antibiotics, and warm water enemas. Lactulose syrup can be instilled in the rectum after the enema to decrease toxin absorption. Some dogs may even need activated charcoal (given by tube through the mouth into the stomach) to absorb toxins if they are comatose.
Long-term medical treatment for dogs with shunts includes a low protein diet such as Hill's L/D, and drugs such as lactulose and neomycin or metronidazole to prevent production and absorption of ammonia and other intestinal toxins. Lactulose, a sugar solution, is extremely safe, although a high dose will cause diarrhea. Some animals do well on medical treatment alone, but most have shortened life spans because of progressive liver shrinkage and loss of function. Medical treatment is normally used to stabilize a patient until it is healthy enough to tolerate the surgery.