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Old 04-25-2015, 03:43 AM   #1
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Default Liver Shunt - Common Questions/Answers by Karen Tobias, DVM

Liver Shunts: Common Questions and Answers
Dr. Karen Tobias, DVM, MS
University of Tennessee College of Veterinary Medicine

What is a liver shunt?
A liver shunt is a blood vessel that carries blood around the liver instead of through it. In some animals a liver shunt is a birth defect (“congenital portosystemic shunt). In others, multiple small shunts (“acquired portosystemic shunts”) form because of severe liver disease such as cirrhosis.

Why do congenital shunts develop?
All mammalian fetuses have a large shunt (“ductus venosus”) that carries blood quickly through the fetal liver to the heart. Since the mother's liver does the work of filtering out toxins, storing sugar, and producing protein for her unborn babies, liver function is not needed in the fetus. This ductus venosus is supposed to close down shortly before or after birth as the baby's liver begins to work. In some individuals the shunt doesn't close down; it is then called a “Patent Ductus Venosus”, or an intrahepatic shunt. In other animals, a blood vessel outside of the liver develops abnormally and remains open after the ductus venosus closes. This is called a congenital extrahepatic shunt.

Why do animals with shunts have problems?
In the normal animal, food and other ingested materials are broken down or digested in the intestines and absorbed into the portal blood stream, where they are carried to the liver. The liver stores some of the food for energy, processes some of it into safe chemicals, and uses some of it to make proteins and other substances. Because the blood bypasses the liver in dogs with shunts, toxins may build up in the bloodstream or kidneys. Additionally, the animal lacks the necessary materials to give it a ready source of energy and to help it grow.

What are the clinical signs of a liver shunt?
Clinical signs are often seen at a young age and include small stature, poor muscle development, behavioral abnormalities (circling, disorientation, unresponsiveness, staring into space, head pressing), seizures, and quiet demeanor. Other less common signs include drinking or urinating too much, apparent blindness, diarrhea, and vomiting. In some animals the signs are associated with eating protein. Other animals are diagnosed when they take a long time recovering from anesthestics (i.e. barbiturates) or sedatives (i.e. acepromazine). Some animals show no signs until they are older, when they develop bladder and kidney infections and stones.

What breeds are commonly affected with shunts?
Small breed dogs tend to have shunts that form outside of the liver (“extrahepatic”). In the United States, Yorkshire terriers have almost a 36 times greater risk of developing shunts than all other breeds combined. Extrahepatic shunts can be seen in any small breed but are also reported commonly in schnauzers, Maltese, dachshunds, Jack Russell terriers, Shih Tzu, Lhasa apso, Cairn terriers, and poodles. Large breed dogs tend to retain the fetal liver shunt (patent ductus venosus), or “intrahepatic” shunts. In the Netherlands, about 2% of Irish Wolfhounds are born with intrahepatic shunts. Intrahepatic shunts can be seen in any large breed dog and have been reported in some small breed dogs (especially poodles); in the United States, we see them most often in Labrador retrievers. Australian shepherds, Australian cattle dogs, Samoyeds, and Old English sheepdogs are also commonly reported.

Are shunts hereditary?
A disease is likely to be hereditary if it occurs more commonly in one breed than others; if it occurs in a family of dogs; or if it or a closely related disease is proven hereditary in other breeds or species. Liver shunts are considered hereditary in Irish wolfhounds, Cocker spaniels, Maltese, and Yorkshire terriers, and are probably hereditary in several other breeds. The affected dog should be castrated or spayed and, because the mode of inheritance is not known, it is best to avoid breeding the parents.

How is a shunt diagnosed?
On blood work, dogs with congenital liver shunts usually have low blood urea nitrogen (BUN) and albumin concentrations. They may be slightly anemic or have red blood cells that are smaller than normal(“microcytosis”). They also may have increases in liver enzymes (“AST”, “ALT”). Their urine may be dilute or infected and contain small spiky crystals (“ammonium biurate”). None of these laboratory changes are specific for a liver shunt; however, when veterinarians see these abnormalities, they will usually measure bile acid or ammonia concentrations to evaluate liver function. A liver shunt cannot be definitively diagnosed by blood work; shunting can only be found with advanced techniques such as scintigraphy, ultrasound, portography, Cat scan (“CT”), MRI, or exploratory surgery.

What are bile acids?
Bile acids are produced in the liver and stored in the gallbladder between meals. They are released into the intestines to help break down and absorb fats, and are reabsorbed and stored again until they are needed. Dogs with liver shunts have increased blood bile acid concentrations because the liver does not get a chance to remove and store these chemicals after they are reabsorbed.

Do all dogs with shunts have high bile acids?
Dog with shunts will almost always have high bile acids 2 hours after eating, and usually at least 95% of dogs will have high bile acids after a 12 hour fast. Samples are taken at both time periods (“fasting” or“preprandial”, and “fed” or “postprandial”) for several reasons. Some dogs normally release bile acids in the middle of the night and therefore naturally have a higher than normal fasting sample. Other dogs may have fat in their blood (“lipemia”) after eating, which can interfere with the test. If only one blood sample can be obtained, it is best to take it 2 hours after eating.

Do all dogs with high bile acids have shunts?
Bile acids can be increased with any liver disease. Bile acids can also be mildly increased in normal dogs, particularly in some breeds (such as Maltese) where chemicals in their blood interfere with the test. Most dogs with liver shunts have fed bile acids over 100 (normal <15-20). If the bile acids are only mildly increased or the animal seems normal, many veterinarians will simply rerun the test in 3-4 weeks.

What is scintigraphy?
Scintigraphy is a nuclear scan that measures blood flow. To evaluate a dog for a shunt, a radioactive material in inserted into the colon (by a high enema) and the animal is scanned with a special camera hooked to a computer. The computer measures the amount of radioactive blood in the heart and in the liver and compares the two. Normal animals have a shunt fraction (amount of blood in the heart divided by amount in the liver) of less than 15%. In other words, at least 85% of the radioactive material ends up in the liver. Dogs with shunts usually have shunt fractions >60%, because most of the blood bypasses the liver and goes straight to the heart. Scintigraphy is safe and quick but does require heavy sedation or anesthesia. Animals must be hospitalized for at least one night after the procedure until they have expelled the radioactive material by defecation and urination. Scintigraphy tells us that shunting is present; however, in most cases the veterinarian cannot tell whether the shunt is inside or outside of the liver, whether there is more than one shunt, or whether the shunt is congenital or acquired.

What is a portogram?
A portogram is an x-ray of the blood vessels to the liver. Because blood vessels are not easily seen on regular x-rays, a contrast material (a liquid that looks white on x-rays) must be injected into a blood vessel in the abdomen. The injection can be performed through a surgical incision into the belly; by injecting the spleen directly through the skin; or by passing a catheter down the jugular vein (in the neck), through the heart, and toward the abdomen. Portograms usually require anesthesia and are more invasive than scintigraphy. They are usually quite safe, however, and are able to provide a picture of the shunt so that the veterinarian can see where it is located and whether there is more than one.

Can a shunt be diagnosed with ultrasound?
Some veterinarians are able to find a shunt by ultrasounding the liver. Diagnosis of a shunt with ultrasound requires lots of experience and usually a specialized machine (“Doppler”) that can detect blood flow. Shunts, particularly those outside the liver, can be easily missed, especially if the dog is small or wiggly, or the ultrsonographer is inexperienced.
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