The nature of liver shunt, its symptoms, how to test for it, and what treatment is available.
by John Dillberger, DVM, PhD
Reprinted from the November/December 2004 Claymore
Please note: Since this article was published in 2004, it is now recommended that every Deerhound be screened for a liver shunt with a bile acid test. This test should ideally be conducted by breeders before puppies go to their new homes. Many labs do bile-acid testing; for more information on liver shunts and testing, go here. There is also a new treatment—for more information, go here.
Some time ago, a Deerhound owner and breeder sent me a letter asking that I devote a column to liver shunts. One of her pups had a liver shunt, and she hoped I could discuss not only the nature of the disorder but also its symptoms, how to test for it, and what treatment was available. As it happens, her problem is not an isolated one in our breed.
In 1994, I reprinted a paper on liver shunts in Deerhounds by Dutch veterinarian Dr. H.P. Meyer. He reported that of 125 Dutch Deerhounds screened for liver shunt, 6 (approximately 5%) were found to have the problem. Dr. Meyer also wrote that liver shunts were most probably an inherited disorder.
This month I’ll revisit the subject of liver shunts to include what has been learned in the last decade.
What is a Liver Shunt?
Among the liver’s many functions is to cleanse blood that is returning to the heart from the intestinal tract. While bathing the intestine, the blood absorbs all sorts of substances, from beneficial nutrients to harmful toxins. The first stop for the blood after it leaves the intestine is the liver, which acts like a sieve to harvest many of the nutrients and strain out all of the toxins. The nutrients are used or stored, while the toxins are rendered harmless or dumped right back into the intestinal tract via the bile. The blood that exits the other side of the liver on its way to the heart is substantially changed and much safer than it was before the liver did its job.
Developing fetuses in all mammals have a large blood vessel called the ductus venosus that “shunts” blood around the liver and directly into the large vein on the other side, called the vena cava. This bypassing of the fetal liver is possible because the dam’s liver has already cleansed the blood of harmful substances before it ever reaches the fetus.
Normally the ductus venosus closes just before or after birth, and the fetal liver takes over the job of cleansing blood. If the ductus venosus doesn’t close down fully, the resulting disorder is called a portosystemic shunt, or liver shunt for short. The term portosystemic shunt is used because the vessel bringing blood to the liver is called the portal vein, and the network of vessels on the other side that serve the rest of the body is called the systemic circulatory system.
Livers shunts that arise in this way, from persistence of the ductus venosus, are called congenital shunts, meaning they were present at birth. This distinguishes them from portosystemic shunts that develop later in life, which are called acquired shunts.
Evidence strongly suggests that congenital liver shunts are an inherited disorder. The mode of inheritance is presumed to be autosomal recessive in all breeds. A study is currently underway to determine the mode of inheritance in Yorkshire Terriers.
Liver Shunt Symptoms
When a dog has a congenital liver shunt, some or all of the blood coming from the intestine bypasses the liver and goes straight into the systemic circulation. That allows toxins produced during digestion to reach the rest of the body, deprives the liver of nutrients and blood supply that it needs for its own growth, and reduces the amount of liver-made building blocks like amino acids needed for growth in general.
Because the shunt is present from birth, clinical signs often show up at a young age. The first thing you might notice is that the pup is a “runt”—that is, it grows more slowly and is smaller and weaker than its littermates.
The brain is especially sensitive to certain toxins, like ammonia and other digestive byproducts, so that behavioral abnormalities are common in dogs with shunts. You might notice that the dog has a quiet demeanor (which can be difficult to tell from the normal Deerhound approach to life), stares into space, is disoriented, turns in circles, presses its head against the wall, or even has seizures. Blindness also can occur. Because the substances causing these symptoms are formed during normal digestion, symptoms may be more common or more pronounced after a high-protein meal.
In dogs with partial shunts, symptoms may not develop until the dog is mature. For instance, a liver shunt may be suspected if a dog takes longer than normal to recover from its first episode of sedation or anesthesia. This happens because the liver is less efficient at extracting and/or metabolizing the sedative or anesthetic.
Certain types of urinary tract stones are more common in dogs with liver shunts. The liver normally removes urates and ammonia from the blood, so that a dog with a congenital shunt will have higher concentrations of these substances in its blood and urine. In the urine, these compounds can react to form ammonium biurate stones in the kidney and bladder.
Occasionally, dogs with congenital shunts will have mildly distended abdomens due to fluid buildup. Other possible signs are vomiting, diarrhea, or excessive salivation.
None of these signs is diagnostic in and of itself for a liver shunt. But they can raise the possibility and prompt further tests.
Diagnosing a Liver Shunt
The current approach to diagnosing a liver shunt is to use a 4-step process:
Step 1: Suspect a shunt exists
This suspicion might be based on symptoms or family history.
Step 2: General tests
For any dog suspected of having a liver shunt, screening tests would include routine blood tests, urinalysis, and probably abdominal X-rays or ultrasound examination. Blood test results that would raise the suspicion of a liver shunt would be a low concentrations of urea (BUN), albumin, or glucose, mildly low red blood cell count, and mildly elevated activity levels of certain enzymes (ALT, Alkaline phosphatase). Urine sediment may contain ammonium biurate crystals. Abdominal examination results that might suggest a liver shunt would be a small liver, large kidneys, or urinary tract stones. In one study published in 2004, the combination of those three findings was 100% predictive for liver shunt.
If screening tests suggest that a liver shunt is present, then further tests are warranted. Screening tests may also reveal that the dog’s trouble is due to something other than a liver shunt. If not, then further tests may be warranted even if all the screening tests are normal.
Step 3: Specialized tests
Blood tests can be done for two substances normally found in only very small amounts: bile acids and ammonia. Bile acids are produced in the liver, excreted into the upper intestine in the bile, reabsorbed from the lower intestine into the blood, and filtered out of the portal blood by the liver. Ammonia is produced by intestinal bacteria as a byproduct of protein digestion. It is absorbed from the intestine but filtered out of the portal blood and chemically de-toxified by the liver.
In a dog with a liver shunt, some of the bile acids and ammonia bypass the liver and go straight into the systemic circulation. That’s why greater-than-expected concentrations of bile acids or ammonia in the systemic circulation strongly suggest a liver shunt. However, other liver disorders can also produce these results, so these blood tests are not definitive.
Step 4: Definitive diagnostic tests
There are four. The least invasive technique for confirming a liver shunt is an ultrasound examination of the abdomen. Sound waves are used to create a picture of the abdominal organs that allows the operator to search for the shunt. This method usually requires no sedation. Its biggest drawback is that it requires a skilled and experienced ultrasound operator. Also, failure to find a shunt does not completely eliminate the possibility, since some shunts are difficult to visualize with ultrasound.
A second technique for finding a liver shunt is called rectal scintigraphy. This requires sequential enemas to empty the dog’s colon. Also, the dog has to lie very still for the procedure, so heavy sedation may be necessary. A short-lived radioactive tracer substance is placed in the colon, from where it is absorbed into the portal blood. A detector measures the uptake of tracer in the heart and liver. In a dog with a liver shunt, some tracer will bypass the liver and reach the heart, so it will show up there first, or at the same time as in the liver. The major disadvantage of this technique (besides enemas and sedation) is its limited availability.
A third technique is called an intravenous portogram. This requires a small incision in the abdominal wall, so the dog must be given general anesthesia. A small amount of contrast medium (dye that shows up on X-ray) is injected into a vein draining the intestinal tract, and then abdominal radiographs are taken at intervals. The contrast medium will clearly highlight the portal vein carrying blood to the liver, and all its branches. If a shunt exists, it will be highlighted, too.
A fourth technique for finding a shunt is to do exploratory surgery and look for it directly. The advantage of this technique is that the shunt can be repaired at once; however, some shunts can be difficult to find, so that failure to see a shunt doesn’t rule out the chance it exists.
One size doesn’t fit all
The exact sequence of tests to diagnose a liver shunt will vary from dog to dog. For instance, if a dog has a family history of shunt and clinical signs highly suggestive of one, then bile acid and ammonia blood tests might be done at once. If those also strongly suggest a shunt, then an ultrasound examination might be done or exploratory surgery might be scheduled.
Treating a Liver Shunt
The only cure for a liver shunt is surgery to close the abnormal blood vessel. If a dog is in fair condition when the shunt is diagnosed, then surgery can be done immediately. If a dog is in poor condition or an owner wants to postpone surgery for any other reason, then steps can be taken to improve the dog’s health before surgery.
Improving the health of a dog with a liver shunt really depends on reducing the amount of toxins absorbed from its intestine. The toxins that do the most damage are produced by normal gut bacteria as they digest proteins in the food. Oral antibiotics can be given to reduce the number of bacteria and therefore the amount of toxins produced. Also, the diet can be modified to reduce the total protein intake to the minimum required. Lactulose, a laxative, can reduce the absorption of toxins in general. Dogs that are dehydrated or have low blood glucose concentrations can be given intravenous fluids that contain glucose.
The surgical closure of a liver shunt seems straightforward enough, but there is a catch. Before closure, blood returning from the intestine flows partly through the shunt and partly through the liver. Closing the shunt will suddenly send all the blood through the liver, which simply can’t handle it. Blood will back up in the intestines (and pancreas) under high pressure, leading to a condition called portal hypertension that can be fatal within hours.
Shunts have to be closed gradually instead of suddenly. One way to do this is with staged surgeries: a first surgery to partly close the shunt and a second surgery 1 to 2 months later to finish the job after the liver has time to adapt and grow. Another way to achieve gradual closure is to place something called an ameroid constrictor around the shunt. An ameroid constrictor is a ring of dehydrated casein with a metal rim. The casein slowly absorbs water and swells. It can’t expand outward because of the metal rim, so it expands inward, gradually constricting the shunt. The process can take from a couple weeks to 3 months.
Complications and Long-Term Outcomes
There are short-term and long-term complications that can occur with shunt closure. Besides the general risks associated with any sort of anesthesia and surgery, the most common short-term complication of shunt closure is portal hypertension. Even when the shunt is closed gradually, as described in the previous section, the liver has to adapt to an increase in blood flow. That takes time, and during that period the blood pressure in the portal venous system will be higher than normal. Sometimes a blood clot forms at the site of partial shunt closure, which further increases portal pressure.
Portal hypertension carries two risks. First, it can lead to damage and even death of the intestinal tract or pancreas, which can be fatal. Second, it can increase the chances of new shunts developing within the liver itself. This phenomenon presumably occurs when small vessels under high pressure stretch, break, and heal to form inappropriate new connections that create a pathway for blood to bypass parts of the liver.
The most recent published data on surgical risk suggests that the chances of surviving liver shunt surgery are greater than 90%. Dogs that do survive will have improved health and usually become completely normal. Unfortunately, about 20% of them will have recurrent symptoms within months to years after surgery.
Dogs with congenital liver shunts (diagnosed when young and presumably present from birth) should not be bred, as the condition is almost certainly inherited. This does not mean that all liver shunts are inherited, especially those that appear later in life; however, it is prudent not to breed a dog with a shunt.
Dogs intended for breeding, and their offspring, can be screened for liver shunts in various ways. Probably the best currently available screening test is to measure blood ammonia concentration. Blood bile acid concentration also can be measured, although this requires a 12-hour fast.
When interpreting blood ammonia concentration, it is important to know that Deerhounds differ from dogs in general. In the paper mentioned in the introduction to this article, Dr. Meyer reported that the blood ammonia concentration in Deerhound pups at 7 to 8 weeks of age averaged almost twice that in other breeds (57 μM vs. 32 μM). A similar greater-than-expected ammonia concentration was found in Irish Wolfhounds. In neither breed was the greater-than-expected ammonia concentration due to small liver shunts, as these were ruled out by rectal scintigraphy (described earlier).
Congenital liver shunts do occur in Deerhounds. The typical affected puppy is a “poor-doer” that doesn’t grow as fast, play as hard, or seem as strong as you would expect. He might sleep more than usual, stare off into space from time to time, or exhibit other strange behaviors. He might have seizures. His symptoms will be worse shortly after he eats.
Fortunately, diagnostic tests can almost always find a liver shunt if that’s the cause of the trouble. Medicine and diet management can provide temporary improvement, but the only cure is surgery to close the shunt. The procedure is expensive and the risks aren’t trivial, but most dogs (more than 9 out of 10) survive and 80% of those are permanently cured. Nevertheless, dogs that are cured should not be used for breeding purposes, as liver shunts are an inherited disorder.