sign in sign up
<<
Home , Portosystemic shunt, Shunt (medical)

Chihuahua Vol. 5, No. 1 ■ January 2006

Understanding Portosystemic Shunts in Chihuahuas

healthy is critical to proper those caused by a portosystemic Dandie Dinmont Terrier, Pug, and metabolism and filtering of include hypoglycemia, or low blood Miniature Schnauzer top the list, Atoxins. Unfortunately, some sugar, epilepsy, upper neck problems, while Chihuahua ranks 22. breeds of dogs — including Chihuahuas and hepatic microvascular dysplasia, Tobias found that PSS occurred in — are more likely to have a vascular where microscopic blood vessels in the 0.18 percent of all dogs. “The annual defect, called a , liver do not develop properly. proportion of dogs diagnosed with that diverts blood from flowing cor- PSS increased tenfold from 0.05 per- rectly through the liver. Consequently, Study Reports on Risk cent (five out of 10,000 dogs) in 1980 unfiltered blood from the gastroin- Liver shunts can be congenital, to 0.5 percent (five out of 1,000 testinal system bypasses the liver and meaning they are inherited and are dogs) in 2001,” she says. “Better circulates throughout the body, result- present at birth, or they can be acquired, recognition of signs — by veterinari- ing in a buildup of toxins, especially meaning they develop later in life as ans and breeders alike — is thought after a high-protein meal. a result of another illness such as to be a factor in the upswing of liver The accumulation of toxic materi- shunt diagnoses.” al, particularly , a byproduct Shunts are seen in 0.25 percent, of protein digestion, can trigger a host THE ACCUMULATION OF TOXIC or one in every 400, of Chihuahuas of neurological signs and unusual presented to referral hospitals. “The MATERIAL, PARTICULARLY AMMONIA, behavior in as little as 15 to 45 min- odds of a Chihuahua having a shunt utes after a dog has eaten. Signs may A BYPRODUCT OF PROTEIN is 4.9 times greater than that for a include stupor, lethargy, circling, con- mixed-breed dog, where the incident fusion, blind staggers, head pressing, DIGESTION, CAN TRIGGER A HOST rate is one in every 2,000 dogs,” staring, or seizures. Less common signs Tobias says. “Thus, it is highly likely OF NEUROLOGICAL SIGNS AND include excessive drinking, urinating, that the condition is hereditary in diarrhea or vomiting, even blindness. UNUSUAL BEHAVIOR IN AS LITTLE Chihuahuas.” General ill health, as evidenced by stunted growth and a poor coat, also AS 15 TO 45 MINUTES AFTER Identifying a Liver Shunt can be an indicator of a liver shunt, as Clinical signs of a liver shunt are A DOG HAS EATEN. SIGNS MAY well as urinary crystals, specifically often seen at a young age, usually ammonium biurate crystals. INCLUDE STUPOR, LETHARGY, before an animal is 1 year old, and “The classic presentation we see is sometimes as early as 3 months of a dullness or mental depression that CIRCLING, CONFUSION, BLIND age. Some dogs show no clinical signs occurs after eating,” says Tony Mann, until they are older, when they develop STAGGERS, HEAD PRESSING, D.V.M., DACVS, associate professor and bladder and kidney problems, such as director of small animal emergency STARING, OR SEIZURES. infections and stones. and critical care at the University of Some dogs are diagnosed after a Missouri. “That’s because the blood prolonged recovery from anesthesia. bypasses the liver so it doesn’t have or . The shunts are “Historically, we’ll see these animals the opportunity to filter out toxins it intrahepatic, located inside the liver, after they’ve been spayed or neutered,” normally does. Some of the toxic or extrahepatic, located outside the Mann says. “One of the complaints chemicals that remain circulating in liver. Large dogs tend to have intra- we hear is that it took the dog for- the bloodstream are depressants. They hepatic shunts, which involve small, ever to wake up from anesthesia. That’s make the animal appear depressed, multiple and are more difficult to because they don’t metabolize the drunk or even sleepy.” treat. Small dogs, including Chihuahuas, drugs very well.” Neurological signs seen with shunts are more likely to have extrahepatic Diagnosis of a suspected portosys- also can be mimicked by other med- shunts involving one of two large temic shunt includes blood tests. “Dogs ical conditions, some common in vessels. This type is usually easier to with congenital liver shunts usually Chihuahuas. For example, hydro- correct surgically. have low blood urea nitrogen (BUN) cephalus, an abnormal accumulation Karen Tobias, D.V.M., professor of and albumin concentration levels,” of in the brain, can small animal surgery at the University Tobias says. “They may be slightly manifest as a dulled mental state and of Tennessee, examined records of anemic or have red blood cells that can cause seizures. Chihuahuas pre- 2,400 canine cases of PSS at 27 are smaller than normal. They also may senting with signs of a liver shunt institutions across the United States have increased liver enzymes (AST could actually have , over a 22-year period. The study noted and ALT).” Mann says. that 33 breeds are at increased risk When these abnormalities are Other medical conditions that pro- of having portosystemic shunts (PSS). duce neurological signs similar to Havanese, York shire Terrier, Maltese, Continued on page 2

19 Portosystemic Shunts treated medically will live a relatively Not all dogs are good surgical can- continued from page 1 long life,” Tobias says. “Dogs that didates. The liver metabolizes some tend to do well with long-term med- anesthetics, and surgery can put undue present, bile acid or ammonia con- ical management are usually older at stress on an already frail animal. In centrations should be measured to the time of diagnosis, have more addition, intrahepatic shunts, those evaluate liver function. Bile acid should normal blood work and less severe within the liver tissue, can be difficult be measured after eating and after a clinical signs.” to find and repair. These dogs are more 12-hour fast. “Dogs with shunts will Medical management aims to reduce likely to suffer post-surgical compli- almost always have high bile acids the amount of ammonia circulating in cations and require additional care. two hours after eating, and usually the body. Lactulose, a type of carbo- Post-operative care often at least 95 percent of dogs will have hydrate utilized by certain intestinal includes a lower protein diet for at high bile acids after a 12-hour fast,” bacteria that changes the pH in the least eight to 12 weeks. Lactulose can Tobias says. “However, some breeds large intestine, can be helpful. The be continued as well, or gradually and individual dogs may have increased change in pH decreases absorption of decreased. Most dogs do not require bile acids without a liver shunt.” ammonia and other toxins, Tobias says. unless they have infections. A definite determination of a shunt Antibiotics may also be prescribed to “The survival rate depends partly on can only be made using radiography decrease the toxin-producing bacteria the health of the dog,” Tobias says. to locate and determine the extent of in the intes tines. Dietary manage- “Because Chihuahuas — particularly the shunt. Testing includes scintigraphy, ment often involves feeding a lower those with shunts — are so small, it portography, ultrasound, CT scan or protein food that is easily digested, is important to have the surgery per- MRI, or even exploratory surgery. rich in antioxidants and vitamins, and formed by a board-certified veterinary Scintigraphy measures and com- low in copper and iron. The “right” pares blood flow between the liver and level of protein is based on an indi- “MOST ANIMALS IMPROVE heart through the rectal insertion of vidual case. The clinician must try to radioactive material that is followed balance meeting the dog’s protein IMMEDIATELY WITH PROPER needs while minimizing the risk of worsening the blood ammonia levels. DIET AND , AND ABOUT A DEFINITE DETERMINATION Surgery provides the best chance ONE-THIRD OF DOGS TREATED OF A SHUNT CAN ONLY BE for long life in most dogs with a por- tosystemic shunt, Tobias says. Surgeries MEDICALLY WILL LIVE A MADE USING RADIOGRAPHY that are successful are more common in dogs with extrahepatic shunts, those RELATIVELY LONG LIFE.” TO LOCATE AND DETERMINE located outside of the liver, which are KAREN TOBIAS, D.V.M., THE EXTENT OF THE SHUNT. the type most common in Chihuahuas. Surgery involves finding the abnormal PROFESSOR OF SMALL ANIMAL SURGERY AT THE UNIVERSITY OF TENNESSEE TESTING INCLUDES SCINTIGRAPHY, blood vessel and closing it off by rerouting the blood through the liver. PORTOGRAPHY, ULTRASOUND, Two devices used to close off the surgeon who has experience with CT SCAN OR MRI, OR EVEN abnormal blood vessel are cellophane toy dogs and who can perform the bands or ameroid constrictor rings. surgery quickly.” EXPLORATORY SURGERY. When cellophane bands are used, thin Generally, “the long-term progno- strips about 5 millimeters wide are sis following surgery is good if the with a special camera connected to placed around the shunt and secured animal gets through the short-term a computer. Though scintigraphy tells with surgical strips. Scar tissue forms consequence,” Mann says. whether shunting is present, in most gradually over the band, which com- Unfortunately, in some cases, eutha - cases the veterinarian cannot tell pletely closes the shunt. nasia is the only option. Uncontrollable whether the shunt is inside or outside The ameroid constrictor is a metal neurological signs, such as seizures, of the liver, whether there is more than band with an inner ring of casein, a behavior changes or progressive liver one shunt, or whether the shunt is protein found in milk. Once in place, damage are examples of when congenital or acquired. A new method the casein absorbs naturally occurring eutha nasia may be best for a dog of scintigraphy, where the radioactive liquid from the abdomen and expands. with a shunt. material is injected directly into the As the casein swells, it presses on the spleen, is proving more useful for abnormal vessels, causing scar tissue Breeding a Dog with PSS differentiating the number and type reaction that seals the shunt in a few In Chihuahuas, it is not known of shunts, Tobias says. weeks. Compared to other techniques, definitely whether portosystemic Portography uses a contrast dye complications are fewer when shunts shunts are hereditary, but it is possible to produce a radiograph of the blood are gradually occluded, or closed, because of the increased incidence, vessels. While ultrasound is one of the Tobias says. Tobias says. In some breeds, such as least invasive ways to confirm the The most serious surgical compli- Irish Wolfhounds, Cocker Spaniels, presence of a shunt, it is also one of cation is , which, if Maltese and Yorkshire Terriers, shunts the most difficult. Shunts can be acute, can be fatal. Death from hyper- are considered hereditary. easily missed, especially if the dog is tensive shock typically occurs within Since it is not definite that shunts small or wiggly, Tobias notes, adding 12 to 24 hours after surgery. Portal are hereditary and since the mode of that the experience level of the ultra- hypertension rarely occurs in dogs that inheritance is not known, it is best to sonographer often has the greatest undergo gradual occlusion. Another avoid breeding dogs that produce off- impact on the success of the diag- uncommon but serious complication is spring with shunts, Tobias says. All nostic test. CT scans are being used postoperative seizures. These seizures affected animals should be neutered more frequently to diagnose por- can be difficult to control and fatal. or spayed. tosystemic shunts, but they also Milder but more common complica- By not breeding dogs likely to pass require expensive equipment and tions include low blood sugar, pain, the condition on to their offspring, special expertise. low body temperature, and bloating. breeders are helping to end the pro- Most of these conditions occur liferation of shunts in Chihuahuas. Managing a Shunt within the first 24 hours after surgery Meanwhile, with proper medical and Medical and dietary management and can be medically managed. Dogs dietary management, and possibly are used to help a dog with a portosys- with low blood protein concentrations, surgery, owners often can help those temic shunt. “Most animals improve high white blood cell counts, or severe dogs diagnosed with portosystemic immediately with proper diet and medi- neurological signs are more likely to shunts live a long, healthy life. ■ cine, and about one-third of dogs have problems following surgery.

20