First, from http://www.vetinfo.com/dshunt.html, an explanation of how shunts develop:

Liver shunts are a congenital problem in some dogs. During gestation the placenta delivers blood with food and oxygen from the mother through the umbilical vein. This means that in the fetus, circulation is the reverse of circulation after birth, because the fetus' veins have the oxygenated blood and arteries return unoxygenated blood to the heart. In order to make this work, there is a shunt from the liver venous circulation to the arterial circulation. At birth, the pressure within the circulatory system changes as respiration occurs and this shuts the shunt, which eventually disappears. If this reverse in circulation doesn't happen for some reason, the liver is deprived of a blood supply and doesn't develop properly after birth. Many puppies can live with the small functioning portion of the liver for some time but eventually have problems and usually die if the situation is uncorrected. It is possible to surgically close the shunt and the surgery works well. I can remember hearing of one sheltie that was 6 years of age (or possibly older) before a congenital liver shunt was recognized, so some dogs can live a long time with this problem.

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From: http://www.cvm.uiuc.edu/ceps/petcolumns/shunt.htm

IMPROPER BLOOD FLOW AROUND THE LIVER CAN POSE SERIOUS HEALTH RISKS FOR YOUR PET

PET COLUMN FOR THE WEEK OF December 16, 1996

CEPS/Veterinary Extension
2938 Vet. Med. Basic Sciences Bldg.
2001 S. Lincoln Ave.
Urbana, Illinois 61802
Phone: 217/333-2907

By Joseph Hahn, Information Specialist, University of Illinois, College of Veterinary Medicine

In railroad tracks and electrical circuits, shunts are useful things, allowing train cars or the flow of current to be diverted from one pathway to another. But a portosystemic shunt, which allows blood to flow abnormally around instead of to the liver, is a serious health problem in pets. Both dogs and cats can have portosystemic shunts, although it is much more common in dogs.

Normally, the blood carries toxins and toxic by-products of metabolism from the stomach and intestines to the liver, where the toxins are removed. "In animals with portosystemic shunts, the blood bypasses the liver and is diverted to another blood vessel, allowing toxins to circulate through the body," says Dr. Jennifer Brinson, a veterinarian who specializes in internal medicine at the University of Illinois College of Veterinary Medicine Teaching Hospital in Urbana.

Shunts can be either congenital--a condition the animal was born with—or acquired--a condition that developed later in life. Congenital shunts are generally diagnosed in animals less than one year old. Acquired shunts can occur at any age and are often caused by liver disease. Shunts are also categorized as intrahepatic (within the liver) or extrahepatic (outside the liver).

"Congenital shunts are most commonly seen in small breed dogs, such as Yorkshire terriers or toy poodles. These dogs generally get single, extrahepatic shunts," says Dr. Brinson. "This disease can also affect large breed dogs. Unfortunately, larger dogs typically get intrahepatic shunts, which are much more difficult to treat."

Dogs with congenital shunts tend to be small for their age and breed. Other signs of shunts include excessive drinking, frequent urination, and a condition known as hepatic encephalopathy. This condition arises shortly after eating and may appear as depression, muscular incoordination, coma, and seizures--signs caused by ammonia (a by-product of protein digestion) reaching the brain instead of being cleared by the liver.

Diagnosis of a suspected portosystemic shunt is often done in three stages. The first stage is checking a blood and urine sample. If these samples are suggestive of a shunt, second stage tests, consisting of a pre- and post-bile acid test and an ammonia challenge test, are performed. These two tests help determine the functional capacity of the liver. Finally, an ultrasound or nuclear scan may be used to try to locate and determine the extent of the shunt.

Treatment and prognosis of shunts depend on their location and severity. "A congenital, single, extrahepatic shunt that is caught early is a good candidate for surgery," says Dr. Brinson. "Intrahepatic shunts commonly must be treated medically."

The medical treatment for portosystemic shunts is aimed at reducing the amount of ammonia circulating in the body and decreasing the symptoms. A low-protein diet and lactulose to reduce absorption of ammonia are prescribed. In emergency cases, enemas with water or lactulose are used to reduce ammonia absorption immediately. If portosystemic shunts go untreated, the symptoms will get progressively worse and eventually the pet may die.

If you would like further information, contact your local veterinarian.

From "Health Research" by Vicki Cooper reprinted from the American Shih Tzu Club (ASTC) Bulletin, Nov- Dec 94.

Those who engage in the dog breeding game of genetic wheel of fortune are challenged to arm themselves with information that is complex as well as mysterious. When the idiosyncrasies of breeding dogs confront me, I attack with the fervor possessed only by Jurassic Park's flesh eating dinosaurs. In this way, I attempt to convert road blocks into stepping stones. I believe that knowledge is empowering, and I am accountable to share my knowledge with my peers. This dinosaur recently dined on a strict diet of books and articles regarding canine portal systemic shunt. Therefore, contained in this study with be a brief description of normal liver function, followed by a explanation of liver shunt disease, and included will be a discussion of the cause, symptoms and treatments concerning dogs with portal systemic shunts (PSS).

According to Dr. Karen Tobias, a veterinary surgeon at Washington State University, in 1974 an increase of PSS was discovered at California Davis Veterinarian School. At that time it was questioned whether the frequency had increased or if knowledge for identifying the disorder had improved. The portal vein transports blood from the gastro-intestinal system to the liver where it is ''detoxified'' before re-entering the circulatory system. "Normally ammonia that has been absorbed through the intestine is carried to the liver by portal circulation, where 81 to 87 percent is converted to urea" (301). "With PSS...ammonia is no longer adequately metabolized and is diverted to systemic circulation" (301). Stephen J. Ettinger (1989) reports, that "no...sex predisposition has been identified for animals with PSS (1499). There is a difference in the type of shunt that develops for large and small dogs. Large dogs tend to have shunts inside the liver (intrahepatic), while small dogs will have them outside (extrahepatic). Shunts develop as small side branches connecting the hepatic portal vein and major gastro-intestinal veins. Blood is diverted around the liver without undergoing hepatic detoxification. Shunts involve a single large vessel or multiple small veins. The most common are outside of the liver and affect small breeds of dogs such as Yorkshire terrier, schnauzer, poodle, Maltese, Shih Tzu and dachshund (299).

Maddison (1988) states, "The cause of the development of congenital shunt has not been determined, but the apparent breed predilections may suggest a genetic etiology" (248). Tobias (1993) states "The ductus venosus is the embryonic channel that carries oxygenated placental blood from the umbilical vein to the caudal vena cava" (298). The ductus venosus should undergo functional closure within the first three days after birth. Ettinger (1989) believes the shunt is most often a remnant of the fetal duct system that remains open instead of closing at birth. According to Ettinger, the severity of symptoms will vary according to the volume of blood that bypasses the liver, and clinical signs vary but all result in hepatic encephalopathy (disturbances of consciousness that may progress to deep coma). "In many cases the clinical signs have an episodic nature; they are present for a few hours to a day or two and the animal returns to normal" (1499). A large portion of cases are diagnosed before one year of age and many animals are referred to as "chronic poor doers". Many are stunted, thin, depressed and have trouble gaining weight. Nearly all have some type of central nervous system signs that may be the best clue to identify the problem. Particularly bizarre behavioral signs or loss of intellectual function, unpredictable bouts of aggression, staggering, pacing, circling, head pressing (which is pushing the head against a solid object), blindness, deafness, tremors, seizures and coma may be seen. Other signs include pica (bizarre cravings for unnatural foods) and polyphagia (excessive eating). Both are considered distinctly characteristic for PSS. These symptoms are corroborated by Tobias (1993), who reports symptoms such as general poor growth rate, weight loss, anesthetic or tranquilizer intolerance, lethargy, depression, ataxia (reduced muscular coordination), behavioral change such as head pressing, circling and the development of a head tilt. According to Ettinger, "Hepatic encephalopathy has been recognized in animals with PSS. Clinical signs include depression, dementia, stupor and coma" (1499). The cause of hepatic encephalopathy is not known, but probably is a result of toxins affecting the brain. Gastro-intestinal abnormalities include: anorexia, vomiting, diarrhea, excess excretion of saliva, difficulty swallowing and excessive ingestion of food. It appears that as high as 52 percent of dogs with liver-shunt problems will have polydipsia (excessive drinking of water) and polyuria (excessive urination). Ettinger reports, "Recurrent urinary calculi [crystals] is another presenting complaint" (1500). "Normal hepatic function is essential for converting ammonia to urea" and 'a decrease in detoxification and uric acid metabolism result[s] in increased excretion of ammonia and uric acid, with eventual discharge of ammonium biurate crystalluria.’" Tobias reports that ammonium crystals are seen in as much as 74 percent of reported cases. A routine urine sediment examination will reveal the crystals and the presence of the crystals is specifically distinctive or characteristic of the disease. These crystals are different from normal urinary crystals which are square or rectangular in shape (Tobias 1994). According to Tobias (1993), "Ammonium biurate crystals have a thornapple shape and golden color" and in "Any breed except in Dalmatians ammonium biurate crystalluria is suggestive of PSS'' (300). "In one study 50 percent of affected males were cryptorchid'' (298) indicating that other deformities coexist along with the liver-shunt (298).

It has been suggested that type of shunt may influence the age of onset and severity of clinical signs. "It could be speculated that prior to onset of clinical signs, the majority of portal blood passed through the liver rather than the shunt. With time there may have been an increase in intrahepatic resistance which causes a higher proportion of blood to shunt to the systemic circulation resulting in [late life] clinical signs..." (Laflamme, 1988, 136). VanGundy (1988) substantiates this theory by reporting the sufficient blood flow though the portal system may maintain a degree of hepatic function for the first years of life. Perhaps a secondary mild insult [to the liver] resulted in loss of function.

Tobias (1993) reports that while history, physical examination and routine laboratory tests may be helpful in diagnosing PSS, liver function tests such as the plasma clearance of certain dyes (BSP), the ammonia tolerance test (ATT) and evaluation of serum bile acid concentrations are more reliable for identifying liver dysfunction associated with liver shunt. "Angiography provides a definitive diagnosis of PSS and is helpful in determining shunt extent and locations" (302). "Urinalysis abnormalities include low specific gravity and ammonium biurate [crystals]" (303). According to Ettinger (1989), an association between the onset of symptoms and ingestion of high protein meals is helpful diagnostically, and a dog that shows signs of dementia coupled with stunted growth and a profound state of ill health is a strong indication. "Small liver size coupled with bizarre neurologic signs in a young dog... is strong supportive evidence that shunt exists, and indicates other diagnostic tests are needed" (1502). Maddison (1988) states, "The presence of a small liver and enlarged kidneys…is highly suggestive of the presence of a portal caval shunt" (248).

In CLINICAL DIAGNOSIS OF HEPATIC PORTO-SYSTEMIC SHUNTS,

Larry Snyder, DVM, provides a good summary of how liver shunt is diagnosed ( http://www.edwards1.com/rose/lacy/livershunt.htm#shunt).

Laboratory Findings: Routine performed serum chemistries are fairly nonspecific toward confirming the diagnosis of porto-systemic shunts, but there may be a decreased total protein (primarily albumin), decreased blood glucose, decreased cholesterol, & decreased blood urea nitrogen (BUN). The uric acid levels may be elevated in a significant number of affected individuals. Acid levels are extremely important in the diagnostic screening of symptomatic potential shunts. Fasting and 2-hr. post meal blood samples are evaluated for bile acid levels. In virtually all porto-systemic shunts there will be a significant rise in the bile acid levels over normal. The use of bile acids in screening clinically normal dogs for liver shunts is not currently being advised due to the variation of normal bile acid levels in Yorkshire Terriers, & other breeds as well. Reports of recent vaccination with modified-live vaccines causing high serum bile acid levels in normal animals have not been confirmed as of this time. Liver function testing with Bromosulfaphthalein (BSP) or ammonia tolerance testing are sensitive & reliable if performed correctly. These tests measure the liver's ability to excrete/detoxify known agents, and thus measure liver function accurately.

Radiography (X-rays). Radiography is one of the most important methods of establishing a diagnosis of porto-systemic shunt, & is currently the only universally accepted method of confirming a shunt, short of major surgery.Injection of a radiopaque dye into the spleen (Splenoportograpy) will show the shunt on radiographs & allow accurate assessment for surgical correction.

Nuclear Medicine. The placement of a radiopharmaceutical agent (radioisotope) specific for the liver into the colon for absorption through the mucosa has been gaining favor because of its noninvasive diagnostic value. This procedure requires expensive equipment & the diagnosis is based on the distribution of the radionuclide in the lung or heart compared to that in the liver. This procedure also does not identify the exact location of the shunt for surgical correction if required.

Ultrasound. Until recently, ultrasound was fairly unreliable for nonsurgical diagnosis of porto-systemic shunts. With the advent of Color Flow Ultrasound, there is the potential for diagnosis of this condition on non-anesthetized animals. At the present time, this technology appears to be the diagnostic procedure of choice. If currently undertaken research confirms its value, Color Doppler Ultrasound will soon be the preferred screening and diagnostic tool.

Cooper resumes in her article: Therapy can be medical or surgical. Usually medical therapy is used to improve preoperative health, or for owners that are unable to consider surgery (Ettinger, 1989). Medical management of PSS includes the restriction of protein, and using non absorbable intestinal antibiotics which are effective against colonic bacteria (Tobias, 1993). The "goal [in dietary management] is to provide sufficient nutrients to allow proper growth and maintenance, without stimulating an increase in toxic metabolites" (Laflamme, 1989, 199). Medical therapy helps to maintain a quality of life, but the disease will continue to progress and the animal will succumb to the disease. "Hepatic functional failure tends to progress in most animals that are diagnosed before one year of age...They eventually become refractory to medical management and succumb to their disease" (Ettinger 1503). "They will require medical management for life" (1505).

According to Tobias (1993) the preferred treatment for single congenital PSS is surgical closure or partial closure of the shunt. Successful surgeries can result in complete reversal of symptoms. Surgery is used to ligate the shunt flow that will redirect the blood through the liver. When blood is re-routed though the liver the pressure on the liver must be stable. Death from hypertensive shock will occur within 12-24 hours if the pressure is too high. The surgeon is challenged to find the correct pressure by returning to surgery to loosen or tighten the ligature. This explains the elevated cost and threat to life. "If shock is avoided serious postoperative complications are uncommon" (Ettinger 1989, 1504). Successful surgeries are more common in shunts located outside the liver than inside, and it is logical to hypothesize that surgical correction of the shunt would result in a spontaneous dissolution of the uroliths (crystals).

In conclusion, the portal systemic shunt appears to be growing in frequency; however, increased clinician skill at detecting the liver-shunt is apparent. Furthermore, advances in technological ability to detect a liver-shunt though angiography may also be responsible for increased reports of the disease. While the origin of the disease is obscure, the disease is considered to be congenital; however, breed predisposition does exist, indicating a hereditary origin. The shunt most likely develops because a fetal duct fails to close after birth. Symptoms vary but all result in central nervous system disorders called hepatic encephalopathy. Puppies that fail to maintain the same growth rate as their siblings are suspect. Specifically shaped urinary crystals develop due to increased concentrations of ammonium. The degree of vessel occlusion may explain animals that are without symptoms until late in life. Small liver size, coupled with bizarre neurological signs in young dogs, is highly suggestive of the disease. Medical or surgical treatment is available; however, without corrective surgery, an animal will eventually succumb to the disease. If the animal survives surgical intervention, complete reversal of the symptoms can be expected.

Ettinger S. (1989).Textbook of Veterinary Internal Medicine. Philadelphia: W.B. Saunders Company.

Laflamme D.P. (1989). Hepatoencephaopathy Associated with Multiple Portal Systemic Shunts in a Dog. The Journal of the American Animal Hospital Association, 25 (2) 199-202.

Maddison J.E. (1988). Department Veterinary Clinical Studies, the University of Sydney New South Wales 2006. Australian Veterinary Journal , 65(8) 245-249.

Tobias, K. (1993). Disease mechanisms in Small Animal Surgery: Portosystemic Shunts. Philadelphia: Lea & Febiger.

Tobias, K. (1994). Personal Communication.

VanGundy, T.E. (1987). Congenital Portacaval Shunt in a Seven-Year-Old Dog. California Veterinarian, 41(3), 19-28.