Canine Portal Caval Shunt

from "Health Research" by Vicki Cooper reprinted from the 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, 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, were 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 gesture-intestinal veins. Blood is divertedaround 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 effect 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 cave" (298). The ductus venous 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 "chronic poor doers". Many are shunted, 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, 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 poorgrowth 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 now known, but probably is a result of toxins effecting the brain. Gastro-intestinal abnormalities include: anorexia, vomiting, diarrhea, excess excretion of saliva, difficulty swallowing and excessive ingestion of food may be seen. It appears that as high as 52 percent of dogs with liver-shunt problems will have polydipsia (excessive drinking of water) and plyulia (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 form 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 "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 neuralogic signs in a young dog... is strong supportive evidence that shunt exists, and indicates other diagnostic tests are needed" (1502). Maddison (1988) confines, "The presence of a small liver and enlarged kidneys. . . is highly suggestive of the presence of a portal caval shunt" (248).

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 intestinal antibiotics that are non absorbable 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 has grown. 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 an 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 isavailable 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.

References

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

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