Portosystemic shunt
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A portosystemic shunt (PSS), also known as a liver shunt, is a bypass of the liver by the body's circulatory system. It can be either a congenital or acquired condition.
Congenital PSS is a hereditary condition in dogs and cats, its frequency varying depending on the breed. The shunts found mainly in small dog breeds such as Miniature Schnauzers and Yorkshire Terriers, and in cats such as Persians, Himalayans, and mix breeds are usually extrahepatic, while the shunts found in large dog breeds such as Irish Wolfhounds and Labrador Retrievers tend to be intrahepatic.[1]
Acquired PSS is uncommon and is found in dogs and cats with liver disease causing portal hypertension, especially cirrhosis.
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[edit] Pathology
Congenital PSS is caused by the failure of the fetal circulatory system of the liver to change. Normally, the blood from the placenta bypasses the liver and goes into circulation via the ductus venosus. A failure of the ductus venosus to close causes an intrahepatic (inside the liver} shunt, while extrahepatic shunts are usually a developmental abnormality of the vitelline system, connecting the portal vein to the caudal vena cava. Thus, blood from the intestines only partly goes through the liver, and the rest mixes into general circulation. Toxins such as ammonia are not cleared by the liver. Congenital shunts are usually solitary. Acquired shunts are usually multiple, and are caused by portal hypertension in dogs with liver disease.
[edit] Symptoms and diagnosis
Symptoms of congenital PSS usually appear by six months of age[1] and include failure to gain weight, vomiting, and signs of hepatic encephalopathy such as seizures, depression, tremors, drooling, and head pressing. Urate bladder stones may form because of increased amounts of uric acid in circulation and excreted by the kidneys. Initial diagnosis of PSS is through laboratory bloodwork showing elevated serum bile acids after eating. Rectal portal scintigraphy using 99mtechnetium pertechnetate demonstrates the blood vessel bypassing the liver. Surgery definitively shows the shunt if it is extrahepatic.
[edit] Treatment
Surgical treatment is best when it can be performed, such as on extrahepatic shunts. Pressure within the portal vein is measured as the shunt is closed, and it must be kept below 20 cm H2O or else portal hypertension will ensue.[1] When surgery is not an option, PSS is treated as are other forms of liver failure. Dietary protein restriction is helpful to lessen signs of hepatic encephalopathy, and antibiotics such as neomycin or metronidazole and other medicines such as lactulose can reduce ammonia production and absorption in the intestines. The prognosis is guarded for any form of PSS.
[edit] Heredity
The intrahepatic shunts found in large dog breeds are passed on in a simple autosomal recessive way, while the extrahepatic shunts of the small breeds are inherited on a polygenic base.[2]
[edit] Reference
- ^ a b c Ettinger, Stephen J.;Feldman, Edward C. (1995). Textbook of Veterinary Internal Medicine, 4th ed., W.B. Saunders Company. ISBN 0-7216-6795-3.
- ^ Rothuizen, Jan (2002). Molecular Genetics-Diseases of the Liver. Proceedings of the 27th WSAVA Conference. Retrieved on 2006-11-14.
