Information about Portosystemic shunt (PSS)
Portosystemic shunt (PSS)
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 (outside the liver), while the shunts found in large dog breeds such as Irish Wolfhounds and Labrador Retrievers tend to be intrahepatic (inside the liver).
Acquired PSS is uncommon and is found in dogs and cats with liver disease causing portal hypertension, especially cirrhosis.
Symptoms and diagnosis
Symptoms of congenital PSS usually appear by six months of age 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 either elevated serum bile acids after eating or elevation of fasting blood ammonia levels, which has been shown to have a higher sensitivity and specificity than the bile acids test. Rectal portal scintigraphy using 99mtechnetium pertechnetate demonstrates the blood vessel bypassing the liver. Surgery definitively shows the shunt if it is extrahepatic.
Surgical treatment is best, when it can be performed. 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. Complete closure of extrahepatic shunts results in a very low recurrence rate, while incomplete closure results in a recurrence rate of about 50 percent. However, not all dogs with extrahepatic shunts tolerate complete closure (16 to 68 percent).
Intrahepatic shunts are much more difficult to surgically correct than extrahepatic shunts due to their hidden nature, large vessel size, and greater tendency toward portal hypertension when completely closed. 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.
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.