Also known as a portosystemicor hepatic shunt, a portosystemic shunt (PSS) is caused by an abnormal vascular connection between the portal vein and the liver, shunting the vein directly into the systemic circulation bypassing the essential detoxifying process in the liver.
Normally the portal vein transports blood from the gastrointestinal tract, spleen, gallbladder, pancreas, and stomach to the liver, whose role is to act as the cat’s waste treatment plant. These shunts are commonly caused when the fetal blood vessels (which bypass the liver during development) have failed to close.
The blood arriving via the portal vein is rich in nutrients extracted from the food as well as bacteria, toxins (and drugs) your cat has eaten and hormones produced by various glands in the cat’s body. The liver processes these nutrients, toxins/drugs and hormones before releasing the blood to the rest of the body via the hepatic vein, into the vena cava and up to the heart which then circulates it around the body.
Due to the lack of normal detoxification of blood performed by the liver, toxic substances that should have been removed build up in the bloodstream and are circulated around the body. Some of these are toxic to the brain (known as neurotoxins). Ammonia is a neurotoxin which is able to cross the blood-brain barrier, as well as that, it is also believed it increases blood-brain barrier permeability, allowing other toxins such as phenols and bile salts to enter the brain, this leads to central nervous system dysfunction, this is known as hepatic encephalopathy (HE).
The lack of nutrients in the blood which would normally go to the liver also affects normal liver growth, the liver is small (hypoplastic), which further adds to the problem.
Portosystemic shunts may be congenital (present at birth) or acquired (occurring as a result of secondary liver conditions), almost all shunts in cats are congenital who typically demonstrate clinical signs within 12 months of age. Persians, Himalayans, and Siamese are over-represented and it is seen more commonly in males. Shunts may be intrahepatic or extrahepatic. Intrahepatic shunts are located within the liver, extrahepatic shunts are located on the outside of the liver. Cats are more prone to developing extrahepatic shunts.
a) Shows a normal liver with no connection between the portal vein (bottom) and the hepatic vein (top). Blood reaches the liver via the portal vein where it is detoxified exiting the liver via the hepatic vein and into the systemic circulation.
b) Intrahepatic shunt: Shows an abnormal connection between the portal vein and the hepatic vein inside the liver, meaning the blood bypasses normal detoxification.
c) Extrahepatic shunt. This image shows an abnormal connection located outside the liver which shunts blood directly to the vena cava, bypassing normal detoxification.
What are the symptoms of a liver shunt?
Central nervous system disorders are commonly seen in cats with a portosystemic shunt due to the build-up of toxins in the cat’s brain. Liver insufficiency, central nervous system and urinary disorders can all occur in cats with portosystemic shunts. Common symptoms may include:
Straining to urinate due to ammonium biurate bladder stone formation
Blood in urine due to ammonium biurate bladder stones
How is a portosystemic shunt diagnosed?
Your veterinarian will perform a complete physical examination of your cat and obtain a medical history from you. There may be a history of prolonged recovery after anesthesia or the pet owner/veterinarian may have noticed the cat is on the small size or losing weight. If your cat is suffering from hepatic encephalopathy neurological signs may be observed during the examination.
A number of medical tests will help your veterinarian to diagnose a liver shunt. These may include:
Complete blood count– In most cases, this test will not show any abnormalities, sometimes abnormally small red blood cells (microcytosis) may be found or mild, nonregenerative anemia.
Biochemical profile– This may reveal slightly elevated alanine aminotransferase (ALT), a liver enzyme, decreased BUN (blood urea nitrogen), hypoglycemia (low blood sugar), hypoalbuminemia (low albumin), hypoproteinemia (low blood protein levels), hyperglobulinemia (low levels of globulin), hypocholesterolemia (low cholesterol).
Urinalysis– May reveal the presence of ammonium biurate crystals and possibly diluted urine. If this is the case, there may also be blood (hematuria), protein (proteinuria) and pus in the urine (pyuria).
Bile acid test. Blood is taken from the cat and he is then fed a fatty meal. Two hours later a second blood sample is taken. When a cat eats, bile (which is secreted by the liver and stored in the gallbladder until needed) is released into the small intestine to help with the digestion of lipids (fats). The bile is then absorbed by the intestine and returned by portal back to the liver where it is removed from the bloodstream. In a cat with a PSS, this doesn’t occur as it should and the bile acid test will reveal high levels of bile acids in the blood.
Ultrasound may reveal the abnormal vessel(s). Not all abnormal vessels may be visible via this method, with intrahepatic vessels being more readily seen than extrahepatic vessels. Ammonium biurate stones may be seen in the bladder during the ultrasound.
X-Raymay show a smaller than normal liver (microhepatica), sometimes the kidneys will be enlarged.
Contrast portography – This test may be used to reveal intrahepatic shunts. A contrast dye is injected into one of the veins draining into the portal vein and an x-ray taken to reveal the abnormal vessel(s).
Laparotomy – This may be performed if there is a strong index of suspicion of a portosystemic shunt. An incision is made in the abdomen and the liver is viewed. Advantages of this procedure are that the shunt can be treated immediately.
How is a portosystemic shunt treated?
The treatment of choice is surgery to cure the condition. Before surgery can be performed, your cat may need to be stabilised so that he is physically able to cope with the anesthesia and surgery as well as treat hepatic encephalopathy. The goal is to decrease the production of ammonia and its absorption from the intestine and make your cat well enough to undergo anesthesia and surgery. Treatment to stabilise your cat may include:
Antibiotics such as neomycin to reduce the number of ammonia causing bacteria.
IV fluids to correct dehydration.
Lactulose administration. This laxative medication lowers levels of ammonia in the bloodstream. If your cat is conscious, this is administered orally, if he is in a coma, it will be administered by enema.
Feeding a high-quality diet which is low in protein.
Anti-seizure medication if necessary.
The benefit of surgery is there is usually a better outcome for the cat as well as removing the need for ongoing medical treatment in cats who don’t have the surgery. The downside is that it can be expensive.
Surgical ligation (tying off to stop blood flow) so that blood flow is redirected to the liver is the treatment of choice where possible. Extrahepatic shunts are easier to treat surgically than intrahepatic shunts due to their location. Surgery is usually performed at a veterinary referral centre.
If surgery is not possible, or if the owner can not afford surgery, medical management will be necessary. This is similar to the treatment prescribed when stabilising your cat. The long-term survival rate of cats managed this way is between 2 months to 2 years. Medical management is palliative and not curative.
A prescription diet which is restricted in protein may be recommended. Ammonia comes from the breakdown of proteins and amino acids in the gut.
Antibioticsto reduce the number of ammonia causing bacteria in the colon.
Lactulose to lower levels of ammonia in the bloodstream.
Intravenous fluids may be administered to treat cats who are dehydrated due to vomiting and diarrhea.
Liver image courtesy of Frank G van Steenbeek, Lindsay van den Bossche, Peter A J Leegwater, Jan Rothuizen Inherited liver shunts in dogs elucidate pathways regulating embryonic development and clinical disorders of the portal vein. Mamm. Genome: 2012, 23(1-2);76-84