Also known as liver or hepatic shunt, a portosystemic shunt (PSS) is caused by an abnormal connection between the portal vein (a blood vessel which carries blood from the gastrointestinal tract to the liver) and the systemic circulation. Blood vessels usually go from the intestines to the liver where toxins are removed from the blood, but in the case of a portosystemic shunt, the liver is bypassed. Due to the lack of normal detoxification of blood performed by the liver, toxins build up within the body.
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 and cats 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.
Symptoms of portosystemic shunts in cats:
Central nervous system disorders are commonly seen in cats with a portosystemic shunt. Common clinical signs include:
- Poor/stunted growth.
- Lack of appetite.
- Intermittent blindness.
- Mental and neurological abnormalities such as sudden aggression, mental dullness, pressing head against objects
- Straining to urinate due to ammonium biurate bladder stone formation.
- Blood in urine due to ammonium biurate bladder stones.
- Copper coloured irises.
Diagnosis of portosystemic shunts:
- 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), 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-Ray may 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.
Treatment of portosystemic shunts:
- Surgical ligation (tying off to stop blood flow) 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. 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. This involves the same treatment set out below under "medical management".
If surgery is not possible, or if the owner can not afford surgery, medical management will be necessary. This is aimed at
- A prescription diet which is restricted in protein may be recommended.
- Antibiotics to reduce the number of ammonia causing bacteria in the colon.
- Lactulose is a laxative and also in patients with liver disease by lowering levels of ammonia in the bloodstream.
- Intravenous fluids may be administered to treat cats who are dehydrated due to vomiting and diarrhea.