You may have heard of a liver shunt in the context of a young dog or cat. These are puppies or kittens that have been born with an abnormally structured liver. This means that blood coming from the gastrointestinal system is re- circulated around the body, bypassing the liver. This blood hasn’t been processed by the liver, and therefore is high in toxins. It is these toxins entering the nervous system which cause neurological signs such as seizures, lethargy and even blindness. These signs are often worse after eating, as this is when the toxin load floods in from the gastrointestinal system. This kind of liver shunt is congenital – meaning the animal was born with it. However, a second type of liver shunt exists – the acquired liver shunt, which is what we will cover in this article.
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What does the liver do?
The liver is a vital organ in all animals, it holds many vital roles, including processing nutrient rich blood coming from the intestines, producing clotting factors to stop bleeding, as well as metabolising vitamins and minerals. The liver is also responsible for the metabolism of carbohydrates, fats and proteins, all of which are essential to life. The liver’s blood supply is unique in that it is the only organ to mix oxygenated and non- oxygenated blood, it does this by mixing oxygen rich blood from the hepatic artery and non-oxygenated blood arriving from the intestines via the portal vein which makes up the majority of the liver’s blood supply. This mix of blood is then filtered and processed, and is drained out of the liver via the hepatic vein, which then goes back to the heart, ready to be re- oxygenated by the lungs.
The liver is also responsible for producing bile acids, these are collected within the biliary tree and are stored in the gallbladder, until the bile is needed. Bile is required often after eating, as this is when it is needed to help process fats within the small intestine.
Where does it go wrong in a liver shunt?
A congenital shunt can form through a persistent blood vessel of the liver that should close after birth, but doesn’t.
This is a different case to acquired portosystemic shunts, where the underlying disease process is anything causing “portal hypertension”. This is high pressure within the portal vein, pushing blood that should be going into the liver, back away from the liver towards the rest of the body. Acquired shunts therefore are often bundles of smaller vessels, joining the portal vein to the vena cava, and therefore sending non-detoxified blood back into main circulation. There are many disease processes that can cause portal hypertension, such as chronic liver disease like cirrhosis, blood clots of the portal vein, or any stricture or narrowing that can form within or around the portal vein.
How is a liver shunt diagnosed?
A combination of blood tests and advanced imaging may be required for diagnosis of this disease. Blood tests may show increased liver enzymes, and if the vet suspects a shunt, they will usually carry out a bile acid stimulation test. In this test, the bile acids will be abnormally high in an animal with a shunt. Advanced imaging, such as ultrasound and CT scans, can then show where the abnormal vessels are.
How can a liver shunt be treated?
Again, there are large differences between congenital and acquired shunts.
Surgery is usually the best option for congenital shunts. However, it is frequently contraindicated in acquired shunts as it can make the underlying liver disease worse.
Medical management is the main option for acquired shunts, this can include a low protein diet and laxatives to reduce the toxin build up. Depending on the clinical signs the animal is showing, appropriate symptomatic treatment may be needed, such as anti-sickness medicines in animals with nausea. If at all possible, the underlying liver disease needs to be treated as soon as the patient is stabilised.
If you want to learn more about this disease:
- Acquired Portosystemic Shunts in Small Animals – Digestive System – MSD Veterinary Manual
- Structure of a liver lobule | eClinpath
- Portosystemic Shunts Fact Sheet | Davies Veterinary Specialists
- Physiology of the liver | eClinpath
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