In healthy animals, blood from the stomach, intestines, spleen, and pancreas drains into the liver through a large vein called the portal vein. In the liver, blood is detoxified before it passes into the general circulation.
Portosystemic shunts (PSS) are caused by congenital abnormalities of the veins in the abdomen, specifically affecting liver bloodflow. In animals with PSS, some of the blood draining from the portal system bypasses the liver and enters general circulation. Because the liver does not have the opportunity to detoxify substances absorbed from the intestines, these accumulated substances can affect the brain and cause seizures and abnormal behavior.
Portosystemic shunts are generally classified as intrahepatic (within the liver) or extrahepatic (outside of the liver). Extrahepatic shunts are more common than intrahepatic shunts, and are generally found in toy breed dogs.
Whenever your pet is showing signs of a health issue, your first step is to contact your primary care veterinarian. If a portosystemic shunt or other serious condition is confirmed, a qualified veterinary specialist is available at an ExpertVet certified hospital.
There are a number of classifications of portosystemic shunts based on their causes and their location within the body. Most are congenital, which means the defect existed at birth and may have a genetic basis. Because congenital liver shunts are likely an inherited problem, dogs and cats that have them should not be bred. Portosystemic shunts are rare in cats, but frequent in dogs, especially small breeds.
The shunts that occur most frequently are single extrahepatic shunts, single intrahepatic shunts, multiple extrahepatic shunts, and microvascular dysplasia.
Of these, the single extrahepatic shunt is the most common, and it is usually congenital. It occurs when there is a single shunting vessel that is located outside the liver. A single intrahepatic shunt is a single shunting vessel within the liver, typically a congenital condition that occurs most often in large-breed dogs.
Multiple extrahepatic shunts are a group of shunting vessels that are not located within the liver. They are caused by underlying liver disease and are usually not present at birth. Microvascular dysplasia, in which there are numerous capillary shunts within the liver, is also known as portal vein hypoplasia with sinusoidal shunting. This condition cannot be surgically corrected like the other types of portosystemic shunts, but many pets with this condition seem to do well for many years. This is a congenital defect mostly seen in small breeds of dogs.
Symptoms of portosystemic shunts vary significantly. You may notice your pet experiencing neurological abnormalities, such as head pressing, seizures, abnormal behavior, lack of coordination and aimless walking. Furthermore, your pet may suffer from lethargy, failure to grow, weight loss, poor appetite, diarrhea, vomiting, or excessive salivation. Portosystemic shunts can also cause a specific type of crystals or stones to form in the urinary bladder resulting in excessive drinking and urinating, difficulty passing urine, and urine that may appear bloody. Clinical signs can worsen after eating, and tend to be worse with a high protein diet. Your veterinarian may notice difficulty recovering from the anesthesia after routine surgery (spay/neuter, dental). The diagnosis of a portosystemic shunt is made with screening blood tests and confirmed at surgery.