Portosystemic Shunt
Also known as: PSS, Congenital Portosystemic Shunt, Portocaval Shunt, Portoazygos Shunt
CatDogUrgency: High
Also known as: PSS, Congenital Portosystemic Shunt, Portocaval Shunt, Portoazygos Shunt
In short
A portosystemic shunt (PSS) is an abnormal blood vessel that bypasses the liver, allowing toxins to build up in the bloodstream and causing neurological, digestive, and urinary issues in dogs and cats.
TL;DR. A portosystemic shunt is an abnormal blood vessel that bypasses the liver, preventing the filtration of toxins and leading to neurological, digestive, and urinary complications in dogs and cats.
A portosystemic shunt diverts toxin-rich blood away from the liver, sending it directly into systemic circulation.
In a healthy dog or cat, blood draining from the stomach, intestines, pancreas, and spleen (known as portal blood) travels directly to the liver. The liver acts as a vital processing plant: it filters out toxins, metabolizes nutrients, and clears waste products from the bloodstream before the blood is circulated to the rest of the body.
A portosystemic shunt (PSS) is an abnormal blood vessel that creates a detour, connecting this portal system directly to the main bloodstream. Because of this detour, blood bypasses the liver entirely, carrying raw intestinal absorption products, including toxins like ammonia, straight to the heart and brain.
"Portosystemic shunts (PSS) are abnormal vessels that divert blood from the abdominal viscera to the heart, bypassing the hepatic sinusoids and carrying intestinal absorption products directly to the systemic circulation."
— Current Techniques in Small Animal Surgery
Without proper blood flow, the liver lacks the necessary growth factors and nutrients it needs to develop, resulting in liver shrinkage, a condition known as hepatic atrophy or microhepatica. The accumulation of unfiltered toxins in the bloodstream leads to hepatic encephalopathy (a syndrome of neurological dysfunction) and various urinary tract complications. While this condition is uncommon, it is a serious illness that requires prompt veterinary intervention.
Most portosystemic shunts are congenital, meaning animals are born with them due to abnormal blood vessel development in the womb. Because certain breeds are far more likely to develop shunts than others, genetics play a major role in this condition.
"Increased breed prevalence suggests a genetic basis to the disease, but this has only been investigated in Irish Wolfhounds, in which an inherited basis of patent ductus venosus has been demonstrated, and in Cairn Terriers with extrahepatic PSSs, in which an autosomal polygenic inheritance or monogenic inheritance with variable expression is suspected."
— Internal Medicine
In Irish Wolfhounds, the condition is inherited as a patent ductus venosus (a specific type of shunt inside the liver). In Cairn Terriers, the inheritance of shunts outside the liver (extrahepatic) is suspected to be polygenic (controlled by multiple genes) or monogenic with variable expression.
Other breeds with a known predisposition to portosystemic shunts include Yorkshire Terriers, Maltese, [Miniature Schnauzers](/p/breeds/miniatureschnauzer_dog), Pugs, Shih Tzus, West Highland White Terriers, Collies, Dachshunds, Golden Retrievers, Havanese, Labrador Retrievers, Old English Sheepdogs, Samoyeds, Australian Cattle Dogs, Bernese Mountain Dogs, Australian Shepherds, Border Terriers, and Schnauzers. Among cats, Persians, Himalayans, and Siamese cats are predisposed.
Clinical signs of a portosystemic shunt typically involve the nervous system, the gastrointestinal tract, and the urinary system. Symptoms often fluctuate and may seem worse shortly after a high-protein meal.
Head pressing is a common neurological sign of toxin accumulation in pets with a portosystemic shunt.
Diagnosing a portosystemic shunt requires a combination of blood tests and advanced imaging to locate the abnormal vessel.
Your vet will start with baseline bloodwork and functional liver tests. The most common screening test is a fasting and 2-hour postprandial bile acids test, which measures how well the liver clears bile acids from the blood after a meal. An ammonia tolerance test or measuring blood ammonia levels can also confirm the presence of hepatic encephalopathy. Additionally, evaluating Protein C activity can help distinguish a shunt from other forms of liver disease.
To visualize the shunt, your vet will recommend imaging studies:
Ultrasonography is a key diagnostic tool used to visualize the abnormal blood vessel and assess liver size.
Treatment for a portosystemic shunt is divided into medical stabilization and surgical correction.
Medical therapy does not cure the shunt, but it helps manage the symptoms by reducing the production and absorption of toxins like ammonia in the digestive tract.
"Therefore the treatment plan includes immediate removal of this protein source with lactulose enemas and reducing ongoing ammonia production and absorption with antibiotic therapy and lactulose."
— Small Animal Critical Care Medicine
Surgery is the preferred treatment for most congenital shunts. Because closing the shunt too quickly can cause a sudden, fatal backup of blood pressure in the liver (portal hypertension), surgeons use specialized devices to close the vessel gradually over several weeks. These devices include ameroid constrictors (rings that slowly swell and close) or cellophane bands that stimulate gradual scarring.
Because of their liver compromise, pets with shunts are highly sensitive to anesthetics. Your veterinary team will carefully select and monitor anesthetic protocols using agents like Propofol, Isoflurane, or Sevoflurane, while avoiding or minimizing drugs like Acepromazine.
The long-term outlook for pets with portosystemic shunts depends heavily on whether they undergo surgery.
Surgical treatment offers a good long-term survival rate, with studies showing an 88% survival rate following successful gradual occlusion. Most surgically treated pets go on to live normal, healthy lives with fully restored liver function.
In contrast, medical management alone carries a much poorer outlook. The long-term survival rate for dogs managed only with medical therapy is 51%, and over half of these medically managed dogs are euthanized within 10 months of diagnosis due to progressive, uncontrollable neurological or urinary signs.
Because congenital portosystemic shunts are inherited genetic anomalies, there is no lifestyle change or medication that can prevent them from developing. Prevention relies entirely on responsible breeding practices.
Dogs and cats diagnosed with a congenital portosystemic shunt should be spayed or neutered and never used for breeding. For high-risk breeds, such as Irish Wolfhounds and Cairn Terriers, breeders should utilize screening tools like bile acid testing to identify affected puppies before they are placed in homes.
If you have a young puppy or kitten of a predisposed breed that is growing slowly, seems unusually quiet, or frequently drools or vomits, schedule an appointment with your veterinarian.
Seek emergency veterinary care immediately if your pet exhibits any of the following red-flag signs:
Treatment must be prescribed by a licensed veterinarian based on your pet. Specific drug doses are intentionally not shown here.
A portosystemic shunt (PSS) is an abnormal blood vessel that bypasses the liver, allowing toxins to build up in the bloodstream and causing neurological, digestive, and urinary issues in dogs and cats.
Anorexia、Ataxia、Behavioral changes、Depression、Diarrhea、Hypersalivation、Lethargy、Seizures
Ammonia tolerance test、Angiography、Computed tomography、Exploratory laparotomy、Fasting and 2-hour postprandial bile acids、Magnetic resonance angiography
Treatment must be prescribed by a licensed veterinarian based on your pet. Specific drug doses are intentionally not shown here.
This article is for general education and is not a substitute for professional veterinary advice. If your pet is unwell, please consult a veterinarian.
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