Portosystemic Shunt in Dogs: Signs, Tests and Treatment

A portosystemic shunt, often called a liver shunt, is a blood vessel that lets blood from a dog's gut skip the liver. The liver never gets to clean that blood, so toxins such as ammonia travel straight to the brain, and the liver itself stays small.

Most dogs with a shunt are born with it. The classic picture is a small-breed puppy that never grew quite right and has strange spells: staring, wobbling, pressing its head against the wall, sometimes after a meal. Bladder stones can be the only clue. A bile acid blood test raises the suspicion, imaging confirms it, and a single congenital shunt can often be closed with surgery.

What a Liver Shunt Does to the Blood Flow

In a healthy dog, blood leaving the stomach, intestines, spleen and pancreas collects in one large vessel, the portal vein, which carries it into the liver. The liver filters out toxins and processes nutrients, then sends the cleaned blood on to the heart.

A shunt is a detour. Part of that blood takes a side road into the general circulation, usually the caudal vena cava (the big vein running back to the heart), and never passes through the liver. The American College of Veterinary Surgeons (ACVS) describes two results:

• The liver stays small. Portal blood carries growth factors the liver needs. Without them it never reaches normal size and works less well.
• Toxins reach the brain. Ammonia and other substances from digestion are no longer cleared, and they disturb brain function. Vets call this hepatic encephalopathy (HE).

Congenital vs Acquired Shunts

Most shunts are congenital. Some develop later because of liver disease, and the treatment is completely different.

Congenital shunts come in two types. Extrahepatic shunts run outside the liver, joining the portal system to the vena cava or azygos vein, and are seen mostly in small and toy breeds. Intrahepatic shunts sit inside the liver and are seen mostly in large breeds. Usually the ductus venosus, a vessel that carries blood through the liver of the unborn puppy, fails to close after birth.

Acquired shunts open when pressure in the portal vein rises, most often from chronic liver disease with scarring. Clots in the portal vein or abdominal masses pressing on it can do the same. These vessels relieve the pressure, so, as the Merck Veterinary Manual stresses, surgeons must never tie them off.

Which Breeds Are at Risk?

Purebreds are affected more often, but any dog, mixed breeds included, can have a shunt. The breed pattern follows the shunt type:

• Small breeds, shunt usually outside the liver: Yorkshire Terrier (overrepresented in most reports), Maltese, Shih Tzu, Havanese, Pug, Miniature Schnauzer, Cairn Terrier, Papillon, Norfolk Terrier and Tibetan Spaniel.
• Large breeds, shunt usually inside the liver: Labrador Retriever, Golden Retriever, Irish Wolfhound (inheritance has been shown in related dogs) and Old English Sheepdog. Australian Cattle Dogs, German Shepherd Dogs and Doberman Pinschers are also reported.

There are plenty of exceptions, and predisposed breeds differ from country to country, notes a 2023 review (Konstantinidis et al., Veterinary Sciences). No single gene has been found, so dogs diagnosed with a congenital shunt should not be bred.

Signs of a Portosystemic Shunt in Dogs

Here is what to watch out for. Signs often come and go, and many affected dogs simply seem quiet or dull. They usually show before 1 year of age, but some shunts are found at 5 or even 10 years, and Merck notes that 10% to 20% of affected animals show no signs at all.

• Growth: smaller than littermates, poor muscle, not gaining weight.
• Brain and behavior: dullness, staring into space, disorientation, pacing, circling, pressing the head against walls, wobbly walking, aggression, blindness that comes and goes, drooling, seizures, collapse and, in severe episodes, coma.
• Urinary: drinking and peeing more, blood in the urine, straining, and ammonium urate stones that can block the flow of urine.
• Stomach and gut: vomiting, diarrhea, poor appetite, weight loss, eating non-food items.
• Anesthesia: a slow recovery from sedation, because the liver normally clears many of these drugs. A long recovery after a spay or neuter is sometimes the first hint.

Worse after meals? Often, not always. A meal sends a fresh load of ammonia from the gut, so signs that flare after eating point strongly toward a shunt. But only 30% to 50% of affected dogs show a clear link with food, according to the 2023 review, so a dog without "after dinner" episodes can still have one.

Bladder stones are sometimes the only reason a shunt is found. These urate stones often don't show on plain X-rays, and, as Merck notes, they don't dissolve with medicine or diet.

When to Call the Vet

Call now (emergency):

• A seizure, collapse, or your dog can't be roused
• Straining to pee with little or no urine coming out
• After shunt surgery: a seizure, a swollen or painful belly, bloody diarrhea, or sudden weakness

See your vet soon:

• Episodes of staring, wobbling, circling or head pressing, especially after meals
• A puppy that is clearly smaller than its littermates or isn't gaining weight
• Blood in the urine, straining, or crystals on a urine test
• An unusually slow recovery from anesthesia or sedation
• Drinking and peeing much more than usual

Mention at your next visit:

• You have a Yorkie, Maltese or other small-breed puppy prone to liver blood-flow problems. Merck suggests a bile acid test for these puppies at around 6 months, so that a high result later in life can be read correctly.

What Can Make Signs Flare Up

In a dog with a shunt, everyday situations can set off an episode. Merck lists high-protein meals (especially red meat and fish), dehydration, vomiting or not eating, constipation, infections, bleeding into the gut, low blood sugar and some medicines, including certain sedatives.

So don't give treats, supplements or over-the-counter medicines your veterinarian hasn't approved, and tell every vet and emergency clinic about the shunt before any sedation or new prescription.

How Vets Diagnose a Liver Shunt

Your veterinarian may start the workup and refer you to a specialist center for imaging and surgery.

Blood and urine tests

Routine bloodwork often hints at a shunt. Typical findings:

• Small red blood cells, sometimes a mild anemia, and target cells on the blood smear
• Low albumin, low BUN (blood urea nitrogen) and low cholesterol, because the small liver makes less of them
• Low blood sugar, mainly in very small toy-breed puppies
• Liver enzymes that are normal or only mildly raised (see our guide to elevated liver enzymes in dogs)

A urinalysis often shows dilute urine and ammonium biurate crystals.

Bile acid test

When blood bypasses the liver, bile acids that should be cleared stay in the bloodstream. The test uses two blood samples: one after an overnight fast and one about 2 hours after a meal. With a shunt, one or both are usually high.

A high result shows that liver function or blood flow is abnormal, not that a shunt is definitely there: other liver diseases raise bile acids too, and healthy Maltese can have raised bile acids without any liver problem. Some Yorkies, Maltese and other small dogs have high bile acids because of microvascular dysplasia (MVD), a look-alike problem in the liver's tiny vessels that usually causes no signs and needs no treatment, according to Merck.

Imaging

Blood tests suggest a shunt; imaging finds it.

• Abdominal ultrasound is the usual first scan and needs only sedation. In skilled hands it can show the abnormal vessel, a small liver and stones, but shunts outside the liver can be missed.
• CT angiography (a CT scan with contrast injected into a vein) is now the imaging test of choice. It maps the shunt for surgery but needs general anesthesia.
• Scintigraphy (a nuclear scan) gives a yes-or-no answer about shunting but doesn't show where the vessel is.

Treatment

Medical management comes first

Almost every dog with a shunt starts on medical management. It doesn't close the shunt, but it lowers the toxins the gut produces and absorbs, before surgery, during recovery, or long term when surgery isn't an option. Your veterinarian chooses the products and doses:

• A liver-friendly diet. Highly digestible food, often a prescription liver diet, in small, frequent meals. Cutting protein hard is outdated advice: dogs lose muscle, which can make ammonia problems worse, so dogs without active signs get as much quality protein as they tolerate. Dairy and vegetable proteins are often preferred over red meat.
• Lactulose. A sugar the gut can't absorb, which traps ammonia in the colon so it leaves in the stool. The vet sets and adjusts the amount; too much causes diarrhea.
• Gut-acting antibiotics, added when needed to reduce the bacteria that make ammonia.
• Anti-seizure medicine, if seizures are part of the picture.

Surgery to close the shunt

For a single congenital shunt, surgery is the treatment of choice, say the ACVS and Merck. Closing the abnormal vessel sends blood through the liver again so it can grow.

Most dogs can't handle having the shunt shut all at once. Their portal veins inside the liver are underdeveloped, and sudden full closure can cause portal hypertension, a dangerous pressure rise with a swollen belly, bloody diarrhea and shock. So surgeons usually close shunts gradually:

• Ameroid constrictor: a steel ring lined with casein (a milk protein) placed around the shunt. The casein slowly swells, and with the scar tissue it triggers, the shunt closes over roughly 2 to 5 weeks.
• Cellophane band: a strip of special film around the shunt that closes it through slow inflammation over weeks to months.
• Coil embolization: a minimally invasive option, mainly for shunts inside the liver. Coils placed through a neck vein make the shunt clot and narrow.
• Suture ligation: tying the vessel off fully or partly. After partial ligation about half of dogs need a second surgery, so it is used less often now.

Shunts inside the liver are harder to reach than those outside it, and results are less predictable. If bladder stones are present, the surgeon may remove them during the same anesthesia.

Recovery After Shunt Surgery

Most dogs stay in the hospital for a few days, then go home with restricted exercise and the same diet and medicines as before. The vet tapers those medicines as the liver grows, guided by follow-up liver tests, often about 2 to 3 months after surgery.

The first days carry the main risks. Call immediately about:

• Seizures or other neurological signs. The 2023 review reports them in about 4% to 12% of dogs, mostly small breeds, usually within 3 days of surgery. Some specialists give anti-seizure medicine before surgery to try to prevent this, but sources differ on whether it helps: the ACVS says it may, while Merck cites a large study that found no benefit. Your surgeon decides.
• Portal hypertension: a swollen or painful belly, vomiting, bloody diarrhea, weakness.

Outlook for Dogs With a Liver Shunt

When a single congenital shunt is fully closed and the dog gets through the first days after surgery, the ACVS calls the prognosis excellent. Many of these dogs live a normal life without daily medicine.

Surgery often gives the best long-term outcome for a single shunt outside the liver, but sources differ. The ACVS and the 2023 treatment review say dogs treated with surgery tend to live longer, while Merck notes that some dogs with mild signs live well on diet and medicine alone. Your vet weighs your dog's case. Dogs managed without surgery stay at risk of flare-ups.

A few dogs keep a little blood flowing through the shunt after surgery, mostly without signs, and a few develop acquired shunts later and need lifelong medical care. With acquired shunts, the outlook depends on the liver disease behind them.

Questions to Ask Your Vet

• Do the bloodwork and bile acids point to a shunt, or could it be microvascular dysplasia?
• Is the shunt likely outside or inside the liver, and what does that mean for surgery?
• Which diet and treats are safe, and what should I do if an episode starts?

Sources

• American College of Veterinary Surgeons: Portosystemic Shunts
• Merck Veterinary Manual: Hepatic Portal Venous Hypoperfusion in Small Animals (portosystemic vascular anomalies and microvascular dysplasia)
• Merck Veterinary Manual: Acquired Portosystemic Shunts in Small Animals
• Merck Veterinary Manual: Hepatic Encephalopathy in Small Animals
• Merck Veterinary Manual (dog owners): Disorders of the Liver and Gallbladder in Dogs
• Konstantinidis et al. (2023), Congenital Portosystemic Shunts in Dogs and Cats: Classification, Pathophysiology, Clinical Presentation and Diagnosis, Veterinary Sciences
• Konstantinidis et al. (2023), Congenital Portosystemic Shunts in Dogs and Cats: Treatment, Complications and Prognosis, Veterinary Sciences
• MSPCA-Angell: Imaging of Congenital Portosystemic Shunts
• Willows Veterinary Centre: Portosystemic (Liver) Shunt

This article is for general information and is not a substitute for advice from your veterinarian.