What Is a Portosystemic Shunt and How Is It Treated?

A portosystemic shunt is an abnormal blood vessel that allows blood from the digestive organs to bypass the liver and flow directly into the body’s general circulation. Because the liver normally filters toxins, metabolizes nutrients, and processes waste products from the gut, a shunt that skips this step lets harmful substances like ammonia build up in the bloodstream. The condition is best known in veterinary medicine, where it is one of the more common vascular malformations in dogs and cats, but it also occurs in people, either as a rare congenital defect or as a consequence of severe liver disease.

How Blood Is Supposed to Flow Through the Liver

To understand why a portosystemic shunt causes problems, it helps to know what the portal system does. Blood leaving the stomach, intestines, spleen, and pancreas collects into the portal vein, a large vessel that carries nutrient-rich but toxin-laden blood into the liver. Inside the liver, this blood passes through a dense network of tiny vessels where liver cells extract nutrients, break down ammonia into harmless urea, metabolize drugs, and clear bacteria. The cleaned blood then exits through the hepatic veins and joins the general circulation. When a shunt exists, some or all of that portal blood takes a shortcut directly into the systemic veins, skipping the liver’s filtering entirely.

Congenital and Acquired Shunts

Portosystemic shunts fall into two broad categories depending on how they arise. Congenital shunts are present from birth and typically involve a single abnormal vessel. In dogs, they are thought to result from abnormal development of the embryonic venous system. Intrahepatic congenital shunts stem from failure of the ductus venosus, a fetal blood vessel, to close after birth, while extrahepatic congenital shunts form abnormal connections between the splanchnic vasculature and the vena cava or azygos vein outside the liver.¹ Congenital shunts are usually solitary, and affected animals typically lack fluid buildup in the abdomen.

Acquired shunts, by contrast, develop later in life as a response to portal hypertension, which is chronically elevated blood pressure in the portal vein. When the liver becomes scarred or obstructed, blood finds alternative routes around it, opening up multiple small collateral vessels. The hallmarks of acquired shunts are the presence of several abnormal vessels rather than just one, abdominal fluid, and an identifiable underlying cause of portal flow obstruction.² In human medicine, acquired portosystemic shunting most often develops in patients with cirrhosis, where common shunt types include splenorenal, gastrorenal, and dilated paraumbilical veins.³

What a Portosystemic Shunt Looks Like Clinically

The signs of a portosystemic shunt revolve around what happens when the liver can’t do its job. The most dramatic set of symptoms falls under the umbrella of hepatic encephalopathy, a brain condition caused by ammonia and other toxins circulating in the blood instead of being cleared by the liver. In dogs and cats, this often shows up as disorientation, aimless wandering, head pressing against walls, circling, staring into space, or seizures. These neurologic episodes tend to come and go, sometimes worsening after a protein-rich meal because digesting protein generates more ammonia.

Beyond the brain, affected animals often grow poorly. Puppies with congenital shunts are frequently the runts of their litters, underweight and slow to thrive. Gastrointestinal symptoms like vomiting, diarrhea, and excessive drooling are common. Another underappreciated consequence is urinary stone formation. Because ammonia and uric acid are metabolic waste products normally handled by the liver, blood levels of both rise when the liver is bypassed. The kidneys then excrete high concentrations of these substances, creating ideal conditions for ammonium urate crystals and stones to form in the bladder or kidneys.⁴ Some dogs are diagnosed with a shunt only after showing up for what initially looks like a urinary problem.

In humans, congenital portosystemic shunts are rare but can produce a similar range of problems including hepatic encephalopathy, pulmonary hypertension, and liver tumors. An international study of patients with congenital extrahepatic shunts, sometimes called Abernethy malformation, found that roughly a quarter developed hepatic encephalopathy over long follow-up, and a substantial number developed benign liver nodules, adenomas, or even hepatocellular carcinoma.⁵

How Shunts Are Diagnosed

Suspicion usually starts with blood work. The two most commonly used screening tests in veterinary medicine are fasting blood ammonia and serum bile acids. Both rise when portal blood bypasses the liver, but they have different strengths. In one large study of over 300 dogs, fasting ammonia was both highly sensitive and fairly specific for detecting portosystemic shunting, while bile acids were somewhat less sensitive and considerably less specific, particularly in dogs that already had liver disease.⁶ A separate study that looked at combined test performance found that having both bile acids and ammonia elevated together gave a very high positive predictive value, essentially making the shunt diagnosis near-certain.⁷

Blood tests flag the problem, but imaging nails down the anatomy. Ultrasound is often the first-line imaging tool because it is widely available and noninvasive. However, it has real limitations. In a study comparing ultrasound to CT angiography in dogs, CT angiography detected shunts with about 96% sensitivity versus 68% for ultrasound.⁸ CT angiography was also far better at identifying exactly where the shunt originated and where it connected, information that matters enormously for planning surgery. For that reason, CT angiography has become the gold standard for shunt mapping in many referral centers.

Nuclear scintigraphy is another option. In this procedure, a radioactive tracer is given rectally, and a gamma camera measures how much of it bypasses the liver. Animals with portosystemic shunts had an average shunt fraction of about 84%, meaning that proportion of portal blood was skipping the liver, compared to around 5% in healthy controls.⁹ Scintigraphy confirms that shunting is happening and quantifies how severe it is, but it doesn’t provide the fine anatomic detail that CT does.

Medical Management

Whether or not surgery is planned, medical treatment typically comes first. The goal is to reduce the ammonia and other toxins that are reaching the brain. The most widely used drug is lactulose, a synthetic sugar that acts in the gut. It draws ammonia into the colon, traps it as ammonium, and moves it out with the stool. Lactulose combined with the antibiotic rifaximin, which reduces ammonia-producing bacteria in the gut, is a well-established combination for managing hepatic encephalopathy in cirrhosis patients.³ In veterinary practice, metronidazole or neomycin are sometimes used in place of rifaximin for the same purpose.

Diet is another important pillar. The aim is to feed enough protein to maintain body condition and liver repair, but not so much that excess ammonia overwhelms the system. Expert consensus recommends that patients with hepatic encephalopathy get adequate calories and protein, ideally spread across multiple small meals throughout the day rather than one or two large ones. A late-night snack of complex carbohydrates can help prevent the body from breaking down its own muscle for energy overnight, a process that also generates ammonia. Diets emphasizing vegetable and dairy protein rather than red meat tend to be better tolerated.¹⁰ In veterinary patients, commercially available liver-support diets serve a similar role.

Medical management alone can keep some animals and people comfortable for months or years, but it has limits. It does not fix the shunt; it manages the fallout. Over time, the liver may continue to atrophy from lack of blood flow, and clinical signs can progressively worsen despite treatment.

Surgical Treatment for Congenital Shunts

Surgery is the definitive treatment for most congenital portosystemic shunts, and the specific approach depends on whether the shunt is inside or outside the liver.

For extrahepatic shunts, the classic technique is surgical ligation, meaning the abnormal vessel is tied off with suture material. When the shunt can be safely closed all at once, complete ligation is performed. In many cases, however, closing the vessel abruptly would force too much blood into a liver that has been underdeveloped and unprepared for full portal flow, causing dangerous spikes in portal pressure. To avoid this, gradual occlusion devices have become popular. Ameroid ring constrictors and cellophane bands are placed around the shunt vessel during surgery. The ameroid ring slowly swells as it absorbs body fluid, gradually narrowing the vessel over weeks. Cellophane bands cause a slow inflammatory reaction that achieves the same progressive closure.¹¹ Both methods give the liver time to grow and adapt to increasing blood flow.

For intrahepatic shunts, which sit deep within the liver tissue and are harder to reach surgically, minimally invasive techniques have gained ground. Percutaneous transvenous coil embolization (PTCE) involves threading a catheter through a vein, navigating it into the shunt, and deploying metal coils that block blood flow. In a study of 20 small- and toy-breed dogs treated with PTCE for intrahepatic shunts, clinical signs resolved in 95% of cases, with one- and two-year survival rates of 92%.¹² Embolization techniques are also used in human patients; one series showed complete or near-complete obliteration of intrahepatic shunts in all treated patients, with improvement in encephalopathy symptoms.¹³

Post-Operative Risks

Surgery for portosystemic shunts carries meaningful risks, and the most feared complications relate to either the portal system or the brain.

Portal hypertension after shunt closure occurs when the liver can’t yet handle the volume of blood being redirected into it. In one study of dogs treated with suture ligation, about 8% developed post-operative portal hypertension, presenting with abdominal fluid, low protein levels, or other signs of vascular overload. Most resolved without additional surgery, but one dog was euthanized due to persistent, unresponsive effusion.¹⁴ This risk is why gradual occlusion methods were developed and why surgeons measure portal pressures during the procedure before deciding how much to close.

Post-attenuation neurologic signs (PANS) are a separate and sometimes devastating complication. Within the first few days after surgery, some dogs develop seizures, blindness, tremors, or status epilepticus that can be fatal. The exact cause remains debated, but risk factors include having hepatic encephalopathy before surgery, being older at the time of surgery, and possibly certain breeds or shunt types.¹⁵ There was hope that giving anti-seizure medication before surgery might prevent PANS, but a systematic review of the evidence found that prophylactic levetiracetam does not reliably reduce seizure rates after extrahepatic shunt surgery.¹⁶ Many surgeons still use it empirically, but the evidence isn’t strong.

Surgery Versus Long-Term Medical Care

For congenital shunts in dogs, the data consistently favor surgery when it’s feasible. One study found that medically treated dogs were roughly three times more likely to die during the follow-up period compared to surgically treated dogs.¹⁷ A second, longer-term study painted an even starker picture: surgically treated dogs had dramatically better survival, with a hazard ratio of about 8, meaning medically managed dogs were roughly eight times more likely to die during the study period. Surgical dogs also had fewer ongoing clinical signs, and that gap widened over the years.¹⁸

That said, medical management is not a dead end. Some dogs, particularly those that are poor surgical candidates due to age, multiple shunts, or concurrent conditions, can live for years on medication and diet alone. The research frames it as an acceptable first-line option when surgery isn’t possible, rather than a preferred alternative to it.

TIPS and Intentionally Created Shunts in Human Medicine

In a twist that might seem paradoxical given everything above, doctors sometimes create portosystemic shunts on purpose. The transjugular intrahepatic portosystemic shunt, or TIPS, is a procedure in which an interventional radiologist threads a catheter through the jugular vein and creates an artificial channel through the liver connecting the portal vein to a hepatic vein, then holds it open with a metal stent. This deliberately diverts some portal blood away from the liver to relieve dangerous portal pressure.

TIPS is primarily used for complications of cirrhosis that haven’t responded to other treatments, especially variceal bleeding (life-threatening hemorrhage from swollen veins in the esophagus or stomach) and refractory ascites (abdominal fluid that won’t go away with diuretics). The procedure is effective at lowering portal pressure, but it creates the same fundamental trade-off as a naturally occurring shunt: diverting blood away from the liver means more toxins reach the brain. Hepatic encephalopathy after TIPS has been reported in anywhere from 7% to 61% of patients, depending on the study and the population.¹⁹ Risk factors include older age, worse liver function, and low sodium levels.

Early TIPS procedures had significant problems with stent clogging over time. The introduction of covered stents improved long-term patency considerably, but hepatic encephalopathy and occasional worsening of liver function remain ongoing issues.²⁰ A study of TIPS placed specifically for portal vein thrombosis in cirrhosis patients found a one-year encephalopathy rate of 25%.²¹ TIPS is not a cure for liver disease; it is a calculated intervention that accepts one set of risks to prevent a more immediately lethal complication.

Abernethy Malformation in Children and Adults

While most congenital portosystemic shunts in humans are discovered incidentally or through investigating vague symptoms, the condition known as Abernethy malformation can cause serious complications that evolve over a lifetime. It comes in two main types. In the more severe form, the portal vein is completely absent, and all splanchnic blood drains directly into the systemic circulation. In the more common partial form, the portal vein exists but a large anomalous vessel diverts most of the flow. Because these shunts are present from birth, the liver develops in the absence of its normal blood supply, and the consequences can be varied and surprising.

A case series from Slovenia illustrates how different the presentations can be even among children from the same country. One child presented at age six with gastrointestinal bleeding and elevated ammonia, was treated with percutaneous embolization of the shunt, and has stayed symptom-free since. Another infant had elevated bilirubin at birth, but the intrahepatic shunt spontaneously closed on its own. A third child developed pulmonary hypertension and required liver transplantation at age five. Two other patients were found to have benign liver nodules but remained stable on conservative therapy.²²

The international observational study found that complications can emerge decades after birth. Liver tumors, both benign and malignant, developed at median ages of 18 and 39 years respectively, and pulmonary hypertension affected multiple patients. Shunt closure, whether surgical or endovascular, improved or stabilized disease in the patients who received it, leading the authors to recommend it both therapeutically and preventively.⁵ In a recently reported case from Kazakhstan, endovascular shunt closure in a child with severe hepatopulmonary syndrome raised oxygen saturation from 60% to 98% and produced dramatic clinical improvement sustained over five years of follow-up.²³

Hepatic Microvascular Dysplasia and Near-Miss Diagnoses

Not every dog suspected of having a portosystemic shunt actually has one. A condition called hepatic microvascular dysplasia, also known as portal vein hypoplasia, mimics many of the same blood test abnormalities. In a retrospective analysis of 140 dogs taken to surgery for suspected shunts, about 10% turned out not to have a visible shunt at all. Most of those dogs were ultimately diagnosed with microvascular dysplasia, and it was especially common in Yorkshire Terriers.²⁴ In microvascular dysplasia, the problem is at the microscopic level within the liver, so standard imaging won’t reveal an abnormal vessel. The condition is generally milder, and many affected dogs can be managed long-term with diet and medications alone. Awareness of this look-alike diagnosis matters because it can prevent unnecessary surgery and set more realistic expectations for treatment.