A portosystemic shunt (PSS) is an abnormal connection between the portal circulation and the systemic circulation. In many cases, these connections occur between the splenic, phrenic, cranial mesenteric, caudal mesenteric, gastric, or gastroduodenal veins and the caudal vena cava or azygos vein. 

Congenital shunts occur when the fetal circulatory system fails to mature normally, similar to a patent ductus arteriosus or persistent right aortic arch. Intrahepatic congenital shunts are most commonly observed in large breed dogs, while extrahepatic congenital shunts are typically seen in small breed dogs (although there are certainly exceptions in both cases). Single congenital shunts can often be treated surgically.

Some shunts that are diagnosed at an early age are not truly congenital. Microvascular dysplasia (MVD), which is most common in small breed dogs, is actually an effect of congenital portal vein hypoplasia (PVH). Although MVD is often diagnosed at an early age, it is not a true congenital condition. Microvascular dysplasia is not surgically treatable. 

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Extrahepatic multiple acquired shunts are another type of acquired shunt. They develop later in life, as a result of end-stage liver disease. In this situation, the extra vessels develop as a means to overcome insurmountable blood pressures within the liver. These shunts cannot be corrected surgically. 

Clinical Signs of PSS 

The typical canine PSS patient is small and underweight, often appearing generally “unthrifty.” These dogs may be smaller than their littermates because their growth has been stunted. If anesthetized, they tend to recover slowly. Neurologic abnormalities, such as seizures, depression, ataxia, and behavioral changes may be observed. Vomiting and diarrhea may be noted and, in severe cases, ascites may develop. 

Suspicious Laboratory Findings 

In many cases, the suspicion of a PSS first arises when a young dog has preanesthetic blood work performed prior to spay or neuter surgery. In many cases, this leads to surgery being postponed until further diagnostic testing can be performed. 

Lab findings suggestive of PSS include: 

• Elevated liver enzymes (ALP, ALT, and/or AST)
• Microcytic anemia
• Decreased BUN
• Hypoglycemia
• Hypoalbuminemia
• Isosthenuria/hyposthenuria

Seeing one or more of these abnormalities, especially if combined with clinical signs suggestive of a PSS, may lead a veterinarian to recommend additional liver diagnostics.

Bile Acids Testing

If a PSS is suspected, bile acids testing is typically the first step in the diagnostic workup. While fasting and postprandial bile acids testing can both provide useful information, they are often combined into pre- and post-prandial bile acids tests. In this test, a blood sample is drawn, the dog is fed a meal, and a postprandial blood sample is drawn two hours later. This test is non-invasive and relatively low-cost, which makes it a valuable screening test to help determine whether a further workup is warranted.

In normal dogs, fasting and postprandial bile acids typically range from 5-15 umol/L. Fasting bile acids >25 umol/L suggest a shunt or other liver disease. Most dogs with PSS or another functional liver abnormality will also have markedly increased postprandial bile acids, with values often >75 umol/L. Importantly, elevated bile acids are not specific for a PSS; they can be observed with any liver disease. However, in a puppy that belongs to a predisposed breed, a PSS is often the most likely differential diagnosis. 

Unfortunately, bile acids testing is not entirely sensitive for the presence of a PSS. In one study, it was estimated that over 10% of dogs with PSS will have normal fasting and post-prandial bile acids.¹ Therefore, further testing may be necessary even in the presence of normal bile acids, if the dog’s clinical signs and laboratory tests strongly suggest the possibility of a PSS. 

Imaging 

Imaging is typically used to definitively diagnose a PSS. It’s important to confirm the diagnosis and not stop with just bile acids testing, so the owner can make educated decisions regarding treatment. When you talk to owners about bile acids testing, it’s important to explain that additional testing may be required, depending on the results of the bile acids test. 

Ultrasound is often the first step in the workup of a suspected shunt. Intrahepatic shunts, in particular, often involve large vessels that can be easily visualized on ultrasound. Other shunts may be more difficult to detect, but their presence may be suggested by ultrasound abnormalities including decreased portal vein blood flow, turbulent hepatic blood flow, bilateral renomegaly, and visible sediment within the bladder and kidneys. 

If ultrasound alone cannot confirm the diagnosis, advanced imaging is recommended. These tests may include contrast portography (with contrast medium injected into the jejunal or splenic vein), nuclear scintigraphy, or computed tomography (CT). These tests provide a clearer view of the hepatic circulation, allowing the definitive diagnosis of a shunt. 

Careful Communication is Essential 

One of the biggest challenges in these cases isn’t the medicine; it’s the client communication. Depending on the practice in which you’re working, you may play a significant role in that communication, talking to owners about their pet’s laboratory findings and/or recommending tests such as bile acids testing or advanced imaging. 

In many cases, a suspected PSS comes to our attention when the dog receives preanesthetic blood work prior to spay or neuter surgery. The owner drops their dog off at the practice with no idea that there’s a problem, then suddenly we’re calling them to recommend postponing surgery and testing for a liver condition that wasn’t even on their radar. This can lead to one of two responses: denial or panic! Handling this scenario requires a delicate balance between leaving the client too concerned or not concerned enough, but walking that tightrope becomes easier with experience! 

References

1. Tobias, KM. Portosystemic Shunts.