A portosystemic shunt (PSS) is an abnormal connection between the portal vascular system and systemic circulation. Blood from the abdominal organs which should be drained by the portal vein into the liver is instead shunted to the systemic circulation by the PSS. This means that a portion of the toxins, proteins and nutrients absorbed by the intestines bypass the liver and are shunted directly into the systemic circulation. There are two categories of congenital shunts, extrahepatic (outside the liver) and intrahepatic (inside the liver). While most portosystemic shunts are congenital (the dog or cat is born with the shunt), under certain circumstances portostystemic shunts may be acquired secondary to another problem with the liver (acquired shunts).
In a normal pet, the blood that exits the intestines, spleen, and pancreas enters the portal vein, which then takes blood to the liver. The liver metabolizes and detoxifies this blood. If a shunt is present, the liver is deprived of factors that enhance liver development (hepatotrophic factors), which results in failure of the liver to reach normal size (hepatic atrophy). A common result of hepatic atrophy is hepatic insufficiency, which then combined with the circulating toxins proteins and nutrients frequently results in hepatic encephalopathy (a clinical syndrome of altered central nervous system function due to failure of normal liver function).
The genetic basis of PSS in dogs is unknown, but it is considered congenital and breeds affected include:
- Miniature schnauzers
- Yorkshire terriers
- Irish wolfhounds
- Cairn terriers
- Australian cattle dogs
- Golden retrievers
- Old English sheepdogs
- Labrador retrievers
Single extrahepatic shunts are typically congenital and affect small and toy breeds whereas single intrahepatic shunts affect large breeds. Cats nearly always have extrahepatic shunts and the left gastric is the most common.
Acquired PSS are almost always multiple vessels, which develop in response to hepatic hypertension. They can occur in any breed or age of animal. They are a compensatory mechanism to prevent or delay liver failure. As such, they cannot be ligated without causing severe symptoms, and medical management is the only option for treatment.
Animals with congenital portosystemic shunts may present for:
- small body stature
- anesthetic intolerance - prolonged recovery following an anesthetic event
- behavioral abnormalities
The signs are often episodic and may be more noticeable after eating. These neurological signs are due to the hepatic encephalopathy syndrome. Signs of abnormal neurologic function include:
- ataxia (swaying as if intoxicated)
- head pressing
Other signs may include:
- anorexia (loss of appetite)
- ptyalism (hypersalivation) – most frequently seen in cats
- polyuria/polydipsia (excessive urination/drinking)
- stranguria (difficulty urinating)
- hematuria (blood in the urine)
If your primary care veterinarian suspects that your pet has a portosystemic shunt, a full diagnostic work-up is advised. Some of these diagnostics may be completed by your primary care veterinarian, but you may also be referred to an ACVS board-certified veterinary surgeon or veterinary specialty center for additional diagnostics. A full work-up may include:
- blood work
- liver function tests (bile acids and ammonia) Bile acids are measured after an overnight fast ("preprandial" or fasting) and then 2 hours after eating ("postprandial"). In dogs with PSS, one or both sets of bile acids are increased. Bile acids can increase with any liver disease, so high bile acids are not specific to congenital portosystemic shunts.
- ultrasound (Figure 1)
- nuclear scintigraphy (a non-invasive technique involving colonic administration of a radioisotope)
- portography (an x-ray dye study that specifically highlights the portal system, (Figures 2 and 3))
- CT scan with intravenous contrast
Routine postoperative management includes intravenous fluids and pain medications. Lactulose and diet modification are continued, as it takes time for the liver cells to regenerate and adjust to the new circulation. These medications may be tapered depending on follow-up bile acid test results. Because serum bile acids values may or may not improve, some dogs may need long term treatment whereas others may only need some dietary restrictions or no medical restrictions. After ligation, the liver should regenerate. Failure of the procedure can occur for any of the following reasons:
- failure of shunt to close
- recanalization of the shunt (the shunt reopens)
- the presence of a second, unrecognized shunt (extremely unlikely)
- the development of multiple acquired PSS secondary to portal hypertension or fibrosis (scarring) of the liver
Complications after surgery include portal hypertension, which can lead to loss of proper blood circulation to abdominal organs and death. Animals may show signs of:
- ascites (fluid distension in the abdomen)
- respiratory distress
Use of gradual occlusion devices has substantially reduced the chance of death by portal hypertension.
One of the most problematic but rare complications is development of seizures that are refractory to treatment. This occurs most frequently in toy breed dogs, in the first 1-2 days after surgery. The cause of these seizures is unknown. Seizures may be managed with anti-seizure medication (Keppra –as mentioned previously). In a recent study Keppra (Levetiracetam) was administered to 33% (42/126) of dogs. No dog treated with LEV experienced postoperative seizures, whereas 5% (4/84) of dogs not treated with LEV experienced postoperative seizures. In severe cases, intravenous administration of anti-seizure or anesthetic agents may be required to control seizures. Development of seizures that are poorly controlled by medication is a very poor prognosis.
The prognosis is excellent if the animal survives the immediate postoperative period and full ligation of the shunt is achieved. With partial ligation, the prognosis is not as good. In many cases, full ligation is possible in animals that were partially ligated 4-6 months previously, so follow-up bile acids tests and portal scintigraphy should be done to monitor for shunt function. should be done to monitor for shunt function.