Portosystemic Shunt

A portosystemic shunt (PSS) is an abnormal vascular connection between the portal venous system and the systemic circulation. Normally, blood draining from abdominal organs should travel through the portal vein into the liver for processing. With a PSS (the shunting vessel), this blood is diverted into the systemic circulation instead. As a result, some of the toxins, proteins, and nutrients absorbed from the intestines bypass hepatic filtration, leading to reduced blood flow through the liver, impaired normal hepatic metabolic activity, and direct delivery of these substances into the body’s general circulation.

Congenital shunts are classified into two main types: extrahepatic (outside the liver) and intrahepatic (within the liver). Most PSS are congenital (present at birth in dogs and cats), but in certain situations, portosystemic shunts can also be acquired as a consequence of another liver disorder. In a normal animal, blood leaving the intestines, spleen, and pancreas enters the portal vein and then flows to the liver for detoxification and metabolism. When a shunt exists, the liver is deprived of hepatotrophic factors that support normal liver development, which can prevent the liver from reaching a normal size (hepatic atrophy). A frequent consequence of hepatic atrophy is hepatic insufficiency, and when combined with circulating toxins, proteins, and nutrients, this often leads to hepatic encephalopathy—a clinical syndrome of altered central nervous system function caused by inadequate liver function.

The genetic basis of PSS in dogs is unknown, but the condition is considered congenital, with breeds commonly affected including:

• Miniature Schnauzers
• Yorkshire Terriers
• Irish Wolfhounds
• Cairn Terriers
• Maltese
• Australian Cattle Dogs
• Golden Retrievers
• Old English Sheepdogs
• Labrador Retrievers

Single extrahepatic shunts are usually congenital and are seen more often in small and toy breeds, while single intrahepatic shunts occur more commonly in large breeds. Cats almost always have extrahepatic shunts, and the left gastric shunt is the most common.

Acquired PSS (Figure 1) are almost always multiple vessels that develop in response to portal hypertension. They can occur at any age and in any breed and represent a compensatory mechanism intended to prevent or delay liver failure. Because of this, they should not be surgically ligated—ligation can cause severe clinical consequences—and medical management is the only treatment option.

Figure 1. Acquired multiple portosystemic shunting.

Pets with congenital portosystemic shunts may be presented for:

• small body size
• anesthetic intolerance, such as prolonged recovery after anesthesia
• behavioral abnormalities

Behavioral changes are often intermittent and may be more evident after meals. These neurologic signs result from hepatic encephalopathy, because the liver can no longer adequately detoxify blood coming from the gastrointestinal tract. Signs of neurologic dysfunction may include:

• ataxia (a swaying gait as if intoxicated)
• seizures
• blindness
• head pressing

Additional signs can include:

• anorexia (loss of appetite)
• vomiting
• diarrhea
• constipation
• ptyalism (hypersalivation), most commonly noted in cats
• polyuria/polydipsia (increased urination/drinking)
• stranguria (difficulty urinating)
• hematuria (blood in the urine)

If your primary care veterinarian suspects a PSS, a complete diagnostic evaluation is recommended. Some tests may be performed by the primary veterinarian, but referral to an ACVS board-certified veterinary surgeon or a specialty center may be needed for advanced diagnostics. A full work-up can include:

• bloodwork
• urinalysis
• liver function testing (bile acids and ammonia). Bile acids are measured after an overnight fast (preprandial/fasting) and again 2 hours after eating (postprandial). In dogs with PSS, one or both measurements are elevated. However, increased bile acids can be seen with many liver diseases, so elevated values are not specific to congenital PSS.
• abdominal radiographs
• abdominal ultrasound (Figure 2)
• nuclear scintigraphy (a non-invasive test using colonic administration of a radioisotope)
• portography (a contrast x-ray study that highlights the portal system; Figures 3 and 4)
• CT scan with intravenous contrast

Figure 2. Abdominal ultrasound identifying a portosystemic shunt (PSS).

Figure 3. Normal portogram showing hepatic vasculature.

Figure 4. Portography demonstrating a PSS (large arrow) and absent portal flow to the liver (small arrows).

Medical management

Before surgery, medical stabilization may be necessary to improve overall condition and reduce anesthesia/surgical risk. Medical management typically includes a low-protein diet plus oral antibiotics and lactulose. The objectives are to reduce intestinal bacterial populations and minimize toxin production. Lactulose is an osmotic laxative that lowers bacterial load in the colon and alters colonic pH to decrease ammonia production and absorption. Antibiotics further reduce bacteria that contribute to toxin formation. The diet should contain high-quality protein, but the amount may need to be moderately restricted depending on the individual animal’s clinical signs. If seizures are part of the presentation, anti-seizure drugs may also be used. Keppra (levetiracetam) has possibly been shown to reduce postoperative seizure occurrence, a rare but potentially devastating complication.

Surgical management

For a single PSS, the preferred treatment is surgical attenuation (narrowing) with eventual closure of the abnormal vessel, or complete ligation if the surgeon determines it is safe. Immediate complete ligation should not be performed in all patients, because it can trigger life-threatening portal hypertension, seizures, and death—particularly in animals with underdeveloped portal vasculature. Attenuation (gradual closure) is achieved with techniques such as an ameroid ring constrictor, cellophane band, hydraulic occluder, or intravascular methods. Because this surgery is technically demanding, referral to an ACVS board-certified veterinary surgeon is common.

If the shunt cannot be located during surgery, an intraoperative portogram is performed (Figures 3 and 4). Once identified, portal venous pressure may be measured (Figure 5) to assess whether complete ligation is feasible. Excessively elevated portal pressure (portal hypertension) can be fatal. Acute portal hypertension may cause abdominal distension, pain, bloody diarrhea, ileus (intestinal stasis with gas accumulation), and endotoxic shock (shock due to bacterial toxins).

Partial ligation may be performed by partially tightening a suture around the vessel up to an acceptable pressure increase. About half of patients treated this way will scar down and fully close the shunt, while about half will continue to shunt and require a second surgery months later once the liver has adapted and can tolerate full ligation. This approach is now rarely used for single extrahepatic shunts due to availability of ameroid constrictors, intravascular coils, and cellophane bands. In intrahepatic shunts, partial ligation or transvenous coils may still be used.

The ameroid constrictor (Figures 6, 7) is made of casein within a stainless-steel C-shaped ring and is placed around the shunt vessel, then secured with a small key. Over subsequent weeks, the casein absorbs fluid and swells, progressively narrowing and occluding the vessel (Figure 7), providing gradual closure.

A cellophane band (Figure 8) can also be used; it triggers an inflammatory reaction, and the vessel closes slowly over months.

Transvenous coiling is typically used for larger, intrahepatic shunts. This minimally invasive procedure places coils within the shunt so that it gradually closes over time; coils are stabilized using a metal or metal-alloy stent. The entire procedure is performed through a small puncture in a neck blood vessel. The intent is to improve hepatic function by directing more blood through the liver.

At the time of shunt attenuation, cystotomy may also be recommended if bladder stones are present.

Figure 4. Manometer for measuring portal pressure
Figure 5. Ameroid constrictor bands
Figure 6. Ameroid constrictor band placed on a shunt vessel during surgery
Figure 7. Cellophane band placed around a shunt vessel during surgery

Standard postoperative care includes intravenous fluids and pain control. Lactulose and dietary modification are continued because hepatic regeneration and adaptation to increased blood flow take time. Medications may be tapered depending on follow-up bile acid testing. Because serum bile acids may or may not improve, some dogs need long-term therapy, while others may require only dietary restriction or no ongoing medical limitations. After successful ligation, hepatic regeneration is expected.

Procedure failure can occur due to:

• failure of the shunt to close/attenuate
• recanalization (the shunt reopens)
• an unrecognized second shunt (extremely unlikely)
• development of multiple acquired shunts secondary to portal hypertension or hepatic fibrosis (scarring)

Postoperative complications include portal hypertension, which can reduce blood delivery to abdominal organs and lead to death. Signs may include:

• ascites (abdominal fluid distension)
• vomiting
• diarrhea
• depression
• respiratory distress

Use of gradual occlusion devices has greatly lowered the risk of fatal portal hypertension.

A particularly concerning but uncommon complication is development of seizures that do not respond to treatment. This occurs most often in toy breeds within the first 1–2 days after surgery, and the cause is unknown. Seizures may be managed with anti-seizure medications such as Keppra. In one recent study, Keppra (levetiracetam) was given to 33% (42/126) of dogs; no dog receiving levetiracetam developed postoperative seizures, whereas 5% (4/84) of dogs not receiving it experienced postoperative seizures. Severe cases may require intravenous anti-seizure drugs or anesthetic agents to control seizures. Poorly controlled postoperative seizures carry a very poor prognosis.

Overall prognosis is excellent if the pet survives the immediate postoperative period and complete shunt ligation is achieved. With partial ligation, prognosis is less favorable. In many cases, animals that previously underwent partial ligation can later tolerate full ligation after 4–6 months, so repeat bile acids testing and portal scintigraphy are recommended to monitor shunt function.

Both medical and surgical approaches can support long-term survival in dogs with congenital PSS (CPSS), but statistical analyses support the commonly held view that surgery is preferable. Although surgery is associated with better long-term survival probability, medical management remains an acceptable first-line approach. Dogs that do not achieve adequate control with medical management can later undergo surgical treatment.