What is a portosystemic shunt?
Portosystemic shunts (PSS) are vascular anomalies which result in direct communications between the portal vascular system and the systemic circulation. They allow blood from the intestines that contain products of digestion to enter directly into the systemic circulation without passing through the liver. The liver is normally a vital organ to help detoxify these various digestive products. If shunting is occurring, the brain is exposed to significantly higher levels of molecular substances that can disrupt normal neural function. These molecular substances include ammonia, methionine, short-chain fatty acids and gamma-aminobutyric acid. These molecules can have a number of effects on brain function, resulting in a syndrome known as hepatic encephalopathy. A variety of bizarre behavioural and nervous signs relate to the effect of these normal digestive by-products on the brain. Other clinical signs relate to the diversion of normal hepatotrophic factors released by the pancreas and intestines from liver, which results in poor liver growth and function.
Are they all the same?
Portosystemic shunts can be single or multiple. They may be large, discrete structures, or shunting may occur at the capillary level (microvascular dysplasia). Single shunts are the most commonly diagnosed and account for approximately 80% of the PSS seen dogs. Single PSS are usually congenital in origin and can be located within the hepatic parenchyma (intrahepatic) or outside of the hepatic parenchyma (extrahepatic). Multiple extrahepatic PSS usually have an acquired aetiology and account for approximately 20% of PSS seen in dogs. The vessels involved in acquired PSS are normal non-functional microvascular connections between the portal and systemic circulatory systems which become patent in response to elevations in portal blood pressure. Microvascular dysplasia is more common in small and toy breeds, with Yorkshire terriers the predominant breed.
Our ability to recognise and detect portosystemic shunts has improved significantly over the last 10-15 years as knowledge of the condition has been more widely recognised, and laboratory detection has become more widely available. Because of this, a greater spectrum of clinical disease syndromes are now becoming apparent. Therefore, although the classic ‘text-book’ presentation of the small, runty puppy with bizarre neurological signs should prompt investigation for a portosystemic shunt, much more subtle gastrointestinal or behavioural signs may occur in less severely affected dogs.
What animals are usually diagnosed with a shunt?
Most single PSS are diagnosed in puppies, kittens and young adults less than one year old. However, middle aged to older dogs and cats may also present with signs consistent with portosystemic shunting. Single extrahepatic PSS are most frequently diagnosed in small breed dogs, especially Yorkshire Terriers, Maltese, and Miniature Schnauzers. The larger breed dogs such as Irish Wolfhounds and Labrador retrievers, and medium-sized Australian Cattle dogs and Australian shepherds, are predisposed to intrahepatic portosystemic shunts. Congenital intrahepatic PSS have been proven to be hereditary in Irish Wolfhounds. Of the small breed dogs, miniature poodles may be more likely to have intrahepatic shunts. Single PSS are also seen in cats, with most cats having extrahepatic PSS. Multiple extrahepatic PSS are usually diagnosed in dogs and cats between the age of 1 and 7 years, but may be seen in younger puppies. Breeds of dogs most commonly affected with multiple PSS include German Shepherd Dogs, Doberman Pinschers, and American Cocker Spaniels.
What are the signs of a portosystemic shunt?
Clinical signs of PSS are the result of hepatoencephalopathy and/or functional impairment of the liver. Clinical signs are frequently intermittent. Neurologic signs include mild disorientation, bizarre behaviour, seizures, blindness, or coma. Gastrointestinal signs include vomiting, diarrhoea, ptyalism, abdominal pain, intense borborygmus or anorexia. Other abnormalities that may be seen include weight loss, a failure to thrive, polydipsia, polyuria, ascites, and formation of urate urinary calculi.
Clinical signs in cats can be less dramatic than those observed in the dog, and may be intermittent in nature. Neurological signs include abnormal mentation, behavioural changes, aggression, ptyalism, seizures and other neurologic signs, and blindness. Gastrointestinal signs are less frequently observed, but ptyalism is commonly seen (75% of cases). A few cats will exhibit polydipsia, polyuria, and intolerance to sedatives. Some cats with congenital PSS will have unusual copper-coloured eyes.
How is a portosystemic shunt diagnosed?
An appropriate history and characteristic clinical signs are supportive of a diagnosis of PSS. Although young age is often an important clue to the potential diagnosis, animals with PSS (especially those with acquired shunts or microvascular dysplasia) can present at any age.
Clinical investigation is usually commenced with a complete blood count and biochemical evaluation. Many shunts will demonstrate a classic ‘foot-print’ on routine blood screening which will provide further support for the diagnosis to prompt more in-depth investigation. The ‘footprint’ of a PSS includes anaemia, mild-moderate elevations of liver enzymes, low urea, and low protein. Dogs with intrahepatic PSS often have significantly higher ALP levels then those with extrahepatic shunts. Ammonium biurate crystals may be seen on routine urinalysis and urate calculi may be present in the urinary tract.
Specific diagnostic testing for portosystemic shunting includes fasting and postprandial bile acid measurement, abdominal ultrasound evaluation, and transcolonic nuclear scintigraphy.
Bile acid measurement is a relatively straightforward diagnostic test and detects circulating levels of bile acid pre- and two hours post- feeding. Bile acids should not normally enter systemic circulation as they are rapidly cleared from the portal circulation on the ‘first-pass’ through the normal liver. It is important that both pre-prandial and post-prandial samples are obtained, as fasting levels may be normal in some animals with a portosystemic shunt. In addition, provision of an appropriate ‘fatty’ meal is necessary to promote adequate gall-bladder contraction. However, care must be taken not to precipitate an encephalopathic crisis by feeding too generous a meal. Ideally, feed a protein restricted meal laced with a small quantity of corn oil.
Does an X-ray help with diagnosis?
Plain radiographs of the abdomen may reveal microhepatica and enlargement of the kidneys. Microhepatica is suggested by the cranial displacement of the gastric axis, and a liver shadow contained well within the costal arch. Urate calculi are radiolucent and may not be visible on plain radiographic assessment only.
Small liver size may again be supported by ultrasound examination. The shunting vessel may also be able to be visualised with abdominal ultrasound, but this largely depends on the expertise of the ultrasonographer. However, large intra-hepatic shunts can often be identified without too much difficulty. The smaller size, variable location, and the presence of overlapping structures and gas-filled viscus can complicate detection of extra-hepatic shunts. Sometimes, evidence to support disturbed blood flow in either portal or vena cava may be detected with Doppler techniques, even if the individual vessel cannot itself be visualised. Flow through all blood vessels is usually laminar, so the observance of turbulent flow is suggestive of a disruption to normal anatomical structures. Ultrasound examination is also useful to allow detection of the presence of renal or bladder calculi.
Contrast portography (CT scan)
Venous portography remains the most definitive diagnostic test to confirm or refute the presence of a PSS. Whether a portogram is performed depends to some degree on the experience and knowledge of normal abdominal vascular anatomy by the individual surgeon, and pre-operative assessment as to the likelihood of the shunt being intra-hepatic or extra-hepatic in location. Most experienced surgeons can identify extra-hepatic shunts on direct visual assessment at laparotomy, and therefore will not do a portogram prior to surgery. However, if the PSS cannot be directly visualised, facilities to perform a portogram are immediately available. This approach can spare the patient an additional procedure, and also limits the length of a possible anaesthetic in these very compromised patients. Pre-operative performance of a portogram is recommended if the shunt is suspected to be intrahepatic in location, or if the surgeon is experienced with normal portal vascular anatomy and direct shunt detection.
The presence of portosystemic shunting can be confirmed with a high degree of accuracy with transcolonic nuclear scintigraphy. This test involves giving the patient an enema with a radionucleide (99mtechnetium-pertechnetate), a compound that is rapidly absorbed, unchanged, across the colonic mucosa. Dynamic imaging in the first 1 to 2 minutes following colonic administration provides a nuclear angiogram of the portal system. In an animal that does not have portosystemic shunting of blood away from its liver, liver activity will precede heart activity by several seconds. In patients with PSS, this situation is reversed. Blood shunted through the PSS bypasses the liver and enters the systemic circulation directly allowing nuclear activity to be detected in the heart and lungs before it is detected in the liver. Nuclear scintigraphy cannot differentiate between single and multiple PSS, and does not help identify the anatomic location of the PSS. Nevertheless, nuclear scintigraphy is the only diagnostic test which permits some quantification of the degree of shunting to be calculated. Furthermore, this shunt index can be remeasured repeatedly in the post-operative period to allow progress to be monitored objectively. Normal values for shunt index are determined to be less than 15%. Pre-operative shunt indices usually range from 67-93%.
How are portosystemic shunts treated?
Surgical treatment permits an opportunity to completely occlude the shunt vessel, and therefore permanently resolves the effects of the anatomical anomaly. For this reason, surgery should be recommended for almost all patients with PSS. Medical management will allow reasonable control of clinical signs, and dogs can be managed successfully for many years on medical treatment alone. However, the potential for complications, crises and premature death remain a constant feature if the vessel remains patent.
The goal of surgical treatment of PSS is to completely ligate the shunting vessel to restore normal hepatic blood flow. Complete ligation of the vessel at the time of exploratory surgery is usually not possible in the majority of patients. In these patients, achieving slow occlusion of the PSS with either an ameriod constrictor or cellophane banding is preferable as permanent occlusion remains a potential goal with these treatments.
Prior to surgical management, it is considered important the patient is stabilised on medical therapy for a period of 2-4 weeks prior to attempting shunt occlusion. This period of medical treatment has several purposes.
Firstly, it improves the nutritional and systemic health of the patient. Many of these animals have been vomiting, anorectic and depressed for several weeks/months prior to diagnosis. With aggressive medical management, many clinical signs can be relieved. This allows the patient to better tolerate the effects of anaesthesia and surgical management.
Secondly, it allows the owners to recognise the potential renewed health of their pet whilst on medical therapy. If complete occlusion of the shunt is not possible at the time of surgery, continued medical management will therefore be necessary. Some clients may choose not to recover their animal from surgery unless permanent resolution of the condition can be attained.
Thirdly, medical stabilisation is thought to help reduce the incidence of post-operative seizures, which can be a life-threatening complication of shunt ligation. Many investigators consider that an imbalance in the ratio of branched and aromatic amino acids is responsible for many signs of hepatic encephalopathy. Furthermore, sudden alteration in the ratio of these various amino acids following correction of the shunt may be significant factor in the occurrence of post-operative seizures. By feeding an appropriate ‘liver disease’ diet (that contains high levels of branched chain amino acids) for several weeks prior to surgery, it is considered that an appropriate balance of essential amino acids will be available for the patient.
Is there an alternative to surgery?
Due to the technical difficulties, perioperative complication and mortality rate associated with surgical correction of an intra-hepatic portosystemic shunt, an alternative technique has been developed called percutaneous transvenous embolisation (PTE).
PTE is typically started by accessing the right jugular vein and placing a 12F vascular sheath. A guide wire and angiographic catheter is the advanced under fluoroscopic guidance, via the caudal vena cava, into the shunt and, if possible, the portal vein. A laser cut stent is then deployed within the vena cava across the shunt entrance. Following stent placement a hydrophilic guide wire and catheter combination is used to access the shunt vessel through the interstices of the stent and deploy stainless steel coils (typically around 12 per case; range 1-18 in a recent study). The coils are thrombogenic and will result in gradual further occlusion of the shunt over the following weeks and should therefore allow staggered withdrawal of any medical and dietary management.
The major complication following PTE is the development of neurological signs including seizures (6% of cases). Around 15-20% of patients may require additional coils to be placed during a second procedure due to ongoing clinical signs and/or failure to improve in biochemical markers. A recent study evaluating the efficacy of PTE showed an improvement >50% in urea, cholesterol and albumin would be expected following successful PTE. While reduction in pre and post bile acids following the procedure was seen in 52% and 29% of dogs respectively they still remained greater than normal in 76% and 94% of dogs.
In summary PTE is a safe, fast and effective technique when performed by a clinician with appropriate training. If offer a similar long-term outcome to traditional surgical options but with reduced morbidity and mortality.
What care is required after surgery?
After surgery, patients should be placed quickly into a warm environment and monitored carefully for evidence of portal hypertension. As stated previously, the risk of this occurring is now reduced as the intention cause maximal attenuation of the vessel at the time of surgery have been replaced by other techniques.
Post-ligation seizures (usually status epilepticus) may develop 1-3 days after surgery. The cause of these is unknown and can be intransigent to management. Long term anticonvulsant therapy is often required.
What is the prognosis for animals with a portosystemic shunt?
The outcome for patients with extrahepatic PSS is dependent on several variables. If complete ligation is accomplished (either directly at surgery, or through the use of ameroid or cellophane banding), and the patients survives the immediate postoperative period without complications, a good long-term outcome should be expected. In one study, 85% of the dogs were able to return to an adult maintenance or light diet after 3 months, whilst about 15% continued to exhibit clinical signs and had to be managed medically. Patients with partial suture ligation have more long-term complications, including recurrence of clinical signs in 35-50%. Recurrence of clinical signs (hepatoencephalopathy) may be due to continued shunting through the PSS or development of acquired PSS. Although bile acid stimulation tests can be repeated to demonstrate continued shunting, the results do not directly correlate with the magnitude of the shunting which is present. Repetition of nuclear scintigraphy scans 2-6 months after surgery can provide objective data to support continued portosystemic shunting.
Acute mortality rates for shunt ligation range from 0-25%, depending on the experience of the surgeon, location of the shunt, method and degree of shunt attenuation, and health status of the patient. Causes of death include peritonitis, thrombosis of the portal vein, infection, DIC, uncontrollable seizures, metabolic disturbances, and cardiovascular collapse secondary to fatal portal hypertension. Higher mortality rates are expected with intrahepatic shunt ligation when inflow occlusion and intravascular techniques are performed, or with acute shunt ligation when standards for acceptable pressure changes are disregarded.
Although the immediate outcome for cats is good following surgery, the long term outcome is often worse than that seen for dogs. Neurologic signs can often persist in many cats, and continued medical therapy is required. In one study, over 30% of cats were euthanised within a year because of the severity of neurologic signs, and only 25% of cats had no neurologic signs one year or more after surgery.