Portosystemic shunts can be present at birth (i.e. congenital) or acquired as the result of another disease process later in life. Congenital shunts are more common, representing approximately 75% of all canine cases, and generally result from anatomic abnormalities of the portal vasculature or persistence of fetal vessels. One or occasionally two vessels are involved, and the shunts are classified according to their location as either outside of (extrahepatic) or within (intrahepatic) the liver.

CONGENITAL SHUNTS

Congenital shunts occur more commonly in purebred dogs than in mixed breeds; miniature schnauzers, Yorkshire Terriers, and Irish Wolfhounds appear to be at increased risk. The prevalence of portosystemic shunts in certain breeds suggests an inherited predisposition. This has been proven only in Irish Wolfhounds, where a number of previously unknown genes appear to be involved.

Extrahepatic shunts are most common, accounting for 61% to 94% of congenital shunts, and are typically seen in small breeds of dogs, such as the Miniature Schnauzer and Yorkshire Terrier. Intrahepatic shunts represent between 6% and 40% of congenital shunts and are more common in large and giant breeds of dogs such as Irish Wolfhounds and Golden Retrievers. The majority of intrahepatic shunts are a result of the embryonic connection between the umbilical vein and the caudal vena cava remaining open; in most dogs this connection closes 3 days after birth but, for unknown reasons, remains open in dogs with intrahepatic congenital shunts.

Hepatic microvascular dysplasia is an unusual form of intrahepatic portosystemic shunting in which no gross vascular abnormality can be identified. This rare condition is associated with somewhat milder clinical signs and appears to be the consequence of a developmental abnormality; it has a higher prevalence in Cairn Terriers, suggesting a hereditary basis.

ACQUIRED SHUNTS

Acquired shunts arise secondary to diffuse liver disease where excessive and sustained pressure at some point within the portal vein causes embryonic, nonfunctional vascular communications to open. These are generally seen in older dogs with cirrhosis, hepatitis, or neoplasia of the liver. In contrast to congenital portosystemic shunts,  a number of vessels are usually affected.

What are the signs of portosystemic shunts?

The clinical signs exhibited by dogs with portosystemic shunts reflect the failure of the liver to eliminate various toxic matter, drugs, and bacteria absorbed from the gastrointestinal tract. Certain materials particularly those produced from dietary protein, are potential neurotoxins (e.g., ammonia, gamma-aminobutyric acid, natural benzodiazepines, and mercaptans), which affect the function of the central nervous system and produce a syndrome called hepatic encephalopathy. Other clinical signs arise from the liver being deprived of portal blood flow, which is essential for the normal development of the liver; as  a result the liver is underdeveloped and it's metabolic, storage, and excretory functions are further impaired...

SIGNALMENT AND HISTORY

Dogs with congenital portosystemic shunts are typically purebred dogs less than 1 year old. The severity of clinical signs varies and is related to the anatomic position of the shunt and the fraction of portal blood that is shunted past the liver. Generally, the lower the fraction of shunting, the milder and later in onset are the clinical signs. Nevertheless, affected animals are often in poor body condition and of small body stature, especially when compared to their littermates. Owners may complain that the animals fail to thrive or grow and that skin and coat condition are poor. Other clinical signs tend to be intermittent or periodic in nature and relate to the central nervous system, gastrointestinal tract, and urinary tract. There may also be a history of an adverse response to sedation or anesthesia.

A significant number of dogs (up to 25% of cases of portosystemic shunts) may develop signs later in life; this may be due to an acquired rather than congenital shunt or because the animal can no longer compensate for a low grade congenital shunt. dogs with acquired shunts have signs similar to those with congenital disease, although the previous history may have been uneventful until the appearance of weight loss and associated clinical signs.

CLINICAL SIGNS

Central nervous system signs are the most common, occurring in over three-quarters of all cases, and may be vague and subtle such as anorexia, depression, and lethargy .(Table 1). More specific signs include episodes of hyperactivity, head pressing and circling, disorientation, temporary blindness, weakness, excess salivation, seizures, and occasionally coma. These signs tend to wax and wane, and their onset may be connected with the recent ingestion of a protein-rich meal that resulted in increased production of neurotoxins within the large intestine.

Gastrointestinal signs

Vomiting and/or diarrhea are present in about two-thirds of cases. Evidence of lower urinary tract disease is present in approximately one-half of cases and is usually due to ammonium urate crystals, which are formed because of excessive excretion of ammonia and uric acid in urine. Some dogs, particularly those that develop signs later in life, have polydipsia and polyuria (excessive drinking and urination). Other less common signs of portosystemic shunting include recurrent fevers and ascites, although the latter is generally seen only in dogs with acquired shunts.

DIAGNOSIS

Although signalment and clinical signs may strongly suggest the presence of a  portosystemic shunt, a series of investigative steps (Table 2) must be taken to:

• Confirm the diagnosis
• Identify the anatomic location and severity of the shunt
• Indicate whether the shunt is congenital or acquired.

Blood chemistry and hematology panels usually show characteristic patterns of mild abnormalities that as a group suggest the presence of a portosystemic shunt. These include mildly elevated liver enzymes, low blood urea nitrogen and total plasma protein concentrations, hypogly- cemia (low blood sugar) and low serum cholesterol. Mild, nonregenerative anemia and microcytosis (undersized red blood cells) may also be present in all cases. Blood ammonia concentrations may also be increased, but samples must be rapidly analyzed soon after collection to detect such an increase. Urinalysis may reveal a low specific gravity and the presence of ammonium blurate crystals. Serum bile acid concentrations taken either after an overnight fast or 2 hours after a meal are usually confirmatory.

The next step is to identify whether the shunt is intrahepatic, extrahepatic, or microvascular. This generally requires specialized imaging techniques, and it is likely that dogs may need to be referred to appropriate specialists. Survey radiographs of the type normally taken in veterinary practices simply indicate the presence of a small liver (Figure 2).

Ultrasonography is a useful, noninvasive tool for the detection of portosystemic shunts. Intrahepatic shunts are easily visualized: the liver usually appears small in size, there is reduced visibility of intrahepatic portal vessels, and an anomalous blood vessel may be obvious (Figure 3).

In dogs with extrahepatic shunts the first two features are usually present, but the detection of the anomalous vessel is not so easy. Application of Doppler ultrasound, an advanced technique, may help in such cases, especially where there is a small extrahepatic shunt.

Contrast radiography, whereby a marker dye is injected into a vein draining the intestine and radiographs are taken immediately, allows ready visualization of the portal vein and shunting. This procedure is usually performed in combination with surgical correction (so that the dog has to be anesthetized only once) and is often referred to as operative mesenteric portography. A loop of small intestine is exteriorized (brought outside of the body) and a tube is placed in a jejunal vein. A water-soluble radiopaque dye is injected via the tube, and lateral and ventrodorsal radiographs are then taken. Where there is a shunt, the abnormal vessel is outlined as blood is diverted into the systemic circulation without appearing in the liver (Figure 4).

Portal or transcolonic scintigraphy is an advanced technique whereby the uptake of radiochemicals from the intestinal tract is monitored. A radiochemical, usually technetium 99m pertechnetate, is administered via the rectum and first accumulates in the liver in normal animals. In dogs with portosystemic shunts the distribution of activity is altered as the radiochemical bypasses the liver and reaches the heart first. Although this does not identify the location of the shunt, it does provide an extremely accurate estimate of the degree of shunting, allowing the clinician to predict the likely success of management options and to follow up the success of surgical management.

Liver biopsy is indicated when there is no obvious shunt or if multiple extrahepatic shunts are identified (as seen in acquired portosystemic shunting). This may reveal hepatocyte atrophy, with small or absent portal vessels, and will allow histopathologic confirmation of micro- vascular disease.

MANAGEMENT

There are two broad management options: surgical ligation of shunts or medical management of the effects of shunting. The decision as to which is most appropriate needs to be made on a case-by-case basis depending on the type and location of the shunt, the age of the animal, and the severity of clinical signs. There may also be significant financial considerations on the part of the owner. Surgery wherever feasible, is generally believed to be the treatment of choice as it suggests the promise of normal liver function. Improvements in dietary and medical manage- ment of hepatic encephalopathy, however, mean that conservative treatments offer a reasonable prognosis for dogs that are not suitable for surgery (Table 3).

SURGICAL MANAGEMENT

Ligation of shunt vessels is an advanced surgical technique requiring a suitably experienced surgeon, careful selection and monitoring of general anesthesia, measurement of blood pressure in the portal vein and systemic circulation, and appropriate critical care support facilities. Such requirements usually necessitate referral to specialist centers, and this is especially true with intrahepatic shunts.

Single extrahepatic shunts are usually identified as tortuous, abnormal vessels. These are ligated close to the vena cava. A potential fatal complication is portal hypertension, which occurs when intrahepatic vessels are unable to cope with the additional volume of blood that is diverted to the liver after closure of the shunt vessel. Guarding against this requires careful monitoring of portal and systemic blood pressures and inspection of the intestines and pancreas for signs of cyanosis. Failure to alleviate the hypertension and pain, bloody diarrhea, and shock leading to death in 2 to 24 hours after surgery.

A 60% to 80% degree of ligation can usually be achieved without complications and is associated with an increase in the amount of portal blood that enters the liver and with improvements in the patient's clinical status. In some cases, the ligation procedure may be repeated on one or more later dates to progressively attain complete ligation. This un- fortunately necessitates additional costs and, with successive surgeries, increases the risk of perioperative, complications. On the other hand, repeat surgery may not be necessary as some partially ligated shunts appear to spontaneously occlude. The prognosis for dogs with partial ligation is guarded because approximately 50% show recurrence of clinical signs at an average of 3 to 4 years after surgery. Ligation of intrahepatic shunts is technically more difficult and is associated with higher risks of fatal complications. Such cases may be best managed conservatively.

MEDICAL AND DIETARY MANAGEMENT

With better understanding of the pathophysiology of hepatic encephalopathy, it has become possible to prescribe specific therapies that provide a reasonable prognosis for those dogs with portosystemic shunts that are not corrected surgically. The primary objective of medical management is to eliminate the clinical signs associated with hepatic encephalopathy. Other goals include minimizing lower urinary tract disease and reducing the metabolic load on the liver. The chief components of medical management strategies are dietary modifications and oral antibiotics.

Medical management is indicated for all dogs with acquired shunts and all dogs with microvascular shunts. It should also be used for a period in those dogs that are about to undergo surgical ligation. This will allow the veterinarian and owner to establish the extent to which the condition can be managed medically, in case it is not possible to completely ligate the shunt at surgery. medical management is also indicated in those dogs whose owners are unable to afford the cost of referral to a specialist surgical facility or whose owners are unwilling to accept the significant risk of perioperative mortality. All dogs undergoing surgical ligation should continue to receive medical therapy for 2 to 4 weeks post- operatively. Finally, some degree of medical therapy may be required in dogs with partially ligated shunts.

Dietary manipulations for the control of hepatic encephalopathy are designed to limit neurotoxin production, which occurs principally in the large intestine, and to reduce the subsequent absorption of these toxins into the portal vein (Table 4). The major toxins are all derived from nitrogenous materials (protein and urea) and are synthesized by bacteria found within the large intestine. The production of these toxins is reduced by limiting the amount of protein fed and ensuring that the dietary protein is high quality and very digestible. These steps reduce the amount of protein that reaches the large intestine; further reductions can be attained by feeding smaller meals more frequently to maximize the digestive capacity of the small intestine.

Specific diets with restricted protein contents are available from veterinarians. These are ideal because they provide a balanced protein-calorie intake, which is important for the stable control of hepatic encephalopathy. Including dietary fiber in the daily ration assists in acidifying the colonic environment and limiting toxin production and also acts as a mild laxative to increase the elimination of toxic factors in feces. Lactulose, a soluble fiber, is often used as a supplement for this purpose and can be readily purchased from pharmacists. Supplementation with zinc salts also im-proves the detoxification of ammonia and the control of hepatic encephalopathy. A veterinary diet specifically designed and tested for the management of liver disease and portosystemic shunts is available in Europe; it is unique in combining a restricted protein content with increased zinc and added dietary fiber. Available in America as Waltham Veterinary Diets Canine Low and Medium Protein, dry and canned.

Antibiotics are used in most cases to reduce the bacteria within the large intestine that are responsible for the production of neurotoxins. Orally administered neomycin is commonly used for this purpose and is often used in combination with lactulose in both the short and long-term medical management of portosystemic shunts.

SUMMARY