Diagnosis and Surgical Management of Portosystemic Shunts
John D. Wooldridge, DVM, Dipl. ACVS
Portosystemic shunts (PSS) are anomalous direct communications between the portal vascular system and the systemic circulation. Therefore, portal blood is allowed to enter the systemic circulation while bypassing the liver. Various clinical signs occur as portal blood, carrying byproducts of digestion absorbed from the intestinal tract, enter the systemic circulation without undergoing hepatic metabolism.
Single PSS account for approximately 80% of the PSS seen in dogs. Single PSS can be either intrahepatic (about 35%) or extrahepatic (about 65%), and are generally congenital in origin. Multiple extrahepatic PSS are usually acquired secondary to chronic hepatic disease and subsequent portal hypertension, and account for approximately 20% of PSS seen in dogs. Most single PSS are diagnosed in young animals, although occasionally older adult dogs and cats are seen. Single extrahepatic PSS are most frequently diagnosed in small breed dogs and in cats. In particular Yorkshire terriers, Maltese terriers, poodles, Lhasa apsos and miniature Schnauzers are predisposed. Large breed dogs such as Irish wolfhounds, Labrador retrievers, Australian cattle dogs and Australian shepherds appear to be predisposed to intrahepatic PSS. Multiple extrahepatic PSS are usually diagnosed in dogs and cats between the ages of 1 and 7 years, and American Cocker spaniels, German shepherds and Doberman pinschers are most commonly affected.
Clinical signs of PSS are the result of hepatoencephalopathy and/or progressive functional impairment of the liver. Neurologic signs can include bizarre behavioral changes, disorientation, blindness or seizures, and are often occur closely related to food consumption (especially a meal high in protein). Gastrointestinal signs are also often seen, and diarrhea, vomiting, ptyalism are common. More general clinical signs include weight loss, failure to thrive (ie. the puppy or kitten is significantly smaller than its littermates), ascites, polyuria, and polydipsia. Formation of ammonium biurate cystic calculi in a non-Dalmation breed is often considered pathognomonic for a PSS. Prolonged recovery in a young animal following anesthesia with agents that require hepatic metabolic clearance should trigger an evaluation of hepatic function. Concurrent congenital abnormalities, such as cryptorchidism, multiple retained deciduous teeth, and body wall hernias can be seen in young animals with PSS.
The definitive diagnosis of PSS is made by surgical identification of the shunt, contrast portography, MRI studies, transcolonic nuclear scintigaphy, or ultrasound. Microhepatica on plain abdominal radiographs can be suggestive of PSS in conjunction with appropriate clinical signs and signalment. Nuclear scintigraphy is the most accurate non-invasive test to diagnose a PSS, but this technique cannot differentiate extra- or intrahepatic location of the PSS, nor single or multiple PSS. Laboratory diagnostics should include a complete blood count, serum chemistry profile, urinalysis and fasting and postprandial bile acid measurement. Common biochemical abnormalities include a low BUN, elevated liver enzymes, low albumen, elevated fasting ammonia levels, and markedly elevated pre- and post-prandial bile acids. The urinalysis may reveal ammonium biurate crystals.
In general, surgical treatment is recommended for all patients with PSS. Medical management may manage clinical signs in the short term, but hepatic function usually continues to deteriorate as long as blood is being shunted away from the liver. There is some recent evidence that the older the dog is at presentation for a PSS, the better its prognosis for favorable response to medical rather than surgical management. Preoperative medical management is intended to correct hepatoencephalopathic clinical signs and to reduce absorption of intestinal bacterial toxins; introducing lower quantity but higher quality protein food, preventing gastrointestinal bleeding, and correcting acid-base and electrolyte abnormalities are all important. The goal of surgery is to identify and to occlude the anomalous vessel to provide complete restoration of normal hepatic blood flow. However, complete ligation at the initial surgery is not possible in all patients, and partial ligation and medical management or re-exploration and further attenuation of the shunt at a later date may be necessary. Ligation of the PSS can be accomplished using sutures, an ameroid ring constrictor or a cellophane band. Measurement of portal pressures prior to and after shunt manipulation/ligation is the safest means of preventing development of portal hypertension, but in practice, many surgeons use delayed ligation devices (ameroid constrictors) and post-operative portal hypertension is a relatively rare occurrence. Anesthetic and post-operative management of the PSS patient can be challenging. In particular, cats often seem to have severe hepatoencephalopathic problems in the post-operative period, even with placement of a delayed constrictor.
Hepatic microvascular dysplasia (HMD) is a condition that can mimic many of the clinical signs of PSS. HMD is characterized histopathologically by small intrahepatic portal vessels and portal endothelial hyperplasia which allows abnormal communication between the portal and systemic circulations within the hepatic parenchyma. HMD can be seen as an isolated condition or in conjunction with PSS. In general, patients with HMD have similar but less severe clinical signs than those with PSS. Bile acids are elevated, but usually are much less significantly elevated than in patients with PSS. Transcolonic nuclear scintigraphy is usually normal in dogs with HMD. Obviously, surgical treatment of HMD is not feasible. It appears that dogs with HMD that have minimal clinical signs can be well managed with conservative treatment.