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What's new with congenital PSS? (Proceedings)

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Nov 01, 2010

Congenital Portosystemic Shunt

Portosystemic shunts (PSS) are vascular communications between the portal and systemic venous systems that allow portal blood to access the systemic circulation without first passing through the liver. Signs of hepatic encephalopathy (HE) dominate the clinical picture because of inadequate hepatic clearance of enterically-derived toxins. Decreased hepatic blood flow and lack of hepatotrophic factors result in hepatic atrophy. Urate urolithiasis, an important complication of PSS, occurs because of increased urinary excretion of ammonia and uric acid. Urolithiasis may be a complication in as many as 50% of animals with congenital PSS. Portosystemic shunts in dogs and cats can be either congenital or acquired. The congenital form is most commonly recognized. Congenital PSS are anomalous embryonal vessels that usually occur as single shunts and are not associated with portal hypertension. Yorkshire terriers are the most commonly reported breed with congenital PSS. Affected lines of Irish wolfhounds and Cairn terriers have also been described. The genetic basis for congenital PSS is unknown, although in Yorkshire and Cairn terriers, inheritance is not simple dominant, simple recessive, or sex-linked. Acquired PSS, which form in response to portal hypertension, are typically multiple extrahepatic shunts connecting the portal system and the caudal vena cava.

Congenital PSS are typically single and may be intrahepatic or extrahepatic. Single intrahepatic PSS are most common in large and medium-sized breeds. Single extrahepatic PSS are more common in cats and small or toy breed dogs. In rare instances, 2 or more vascular communications may be present. Portoazygous or portophrenic shunts may be associated with less severe clinical signs, and consequently, may not be diagnosed until later in life. Dogs with intrahepatic shunts have more severe signs at an earlier age, probably related to the larger volumes of blood diverted through the shunt as compared to extrahepatic shunts. In dogs and cats with congenital PSS, the liver is grossly small and often mottled in appearance. Liver biopsy most consistently reveals hepatocyte atrophy with small or absent portal veins and arteriolar hyperplasia, although biopsy abnormalities may be subtle. Biopsy findings are typical for decreased portal vein flow and are indistinguishable from findings in congenital portal vein hypoplasia (see following section). Liver biopsy findings at the time of surgery do not predict long-term outcome in dogs undergoing surgical correction of PSS.

Congenital PSS occur more commonly in purebred than in mixed breed dogs. Breeds at increased risk include Yorkshire terrier, Miniature schnauzer, Irish wolfhound, Cairn terrier, Maltese, Havanese, Dandie Dinmont terrier, Pug, Australian cattle dog, Golden retriever, Old English sheepdog, and Labrador retriever. In contrast, mixed breed cats are affected more commonly than purebred cats. Of the affected purebreds, Persian, Siamese, Himalayan, and Burmese cats appear to be at increased risk. Age is an important diagnostic clue since most animals develop signs by 6 months of age. A congenital PSS should still be a diagnostic consideration in middle-aged or older dogs, since signs may be subtle and some dogs with congenital PSS go undiagnosed until as late as 10 years of age. This is especially true for dogs with urate urolithiasis, who may not have an obvious history of HE. Miniature schnauzers are more likely to be diagnosed at an older age than are dogs of other breeds.

Clinical signs of congenital PSS are referable to the central nervous system, gastrointestinal system, or urinary tract. Signs of HE usually predominate. The most consistent signs of HE are often subtle such as anorexia, depression, and lethargy. Other common findings indicative of diffuse cerebral disease include episodic weakness, ataxia, head-pressing, disorientation, circling, pacing, behavioral changes, amaurotic blindness, seizures, and coma. Bizarre aggressive behavior, seizures, and blindness appear more likely in cats. Hypersalivation is a prominent sign in cats, but also occurs in dogs. Clinical signs of HE tend to wax and wane and are often interspersed with normal periods, reflecting the variable production and absorption of enteric products that are neurotoxic. Signs of HE may be exacerbated by a protein-rich meal, gastrointestinal bleeding associated with parasites, ulcers, or drug therapy, or administration of methionine-containing urinary acidifiers or lipotrophic agents. Clinical improvement in HE after fluid therapy is common and most likely attributed to correction of dehydration and promotion of urinary excretion of ammonia and other toxins. Improvement from broad-spectrum antibiotic therapy reflects the effect of antibiotics on the toxin-producing intestinal flora. Gastrointestinal signs of intermittent anorexia, vomiting, and diarrhea are common non-specific features of hepatic dysfunction and are not necessarily accompanied by overt signs of HE. Dogs with intrahepatic PSS are at increased risk for GI ulceration and inflammation. Many affected animals have a history of stunted growth, failure to gain weight compared to unaffected littermates, or weight loss. Polydipsia and polyuria are common in dogs but not in cats. If urolithiasis is a complicating feature, pollakiuria, dysuria, and hematuria may occur. Some animals are primarily presented for evaluation of urolithiasis without obvious HE or gastrointestinal signs. A history of prolonged recovery after general anesthesia or excessive sedation after treatment with tranquilizers, anticonvulsants, or organophosphates can be attributed to impaired hepatic metabolism of these substances. Physical examination may be unremarkable except for small body stature or weight loss. The neurologic examination is normal, or if overt signs of HE are present, neurologic findings are consistent with diffuse cerebral disease. Many affected cats have copper-colored irises. Ascites and edema are rare findings unless a congenital PSS is complicated by hypoalbuminemia (<1.0 gm/dl) from GI bleeding or severe dietary protein restriction. Ascites is more likely with portal vein hypoplasia that is accompanied by portal hypertension (see below).