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).