Portosystemic Shunts in theCat and Dog

  Portosystemic shunts (PSS) are vascular communications between the portal and systemic venous systems that allow portal blood to reach the systemic circulation without first passing through the liver.  PSS can be either congenital or acquired.  Congenital PSS are usually single shunts that can be either intrahepatic or extrahepatic.  In most cases, congenital PSS represent retained fetal vascular anastomoses, but can also occur when compensation for portal vein atresia results in formation or retention of collateral connections to adjacent veins.  Examples of congenital PSS include persistent sinus venosus and direct portal vein connection(s) to the caudal vena cava or azygous vein.  Acquired PSS are secondary to portal hypertension and are typically multiple extrahepatic shunts that connect the portal system to the caudal vena cava.

  Congenital PSS are most frequently diagnosed in purebred dogs (Yorkshire terriers, miniature Schnauzers, Irish wolfhounds, Old English sheepdogs and Cairn terriers) and mixed breed cats.  Some diagnostic features include central nervous system (CNS) signs (disorientation, ataxia, blindness, seizures), poor growth, nonspecific gastrointestinal signs, cryptorchidism in dogs and cats, polydipsia and polyuria in dogs, and heart murmurs, seizures, ptyalism, and copper iris color in cats.  Large breed dogs usually have intrahepatic shunts whereas small breed dogs more often have extrahepatic shunts.

  Laboratory findings include a mild nonregenerative anemia with microcytosis and poikilocytosis, mildly elevated ALT and ALP, low BUN, hypocholesterolemia, hypoglycemia, hypoalbuminemia, and hypoglobulinemia.  Ammonium biuratecrystalluria and urate calculi may be seen in up to 50% of the PSS cases.

  Diagnostic tests can be used to determine liver function.  These include sulfo-bromopthaleim (BSP) retention testing, fasting ammonia concentrations and ammonia tolerance testing (ATT), and serum bile acids (SBA).  BSP is difficult to obtain and, due to many inadequacies associated with the use of organic anions for estimation of liver function, BSP is not commonly utilized.  A normal fasting ammonia concentration does not rule out PSS since dogs and cats with PSS may have normal values.  If the concentrations are above normal reference values an ATT is unwarranted.  An ATT is a reliable test to detect hepatic insufficiency.  One drawback of the ATT is that it is a labile test which requires immediate assay samples for diagnostic accuracy which is not feasible in all veterinary practices.  ATT is contraindicated in patients with hepatic encephalopathy.  High resting and postprandial SBA concentrations are good indicators of portosystemic shunting.  The postprandial SBA concentration is the most dependable diagnostic test for detection of PSS in routine practice.

   Abdominal survey radiographs may reveal microhepatica and renomegaly.  Abdominal ultrasound, especially with Doppler capabilities, may reveal a small hypovascular liver and a shunt.  Renal calculi may also be detected.  Portography is the gold standard for documentation and anatomical location of the shunt.

  A liver biopsy should be collected to ascertain the presence or absence of hepatic fibrosis and acquired hepatobiliary disease.  When portal blood circumvents the liver, the liver fails to develop normally.  Hepatic hypoplasia is recognized histologically as atrophy of hepatic lobules, compressed hepatic cords with dilated sinusoids, close proximity of portal triads, portal vein hypovascularity, hepatocellular degeneration (vacuolization, lipidosis) and proliferation of the small vessels, arterioles, and lymphatics.  If the animal had hepatic encephalopathy (HE), on necropsy, brain lesions would include bilateral symmetric polymicro-cavitation of the brain stem and diffuse neuronal necrosis throughout the cerebrum and cerebellar cortex.

  Diagnosis of PSS should be made based on historical and physical findings, laboratory findings, and diagnostic tests.

  Treatment of PSS includes medical management (lactulose, neomycin, metronidazole), dietary therapy (high carbohydrate, low protein) and surgical intervention (ameroid ring contrictor, suture attenuation).  Complete surgical ligation of the shunt has an excellent prognosis.  Partial occlusion of the shunt usually results in improvement, but has a more guarded long-term prognosis.  Exclusive medical management results in continuation of signs, but the patient may still survive for years.  In some cases, a combination of surgical, medical, and dietary management may be necessary.

 -by Grace Steenburgen, Class of 2001

 -edited by Evan Janovitz, DVM, PhD,

    ADDL Pathologist