A Review of Congenital Portosystemic Shunting and Hepatic Encephalopathy

Heptic encephalopathy is a neurologic disorder which may develop in animals who have advanced liver disease and/or severe portosystemic shunting. Congenital porto-vascular anomalies, which allow portal blood to circumvent hepatic detoxification in affected dogs and cats, and chronic severe hepato-cellular disease with acquired intra- or extrahepatic portosystemic shunting (PSS) in dogs account for most of the cases of hepatic encephalopathy.

 Congenital PSS is more commonly seen in purebred (Yorkshire terriers and Miniature schnauzers) than in mix-breed dogs. Most animals are presented by 2 years of age, often by 6 months of age, and sporadically at any age. Owners' concerns are commonly related to neurologic, gastrointestinal, and/or urinary tract disorders. Furthermore, affected animals may have a history of stunted growth or failure to gain weight compared with unaffected littermates.

 The laboratory data may be consistent with hepatocellular dysfunction: hypoproteine-mia, hypoalbuminemia, hypoglobulinemia, hypoglycemia, decreased blood urea nitrogen, abnormal bile acid concentrations, mild hypocholesterolemia, and ammonia biurate crystalluria. Hematologic features may include microcytosis, target cells, poikilo-cytosis (especially in cats) and a hypo- or nomo-chromic mild non-regenerative anemia. Hyper-ammoniaemia is also a common finding in animals with PSS, and the ammonia tolerance test is consistently abnormal and equal in sensitivity to postprandial serum bile acid concentrations. However, combined fasting and 2-hour postprandial serum bile acid concentration determination is the test of choice for clinical evaluation of liver function.

 Definitive diagnosis of a portosystemic shunt requires identification of the shunt by ultrasonography, contrast radiography, or exploratory laporatomy.

 Histologic changes in the brains of human and animal patients with hepatic encephalopathy generally are mild and non-specific. Two microscopic changes are recognized: polymicrocavitation and Alzheimer type II astrocytes. Polymicrocavitation has a bilateral and symmetrical distribution. The lesion is located in the white matter of the cerebrum, internal capsule, thalamus, hypo-thalamus, and cerebellar medulla oblongata. Single or small groups of astrocytes with clear, swollen nuclei (Alzheimer type II cells) may also be found within the gray matter.

 Microscopically, the liver contains features of hepatic atrophy, including small hepatic acini with a deficiency or lack of portal venous branches and a proliferation of hepatic arterial branches in the portal triads.

 The pathogenesis of hepatic encephalopathy is multifactorial and not completely understood. The theories that have been proposed are based on two concepts. (1) Hepatic encephalopathy results when toxic metabolites from gastrointestinal bacteria are not removed from the portal circulation by the liver. Hepatic encephalopathy develops because of failure of the diseased liver to synthesize factors that are necessary for normal brain function. Gut-derived substances believed to be involved in the development of hepatic encephalopathy include ammonia, mercaptans, short chain fatty acids, false neurotransmitters, aromatic amino acids, gamma-amino-butyric-acid (GABA) and GABA like agents, and endogenous benzodiazepine ligands.

 A 2.5 pound, female Dachshund puppy, reportedly 2 months old, was submitted dead for postmortem examination. The history included a one week duration of lethargy, biting at the cage and falling over backwards. No clinical pathology (hematologic or biochemical) test results were submitted.

 Gross examination: A venous shunt connecting the mesenteric vein to the caudal vena cava was observed approximately 0.5 to 1 cm caudal to the liver (portal-caval venous shunt).

 Microscopic examination: The brain contained multifocal, locally extensive areas of vacuolation within the white matter of the pons, cerebellar peduncle and cerebrum. In the liver, there was a slight increase in the number of profiles of hepatic arteries within the large portal tracts.

 The microscopic lesions in the brain and liver were consistent with hepatic encephalopathy secondary to the portal-caval venous shunt.

 References available upon request.

 - by Lavun Anothayanontha, DVM - Graduate Student