Portosystemic Shunt

Signalment

• Relatively common in dog, especially small breed dogs
• Purebred dogs are more at risk
• Occasionally seen in cats, horses, cows and pigs

Description

Portosystemic shunts (PSS) are anomalous vascular connections between the portal and systemic venous systems. This allows for some portal blood draining from the stomach, intestines, pancreas and spleen to bypass the liver and drains directly into the systemic circulation. PSS may be congenital or acquired secondary to portal hypertension.

Congenital PSS occurs in approximately 70% of PSS in dogs and majority of PSS in cats. It commonly present as a single, or at most double, extrahepatic or intrahepatic anomalous vessel. Extrahepatic PSS accounts for 63% of single shunts in dog and is more commonly found in miniature and toy-breed dogs. Intrahepatic shunts are usually left-sided, resulting from persistent foetal ductus venosus, and more common in large breed dogs. Right-sided or central intrahepatic shunts are recognised and these may have a different embryological origin.

Acquired PSS occurs in approximately 20% of PSS and often consists of multiple shunts. They arise due to portal hypertension, following an increased resistance to portal blood flow. This leads to opening of some of the numerous normal, non-functional microvascular communications. Underlying causes of portal hypertension included acute fulminant hepatitis, hepatic fibrosis, hepatic neoplasia, portal vein thrombosis, hepatic arteriovenous fistulae and congenital hypoplasia of the portal vein.

The pathophysiology of PSS relates to the shunting of blood directly from the systemic circulation, resulting in hyperammonaemia and Hepatic Encephalopathy.

Diagnosis

Clinical Signs

• Young animals, usually under 1 years of ages, but can sometimes be up to 10 years or older
• Failure to thrive, small body stature or weight loss
• Waxing and waning neurological signs due to hepatic encephalopathy. These are usually most severe an hour or two post prandial but this may not be obvious in all cases
  • Bizarre behaviour
  • Head pressing
  • Seizures
  • Intermittent blindness.
• Ptyalism in cats
• Dysuria, stranguria, haematuria, pollakiuria and urethral obstruction
  • An increase in ammonium concentration in the blood decreases the ability of enzymes to convert uric acid to allantoin, thereby resulting in urate urolithiasis.
• Intermittent vomiting or diarrhoea

Laboratory Tests

Haematology

• Microcytosis

Biochemistry

• Decreased blood urea nitrogen (BUN)
• Hypoalbuminaemia
• Hypocholesterolaemia
• Hypoglycaemia

Other Tests

• Increased postprandial ± preprandial bile acids
• Increased ammonia levels

Diagnostic Imaging

A definitive diagnosis relies on visualisation of the shunting blood vessel. This may be done with either ultrasonography or contrast portography or at surgery.

Treatment

• Surgical ligation of the anomalous vessels
  • Portal hypertension is possible post ligation. For this reason, a partial ligation is performed initally, followed by a complete ligation a few months later.
• Medical treatment of Hepatic Encephalopathy

Prognosis

Excellent prognosis in dog for resolution of clinical signs after total surgical ligation. However, the response of cat to surgical intervention in cats is less promising than in dogs.

References

• Ettinger, S.J. and Feldman, E. C. (2000) Textbook of Veterinary Internal Medicine Diseases of the Dog and Cat Volume 2 (Fifth Edition) W.B. Saunders Company.
• Fossum, T. W. et. al. (2007) Small Animal Surgery (Third Edition) Mosby Elsevier.
• Nelson, R.W. and Couto, C.G. (2009) Small Animal Internal Medicine (Fourth Edition) Mosby Elsevier.
• Watson, P. (1997) Decision making in the management of portosystemic shunts In Practice 19;106 - 120 [1]

From Pathology

• seen in dogs and cats
• Inherited in Irish wolfhounds
  • Not known what mode of inheritance in this breed
• these are vessles that allow the blood in the portal vein to bypass the liver tissue (parenchyma)
• congenital
  • shunting from the portal vein directly into the vena cava, azygos or renal vein
  • this is the common type seen in small dogs and cats - usually a single communication between the vessels, occasionally multiple
  • larger breeds tend to have the shunting to the vena cava take place within the liver itself (persistent ductus venosus)
• acquired
  • due to hepatic fibrosis whcih results in increased resistance of flow of blood into the liver from the portal vein
  • produces hypertension in the portal vein and fluid accumulates in the peritoneal cavity - ascites
  • several thin-walled tortuous vessels may be seen connecting the mesenteric veins to the vena cava, and the liver looks atrophic and fibrosed
• Bacteraemia is a common finding in severe hepatic disease and PSS in humans
  • portal or systemic
  • usually Gram-negatives
  • also seen in dogs with PSS
  • presumably due to reduced effectiveness of phagocytic activity in these livers
  • or due to shunting of blood around the liver

NB: portosystemic shunt is a major cause of hepatic encephalopathy (need link), therefore the affected animals are stunted and seem dull or stupid because of the toxic substances in their systemic circulation