Portosystemic Shunt

Description

Portosystemic shunts (PSS) are anomalous vascular connections between the portal and systemic venous systems. These vessels shunt blood from the hepatic portal vein (deriving from the stomach, intestines, pancreas and spleen) directly into systemic venous system, bypassing the liver.

Portosystemic shunts may be congenital or acquired in disease the cause portal hypertension. Congenital shunts represent approximately 70% of the total number diagnosed in dogs and constitute the majority of those diagnosed in cats. Congenital shunts usually involve a single (or occasionally double) anomalous vessel which may be located outside of the hepatic parenchyma (extrahepatic) or within it (intrahepatic). Extrahepatic shunts accounts for 63% of single shunts in the dog and they are most commonly found in miniature and toy breeds. Intrahepatic shunts are usually located within the left lobes of the liver and occur due to persistence of the foetal ductus venosus. This form of shunt is most common in large breed dogs and patent ductus venosus is an inherited condition in Irish wolfhounds. Intrahepatic shunts running through the central or right liver lobes are recognised and these may have a different embryological origin.

Acquired shunts represent approximately 20% of those diagnosed in dogs and they often consist of multiple small vessels. They arise due to portal hypertension, following an increased resistance to portal blood flow. The increased pressure in the portal vein and its tributaries causes numerous non-functional microvascular communications with the systemic venous system to open. The causes of portal hypertension are numerous and they may be divided into pre-hepatic, hepatic and post-hepatic:

• Pre-hepatic
  • Right-sided congestive heart failure, including cardiac tamponade.
• Hepatic
  • Acute fulminant hepatitis, hepatic lipidosis or neoplasia
• Post-hepatic
  • Thrombosis of the portal vein, 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.

Signalment

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

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

Also known as:

Portocaval Shunt Patent Ductus Venosus