Shunts in dogs - Veterinary Articles on Congenital Shunts in dogs

Introduction

The first case of liver insufficiency in a dog due to a portal vein (the vein that carries blood that has been circulating in many of the abdominal organs to the liver) abnormality appeared in the veterinary literature in 1974. A portosystemic shunt is an abnormality of the blood circulation system, resulting in blood from the heart bypassing the liver and entering the general circulation. shunts may be either congenital or acquired. The congenital form is however most commonly recognised and occur more commonly in purebred than in mixed breed dogs. A slight predilection for female dogs has also been suggested. In miniature dog breeds, congenital shunts are usually single and extrahepatic (occur outside the liver), while in large dog breeds, congenital shunts are usually single and intrahepatic (occur inside the liver). Although the genetic basis for congenital portosystemic shunts is unknown, affected lines have been recognised in Miniature schnauzers, Irish wolfhounds, Old English sheepdogs, and Cairn terriers.

Normal liver function

The liver orchestrates all aspects of metabolism (directly or indirectly) and has over 1500 functions. Liver disease will therefore virtually affect every homeostatic function in the body. Homeostasis is the ability of an organism or cell to maintain internal equilibrium by adjusting its physiological processes.

The functions that the liver performs include, but not are limited to:

Lipid (fat) carbohydrate, and protein metabolism;
Storage and metabolism of vitamins;
Storage of minerals, glycogen (form in which carbohydrates taken in the food are stored in the liver and muscle before they are converted into glucose when needed) and triglycerides;0
Production of albumin;
Gluconeogensis (production of glucose)
Breaking down of old and worn-out red blood cells;
Synthesis (production) of several blood coagulation factors (Factors I, II, V, VII, VIII, IX, X, XI, XII);
Contributes to digestion by producing bile acids. Bile consists primarily of water with bile acids, bile pigments, cholesterol and inorganic salts. Bile aids in the absorption of fats and fat-soluble vitamins, but also has some laxative action and stimulates intestinal peristalsis. More than 90% of bile acids are re-absorbed from the small intestine and transported via the portal vasculature to the liver, where it is actively removed and re-excreted by the liver. This process is called entero-hepatic circulation;
Breaking down of exogenous toxic substances from the tissues of the body as well as exogenous (from outside the body e.g. anaesthetic drugs).

Clinical signs due to portosystemic shunt(s) in dogs

A history of prolonged recovery after general anaesthesia or excessive sedation after treatment with tranquilizers can be attributed to impaired hepatic metabolism of these substances. Clinical signs of congential portosystemic shunts are referable to the central nervous system (brain), gastrointestinal system (stomach and intestines), and urinary system (bladder and kidneys).

Central nervous system signs

Signs of hepatic encephalopathy usually predominate. Hepatic encephalopathy is a complex of neurological signs due to the shunting of blood past the liver as well as a reduction in the the functional liver mass, resulting in defective metabolic processes in the liver. the most consistent signs of hepatic encephalopathy are often subtle and include anorexia (loss of appetite), depression and lethargy. Other signs of hepatic encephalopathy include episodic weakness, ataxia (loss of power of governing movements), head pressing, disorientation, circling, pacing, behavioural changes, seizures, coma, and impaired vision or even loss of sight (amaurosis). Clinical signs of hepatic encephalopathy however wax and wane and are often interspersed with normal periods, reflecting the variable production and absorption of enteric (intestinal) products that are neurotoxic (toxic to the central nervous system).

Gastrointestinal system signs

Intermittent anorexia, vomiting and diarrhoea, are common non-specific signs due to hepatic (liver) dysfunction. Vomiting result from toxins that are not cleared by the liver, that stimulate the vomiting centre in the brain. Diarrhoea results from reduced fat and water absorption from the intestines. Many affected individuals also have a history of stunted growth, and thus a failure to gain weight compared with unaffected littermates, or weight loss.

Urinary system signs

Polyuria (much greater amount of urine is passed than normal) and ploydipsia (excessive thirst) are common in dogs with portosystemic shunts. These signs are due to unfiltered endotoxins affecting antidiuretic hormone receptors and low urea in the renal medulla (inner core of the kidney).

Age is an important diagnostic clue, as most animals develop signs by 6 months of age. A congenital portosystemic shunt should however still be a diagnostic consideration in middle-aged or older dogs, as signs may be subtle and some dogs with congenital portosystemic shunts go undiagnosed until as late as 10 years of age.

The clinical examination may be unremarkable except for a stunted appearance or weight loss. Routine haemotology (blood tests) is also often unremarkable in dogs with congenital portosystemic shunts.

In dogs with suspected congenital portosystemic shunts, serum bile acid concentrations should be determined to document hepatic dysfunction. Survey abdominal radiographs are taken to evaluate liver size, as well as to evaluate the urinary system and the gastrointestinal system. Additional radiographic imaging techniques including ultrasonography, contract portography (radiographs of the portal circulation following the injection of a contract medium into the mesenteric vein, portal vein or splenic vein), or transcolonic portal scintillation camera is then used to obtain a two-dimensional image of the distribution of the readioactivity in tissues) can provide important information about the presence, location and type of portosystemic shunt.

The treatment of choice for dogs with a congenital portosystemic shunt is surgical ligation (tying-off) of the abnormal vessel.

Prognosis

The prognosis in dogs after surgical ligation of the shunt is excellent if the dog survives the immediate postoperative period. However, in dogs with partial shunt ligation, the prognosis is not as good.

For more information on other inherited defects in dogs:

Dog health conditions - A chart of many inherited dog health conditions

Deafness in Dogs

Canine Leukocyte Adhesion Deficiency - CLAD

Defects of the vertebral column in dogs - Congenital conditions

Defects of the Nervous System of Dogs

Return from Shunts in dogs to Veterinary Articles

Veterinary articles supplied by Dr S Strydom and published with kind permission of DIA Publishing - publishers of KUSA Dogs in Africa Magazine

Dr Sunelle Strydom qualified as a veterinarian in 2004 at the Faculty of Veterinary Science, University of Pretoria. She has a passion for writing and the sharing of knowledge to promote animal health and welfare. She has written several articles for KUSA Dogs in Africa as well as for Vra vir Faffa on the Landbouweekblad website at www.landbou.com

References

Boden E 1998. Black's Veterinary Dictionary 19th Edition. A & C Black, London
Brockman D J. 2007. Diagnosis of Portosystemic shunts. Scientific proceedings: Companion Animals Programme, Voojaardagen European Veterinary Conference 2007 - Amsterdam. International Veterinary Information Service (www.ivis.org), Ithaca, New York, USA.
Ettinger S J & Feldman E C. 200. Textbook of Veterinary Internal Medicine 5th Edition. W.B. Saunders Company, USA.
The Free Dictionary by Farlex (website). Available from http://www.thefreedictionary.com/homeostatis as accessed on 29 September 2008
BVSc lecture notes: Small Animal Clinical Studies 470

Go from Shunts in dogs to About Small Dog Breeds Home Page

Congenital Portosystemic Shunts in Dogs

Congenital Portosystemic
Shunts in Dogs