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Portosystemic Shunts

Portosystemic Shunts

What is a portosystemic shunt?

Portosystemic shunts (PSS) are abnormal blood vessels which exist either within, or outside of the liver.

In normal dogs, blood coming from the gastrointestinal tract should pass through the liver in order for toxins to be removed and certain nutrients to be processed, before this blood then travels back to the heart to continue around the general circulation.  In dogs with a PSS a proportion of the blood from the intestines does not pass through the liver, and instead passes straight back into the general circulation without the toxins being removed. 

There are different types of PSS.  Shunts can be congenital, meaning they are present from birth, or acquired, in which case they develop later in life as a result of another problem, such as liver disease.  Congenital PSS are suitable for surgical management, whereas acquired shunts are not.  Shunts can also be categorized based on their location as they can occur either within the liver (intrahepatic), or outside of the liver (extrahepatic).  Typically, small dogs have extrahepatic shunts, whilst large breed dogs have intrahepatic shunts. 

What are the signs of a PSS?

Most of the clinical signs of a PSS are a result of the blood from the intestines not passing through the liver, and therefore toxins build up within the circulation.  These toxins will pass to the brain and can result in neurological changes such as restlessness, lethargy, hyperactivity, wobbliness, blindness and seizures.  These signs will sometimes be worse after eating.  Other clinical signs can include poor growth, vomiting, diarrhoea, cystitis and general poor condition. 

How is a PSS diagnosed?

There will often be a suspicion of a PSS based on the clinical history and general examination, however confirmation requires further testing.  Routine blood tests will often show changes suggestive of a shunt, at which point more blood specific testing can be performed.  An abdominal ultrasound will often be performed which may show the shunt, or if the shunt itself can not be seen then changes associated with a shunt sometimes can.

The gold standard method for imaging of a PSS requires a contrast CT scan under general anaesthesia.  During the CT scan a contrast agent is injected into the circulation which then highlights the abnormal vessels.  This allows us to assess the location of the shunt, as well as assessing the vessels which are passing through the liver.  Dogs with liver shunts are often unwell when they first come to us, and so it is common for dogs to be started on medication following their blood tests and ultrasound scan, and the CT scan is then performed on the day of surgery. 

How are PSS treated?

Medical management is aimed at reducing the clinical signs associated with the shunt, however surgery is the treatment of choice for congenital shunts.  Altering the protein intake in the diet is often the most important change, and this is usually achieved by feeding a specific diet.  Antibiotics and lactulose are also used to reduce the amount of toxins both produced and absorbed by the intestines.  Medical management is usually started for several weeks prior to surgery in order to get patients as stable as possible. 

PSS surgery can be a challenging procedure.  It aims to locate the shunt and close the abnormal blood vessel, thus allowing blood from the intestines to travel correctly through the liver where nutrients and toxins can then be processed.  However, as the blood vessels through the liver are not used to this higher volume of blood, sudden changes in blood flow and blood pressure within the liver can cause major complications, and in worst cases can result in death.  The aim of surgery is therefore to only partially close the vessel and allow the patient to recover from surgery.  Over the following weeks/months the shunt will gradually close down, and more blood will start to pass through the liver instead of the shunt.  In some instances, the shunt may never close completely but these patients will often still do very well - they may however, require some degree of ongoing medical management. 

What happens after surgery?

Following surgery patients will usually remain in the hospital for 24-48hours for close monitoring.  They will remain on antibiotics and lactulose as well as their specific diet for the first month after surgery.  At 4 and 12 weeks after surgery we will take blood samples before and after eating, and medication will be altered depending on the results of these blood tests.

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