Porto-systemic shunts (Liver shunts)
Professor EJ Hall
A porto-systemic shunt (PSS) is an abnormal vessel that bypasses the liver so that blood which would normally drain from the intestines (via the portal vein) to the liver is 'shunted' directly into the general circulation. This causes significant ill health because of toxins from the gut reaching the brain. Ideally the shunt is surgically corrected.
Shunts are being recognised with increasing frequency in pedigree dogs (and occasionally in pedigree cats). They are most common in giant and toy breed dogs. Occasional cases have been seen in Irish setters although fortunately this is not (yet) a well known problem in the setter world.
We know that shunts are an inherited condition in the Irish wolfhound, but because of the prevalence in other specific breeds we suspect it is inherited in most cases. The exact genetic defect is not known yet, but work is underway in the USA. However, the mode of inheritance is not simple and parents and littermates may not be affected. However, breeding from parents that have produced affected offspring, or from affected animals cannot be recommended.
This article was written at the request of the Breed Club Health Coordinators with the aim of both raising awareness of this condition, so that cases are recognised and successfully treated, and ensuring appropriate measures to control breeding are applied.
This article draws on a client FAQ sheet given by the author to owners of affected dogs referred to Bristol Veterinary School.
What causes a shunt ?
This is a congenital problem, but although a patient is born with the PSS, signs usually only begin to develop weeks or even months after weaning, as the protein content of the diet increases. It is likely an inherited condition and breeding from affected animals is not recommended.
What does a PSS do to the animal?
A PSS can have a number of consequences:
1. Toxins [including ammonia (NH3)] produced by bacterial fermentation of protein in the intestines are not filtered by the liver and affect the brain. Variable neurological signs of 'hepatic encephalopathy' may occur e.g. restlessness, intermittent blindness, aimless wandering, head pressing, disorientation, increased thirst and even fits (seizures) and coma in severe cases.
2. Nutrients are not metabolised by the liver, which remains small. This can lead to stunting of the animal.
3. If the liver fails to produce adequate blood proteins, fluid may accumulate in the abdomen ('ascites') giving a pot-bellied appearance.
4. Sometimes the abnormal liver function leads to formation of stones in the kidneys and/or bladder and signs of blood in the urine or even obstruction.
5. Occasionally bacteria escape from the gut and, having evaded the liver, enter the circulation causing periods of ill health and raised temperature.
Where is the shunt ?
There are many anatomical variations on a theme, but in general there are two main types:
1. Intra-hepatic – the vein draining the intestine passes through the liver without dividing. This arises most frequently from failure of a vessel normally only present in the foetus to close. It is most commonly seen in giant breed dogs, and is a surgical challenge to correct.
2. Extra-hepatic – the shunt completely bypasses the liver and enters the general circulation directly via one of several possible routes; porto-caval is the most common type. Extra-hepatic shunts are more amenable to surgical correction.
What is the ideal treatment ?
In an ideal world the PSS is tied off (ligated) surgically, and this can be curative. The success rate varies between 50 and 85% depending on the type of shunt and surgical expertise. At Bristol Vet School, we can also now attempt to treat intra-hepatic shunts by placing an occluding coil via a venous catheter. There is still a risk with this new procedure but even riskier open surgery is not required
In some cases, ligation is not possible, for either medical or financial reasons. These patients are managed medically to control the signs of hepatic encephalopathy. Medical treatment merely reduces the production of toxins and does not correct the shunt.
What can go wrong ?
The aim of surgery is to completely close the shunt. Regrettably it is not always that straightforward:
The shunt may be impossible to find
There may be inadequate veins going to the liver (or even none) so that complete closure of the PSS causes excessive back-pressure on the intestines.In mild cases this may cause temporary accumulation of fluid (ascites). In severe cases it can lead to death of the patient, and so the surgery has to be reversed.
There is a risk of serious haemorrhage, especially with intra-hepatic shunts, which may have to be dissected free of surrounding liver tissue. Placement of a coil by venous access is less risky but not widely available.
If the shunt is found but complete closure is not possible, a partial ligation may be performed. Alternatively a sterile cellophane band placed around the shunt, in order to cause scarring and gradual closure to allow time for the vessels to the liver to regrow.
What is medical management ?
The aim is to reduce intestinal production and absorption of toxins such as ammonia, and so reduce signs of hepatic encephalopathy. Medical treatment is indicated for:
· For short-term stabilisation of patients before surgery
· Patients where ligation of the shunt fails because of a lack of normal vessels going to the liver to cope with the revised blood flow
· Patients where surgery is declined for whatever reason.
There are three lines of treatment
1. Dietary management
A restricted protein diet with carbohydrates as the main energy source should be fed. Veterinary diets such as RCW Hepatic Support or Hill's l/d are suitable. Alternatively a home-prepared diet consisting of equal parts of boiled rice, pasta or potatoes with low-fat cottage cheese may be fed. IIf blood proteins are low, protein should not be restricted severely, and other methods must be used.
This synthetic sugar is a laxative that helps remove the intestinal contents rapidly before significant fermentation occurs. It also decreases the absorption of ammonia. The effect is quite variable, and the dose has to be tailored until the patient produces 2-3 soft motions per day.
3. Oral antibiotics
These help reduce the number of ammonia producing bacteria in the gut lumen.
Treatment is tailored by trial and error to each individual patient until signs of hepatic encephalopathy are controlled. Mild cases may do well on dietary management alone, whilst severe cases may require all three medications.
NB. Cases of PSS must be referred by their vet to other centres offering surgery (including Bristol Veterinary School); owners cannot make arrangements directly