WHAT IS A PORTOSYSTEMIC SHUNT AND HOW DOES IT OCCUR ?
Portosystemic shunts are abnormal vascular connections between the stomach/intestines and the general circulation. Most people know that each part of the body is supplied by blood carrying oxygen and nutrients in arteries from the heart. This blood is returned to the heart, for recirculation, in veins and carries with it waste products from the body tissues. In the case of the intestine the rest of the body needs protection from the extra burden of waste and food digestion products which this organ deposits in the blood circulating through it. For this reason the veins carrying blood from the intestines do not carry it directly to the general circulation but pass it into the liver' s own circulation via a special filtration bed of liver cells. This is the back door or 'portal' liver circulation. Before birth blood vessels are present in the foetus to allow blood to bypass the portal circulation. This is similar to the foetus's bypass system which allows blood to avoid unnecessary circulation through the lungs before birth. Both the lung and liver blood bypass systems should and usually do close off shortly after birth. If the portal bypass does not close blood continues to take this line of least resistance and is 'shunted' from the 'portal' to the 'systemic' circulation. This is a portosystemic shunt. In this example the shunt would be present at birth or 'congenital'. Acquired during life or, “acquired” portosystemic shunts arise when due to certain types of liver disease the local blood pressure in the liver portal circulation is elevated. This causes one or more blood pathways through the liver filter system to open into larger vessels. These allow blood routes of less resistance that bypass the liver filter system.
For the purpose of this description we will ignore acquired shunts. The congenital Portosystemic shunt patient has inadequate blood supply to its liver. This causes:
1) Failure of the liver to develop to its normal size and function.
2) Generalized effects on the body because the liver fails to process nutrients and waste products from the gut into safe materials.
WHAT SIGNS CAN BE EXPECTED ?
Most dogs which have a shunt present from birth that persists beyond a few weeks of age develop abnormal signs within the first months of life. Usually the diagnosis is made by one year of age. Exceptionally it may not be apparent for some years. In these cases it is likely that the animal is only slightly affected by the abnormality until some other problem such as damage to the liver "tips the balance".
These abnormalities can produce a wide variety of symptoms. Portosystemic shunts in Deerhounds have produced the following particular signs early in the progress of the illness:
1) Excessive drinking and urine production. This can start from 3 months of age.
2) The urine is often noted to be abnormal. Dark yellow and strong smelling.
3) A tendency to loose or runny stools.
4) Weakness or lack of co-ordination in movement. Hind leg problems are more likely to be noticed.
There is a wide range of other possible symptoms that are often vague and frequently seem unconnected. AIl may be intermittent or episodic.
As the effects of the shunt worsen the whole body suffers a progressive deterioration and any of the following may be noticed:
1) Stunted growth, Poor body condition.
2) Depression, Anorexia (lack of appetite), Lethargy.
3) Nausea, Vomiting, Diarrhoea.
4)Behavioural changes.
5) Ataxia (Ioss of co-ordination/ balance ).
6) Circling, Aimless wandering or pacing, head pressing, Stupor.
7) Excessive salivation, Drooling.
8) Dementia, Seizures, Coma.
9) Apparent blindness.
10) Excessive drinking, Excessive urine production.
11) Kidney/ Bladder stones.
12) Intolerance to drugs (e.g. prolonged effects of sedatives and anaesthetics).
ARE PORTOSYSTEMIC SHUNTS BECOMING MORE COMMON ?
It would be possible for any mammal to suffer from a shunt. The condition has been diagnosed in many species including humans and cats. They do not occur very commonly in any particular breed of dog and are diagnosed sporadically in all breeds (and are seen in mixed breeds). The frequency of occurrence appears to vary from breed to breed. The condition is now being diagnosed more frequently in both dogs and cats. Possible reasons for this which must be considered other than a real increase in frequency of occurrence are:
1) Each year a higher proportion of practising vets are familiar with the problem.
2) lmprovements in veterinary medicine have increased the use of diagnostic laboratory and clinical tests.
3) Pet insurance encourages full investigation of very sick dogs that would once have been euthanased.
CONFIRMATION OF DIAGNOSIS
After or even before the condition is suspected in a puppy or adult the following may be used help with diagnosis:
Blood tests: Changes in the blood celI types and numbers. Changes in blood biochemistry .
Urine tests: To look for abnormal crystals (stones).
Ultrasound scans: To determine liver size and look for Iarge shunt vessels.
X-rays: To determine liver size.
Angiography: Injecting X-ray dense materials into gut blood vessels: to trace abnormal vessels from the gut.
Scintigraphy: Using radioactive labelIed materials into the bowel: to trace their path from gut to liver.
SPECIFIC DIAGNOSTIC TESTS
Two specific blood tests can be used to confirm the diagnosis of Portosystemic Shunt: the second can be simplified to a cost effective screening test for litters of puppies. This should allow a breeder to sell puppies with confidence that they are not affected. Puppies which yield inconclusive or positive results can then be held back for further tests and investigations.
POST FEEDING BLOOD AMMONIA TEST
This was the first established of the two diagnostic tests. The level of Ammonia is estimated in a very fresh bIood sample taken after feeding. Ammonia is produced in the gut when protein food is broken down. Normally this is absorbed and passes into the portal veins but is filtered out of the circuIation by the liver to protect the rest of the body from its effects. Where, in the case of a Shunt, the blood misses the liver filtration ammonia will reach higher than normal levels in the general blood circulation. This ammonia is very toxic to the brain and is the cause of many of the behaviouraI and nervous system effects of a shunt.
This test is no longer preferred as it is difficult to produce accurate reliable results. The blood sample must be analysed almost immediately after it is taken. It is interesting that resting levels of ammonia in blood samples from some normal lrish WoIfhounds is higher than that of most other dogs. Some have suggested that Deerhounds may be simiIar.
DYNAMIC BILE ACID TEST
Bile acids are excreted by the liver via the bile ducts and gall bladder into the bowel where they aid in the digestion of food. Some become reabsorbed by the gut and pass via the portaI circulation to the liver where they are filtered out of the blood. These are then recirculated into the bile system. Feeding causes the gall bladder to empty thus increasing the amount of bile acid entering the gut. When this is absorbed the level in the portal vein also rises. Where a shunt exists these bile acids will not be adequately cleared from the blood by the liver. The after feeding blood bile acid rise is therefore exaggerated.
To perform the test a resting blood sample is taken after fasting. A meat meal containing a significant fat content is then fed. A second blood sample is taken 90 to 120 minutes after feeding.
False negative results should not occur. Provided the puppy has eaten the meal a puppy with a negative result can be reasonably considered not to have Congenital Portosystemic shunt.
Positive results not related to shunts can occur in cases of severe acquired liver disease in the adolescent or adult. However in the young (less than 4 months old), clinically normal (i.e. showing no abnormal signs of illness) puppy it can be considered almost diagnostic of a shunt if a very marked positive result is achieved. This test can therefore be used as a reliable screening test for shunts in puppies old enough to be fasted (i.e. weaned) and then fed an appropriate meal. It is also useful as a more specific and sophisticated test where the following screening test has positive or inconclusive results.
SINGLE POST FEEDING BILE ACID TEST
This is the test that we recommend for screening litters of Deerhound puppies prior to sale.
In a puppy with a persisting shunt the second sample in the Dynamic Bile Acid Test should give a very high result. In most normal puppies the second sample will give a normal result. It is therefore possible to take and analyse only an after feeding sample as a screening test. This will likely be less expensive and easier to perform if a whole litter is to be screened. Low (negative) results can be considered normal with better than 99% certainty. Both high and inconclusive results can be further investigated by repeating the test, with a full Dynamic Bile Acid Test or other investigations as necessary.
TREATMENT OF SHUNT PATIENTS
Less severe cases (where less of the portaI blood is shunted) can sometimes be maintained adequately on appropriate medication and a very carefully managed diet. They are however not normal and such cases are exceptional. More severely affected dogs usually require surgical treatment to partially, fully or progressively close the shunt vessel or vessels. This sounds simple but the immediate consequences are very serious and very life threatening. Many animals have died in the immediate post operative period. Fully successful cases may not be in the majority even if surgical candidates have been well chosen.
WHY SHOULD BREEDERS BE CONCERNED?
A significant number of cases have been recorded in Deerhounds. Responsible breeders should be concerned for the following reasons:
1) The condition Ieads to considerable suffering in the patient.
2) Unimaginable worry and distress has been suffered by families with affected dogs.
3) lt is possible that in law a breeder might be considered to be liable for Ioss, distress and suffering caused if it was determined that the condition occurred more than sporadically in the breed and that reliable screening of puppies was available.
4) Even if a fully investigated patient is insured to cover veterinary fees some insurance policies reserve the right to exclude payment for congenital (present at or from birth) conditions.
Copyright 2004 - Sue Finnett & Hector Heathcote
hyndsight@gayton.co.uk |
