The hunt for shunts

We see some really interesting cases in emergency practice, which is one of the reasons I enjoy this work so much.  There is lots of variation and challenging presenting signs to get the brain thinking.  However, we do use “pattern recognition” in some cases, where you see a patient with unusual presenting signs that might just happen to remind you of a case you saw previously.  This can help in diagnosis, but we do need to be careful about jumping to the wrong conclusion!

I recently saw a gorgeous young cattle dog named Molly who presented to me after a period of waxing and waning, but gradually deteriorating, clinical signs.  At 10 months of age, she was a little smaller than she should be and at the hospital had arrived looking very out of it mentally, almost as though hallucinating.  She was drooling profusely had been doing so more over the previous few days.  She was very sensitive to stimulation but quite dopey in between stirring and when placed on the floor, would proceed to pace with a wobbly back end.

20130404-062442.jpg

Molly, our patient with the shunt, was a little smaller than would be expected for her breed.

When I spoke to her parents, they explained how she had taken three days to recover from her anaesthetic when she was neutered four months prior.  Definitely not normal unless her vet had used very heavy doses of long acting anaesthetics, which I was fairly certain was not the case!  Since her desexing, she had episodes where she would stare into space vaguely and then snap out of it when disturbed, and sometimes would pace around the perimeter of the yard.  Her signs had been deteriorating over the previous week so her owners had taken her to their vet the day prior, bloods had been sent to the lab for assessment, and on this day she was a little worse, so came to see us.

Pattern recognition lead me to think that one of the possible causes of Molly’s signs might be a Porto-systemic Shunt (PSS).  “A what?” you might say.  A porto-systemic shunt is a little difficult to describe, but I guess I could make an analogy.  Lets say you were taking the highway into the city (Brisbane is a good example of this) and you decided to take the inner city by-pass rather than all the smaller more congested roads into the city from the highway.  The caudal vena cava (the large vein returning blood from the body) would be the main highway and the small congested roads would be the branches of this vessel taking blood to the liver for detoxification before returning it to the heart (the city).  The inner city by-pass is the shunt, taking the shortest route from the highway to the city and avoiding the liver and all those small houses and suburbs where important things happen.

There are intrahepatic shunts, which is where the by-pass goes through the body of the liver, and extrahepatic shunts, where the by-pass goes around the liver.  While it is not always the case, intrahepatic shunts are more common in certain pure-breed dogs, and one report I read said that Australian cattle dogs are an over-represented breed.  In their case, it is generally a developmental, or congenital, abnormality.

Extrahepatic shunts tend to be more common in the smaller terrier breeds or older dogs of any breed.  They are thought to be congenital, even hereditary, in Cairn and Yorkshire terriers and possibly other small breeds, but can be acquired, where they develop over time as a result of underlying disease processes.  Acquired shunts may be seen in small and large breed dogs.

Clinical signs of shunts are not always obvious, but the evidence of changes to mentation/behaviour, failure to grow normally, and the history of poor anaesthetic tolerance were highly suggestive of a shunt, and most likely an intrahepatic shunt given Molly’s breed.

The sometimes bizarre signs seen in shunt cases are due to the failure of blood to flow through the liver tissue.  The liver is wonderful at sorting out the rubbish in the bloodstream, detoxifying it and returning it to the body, clean and healthy.  If the liver is by-passed, toxins such as ammonia, which is absorbed from digested food and used to make proteins in the liver, builds up in high concentrations.  Ammonia is quite toxic to brain cells, and hence, abnormal neurological signs are seen.  The medical name for these abnormal signs is ‘hepatic encephalopathy’ (HE), denoting that the brain/encephalopathic signs are originating from liver (hepatic) malfunction.

Shunts can be tricky to diagnose.  The first thing I did was to try to access the bloods that Molly’s vet had sent off the previous day, and fortunately the lab was very helpful there.  Then we ran a blood ammonia level, which needs to be performed in a specific manner to obtain a reliable result.  As expected, Molly’s blood ammonia level was off the scale.  The next step was to have an abdominal ultrasound performed and the sonographer identified an abnormal blood vessel shunting through the liver (confirming an intrahepatic shunt), as well as a very small liver which had not been very active through her life.  The next step will be to perform a CT angiogram on Molly, running dye through the blood vessels of the liver to further track the path of the shunt, so that a plan can be made for potential surgical management.  This will require an anaesthetic, so Molly will receive medical management in the meantime to help improve her chances of tolerating anaesthesia.

Other tests can be performed if these tests do not conclusively diagnose a PSS, and include other bloods such as bile acids and ammonia tolerance testing, nuclear scintigraphy and MRI angiography as an alternative to CT angiography.

normalliver

In the normal liver, blood from the gut goes via the portal vein through the liver then into the vena cava

http://www.powerscourt-cockers.co.uk/advice_pages/health_liver_shunt.htm

The intrahepatic shunt runs from the portal vein directly to the vena cava and by-passes the liver

How can Molly be fixed?  There are two options available, however, the chances of Molly living a long and healthy life are less than her house mate who does not have a shunt, regardless of which option is taken.  Though this is not to say she wont have a good quality of life if everything goes well.  The treatment of choice is surgery.  Surgery involves attenuating (or slowly blocking off) the shunt by placing a band around it which over time will cause constriction of the shunt, hopefully as the normal blood flow to the liver develops or re-develops.  Surgery can be technically difficult because we don’t want the shunt to close too early before the smaller vessels have opened.  This would lead to too much back pressure in the organs behind the liver, such as the spleen, intestine and kidneys and can cause fatal damage to these organs.  Accessing an intrahepatic shunt is also quite difficult as it involves cutting open the liver and finding the shunt to attenuate without causing fatal haemorrhage.  Very stressful surgeries and anaesthetics I can tell you!

Medical management can be performed if surgery is not an option for the patient.  There are conflicting opinions regarding the outcome for medical management, with some clinicians saying it may offer similar results for long term survival as surgery, however, the evidence to support this viewpoint is lacking.  Medical management is important to help stabilise the patient prior to surgery.  It involves supportive care with fluids, feeding a low protein/liver support diet, and providing medications to help reduce ammonia production in the gut and remove the ammonia that is liberated before it reaches the blood stream.  Gastric protectants may also be required if there are signs of stomach upset or ulceration.

Problems can occur during recovery from surgery.  While I wont elaborate here, what I will say is that this is not a straightforward procedure, it is complicated and there are risks involved.  However, if successful and the patient responds appropriately, the long term outlook for the animal is far greater than without having had the surgery.

Molly responded really well to her initial medical management.  When I saw her 24 hours after I admitted her, I was surprised to see a normal, bright and happy dog in the ICU, off of intravenous fluids, eating well and looking to go home the next day.  She will continue on her medications and come back to see the specialists for further investigation and planning in a couple of weeks.  Hopefully we will see more good results down the track because she is a lovely dog with very dedicated owners who want to see the best outcome for her.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s