Porto-systemic Shunts
Written by Shula Berg BVSc CertAVP(GSAS) GPAdvCert(SASTS) MRCVS
Clinically reviewed by Elizabeth McLennan-Green BVM&S CertAVP(SAM) MRCVS
Table Of Contents
- Overview
- Symptoms
- Diagnosis
- Treatment
- Outlook
What is a porto-systemic shunt?
In a normal abdomen, blood flows past the gastrointestinal tract and pancreas into the liver, through a vessel called the portal vein. The liver filters nutrients and toxic waste products out of the blood, before it returns to the heart and is pumped on around the body. A porto-systemic shunt (PSS) is an abnormal blood vessel connecting the portal vein with the vessels leading directly to the heart. This abnormal vessel causes the majority of the blood to bypass the liver. The PSS can be inside or outside the liver. If it is present from birth, it is known as a congenital shunt. Shunts outside of the liver are usually congenital. Those inside the liver can be congenital or acquired.
PSS are more common in terriers, Bichon Frises, Shih Tzus, Miniature Schnauzers, Irish Wolfhounds, Border Collies and Golden Retrievers, as well as Siamese, Himalayan and Persian cats.
What are the symptoms of porto-systemic shunts?
When the blood bypasses the liver, it causes two main problems. Firstly, the liver isn’t able to filter toxic waste products out of the blood, and secondly, the liver has reduced blood flow compared to normal. Most cases of PSS are congenital and, in affected animals, the liver is often under-developed. Pets with a congenital PSS are often small for their age and underweight, despite normal appetite.
If there is a shunt present, the liver cannot filter waste products. Toxins, such as ammonia, build-up in the blood and make the pet feel unwell. If toxin levels get too high they can affect the brain, known as hepatic encephalopathy. Symptoms can include vomiting and diarrhoea, blood in urine, increased thirst and/or urination, strange behaviour including loss of coordination, weakness, blindness and even seizures. High levels of ammonia can also lead to bladder stone formation.
Which tests are used to diagnose porto-systemic shunts?
Often there is a high suspicion of PSS based on the history and clinical examination, but it is important to rule out other possibilities and confirm the diagnosis.
General blood tests
General blood tests are usually performed initially. These cannot diagnose PSS, but may be suggestive and are important to rule out other conditions.
Specific blood tests
A bile acid stimulation test (BAST) involves blood samples before and after a meal, and measures the liver’s response to food. The result will be high if the liver is not functioning normally. It is not specific for PSS: this means that the BAST will always be high in cases of PSS, but not all animals with a high BAST have a PSS. Some clinics are able to measure ammonia levels, but this requires specific equipment. The next step is to measure ammonia levels. Again, other conditions can cause high ammonia, but the presence of high ammonia and a high BAST are very suspicious for a PSS.
Imaging
The only way to definitively diagnose a PSS is with imaging, usually either an advanced ultrasound scan or a CT scan. Identifying a shunt on ultrasound can be extremely challenging, and specialist referral is often required.
How are porto-systemic shunts treated?
Initially, most pets will be treated medically. This consists of medication to support the liver, an osmotic laxative known as lactulose used to bind ammonia in the intestines, and sometimes antibiotics to reduce levels of ammonia-producing bacteria in the guts. If hepatic encephalopathy is present, causing behavioural changes or seizures, anti-seizure medication or more intensive care may be needed.
The only method for definitive resolution of the PSS is surgery to close the abnormal vessel. This is an extremely complex surgery, and is almost always performed at a specialist centre. If the shunt is within the liver, surgery is even more complicated and may not be possible. Often, pets are managed medically until they are big enough, and stable enough, for surgery.
Several different techniques are available to close the shunt vessel, and different surgeons will prefer different methods. The main risk is that if the vessel is closed suddenly, the liver is not prepared to receive such a large increase in blood flow and does not cope. As a result, most methods are designed to gradually close the shunt vessel over days or weeks. Intensive care is usually required post-operatively while the body adapts to the new blood flow.
What is the outlook for pets with porto-systemic shunts?
Generally, the prognosis for pets with untreated PSS is guarded, due to progressive liver disease and clinical signs. Medical management can be successful and if surgery is not an option, can sometimes keep pets comfortable for months or even years. Ultimately, the only way to achieve a cure is with surgery. Although the surgery itself has risks, pets who come through the procedure can make a complete recovery and have an excellent prognosis long-term.
Disclaimer
Please note that the content made available on this webpage is for general information purposes only. Whilst we try to ensure that at the time of writing all material is up to date and reflects industry standards, we make no representation, warranties or guarantees that the information made available is up to date, accurate or complete. Any reliance placed by yourselves is done so at your own risk.
Page last reviewed: 12th June 2024
Next review due: 12th June 2026