Porto-systemic Shunts
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.
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.
General blood tests
Specific blood tests
Imaging
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.
