Liver Shunts : What Are They and Why Do We Look For Them?

What are liver shunts?
The liver has multiple functions. One of the most important is to filter blood returning from the intestines before it enters general circulation. The blood coming from the intestines is full of nutrients the body needs to function. However, it is also full of toxic byproducts of digestion. These digestive toxins are supposed to be neutralized and removed by the liver before ever reaching the rest of the body.

Before birth, placental mammals do not need this filtering property of the liver. The embryo gets its nutrients from the placenta and it delivers all its waste products back into the mother’s circulation. As a result, embryonic blood vessels bypass the liver. After birth, the bypass is supposed to close and circulation should be directed through the liver filtration system.

In rare circumstances, the bypass vasculature does not close. This results in a “shunt” around the liver. This shunt allows toxic byproducts of digestion into general circulation where it can cause nervous system malfunctions, called hepatic encephalopathy. In addition, since the liver does not receive the appropriate level of blood flow, it will generally remain small and have difficulty carrying out its other functions, like manufacturing blood proteins.

We call this form of shunting “congenital” since it is a condition present from birth. Congenital shunts can occur in any breed, but are more prevalent in Yorkshire terriers, Maltese, Cairn terriers,Doxy,mini Doxy, Tibetan spaniels, Havanese,Hybrids of these breeds ie maltipoo, Yorkipoo etc and Irish wolfhounds. They are generally diagnosed at a young age, but smaller shunts can persist into adulthood without being detected. The condition is usually detected when investigating delayed growth or abnormal neurologic episodes. Sometimes abnormal blood values on a pre-anesthesia screening for a spay or neuter will tip off a veterinarian to an underlying condition. The Bile Acid Profile is the test commonly used for just this purpose. An abnormal bile acid profile can help determine if there is increased risk to the patient from the anesthesia.

Once detected, a congenital shunt is usually visualized by using an ultrasound machine. If the anatomy of the shunt will allow, a surgical procedure to close the shunt can be undertaken. Some shunts can be completely closed using an constrictor band that slowly swells allowing the liver to adapt to gradually increasing blood flows. Other shunts can only be partially closed, hopefully reducing the frequency or severity of symptoms. There are numerous risks involved with surgical closure of a shunt and not all patients are candidates for the procedure.

Patients with congenital shunts that cannot be treated surgically may respond to medical management of their condition. Low protein foods, medications to trap nitrogen, and selective supplementation of certain vitamins can help reduce some of the symptoms. This is also the method used to treat acquired liver shunts.

Acquired shunts occur secondary to chronic liver disease. Scar tissue, inflammation, blood clots, and abnormal formation of ducts or capillaries within the liver can all lead to shunting. Most commonly these shunts are in multiple small vessels throughout the liver and cannot be surgically corrected. Frequently a liver biopsy is required to identify the cause of the shunting.

The best treatment is to obtain an accurate diagnosis and address the underlying cause. Until that can happen, the medical treatments above are often used.