Portosystemic Shunt Surgery

A portosystemic shunt is an abnormal vascular communication between the blood flow returning from the intestines (called the portal blood flow) and the systemic circulation (for example the blood returning to the heart from the rest of the body). Normally all of the blood returning from the intestine passes through the liver before circulating through the rest of the body to remove digested food and toxins produced by bacteria in the intestine. When a portosystemic shunt is present, the liver is unable to process the blood from the portal circulation effectively. This can cause a wide variety of clinical signs ranging form seizures, blindness, slow growth, vomiting, urinary obstruction to no signs at all. Often signs are worsened after eating a protein rich meal. 

There are four basic forms of portosystemic shunts.

1. A single shunting vessel outside the liver (single extrahepatic shunt)
2. A single shunting vessel inside the liver (single intrahepatic shunt)
3. Multiple shunting vessels outside the liver (multiple extrahepatic shunts)
4. Microscopic shunting vessels inside the liver (microvascular hepatic dysplasia)

A diagnosis of a portosystemic shunt is suspected based on blood tests and clinical signs consistent with a shunt.
Blood abnormalities include:

1. Mild anemia
2. Low BUN
3. Low or normal blood glucose
4. High bile acid test

An ultrasound evaluation of the abdominal vasculature and liver at times can identify a shunt. More recently, MRI evaluations of the abdominal vasculature and liver allow accurate identification and localization of the shunting vessel. 

At this point if a shunt is suspected, an exploratory surgery to identify the shunt can be performed. At surgery, an abnormal vessel or area of liver can be readily identified with a sound knowledge of regional portal vascular anatomy. Occasionally a portovenogram can facilitate identification of a portosystemic shunt. 

The goal of treatment for a portosystemic shunt involves reducing the signs that may be present and directing the blood flow away from the shunt to the liver. In the past, a partial or complete ligation of the shunting vessel was used to suddenly force the blood through the portal circulation. Although this technique is successful, at times the liver is unable to accept the additional blood flow over such a short period of time resulting in portal hypertension. More recently, a vascular constricting device called an Ameroid ring is used to slowly constrict the vessel over a period of months. By slowly shifting the blood flow from the shunt to the portal circulation, serious risk of portal hypertension is avoided. Approximately 85% of patients with a single extrahepatic portosystemic shunt will be alleviated of clinical signs. Intrahepatic shunts are typically managed with partial attenuation of the shunt after identification. Intrahepatic shunt corrections tend to be more complicated with a perioperative mortality rate approaching 20%.