The main blood vessel that brings blood into the liver is called the portal vein. This vein carries blood from the “contaminated organs” such as the intestines. Once the blood enters the liver it is passed through tiny filters, which remove toxins and bacteria.
What is a portosystemic shunt (PSS)?
A fetus normally has a shunt, which directs blood away from the liver to the placenta for cleansing via the mother’s body. Within three days after birth, this vessel typically closes. Occasionally, the vessel will fail to close allowing unfiltered, contaminated blood to reach the venous system without going through the liver. This is called a congenital PSS. The liver is often small in these dogs. A PSS can either be inside (intrahepatic) or outside (extrahepatic) of the liver. A PSS can also be acquired secondary to chronic liver disease.
A congenital extrahepatic PSS is the most common type of shunt and is usually seen in small breed dogs such as Yorkshire terriers, toy poodles, and miniature schnauzers. Intrahepatic shunts are typically found in large breed dogs such as Australian cattle dogs and Labradors. Affected dogs can be seen at any age, although congenital shunts are usually diagnosed before one or two years of age. Clinical signs of a PSS may be more pronounced after the affected pet has eaten a meal of food and may include disorientation, pressing the head on a wall, standing in corners, blindness and seizures. Other signs may include stunted growth, bladder stones or the inability to urinate. Some dogs have no clinical signs of a shunt, and the problem is noted on blood work. Yet others have intermittent signs with variable severity.
A number of tests will be recommended by our specialists to evaluate a patient that may have a PSS. General blood work, including a CBC and serum chemistry panel, and urine testing will be done. The main test that gives supporting evidence of a PSS is a fasting (12 hour) and post-eating (two hours) bile acids test. Ultrasonography is used to evaluate the size to the liver and to check the bladder for stones. In addition, a PSS may be identified by experienced ultrasonographers in about 75% of the cases. Nuclear scintigraphy is a test that can be done to identify patients that have a PSS and is about 85% accurate.
Animals exhibiting clinical signs associated with a PSS require medical management prior to surgical intervention. A low protein diet should be fed to decrease the toxins in the blood from protein metabolism. Lactulose is a medication that decreases the absorption of toxins from the intestines by trapping toxins and decreasing the transit time (laxative effect) of the stool in the intestines. Antibiotics are used to kill blood-borne bacteria that the liver fails to filter in the PSS patient.
Most PSS are visible during surgery. In order to treat the PSS, a device (ameroid constrictor or cellophane band) is placed around the abnormal vessel, which will gradually stop the blood flow through the shunt over a period of six weeks. If a PSS is not found at surgery, a portogram (a dye study of the blood vessels) will be performed to allow visualization of the portal system and identify the shunt if it is present. Biopsies are always taken of the liver to evaluate for the presence of concurrent disease processes. Dogs with abnormal bile acids and no PSS, often have a disease known as microvascular dysplasia. These dogs have many microscopic shunts that require medical treatment to control clinical signs. An intrahepatic PSS is often very large and may sometimes require multiple difficult surgeries.
According to the American College of Veterinary Surgeons, the prognosis is excellent if the animal survives the immediate postoperative period, and the shunt is successfully closed off.
For more information on this subject, speak to the veterinarian who is treating your pet.