One of the liver’s functions is to remove toxins from the blood.
A portosystemic shunt is an abnormal blood vessel that allows toxins in blood to bypass the liver and return to the blood stream of the body without being cleansed.
Pre and postprandial (before and after eating) bile acid assays are used to test for a liver shunt.
Surgery provides the best long term prognosis.
What is it?
A portosystemic shunt is an abnormal blood vessel that allows contaminated blood to bypass the liver and return to the blood stream of the body without being cleared of toxins (see animation below labeled abnormal blood flow - shunt).
A liver is often underdeveloped in dogs that have a shunt.
Shunts can either be inside (intrahepatic) or outside (extrahepatic) of the liver. They can be congenital or acquired secondary to chronic liver disease. Congenital extrahepatic shunts are the most common shunts seen.
Anatomy
The main blood vessel that brings blood into the liver is the portal vein. This vein drains blood from the “contaminated organs” such as the intestinal tract, spleen and pancreas. Once the blood enters the liver it is passed through tiny filters which remove toxins and bacteria (see animation to right labeled "Normal Blood Flow").
A fetus normally has a shunt which directs blood away from the liver to the placenta for cleansing via the mother's liver. When born, these vessels typically close off within three days.
Occasionally, a vessel will fail to close off and allows unfiltered, contaminated blood to reach the venous system without going through the liver, thus the dog has a shunt.
Clinical Signs
Breed:
Extrahepatic shunts – small breed dogs such as Yorkshire terriers, toy poodles, and miniature schnauzers.
Intrahepatic shunts – Typically large breed dogs such as Australian cattle dogs, and Labradors
Shunts of either kind can be seen in cats as well, although the extrahepatic is most common
Age: shunts can be seen at any age, although congenital shunts are usually diagnosed before one or two years of age.
Sex: no sex predilection has been identified
Signs:
Changes in behavior following a eating
Disorientation
Walking into walls, standing in corners
Blindness
Seizures
Inability to gain weight, stunted growth
Inactivity
Bladder stones or the inability to urinate
Clinical signs may be intermittent and variable in severity
Diagnosis
A number of tests will be recommended by our specialists to evaluate a patient that may have a shunt.
General blood work including a CBC, serum chemistry panel, and urinalysis
Pre and postprandial bile acids to evaluate the livers ability to function is done; your pet should be fasted for 12 hours prior to the test; blood is drawn, then a meal is fed and another blood sample is collected two hours later. If either the pre or postprandial bile acids level is greater than 100 U/ml a shunt is strongly suspected.
Ultrasonography to evaluate the size to the liver and to check the bladder for stones. A shunt can sometimes be identified by experienced ultrasonographers. At our practice we usually omit the ultrasound as we do not feel that this test is of great benefit.
Other tests available, but not commonly performed include: blood ammonia assays, ammonia tolerance testing, and nuclear scintigraphy
Most shunts are definitively identified at surgery. If a shunt is not found at surgery, a portogram (a dye study of the portal system) will be performed to allow visualization of the portal system and identify the shunt.
Biopsies are always taken of the liver to evaluate for the presence of underlying or concurrent disease processes. Dogs with abnormal bile acids, but without a shunt, often have a disease known as microvascular dysplasia. This is shunting of blood on a microscopic level and requires medical management.
Treatment
Animals exhibiting clinical signs associated with a shunt require medical management prior to surgical intervention.
Medical management includes:
A low protein diet. Many of the toxins causing the clinical signs are byproducts of protein metabolism. Limiting the protein will decrease the amount of toxins produced.
Lactulose is a medication that decreases the absorption of toxins from the intestines by trapping toxins and increasing the transit time (laxative effect) of the intestines.
Antibiotics are used to decrease the amount of toxin producing bacteria in the intestines and to treat low grade infections if present.
Surgical intervention:
The goal of surgery is to slowly occlude the abnormal vessel.
Definitive treatment requires complete occlusion of the shunt. This needs to be done gradually as the underdeveloped developed liver is often not capable of handling the entire portal blood supply and needs time to accommodate the increased blood flow.
Gradual occlusion is often performed by placing an ameroid constrictor band around the shunt (see animation to the right). This band will slowly swell upon contact with water and gradually close down over 4-6 weeks.
Other surgical options include the use of cellophane banding or multiple surgeries to gradually close the shunt.
Intrahepatic shunts are often very large and may sometimes require multiple surgeries for complete occlusion.
Potential Complications
The blood pressure within the liver can elevate to dangerous levels (portal hypertension) after surgery. This causes blood flow to back up in the organs drained by the portal system. Clinical signs included abdominal pain, vomiting, diarrhea (with or without blood), or abdominal fluid accumulation. This is an uncommon, but potentially fatal complication. By the time external clinical signs are evident the problem is too advanced to be reversed.
Post-Ligation Neurological Dysfunction: This is a syndrome causing variable degrees of neurological abnormalities following surgery. Signs can be mild, such as pacing or vocalization, to life threatening seizures. Most serious problems are noted within the first 3-5 days following surgery. This is also a very uncommon, but potentially serious complication. If animals fail to respond to medical treatment of the seizures, this complication may be fatal.
Some animals may develop additional shunts (acquired extrahepatic) in response to the rerouted blood flow. This can be seen in up to 15% of patients, although clinical signs may not be evident.
Postoperative Care
Medical management with a low protein diet should continue until the bile acids are rechecked two months following surgery. If the bile acids have normalized, then a regular diet can be resumed.
The antibiotics and lactulose should be continued for about two weeks following surgery, depending on how the patient is doing.
Prognosis
About 85% of dogs having shunt surgery respond very well.
Older patients and dogs with poorly controlled clinical signs may have a worse prognosis.
MVS Surgery Team
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