Portosystemic Shunt (Hepatic Shunt) in Dogs
Dr. Erika De Papp
A thorough history and physical exam are important in making the diagnosis of a portosystemic shunt. Behavioral patterns described by the owner can be informative. Additionally, some shunt patients have a history of slow recovery following sedation or anesthesia, failure to grow or gain weight, and small size compared to littermates.
Complete blood count. The CBC provides information regarding red blood cells, white blood cells and platelets. The CBC may be normal, or it may show subtle changes in red blood cell size and shape. Some pets also demonstrate a mild anemia (low red blood cell count).
Biochemical profile. The biochemical profile provides information regarding liver and kidney function, and measures electrolyte levels in the blood. Animals with shunts may have normal or mildly elevated liver enzymes. If liver function is markedly compromised, they may also have low protein, cholesterol, blood sugar, and blood urea nitrogen levels. These parameters are all measured on the biochemical profile.
Urinalysis. Urine of patients with shunts may show abnormal crystal formation. Ammonium biurate crystals and bladder stones are suggestive of liver disease. Animals with crystal or stone formation may be prone to urinary tract infection, which is also assessed by the urinalysis. Additionally, dilute urine can sometimes be a marker for liver disease, and is usually seen in the pets with a history of frequent urination and excessive drinking.
Bile acids. The bile acid test is a special liver function test. It provides more information than the biochemical profile and is an excellent indicator of liver function. This is an important screening test for animals suspected of having a shunt, because the bile acids are markedly abnormal in patients with shunts.
Blood ammonia level. Ammonia is one of the toxins that become elevated in the blood when the liver is not functioning properly. It may be normal, but is often elevated in patients showing signs consistent with hepatic encephalopathy (degenerative brain disease).
Clotting tests. The liver is responsible for making the majority of the body's clotting factors; therefore, these tests are also a good way to assess liver function. Most patients with shunts have normal clotting tests, but this is important to evaluate before considering surgery.
Abdominal radiographs. X-rays of the abdomen will often reveal a liver that is smaller than normal. Additionally, there is sometimes evidence of enlarged kidneys in patients with shunts. Abdominal x-rays are also useful in ruling out intestinal obstruction in patients that are vomiting.
Abdominal ultrasound. Use of ultrasonic waves to create an image is a common diagnostic method used in both human and veterinary medicine. A veterinary radiologist can sometimes find the shunting vessel by performing an ultrasound of the patient's abdomen. An inexperienced operator cannot perform this study, and even the most seasoned clinician may not be able to visualize all shunts by this method. Ultrasonic evaluation of the bladder is also useful to look for ammonium biurate bladder stones, which do not routinely show up on X-rays.
Portography. This test is considered the gold standard for diagnosis of portosystemic shunts. It involves surgically opening of the abdomen and injecting contrast (dye) into one of the veins that drains into the portal vein, and then into the liver. X-rays are then taken to outline the pathway along which the contrast travels. By this method, shunting vessels can be clearly identified. This allows definitive diagnosis of the shunt and also helps the surgeon identify the exact location of the vessel that needs to be ligated. Portography is usually performed by surgical specialists.
Transcolonic scintigraphy. This test provides a non-invasive method to identify the presence of a shunt. A radioactive material is inserted into the rectum. This gets absorbed into the bloodstream and the radioactivity is measured. The rate and degree of radioactive uptake in the liver can be compared to other parts of the body, specifically the heart, to determine if the radioactive blood is being shunted around the liver. This test can only be done at specialized centers that are licensed to deal with use of radioactive material.
Abdominal exploratory surgery. Surgical exploration of the abdomen is yet a third method to prove the existence of a shunt. Depending on the history, clinical signs and lab results, some surgeons prefer to perform surgery in lieu of portography or scintigraphy if the index of suspicion is high for the presence of a shunt. This also provides the added advantage of being able to repair the shunt at the same time. This is a complicated surgery and is almost exclusively done by veterinary surgical specialists.