A portosystemic shunt (PSS), also often called a liver shunt or a portosystemic vascular anomaly, is essentially a defect in which one or more abnormal veins allow blood from the intestines and elsewhere in systemic circulation to circumvent the liver. These veins are actually remnants of normal embryonic blood vessels that for some reason fail to regress normally after the puppy is born. Portosystemic shunts may be congenital (present at birth) or acquired after birth. Congenital portosystemic shunts, which are the most common form of this disorder, typically are evident by the time a puppy reaches 6 months of age; in almost all cases, clinical signs will be obvious to owners before the dog reaches two years of age.
Congenital portosystemic shunts usually involve a single large fetal blood vessel in or close to the liver. When the vein is inside the liver, it is called an "intrahepatic" shunt; when the vein is outside of the liver, it is called an "extrahepatic" shunt. "Hepatic" means pertaining to the liver. Intrahepatic portosystemic shunts are more common in large breed dogs and are more difficult to correct surgically. Extrahepatic portosystemic shunts are more common in small dogs and are more easily corrected. When a portosystemic shunt exists, the blood from normal systemic circulation bypasses the liver. Because the liver is involved in so many essential bodily functions, including detoxification of noxious substances from the blood and the synthesis and breakdown of fats, portosystemic shunts cause a potpourri of problems for affected dogs.
Causes of Congenital Portosystemic Shunts
Congenital portosystemic shunts (PSS) are embryonic blood vessels that do not regress normally before or just after birth. These veins "shunt" blood from the gastrointestinal tract and spleen around the liver, which causes toxins, nutrients and other substances that normally would be filtered, metabolized or modified by the liver to remain in circulation. Many of these substances, especially ammonia, are highly toxic to other tissues, especially to the central nervous system. The severe neurological symptoms that frequently are associated with portosystemic shunts in dogs are caused by a condition called hepatic encephalopathy. When blood does not flow normally through the liver, it is not detoxified as it should be, and the metabolism of fats and other essential nutrients is disturbed. As a result, the blood in circulation accumulates abnormal amounts of ammonia and other substances, which in turn adversely affect the brain and cause it to become spongy and damaged. Other organs, especially the intestines and kidneys, are also damaged. The abnormal shunting of blood around the liver also causes low portal blood pressure.
The exact reasons for congenital portosystemic shunts are not known. They appear to be an anomaly of prenatal development and may be caused by some insult to the puppies in utero. However, because portosystemic shunts seem to be more prevalent in certain breeds, it is thought that there may be a genetic component to them as well.
Prevention of Congenital Portosystemic Shunts
Because the cause of congenital portosystemic shunts is not known, there is no documented way to prevent development of this disorder. However, most authorities recommend that affected dogs (those who are identified as having a PSS) not be used as part of a responsible breeding program.
Congenital portosystemic shunts are the most common congenital portovascular disorder – or disorder of portal circulation - in domestic dogs. "Portal circulation" refers to the normal flow of blood from the gastrointestinal tract and the spleen through the large portal vein into the liver.
How Congenital Portosystemic Shunts Affect Dogs
It is hard to speculate as to how dogs with congenital portosystemic shunts "feel" differently than they would have if they were not born with this anatomical abnormality. However, the primary effects that we see in dogs with this condition are largely neurological, gastrointestinal and/or urological in nature. Neurological means pertaining to the brain and central nervous system; gastrointestinal means pertaining to the digestive tract (stomach and small and large intestines); and urological refers to the urinary tract.
Symptoms of Congenital Portosystemic Shunts
A congenital portosystemic shunt (PSS) can cause a huge variety of clinical signs. About 75% of affected dogs develop signs by the time they reach 1 year of age. Typically, these signs wax and wane over time. Most dogs with PSS only develop a few observable symptoms of their condition. These may include neurological signs that are secondary to a condition called hepatic encephalopathy, which may span a spectrum of severity from mild to extremely severe. Hepatic encephalopathy is a syndrome caused by severe damage to the liver, such as the inadequate blood supply to the liver caused by a PSS. The neurological signs associated with a portosystemic shunt typically are episodic (come and go) and may include one or more of the following:
Ataxia (lack of coordination)
Vision disturbances (apparent blindness)
Gastrointestinal (stomach and intestinal) symptoms also often are present in dogs with congenital portosystemic shunts. These may include:
Lack of appetite (inappetence; anorexia)
Excessive appetite (excessive ingestion of food; polyphasia)
Pica (craving for unnatural materials or articles of food; often involves ingestion of feces)
The urinary tract in dogs with portosystemic shunts can be adversely affected as well. If this happens, the signs may include:
Blood in the urine (hematuria)
Difficulty urinating (dysuria)
Abnormally frequent urination (pollakyuria)
Abnormally large volume of urine (polyuria)
Abnormally large intake of water (polydipsia)
Enlarged kidneys (bilaterally)
Kidney and/or bladder crystals or stones (ammonium urate or biurate uroliths)
Other assorted signs of a PSS may include:
Stunted growth (common)
Itchy skin (pruritis; often intense)
Slow recovery from anesthesia or tranquilizers
Dogs at Increased Risk
Congenital portosystemic shunts are typically diagnosed in young dogs, on average well before two years of age. Female Bichon Frises are roughly 12 times more likely to be born with a PSS than are males of that breed. The reason for this gender-based association is not known. The Maltese Terrier, Yorkshire Terrier and Irish Wolfhound reportedly have an approximately 20 times greater risk of being born with a congenital PSS than do other breeds. Other breeds reported to have a predisposition to being born with portosystemic shunts include the Golden Retriever, Labrador Retriever, Old English Sheepdog, Samoyed, Australian Shepherd, West Highland White Terrier, Miniature Schnauzer, Australian Cattle Dog, Collie, Poodle, Cairn Terrier, Tibetan Spaniel, Havanese, Shih Tzu and Dachshund.
How Congenital Portosystemic Shunts are Diagnosed
Congenital portosystemic shunts (PSS) are usually diagnosed in young dogs (under 2 years of age) as a result of a combination of nonspecific symptoms. The results of a urinalysis and routine blood work (a complete blood count and a serum biochemistry profile) are typically unremarkable in dogs with portosystemic shunts, although there may be an elevation in liver enzymes, and some changes in blood urea nitrogen (BUN) levels. Abdominal radiographs (X-rays of the belly) may reveal a small liver (microhepatica) and/or abnormalities in the kidneys. More definitive diagnosis is made by drawing blood samples and submitting them to a diagnostic laboratory for preprandial (fasted) and postprandial (after a meal) serum bile acid tests.
Abdominal ultrasound done by a skilled ultrasonographer may actually identify a PSS and/or an abnormally small liver. Other advanced diagnostic imaging techniques include portovenography, transcolonic or colorecytal portal scintigraphy and radiographic mesenteric portography. Mesenteric portography is considered to be the gold standard for diagnosing a PSS. A liver biopsy is recommended in almost all cases of suspected portosystemic shunts, to assess the nature and extent of liver damage. A fine needle aspirate can also be taken, but while it is a less invasive procedure than a biopsy it does not provide as good a sample for diagnostic purposes. Radiographic studies using contrast media are also available to diagnose a PSS. Many of these tests are only available at specialized referral centers and at veterinary teaching hospitals.
Puppies of high-risk breeds can be screened for portosystemic shunts by measuring the concentration of bile acids and/or ammonia in their blood. Unfortunately, these tests can have false positive results, and no puppy should be labeled as having a PSS - and certainly should not be euthenized - based solely on the results of a blood test. The best way to diagnose a PSS is through abdominal ultrasound. This technique is highly specific and very sensitive for the diagnosis of a portosystemic shunt.
The main goal of treating a dog with a congenital portosystemic shunt (PSS) is to reverse the neurological signs of hepatic encephalopathy by eliminating the shunting of blood around the liver. Other goals are to relieve the gastrointestinal and urological signs associated with the condition. Congenital portosystemic shunts are typically treated surgically. Before surgery, the dog will be given inpatient supportive care, including nutritional and fluid management, to optimize the success of the surgical procedure. The veterinarian will attempt to occlude or ligate (tie off) all or part of the anomalous vessels that are re-routing blood around the liver, so as to restore normal or near normal blood circulation and liver function. Complete ligation may not be possible, but even with partial ligation the dog may be able to live a happy and full life without further treatment.
However, owners should recognize that when the abnormal vessel is occluded, the blood pressure through the liver may rise so much that it can cause seizures and, in very rare cases, even death. The hepatic (liver) vasculature initially may not be able to accommodate all of the blood that previously was shunted through the anomalous vein. Fortunately, even a partial ligation of the shunting vein resolves many of the dog's symptoms and restores a fairly normal quality and duration of life in roughly 70% to 80% of dogs. Sometimes, however, postoperative complications do occur. These may include seizures, high portal blood pressure, sepsis, endotoxemia, hemorrhage, acute pancreatitis, bloody diarrhea, abdominal pain, elevated heart rate (tachycardia) of unknown origin, high or low body temperature (hyper- or hypothermia).
Good nutritional support is another component of a management protocol for dogs with liver shunts. Many veterinarians recommend a well-balanced, protein-restricted diet to help control the signs of hepatic encephalopathy which commonly accompany a PSS.
If the anomalous blood vessel can be completely ligated (tied off surgically), the dog's prognosis is excellent. If only a partial ligation is accomplished, the prognosis ranges from good to poor. If surgery is not done and only medical management (drug therapy and supportive care) is attempted, the outlook is usually poor. Intrahepatic portosystemic shunts (those located within the liver itself) are much more difficult to correct surgically than are extrahepatic shunts, where the blood vessel is outside of the liver. The ultimate prognosis depends upon how well the remaining liver vasculature functions once the abnormal vessel is occluded. Owners should be aware of the risks of putting a dog with a PSS under general anesthesia; according to some experts, the surgical and anesthetic mortality rate ranges from 5% to 25%.