Portosystemic Shunt (Hepatic Shunt) in Dogs

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A portosystemic shunt is an abnormal communication between blood vessels, which causes blood to bypass the liver. The portal vein is a major vessel in the body which enters the liver and allows toxic components of the blood to be detoxified by the liver. When a shunt is present, the portal vein, or one of its related veins, is inappropriately connected to another vein which creates blood flow around the liver.

The most common type of shunt is a single congenital shunt. This means that the animal is born with the problem. Acquired shunts may occur secondary to liver disease.

Congenital shunts occur in both dogs and cats. Most animals start showing signs by six months of age. However, shunts have been diagnosed in adults as old as 10 years.

Shunts are more common in purebred dogs than mixed breeds. The breeds predisposed to congenital shunts include: miniature schnauzer, Yorkshire terrier, Irish wolfhound, cairn terrier, Maltese, Australian cattle dog, golden retriever, Labrador retriever, and Old English sheepdog.

It occurs more often in female dogs, and of the affected males, there is also an increased incidence of cryptorchidism, in which one or both testicles remain undescended.

The impact of a portosystemic shunt on your pet can present itself in a variety of ways. The most common clinical signs are a result of elevated toxin levels in the blood secondary to failure of removal by the liver. One of the important toxins is ammonia, which causes abnormalities of the central nervous system.

What to Watch For

  • Lack of appetite
  • Lethargy
  • Weakness
  • Disorientation
  • Stumbling
  • Circling or pacing
  • Apparent staring into the corner
  • Pressing of the head against objects
  • Seizures
  • Blindness
  • Behavioral changes
  • Excess salivation (more common in cats)
  • Vomiting
  • Diarrhea
  • Increased thirst and frequent urination (more likely in dogs)
  • Straining to urinate
  • Blood in the urine
  • Failure to grow and thrive
  • Weight loss

    Diagnosis

  • History and physical exam
  • Complete blood count (CBC)
  • Biochemical profile
  • Urinalysis
  • Bile acids
  • Blood ammonia level
  • Clotting tests
  • Abdominal radiographs (x-rays)
  • Abdominal ultrasound
  • Portography (special dye study)
  • Transcolonic scintigraphy to monitor the pattern of blood flow
  • Abdominal exploratory surgery

    Treatment

  • Surgical ligation (closure) of the shunt is the treatment of choice. However, animals must be medically stabilized prior to surgery.

  • IV fluid therapy restores hydration deficits and corrects electrolyte imbalances.

  • Lactulose decreases absorption of intestinal toxins such as ammonia by altering intestinal pH.

  • Antibiotics alter intestinal bacteria such that ammonia production is decreased.

  • Feeding a protein restricted diet since protein in the diet can precipitate neurologic abnormalities.

  • In animals with vomiting and diarrhea, gastric protectants help reduce acidity of the gastrointestinal tract and may alleviate some of the symptoms.

  • Anti-convulsant drugs may be necessary in animals with seizures that are not responsive to the aforementioned medical treatments.

    Home Care and Prevention

    Give all medications as prescribed by your veterinarian. Feed only the prescribed diet. Monitor your pet for recurrence or worsening of the original clinical signs that alerted you to a problem.

    As this is a congenital disorder, there are no known preventative measures for your individual pet. However, any cat or dog with a shunt should never be used for breeding purposes.

  • A congenital portosystemic shunt is a condition that exists when your pet is born. This is a serious disorder because the liver does not receive adequate blood flow, and therefore does not grow normally. Most animals with shunts have livers that are smaller than normal. Because of the inadequate blood flow and improper growth, the liver does not function properly.

    The liver is an enormously important organ that has many functions. The most notable abnormalities that result from a shunt are those affecting:

  • The central nervous system (CNS). The neurologic disorders are collectively referred to as hepatic encephalopathy and can range from lethargy and dullness to seizures, blindness, and erratic behavior.

  • The gastrointestinal system. The most common gastrointestinal signs are anorexia, vomiting and diarrhea.

  • The urinary tract. The most frequently documented urinary signs are straining to urinate and blood in the urine. The urinary problems are a result of ammonium biurate bladder stones that occur secondary to liver dysfunction. Some pets also drink more and urinate more than normal. Your pet may show many of these signs or only a few.

    Although the CNS problems are the most common, some pets show only urinary tract or gastrointestinal signs. Some animals may show a failure to grow normally as the only obvious abnormality. Because the clinical signs can be quite varied, there are a number of other conditions that can cause similar signs, including:

  • Infectious diseases. Distemper is a common viral illness of young dogs. Infections with protozoal organisms such as Toxoplasma and Neospora can occur in dogs . All of these diseases may cause abnormalities of the CNS and gastrointestinal tract, causing signs similar those seen in patients with portosystemic shunts.

  • Toxicities. Ingestion of, or exposure to, certain toxins may produce multiple neurologic abnormalities that can mimic the signs seen with hepatic encephalopathy. Toxins may include ethylene glycol (antifreeze), lead, flea products (organophosphates) and prescription medications. These animals often present with vomiting and/or diarrhea as well.

  • Hydrocephalus. This is a congenital brain defect that may cause seizures or abnormal behavior in young animals.

  • Epilepsy. This is a seizure disorder most commonly seen in dogs, with no known cause.

  • Hypoglycemia. Low blood sugar will often cause weakness and seizures, if severe. Young animals are prone to developing hypoglycemia if they are not eating normally.

  • Urea cycle enzyme deficiency. This is a rare metabolic disorder in which the animal is missing an enzyme necessary for normal ammonia processing. Build-up of ammonia causes encephalopathy, but the patient does not have a shunt.

  • Hepatic microvascular dysplasia. This disorder has only been described in dogs. It is another congenital disorder that causes abnormal liver function and can therefore cause many of the same signs that are seen with shunts. This is not a surgically correctable disorder.

  • Urinary tract infections. Bladder infections or inflammation will cause blood in the urine and straining to urinate. A simple urinary disorder would not cause the other clinical signs often seen in patients with shunts.

  • Gastroenteritis. There are multiple causes of vomiting and diarrhea. Although some pets with hepatic shunts will show only gastrointestinal signs, this is less common.

  • 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.

  • Surgical ligation of the shunt is the preferred treatment. However, animals may need to be medically stabilized before they are good candidates for anesthesia and surgery. In some cases it may not be possible to close the shunting vessel completely; this depends on its location. This is most often a problem in large breed dogs. If surgery is not an option, medical management is the mainstay of therapy. Medical treatment may include:

  • Intravenous fluid therapy. Patients may be dehydrated and have electrolyte imbalances and/or hypoglycemia due to vomiting, diarrhea, or not eating and drinking normally. IV fluids are important to correct metabolic disturbances prior to any invasive procedures.

  • Lactulose. This is a laxative drug that causes acidification of the gut contents in the colon. The acidity causes ammonia to be trapped in the colon, thereby preventing absorption of ammonia into the bloodstream. This helps lower the blood ammonia levels in patients with hepatic encephalopathy.

  • Antibiotics. Antibiotics are often given orally to alter the type of bacteria normally present within the intestines. Certain types of bacteria cause ammonia production. By reducing these bacteria with antibiotic therapy, there is less ammonia production. Antibiotic therapy is often used in combination with lactulose therapy.

  • Protein restriction. Prescription diets that are low in protein or homemade diets are recommended for patients with shunts. High protein content can exacerbate hepatic encephalopathy.

  • Gastric protectants. Drugs that are designed to reduce stomach acidity can be beneficial for animals with shunts, especially if they are showing signs of gastrointestinal upset. Examples of these drugs include H2 blockers such as cimetidine (Tagamet®), famotidine (Pepcid®) and ranitidine (Zantac®). Another drug class that is a potent inhibitor of stomach acid production is the proton pump inhibitors. Omeprazole (Prilosec®) is an example of this type of drug. Sucralfate (Carafate®) is a third class of stomach protectant that binds to stomach ulcers. One or more of these drugs may be useful in alleviating some of the gastrointestinal signs seen in patients with shunts.

  • Anti-convulsant medication. Hepatic encephalopathy is often responsive to treatment with fluid therapy, lactulose, and antibiotics. In cases that are not responsive to these drugs, anti-convulsant drugs may be needed to control seizure activity. Examples of this drug class include phenobarbital and potassium bromide.

    Follow-up

    Optimal treatment for your pet requires a combination of home and professional veterinary care. Follow-up can be critical, especially if your pet does not rapidly improve.

  • Immediately following surgery your pet will remain hospitalized for several days. Complications following surgical correction of the shunt may include portal hypertension and worsening of neurologic abnormalities. Portal hypertension is an elevation of blood pressure in the portal vein. This can occur after surgery and can result in reduced blood flow to other abdominal organs. This in turn may cause abdominal fluid accumulation, vomiting, diarrhea, abdominal pain and in severe cases, shock. New methods of repairing the shunting vessel have minimized this complication. Animals may also develop seizures following surgery, even if they did not have them pre-operatively. This requires anticonvulsant therapy and in-hospital monitoring.

  • Once your pet is discharged from the hospital, careful monitoring at home will be necessary. Observation of seizures, anorexia, vomiting, abdominal distension, or abnormal behavior should prompt a call to your veterinarian.

  • Administer all prescribed medications as directed. Alert your veterinarian if you are having problems treating your pet.

  • Feed only the prescribed diet. Although your pet may eventually be able to eat a normal diet, protein restriction may be necessary for an extended time period or even for life.

  • After hospital discharge, your pet should be re-examined by your veterinarian in 10 to 14 days. At that time the surgical sutures (stitches) will be removed and a physical exam will be performed to evaluate clinical progress. Even if your pet seems to be doing well at home, a careful exam may reveal subtle neurologic abnormalities.

  • Follow-up bloodwork, especially bile acids, can be used to assess improvement in liver function following surgical correction of the shunt. These results often do not return to normal, but are expected to improve after surgery. In certain cases the shunt may not be fully closed at the first surgery. The degree of improvement in clinical signs and lab results may dictate whether a second surgery is needed in the future.

  • Monthly evaluations for the first several months after surgery are important for keeping track of your pet's progress.

  • Many pets can do very well after shunt repair, but it is important to keep a close relationship with your veterinarian for the life of your pet, as this is a complicated and serious disorder.

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