Portosystemic shunt

What is ... ?: 

Portosystemic shunt (PSS) is a birth defect of the circulation in the liver.  In animals with a PSS, there is abnormal blood flow through the liver.  Blood should flow from the digestive tract (stomach, intestines) to the liver via the portal system into the blood vessels of the liver, and then to the caudal vena cava which is the large blood vessel carrying blood back to the heart. This way, blood normally percolates through the liver where it is detoxified.  In a portosystemic shunt, as the name implies, an accidental shortcut is created: portal blood bypasses the liver and goes directly to the systemic venous circulation (caudal vena cava), without having filtered through the liver tissue. This has very important repercussions because one of the main functions of the liver is to eliminate toxins from the bloodstream. In PSS, blood bypasses the liver and therefore these toxins are not cleared, remaining in the circulation throughout the body. The result is symptoms of PSS, many of which are neurological because the toxins alter the brain's capacity to maintain a dog alert and responsive. The complex of neurological and behavioural signs caused by liver dysfunction is called hepatic encephalopathy.

Portosystemic shunts may occur as intra-hepatic or extra-hepatic depending on the location of the blood vessel in relation to the liver.  That is, intrahepatic PSSs are malformations within the liver, whereas extrahepatic PSSs are malformations just upstream from the liver.  The location of a PSS is important because it determines the best way to correct it via surgery.

Most animals with congenital portosystemic shunts show symptoms before 6 months of age. When symptoms are subtle, however, the condition may not be diagnosed until much later. 

How is ... inherited?: 

The exact mode of inheritance is not known, but a genetic basis is clearly indicated by higher-than-average occurrence in certain breeds (see below).

What does ... mean to your dog & you?: 

The symptoms of PSS tend to emerge during puppyhood.  These symptoms generally are associated with the central nervous system, the gastrointestinal tract, or the urinary tract. Most consistently, there are signs of hepatic encephalopathy - neurological and behavioural evidence of diffuse brain dysfunction due to liver dysfunction. Examples include loss of appetite, mental dullness, lethargy and sluggishness, weakness, poor balance, disorientation, blindness, seizures, and even coma. The symptoms may wax and wane, and may worsen after eating a protein-rich meal.

With PSS, a pup's growth may seem to be stunted or slower than the growth of littermates and age-mates.  Indeed, "runts" of litters often turn out to be puppies that have birth defects, and PSS is a very common one of these.
Failure of the liver to clear ammonia means that there will be increased excretion in the urine. This commonly leads to urolithiasis -  kidney, bladder, or urethral calculi (stones) due to the build-up of mineral salts. Any young dog with urolithiasis (stones in the bladder, urethra, or kidneys) should be checked for PSS.

The first sign of PSS in a dog may be a prolonged recovery from anesthesia, or excessive sedation after treatment with some medications. This occurs because with PSS, anesthetics and medications are not filtered out of the blood and broken down as they would normally be by the liver, but instead are recirculated in the body.
The impact of PSS may not be apparent at first.  Symptoms tend to worsen with age, and the decision to treat (via surgery) should be made as early as possible.  Dogs do not outgrow PSS; surgery should be performed when a puppy is still growing, to minimize the risk of permanent damage.  Dogs who are not candidates for surgery, either because they have a form of PSS that is inoperable or because surgery is not an option due to cost or availability, may still benefit from orally-administered medications at home. 
It is very important to realize that the final result of surgery for PSS can only be known weeks after the surgery has been done.  The effects of PSS take time to subside, and the body's ability to adapt back to normal after the surgical correction is different in every dog: many become totally normal and have normal lives, whereas some have irreversible damage associated with the PSS, and surgery only partially corrects these changes.

How is ... diagnosed?: 

Generally, the diagnosis of PSS is suspected based on a combination of the medical history (such as delayed anesthetic recovery or previous surgery for urolith/urinary tract stone removal), symptoms (such as those described above), and results of laboratory tests. The screening test of choice is a routine laboratory panel (complete blood count, serum biochemistry profile, and urinalysis) with serum bile acids, which is a specific blood test that requires a 12-hour fasting period beforehand and takes 2-3 hours. 
The confirmatory test of choice is a high-detail abdominal ultrasound examination by a specialist (radiologist or internist); nuclear scintigraphy (a type of scan) also is highly definitive but is less widely available.  Either test is noninvasive and is generally done without sedation.

The screening blood test, serum bile acids, is highly accurate, with a nearly 100% ability to detect PSS in dogs that have it.  A positive test still requires ultrasound/scan confirmation, because other disorders such as hepatic microvascular dysplasia, which is an incurable, microscopic version of PSS where thousands of small shunts ("shortcuts") cause blood to bypass the liver at the tissue level, may be present instead.  Hepatic microvascular dysplasia can only be confirmed with a liver biopsy, so it is routine for dogs that undergo surgery for PSS to also have a liver biopsy, for identifying whether hepatic microvascular dysplasia is also present.  This alters the long-term outlook: dogs with PSS but without microvascular dysplasia have a better long-term outlook for living free of symptoms and without medications.

How is ... treated?: 

The most definitive way to deal with PSS is surgery.  The surgeon identifies the path of blood bypassing the liver and closes it, forcing the blood to follow the new, normal course through the liver.  This type of surgery is an open-abdominal procedure, meaning general anesthesia is warranted and a period of recovery, typically lasting 1-4 days in the hospital and 2 weeks or so at home, is to be expected.  The success rate of surgery is high (>90%) but not perfect; even in the most experienced hands, some dogs with PSS who also have microvascular dysplasia or other circulatory defect through the liver may not tolerate the operation and may need only partial closure of the shunt.  Other dogs do not tolerate any correction of PSS and this is only apparent during the operation.  To improve the chances of success, surgeons often repair PSS by inserting a device that closes the PSS gradually, over several weeks.  Surgeons also will be careful to monitor a dog's status both before the surgery and during the post-operative period; the liver performs so many essential functions that careful monitoring and medical support, such as with plasma transfusions, antibiotics, or other treatments, are essential.
In some cases, PSS may involve a single shunt that is buried deep within the liver tissue: intra-hepatic PSS.  These situations are difficult to correct in the manner described above, and a better option in such cases is minimally-invasive occlusion (closure) of the shunt through catheter-based techniques.  Briefly, this approach does not involve surgical opening of the abdomen but rather involves a surgeon placing a catheter through a blood vessel in the groin and steering the catheter to the location of the shunt within the liver under fluoroscopic, real-time X-ray guidance.  The surgeon can then deploy a device that occludes (blocks) blood flow at that level, redirecting it into the normal path.  This is an extremely challenging procedure performed only at certain specialist referral hospitals; discussing this possibility with a general practitioner veterinarians is the first step, followed by referral if the features of the PSS are compatible with this procedure.
Finally, many dogs with very mild or no symptoms of PSS, especially if the condition is first identified after 5 years of age, may do well simply by receving oral medications and no surgery at all.  To be clear, definitive (surgical) correction is the best treatment, but if the PSS is very minor and it escapes notice until age 5 years or thereafter, there may be more to be gained with a conservative approach and no surgery. 

For the veterinarian: 

 

  1.      COMPLETE BLOOD COUNT: often unremarkable. Subtle but important clues may include microcytosis, target cells, and/or a mild nonregenerative anemia.
  2.      BIOCHEMISTRY: mild abnormalities suggestive of hepatic dysfunction often are present, such as hypoproteinemia, hypoalbuminemia, hypoglycemia, low blood urea nitrogen, and normal to mild increases in serum liver enzymes. Elevated bilirubin levels are inconsistent with PSS and, if repeatable, suggest a different diagnosis.  Postprandial serum bile acids (SBA) are consistently elevated (>99% sensitivity for PSS when >25 uM/l).
  3.      URINALYSIS: dogs with polyuria and polydipsia often are isosthenuric or hyposthenuric. Ammonium biurate crystals in the urine sediment are an important and common finding. Where urolithiasis occurs, there may also be hematuria, proteinuria, and/or pyuria.
  4.      PLAIN RADIOGRAPHY: microhepatica is common.
  5.      ADDITIONAL IMAGING TECHNIQUES: ultrasonography, rectal portal scintigraphy, and mesenteric portography (less commonly needed nowadays) can provide information about the presence, location, and type of shunt.
Breeding advice: 

Affected individuals and their parents should not be used for breeding.  Siblings should only be used after careful screening. If any affected offspring are born, breeding of the parents should be discontinued.

FOR MORE INFORMATION ABOUT THIS DISORDER, PLEASE SEE YOUR VETERINARIAN.
 

Resources: 

Hart Jr, JR. Portosystemic shunt. In Cote E, ed. Clinical Veterinary Advisor: Dogs and Cats, 2nd ed (St. Louis, MO: Mosby Elsevier, 2011) pp. 905-907.
Berent AC, Weisse C. Hepatic vascular anomalies. In Ettinger SJ, Feldman EC, eds. Textbook of Veterinary Internal Medicine, 7th ed (St. Louis, MO: Saunders Elsevier, 2010) pp. 1649-1672.
 


What breeds are affected by ... ?
Tags: