Portosystemic Shunts (Liver Shunt) FAQ
What
is a liver shunt?
A liver shunt is a blood vessel that carries blood around the liver instead of
through it. In some animals a liver shunt is a birth defect ("congenital
portosystemic shunt). In others, multiple small shunts ("acquired
portosystemic shunts") form because of severe liver disease such as
cirrhosis.
Why
do congenital shunts develop?
All mammalian fetuses have a large shunt ("ductus venosus") that
carries blood quickly through the fetal liver to the heart. Since the mother's
liver does the work of filtering out toxins, storing sugar, and producing
protein for her unborn babies, liver function is not needed in the fetus. This
ductus venosus is supposed to close down shortly before or after birth as the
baby's liver begins to work. In some individuals the shunt doesn't close down;
it is then called a "Patent Ductus Venosus", or an intrahepatic
shunt. In other animals, a blood vessel outside of the liver develops
abnormally and remains open after the ductus venosus closes. This is called a
congenital extrahepatic shunt.
Why
do animals with shunts have problems?
In the normal animal, food and other ingested materials are broken down or
digested in the intestines and absorbed into the portal blood stream, where
they are carried to the liver. The liver stores some of the food for energy,
processes some of it into safe chemicals, and uses some of it to make proteins
and other substances. Because the blood bypasses the liver in dogs with shunts,
toxins may build up in the bloodstream or kidneys. Additionally, the animal
lacks the necessary materials to give it a ready source of energy and to help
it grow.
What
are the clinical signs of a liver shunt?
Clinical signs are often seen at a young age and include small stature, poor
muscle development, behavioral abnormalities (circling, disorientation,
unresponsiveness, staring into space, head pressing), seizures, and quiet
demeanor. Other less common signs include drinking or urinating too much,
apparent blindness, diarrhea, and vomiting. In some animals the signs are
associated with eating protein. Other animals are diagnosed when they take a
long time recovering from anesthetics (i.e. barbiturates) or sedatives (i.e.
acepromazine). Some animals show no signs until they are older, when they
develop bladder and kidney infections and stones.
What
breeds are commonly affected with shunts?
Small breed dogs tend to have shunts that form outside of the liver
("extrahepatic"). In the United States, Yorkshire terriers have
almost a 36 times greater risk of developing shunts than all other breeds
combined. Extrahepatic shunts can be seen in any small breed but are also
reported commonly in schnauzers, Maltese, dachshunds, Jack Russell terriers,
Shih Tzu, Lhasa Apso, Cairn terriers, and poodles. Large breed dogs tend to
retain the fetal liver shunt (patent ductus venosus), or
"intrahepatic" shunts. In the Netherlands, about 2% of Irish Wolfhounds
are born with intrahepatic shunts. Intrahepatic shunts can be seen in any large
breed dog and have been reported in some small breed dogs (especially poodles);
in the United States, we see them most often in Labrador retrievers. Australian
shepherds, Australian cattle dogs, Samoyeds, and Old English sheepdogs are also
commonly reported.
Are
shunts hereditary?
A disease is likely to be hereditary if it occurs more commonly in one breed
than others; if it occurs in a family of dogs; or if it or a closely related
disease is proven hereditary in other breeds or species. Liver shunts are
considered hereditary in Irish wolfhounds, Cocker spaniels, Maltese, and
Yorkshire terriers, and are probably hereditary in several other breeds. The
affected dog should be castrated or spayed and, because the mode of inheritance
is not known, it is best to avoid breeding the parents.
How
is a shunt diagnosed?
On blood work, dogs with congenital liver shunts usually have low blood urea
nitrogen (BUN) and albumin concentrations. They may be slightly anemic or have
red blood cells that are smaller than normal ("microcytosis"). They
also may have increases in liver enzymes ("AST", "ALT").
Their urine may be dilute or infected and contain small spiky crystals
("ammonium biurate"). None of these laboratory changes are specific
for a liver shunt; however, when veterinarians see these abnormalities, they
will usually measure bile acid or ammonia concentrations to evaluate liver
function. A liver shunt cannot be definitively diagnosed by blood work;
shunting can only be found with advanced techniques such as scintigraphy,
ultrasound, portography, Cat scan ("CT"), MRI, or exploratory
surgery.
What
are bile acids?
Bile acids are produced in the liver and stored in the gallbladder between
meals. They are released into the intestines to help break down and absorb
fats, and are reabsorbed and stored again until they are needed. Dogs with
liver shunts have increased blood bile acid concentrations because the liver
does not get a chance to remove and store these chemicals after they are
reabsorbed.
Do
all dogs with shunts have high bile acids?
Dog with shunts will almost always have high bile acids 2 hours after eating,
and usually at least 95% of dogs will have high bile acids after a 12 hour fast
Samples are taken at both time periods ("fasting" or
"preprandial", and "fed" or "postprandial") for
several reasons. Some dogs normally release bile acids in the middle of the
night and therefore naturally have a higher than normal fasting sample. Other
dogs may have fat in their blood ("lipemia") after eating, which can
interfere with the test. If only one blood sample can be obtained, it is best
to take it 2 hours after eating.
Do
all dogs with high bile acids have shunts?
Bile acids can be increased with any liver disease. Bile acids can also be
mildly increased in normal dogs, particularly in some breeds (such as Maltese)
where chemicals in their blood interfere with the test. Most dogs with liver
shunts have fed bile acids over 100 (normal <15-20). If the bile acids are
only mildly increased or the animal seems normal, many veterinarians will
simply rerun the test in 3-4 weeks.
What
is scintigraphy?
Scintigraphy is a nuclear scan that measures blood flow. To evaluate a dog for
a shunt, a radioactive material in inserted into the colon (by a high enema)
and the animal is scanned with a special camera hooked to a computer. The
computer measures the amount of radioactive blood in the heart and in the liver
and compares the two. Normal animals have a shunt fraction (amount of blood in
the heart divided by amount in the liver) of less than 15%. In other words, at
least 85% of the radioactive material ends up in the liver. Dogs with shunts
usually have shunt fractions >60%, because most of the blood bypasses the liver
and goes straight to the heart. Scintigraphy is safe and quick but does require
heavy sedation or anesthesia. Animals must be hospitalized for at least one
night after the procedure until they have expelled the radioactive material by
defecation and urination. Scintigraphy tells us that shunting is present;
however, in most cases the veterinarian cannot tell whether the shunt is inside
or outside of the liver, whether there is more than one shunt, or whether the
shunt is congenital or acquired.
What
is a portogram?
A portogram is an x-ray of the blood vessels to the liver. Because blood
vessels are not easily seen on regular x-rays, a contrast material (a liquid
that looks white on x-rays) must be injected into a blood vessel in the
abdomen. The injection can be performed through a surgical incision into the
belly; by injecting the spleen directly through the skin; or by passing a
catheter down the jugular vein (in the neck), through the heart, and toward the
abdomen. Portograms usually require anesthesia and are more invasive than
scintigraphy. They are usually quite safe, however, and are able to provide a
picture of the shunt so that the veterinarian can see where it is located and
whether there is more than one.
Can
a shunt be diagnosed with ultrasound?
Some veterinarians are able to find a shunt by ultrasounding the liver.
Diagnosis of a shunt with ultrasound requires lots of experience and usually a
specialized machine ("Doppler") that can detect blood flow. Shunts,
particularly those outside the liver, can be easily missed, especially if the
dog is small or wiggly, or the ultrsonographer is inexperienced.
Can
a shunt be diagnosed with a liver biopsy?
In animals with shunts, the liver is smaller than normal because it is
atrophied from poor blood flow. On a liver biopsy, the tissues appear shrunken.
Some of the vessels are very tiny, while others multiply in an attempt to
improve the blood supply and drainage. These changes are called hepatic
microvascular dysplasia. Hepatic microvascular dysplasia (HMD or MVD) can also
occur in dogs without liver shunts; therefore, other tests are needed to be
determine if a shunt is also present.
What
medical management is needed for an animal with a shunt?
Dogs with shunts are usually stabilized with special diets and medications to
reduce the amount of toxins that are produced and absorbed in the large
intestines. Dogs that are severely ill may require intravenous fluids to
restore blood sugar, an enema to remove intestinal toxins before they are
absorbed, and medications such as valium to stop seizures.
Diet: Because many of the toxins produced in the intestines come from protein, it is important to reduce the amount of protein in the diet. Dog food for adults and puppies usually contains 25% and 29% protein, respectively, and may have meat byproducts. Dogs with shunts need high quality proteins made from milk or vegetable, and are restricted to a protein content of 18% or less (on a dry matter basis). The diets should be easily digestible, rich in antioxidants and vitamins, and low in copper and iron.
Lactulose: Much of the toxins absorbed from the intestines are produced by normal intestinal bacterial. Lactulose changes the pH in the large intestines, which decreases absorption of ammonia and other toxins and makes the environment unfavorable for the toxin-producing bacteria. It also encourages the intestinal contents to leave the area more quickly, so that toxins have less time to be absorbed. Lactulose is basically a sugar solution; its primary side effect is diarrhea. Because of this, veterinarians will instruct owners to adjust the dose so that the dog's feces is soft but formed.
Antibiotics: If clinical signs are not controlled with a protein-restricted diet and lactulose, veterinarians will often prescribe antibiotics to reduce the number of toxin-producing bacteria in the intestines. Antibiotics will also be needed if the animal has a urinary tract infection.
Can
dogs with shunts be treated with only medical management?
Most animals improve immediately with proper diet and medicine, and about one
third of the dogs treated medically will live a relatively long life.
Unfortunately, over half of the dogs treated medically are euthanized, usually
within 10 months of diagnosis, because of uncontrollable neurologic signs, such
as seizures and behavior changes, or progressive liver damage. Dogs that tend
to do well with long term medical management are usually older at the time of
diagnosis, have more normal bloodwork, and have less severe clinical signs.
Surgery provides the best chance for a long healthy life in most dogs.
How
is a shunt corrected surgically?
Because shunts inside the liver are more difficult to find and close off,
surgery of dogs with intrahepatic shunts is best performed by a board certified
surgeon (ACVS Diplomate). Surgery for congenital extrahepatic liver shunts is
slightly easier, particularly if the veterinarian has a lot of experience, and
is performed at most veterinary surgery referral centers. The surgeon must find
the abnormal blood vessel and close it off to force blood to flow back through
the liver. Unfortunately, the blood vessels inside the livers of some dogs are
so poorly developed that they will not open quickly. Therefore, most surgeons
will use a device that slowly closes the shunt, such as an ameroid constrictor.
Other options include placement of a suture or cellophane band around the shunt
or coils inside of the shunt. Placement of coils can be performed through a
catheter in the neck ("jugular") vein; however, because they tend to
cause rapid obstruction of the shunt in animals, their use is still being
researched.
How
does an ameroid constrictor work?
An ameroid constrictor is a metal band with an inner ring of casein, a protein
found in milk. In the belly, the inner ring absorbs normal fluid and gradually
swells, pressing on the shunt and encouraging it to scar shut. Shunts usually
close within 3-4 weeks of ameroid constrictor placement. Because of the metal
outer ring, the constrictor will always be visible on x-rays of the belly.
What
are the complications of shunt surgery?
Surgery with ameroid constrictor placement is faster and complications are
fewer compared to other techniques, but the puppies can still get very cold or
develop low blood sugar during or after the procedure. Occasionally dogs will
develop pain and bloating if the constrictor kinks the vessel or if a suture or
a cellophane band is used. This can progress to shock and death, so animals
must be watched carefully for several days after the procedure. A small percentage
of dogs may also have seizures after surgery. Dogs with intrahepatic shunts are
more likely to have complications and usually require several days of intensive
care and possible blood transfusions.
What
care is needed for dogs after shunt surgery?
Dogs are kept on a protein-restricted diet for at least 6-8 weeks after
surgery. Lactulose can be continued as well, or can be gradually decreased over
2-4 weeks. Most dogs do not need antibiotics unless they have infections in the
urine or other sites. The liver will begin to grow as the shunt closes, and
will often be normal sized in 2-4 months. To check liver function, blood tests
(BUN, albumin, liver enzymes, and bile acids) are usually evaluated at 8-12
weeks after surgery. If these are still abnormal, they are repeated in another
3 months. If they are normal, the diet is gradually switched to an adult
maintenance dog food. A scintigraphy can be performed at 3-6 months to confirm
that the shunt is closed.
What
is the prognosis for dogs after ameroid constrictor placement around a shunt?
Survival rate from the surgery is over 95% for dogs with shunts treated by
ameroid constrictor placement, and our long-term prognosis is better with this
technique than with most other methods. Many dogs are clinically normal within
4-8 weeks after the surgery. Long term, about 85% of dogs with liver shunts
closed with ameroid constrictors do well clinically. About 15% continue to have
problems, probably because the tiny blood vessels inside the liver were also
abnormal. Usually these dogs develop multiple acquired shunts and must be
managed with a protein restricted diet and lactulose for life.
Dr. Karen Tobias is an Associate Professor in Small Animal Surgery at the University Of Tennessee College Of Veterinary Medicine.