LIVER SHUNT
Dr.
Karen Tobias is an Associate Professor in Small Animal Surgery at the
University of Tennessee College of Veterinary
Medicine.
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 diagnosied when they take a long time recovering from anesthestics (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
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
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
longterm 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.