“
Portosystemic shunts are abnormal vascular connections between
the hepatic portal vein (the blood vessel that connects the
gastrointestinal tract with the liver) and the systemic circulation.
Such anomalies cause blood in the gastrointestinal track
to be diverted past the liver, there by limiting the liver’s
vital functions, in metabolism and detoxification of compounds
and the body’s defenses against intestinally derived
pathogens. This effectively exposes the body to toxic by-products
of digestion (toxins and bacteria) and mimics the effects
of liver failure.”
T.D.G. Watson, BVM&S, PhD, MRCVS
Waltham Center, United Kingdom
Portosystemic Shunts (PSS) can be classified as Extrahepatic
(outside the liver) or Intrahepatic (inside the liver),
single or multiple, congenital or acquired.
Extrahepatic Shunts are most common and constitute over
60% of all congenital shunts. Extrahepatic Shunts tend
to be
found more commonly in small breed dogs, such as Yorkshire
Terriers, Maltese, Dachshunds, and Schnauzers. Of these,
the Yorkie is 36 times more likely to have congenital
Extrahepatic shunts than the other breeds.
Intrahepatic Shunts account for less than 40% of all
congenital shunts is more often seen in large breed dogs,
such as Irish Wolfhounds and Golden Retrievers.
Within the first three days of life the natural opening of the embryonic
connection closes. In Intrahepatic Shunts dogs the embryonic
connection remains open.
Multiple Acquired Shunts are a result of an increase in liver pressure. This
can be a result of the surgical closing of an Extrahepatic Shunt in surgery.
Multiple Acquired Shunts are not surgically correctable.
CLINICAL
SINGS OF LIVER SHUNT
Dogs with PSS are usually purebred and less than a year old when signs first
develop. Poor coat development, lethargy, long sedation recovery, Urinary Tract
Infections, disorientation, head pressing, anorexia, depression, weakness, excessive
salivating, and temporary blindness have all been connected with Liver Shunt.
Vomiting and diarrhea are reported in roughly two-thirds of Liver Shunt cases
and in older dogs. Less common signs are recurrent fevers and ascites. Ascites
is usually associated only with Acquired Shunts.
DIAGNOSIS
OF LIVER SHUNT
In identifying Liver Shunt the identity of the anatomical
location of the shunt, its severity, and whether the shunt
is congenital or acquired need to be determined. Different
Veterinarians utilize different methods for these procedures.
Initially a round of blood work is usually ordered. Common
blood work orders are: a full blood chemistry panel, a hematology
panel, and a liver profile. It is the combination of elevated
liver enzymes combined with low serum cholesterol, hypoglycemia,
low blood urea nitrogen, and the total plasma protein concentrations
that indicate that a liver shunt is evident.
Serum Bile Acids are also helpful in the making a more definitive
overall picture in diagnosing the presence of a Liver Shunt
and is usually taken when the blood work is indicative there
is a shunt present. It is possible to test the Bile Acids
in young puppies. However, unless the puppy is showing apparent
signs of PSS a Bile Acid Test can throw false readings. It
is recommended an apparently healthy puppy be a minimum of
two pounds and 6 to 9 months old to test the Bile Acids accurately.
Once the blood work has been completed multiple testing procedures
may take place to help identify the severity and location
of the shunt. Three common procedures are: Doppler Ultrasound,
Contrast Radiography, and Scintigraphy. Doppler Ultrasound
is a 95% accurate and noninvasive procedure for the detection
of Portosystemic Shunts.
Contrast Radiography is the application of a dye injected
into the blood stream allowing for easy visualization of
the location of the portal shunting and is often done in
conjunction with surgery so as to minimize anesthesia of
the dog. Scintigraphy is the application of radioactive chemicals
into the rectum allowing for the diagnosis to the degree
of the shunting allowing a more accurate assessment for management
options of the disease.
TREATMENTS
AND MANAGEMENT OF LIVER SHUNT
Several options exist for dogs diagnosed with Liver Shunt.
Many cases can be treated with medical management and diet.
Diet is used to limit the production of neurotoxin production
in the large intestine thus reducing the stress put on the
liver. The limiting of proteins into smaller amounts and
only feeding highly digestible sources does this. Protein
restricted diets such as Hills, l/d, k/d or Royal Canin Hepatic
LS along with lactulose, milk thistle, and occasionally antibiotics
are often times successful in treatment of more minor shunt
cases.
Surgical options include placements of ameroid constrictors, cellophane bands,
and partial suture ligation. 85% of surgical cases of Single Extrahepatic Shunts
are successful in the United States. Of the 15% of cases that are not so successful,
most are a resulting failure due to either Multiple Acquired Shunts developing
from the new pressure being placed on the liver or due to a pre-existing condition
such as Hypolatic Microvascular Dysplasia or Portal Atresia.
Hypolatic Microvascular Dysplasia or MVD/HMD is a congenital
defect where the portal vein breaks into smaller microscopic
vessels in the liver which are poorly formed or undeveloped.
This lack of development causes the liver to atrophy which
in turn inhibits the liver from filtering out toxins and
allowing
for normal growth. Dogs can have both a Liver Shunt and MVD/HMD.
They can also have one condition without having the other.
MVD/HMD livers and shunted liver samples look identical under
the microscope and thus can often be misinterpreted for the
other. However, a dog with both Liver Shunt and MVD/HMD must
have the shunt surgically corrected successfully before a
definite conformation of MVD/HMD can be made.
The sign of MVD/HMD are identical to that of Liver Shunt.
However, in many cases there are no evident signs of a problem
until the dog is 3 to 4 years old. There is currently no
treatment for MVD.
BREEDING
THE FUTURE
Hopefully, in the future simple blood tests will be able
to identify carriers allowing for breeders to eliminate
this
terrible condition from the Yorkshire Terrier and other dog
breeds that are producers of the disease such as Tibetan
Spaniels, Cairn Terriers, Havanese, Shih Tzu, and Maltese.
The Yorkshire Terrier Fanciers Foundation is currently working
with Dr. Sharon Center, DVM, DiplACVIM, Professor and Internal
Medicine Specialist of the department of Clinical Sciences,
College of Veterinary Medicine, Cornell University to help
provide a base in genotyping Liver Shunt. Dr. Center, as
the developer of the Bile Acid Test, believes that Liver
Shunt is an ancient mutation in the dog that involves vasculogenesis
or angiogenesis (embryologic formation of the blood vessels)
and that the need to establish a demonstrated genotype linkage
between the breeds is crucial.
The Foundation has provided a letter of intent to the AKC’s
CHF in support of a grant proposal for the project. Dr. Center
is currently projecting 18-24 months before a DNA Marker
test may become a reality.
As with all genetic and congenital defects, dogs with either
Liver Shunt or MVD/HMD should not be used for breeding and
should be spayed or neutered.
