This article is reproduced with the permission of
  Kal Kan Foods, Vernon, CA,
Pedigree Breeder Forum magazine,
Vol 7 , 1998.
.Copyright © 1999
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  Do not remove anything from these pages without owners permission.

Recognition and Management of Portosystemic Shunts in Dogs.
T.D.G. Watson, BVM&S, PhD,MRCVS
Waltham Centre for Pet Nutrition
Waltham on the Wolds
Leicestershire, LE14 4RT, United Kingdom

Portosystemic shunts are abnormal vascular onnections 
between the hepatic portal vein (the blood vessel that 
connects the gastrointestinal tract with the liver) and
the systemic circulation(Figure 1). 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.
Portosystemic shunts can be present at birth (i.e. congenital) 
or acquired as the result of another disease process later in
life. Congenital shunts are more common, representing
approximately 75% of all canine cases, and generally result
from anatomic abnormalities of the portal vasculature or 
persistence of fetal vessels. One or occasionally two vessels
are involved, and the shunts are classified according to their 
location as either outside of (extrahepatic) or within
(intrahepatic) the liver.
Congenital shunts occur more commonly in purebred dogs
than in mixed breeds; miniature schnauzers, Yorkshire
Terriers, and Irish Wolfhounds appear to be at increased
risk. The prevalence of portosystemic shunts in certain breeds
suggests an inherited predisposition. This has been proven
only in Irish Wolfhounds, where a number of previously
unknown genes appear to be involved.
Extrahepatic shunts are most common, accounting for 61% 
to 94% of congenital shunts, and are typically seen in small
breeds of dogs, such as the Miniature Schnauzer and
Yorkshire Terrier. Intrahepatic shunts represent between 6%
and 40% of congenital shunts and are more common in large
and giant breeds of dogs such as Irish Wolfhounds and
Golden Retrievers. The majority of intrahepatic shunts are
a result of the embryonic connection between the umbilical
vein and the caudal vena cava remaining open; in most
dogs this connection closes 3 days after birth but, for
unknown reasons, remains open in dogs with intrahepatic
congenital shunts.
Hepatic microvascular dysplasia is an unusual form of 
intrahepatic portosystemic shunting in which no gross
vascular abnormality can be identified. This rare condition
is associated with somewhat milder clinical signs and 
appears to be the consequence of a developmental 
abnormality; it has a higher prevalence in Cairn Terriers, 
suggesting a hereditary basis.
Acquired shunts arise secondary to diffuse liver disease 
where excessive and sustained pressure at some point within
the portal vein causes embryonic, nonfunctional vascular 
communications to open. These are generally seen in older
dogs with cirrhosis, hepatitis, or neoplasia of the liver. In
contrast to congenital portosystemic shunts,  a number of
vessels are usually affected.
What are the signs of portosystemic shunts?
The clinical signs exhibited by dogs with portosystemic
shunts reflect the failure of the liver to eliminate various
toxic matter, drugs, and bacteria absorbed from the 
gastrointestinal tract. Certain materials particularly
those produced from dietary protein, are potential
neurotoxins (e.g., ammonia, gamma-aminobutyric acid,
natural benzodiazepines, and mercaptans), which affect the
function of the central nervous system and produce a
syndrome called hepatic encephalopathy. Other clinical signs
arise from the liver being deprived of portal blood flow,
which is essential for the normal development of the liver;
as  a result the liver is underdeveloped and it's metabolic,
storage, and excretory functions are further impaired.
Dogs with congenital portosystemic shunts are typically
purebred dogs less than 1 year old. The severity of clinical
signs varies and is related to the anatomic position of the
shunt and the fraction of portal blood that is shunted past
the liver. Generally, the lower the fraction of shunting, the
milder and later in onset are the clinical signs. Nevertheless,
affected animals are often in poor body condition and of
small body stature, especially when compared to their
littermates. Owners may complain that the animals fail to
thrive or grow and that skin and coat condition are poor.
Other clinical signs tend to be intermittent or periodic in
nature and relate to the central nervous system,
gastrointestinal tract, and urinary tract. There may also be 
a history of an adverse response to sedation or anesthesia.
A significant number of dogs (up to 25% of cases of
portosystemic shunts) may develop signs later in life; this
may be due to an acquired rather than congenital shunt or
because the animal can no longer compensate for a low
grade congenital shunt. dogs with acquired shunts have
signs similar to those with congenital disease, although
the previous history may have been uneventful until the
appearance of weight loss and associated clinical signs.

Central nervous system signs are the most common, occurring
in over three-quarters of all cases, and may be vague and
subtle such as anorexia, depression, and lethargy.... More
specific signs include episodes of hyperactivity, head
pressing and circling, disorientation, temporary blindness,
weakness, excess salivation, seizures, and occasionally coma.
These signs tend to wax and wane, and their onset may be
connected with the recent ingestion of a protein-rich meal
that resulted in increased production of neurotoxins within
the large intestine.
Gastrointestinal signs (vomiting and/or diarrhea) are present
in about two-thirds of cases. Evidence of lower urinary tract
disease is present in approximately one-half of cases and is
usually due to ammonium urate crystals, which are formed
because of excessive excretion of ammonia and uric acid in
urine. Some dogs, particularly those that develop signs later
in life, have polydipsia and polyuria (excessive drinking and
Other less common signs of portosystemic shunting include
recurrent fevers and ascites, although the latter is generally
seen only in dogs with acquired shunts.
Although signalment and clinical signs may strongly suggest
the presence of a  portosystemic shunt, a series of investigative
steps... must be taken to:
    *Identify the anatomic location and severity of the shunt
    *Indicate whether the shunt is congenital or acquired
The latter two criteria are important when considering
whether the shunt is amenable to surgical correction and
the likely outcome of such surgery.
Blood chemistry and hematology panels usually show
characteristic patterns of mild abnormalities that as a group
suggest the presence of a portosystemic shunt. These include
mildly elevated liver enzymes, low blood urea nitrogen and
total plasma protein concentrations, hypogly-cemia (low
blood sugar) and low serum cholesterol. Mild,
nonregenerative anemia and microcytosis (undersized
red blood cells) may also be present in all cases. Blood
ammonia concentrations may also be increased, but samples
must be rapidly analyzed soon after collection to detect such
an increase. Urinalysis may reveal a low specific gravity and
the presence of ammonium blurate crystals. Serum bile acid
concentrations taken either after an overnight fast or 2 hours
after a meal are usually confirmatory.
The next step is to identify whether the shunt is intrahepatic,
extrahepatic, or microvascular. This generally requires
specialized imaging techniques, and it is likely that dogs
may need to be referred to appropriate specialists. Survey
radiographs of the type normally taken in veterinary
practices simply indicate the presence of a small liver
(Figure 2).
Ultrasonography is a useful, noninvasive tool for the
detection of portosystemic shunts. Intrahepatic shunts are
easily visualized: the liver usually appears small in size,
there is reduced visibility of intrahepatic portal vessels,
and an anomalous blood vessel may be obvious (Figure 3).
In dogs with extrahepatic shunts the first two features are
usually present, but the detection of the anomalous vessel
is not so easy. Application of Doppler ultrasound, an
advanced technique, may help in such cases, especially
where there is a small extrahepatic shunt.
Contrast radiography, whereby a marker dye is injected
into a vein draining the intestine and radiographs are taken
immediately, allows ready visualization of the portal vein
and shunting. This procedure is usually performed in 
combination with surgical correction (so that the dog has
to be anesthetized only once) and is often referred to as
operative mesenteric portography. A loop of small intestine
is exteriorized (brought outside of the body) and a tube is
placed in a jejunal vein. A water-soluble radiopaque
dye is injected via the tube, and lateral and ventrodorsal
radiographs are then taken. Where there is a shunt, the
abnormal vessel is outlined as blood is diverted into the
systemic circulation without appearing in the liver
(Figure 4).
Portal or transcolonic scintigraphy is an advanced technique
whereby the uptake of radiochemicals from the intestinal
tract is monitored. A radiochemical, usually technetium 99m
pertechnetate, is administered via the rectum and first
accumulates in the liver in normal animals. In dogs
with portosystemic shunts the distribution of activity is
altered as the radiochemical bypasses the liver and reaches
the heart first. Although this does not identify the location
of the shunt, it does provide an extremely accurate estimate
of the degree of shunting, allowing the clinician to
predict the likely success of management options and to
follow up the success of surgical management.
Liver biopsy is indicated when there is no obvious shunt or
if multiple extrahepatic shunts are identified (as seen in
acquired portosystemic shunting). This may reveal
hepatocyte atrophy, with small or absent portal vessels,
and will allow histopathologic confirmation of micro-
vascular disease.
There are two broad management options: surgical ligation
of shunts or medical management of the effects of shunting.
The decision as to which is most appropriate needs to be
made on a case-by-case basis depending on the type and
location of the shunt, the age of the animal, and the
severity of clinical signs. There may also be significant
financial considerations on the part of the owner. Surgery
wherever feasible, is generally believed to be the treatment
of choice as it suggests the promise of normal liver function.
Improvements in dietary and medical management of hepatic
encephalopathy, however, mean that conservative treatments
offer a reasonable prognosis for dogs that are not suitable
for surgery....
Ligation of shunt vessels is an advanced surgical technique
requiring a suitably experienced surgeon, careful selection
and monitoring of general anesthesia, measurement of blood
pressure in the portal vein and systemic circulation, and
appropriate critical care support facilities. Such requirements
usually necessitate referral to specialist centers, and this is
especially true with intrahepatic shunts.
Single extrahepatic shunts are usually identified as tortuous,
abnormal vessels. These are ligated close to the vena cava. A
potential fatal complication is portal hypertension, which
occurs when intrahepatic vessels are unable to cope with the
additional volume of blood that is diverted to the liver after
closure of the shunt vessel. Guarding against this requires
careful monitoring of portal and systemic blood pressures
and inspection of the intestines and pancreas for signs of
cyanosis.  Failure to alleviate the hypertension and pain,
bloody diarrhea, and shock leading to death in 2 to 24 hours
after surgery.
A 60% to 80% degree of ligation can usually be achieved
without complications and is associated with an increase in
the amount of portal blood that enters the liver and with
improvements in the patient's clinical status. In some cases,
the ligation procedure may be repeated on one or more later
dates to progressively attain complete ligation. This un-
fortunately necessitates additional costs and, with successive
surgeries, increases the risk of perioperative, complications.
On the other hand, repeat surgery may not be necessary as
some partially ligated shunts appear to spontaneously
occlude. The prognosis for dogs with partial ligation is
guarded because approximately 50% show recurrence of
clinical signs at an average of 3 to 4 years after surgery.
Ligation of intrahepatic shunts is technically more difficult
and is associated with higher risks of fatal complications.
Such cases may be best managed conservatively.

With better understanding of the pathophysiology of hepatic
encephalopathy, it has become possible to prescribe specific
therapies that provide a reasonable prognosis for those dogs
with portosystemic shunts that are not corrected surgically.
The primary objective of medical management is to
eliminate the clinical signs associated with hepatic
urinary tract disease and reducing the metabolic load on
the liver.  The chief components of medical management 
strategies are dietary modifications and oral antibiotics.
Medical management is indicated for all dogs with acquired
shunts and all dogs with microvascular shunts. It should
also be used for a period in those dogs that are about to
undergo surgical ligation. This will allow the veterinarian
and owner to establish the extent to which the condition can
be managed medically, in case it is not possible to completely
ligate the shunt at surgery. medical management is also
indicated in those dogs whose owners are unable to afford
the cost of referral to a specialist surgical facility or whose
owners are unwilling to accept the significant risk of
perioperative mortality. All dogs undergoing surgical
ligation should continue to receive medical therapy for 2 
to 4 weeks post-operatively. Finally, some degree of medical
therapy may be required in dogs with partially ligated
Dietary manipulations for the control of hepatic
encephalopathy are designed to limit neurotoxin production,
which occurs principally in the large intestine, and to reduce
the subsequent absorption of these toxins into the portal
vein....The major toxins are all derived from nitrogenous
materials (protein and urea) and are synthesized by bacteria
found within the large intestine. The production of these
toxins is reduced by limiting the amount of protein fed and
ensuring that the dietary protein is high quality and very
digestible. These steps reduce the amount of protein that
reaches the large intestine; further reductions can be
attained by feeding smaller meals more frequently to
maximize the digestive capacity of the small intestine.
Specific diets with restricted protein contents are available
from veterinarians. These are ideal because they provide a
balanced protein-calorie intake, which is important for the
stable control of hepatic encephalopathy.
.Including dietary fiber in the daily ration assists in acidi-
fying the colonic environment and limiting toxin production
and also acts as a mild laxative to increase the elimination
of toxic factors in feces. Lactulose, a soluble fiber, is often
used as a supplement for this purpose and can be readily
purchased from pharmacists. Supplementation with zinc
salts also improves the detoxification of ammonia and the
control of hepatic encephalopathy. A veterinary diet
specifically designed and tested for the management of liver
disease and portosystemic shunts is available in Europe; it
is unique in combining a restricted protein content with in-
creased zinc and added dietary fiber. Available in America
as Waltham Veterinary Diets Canine Low and Medium
Protein, dry and canned.
Antibiotics are used in most cases to reduce the bacteria
within the large intestine that are responsible for the
production of neurotoxins. Orally administered neomycin
is commonly used for this purpose and is often used in
combination with lactulose in both the short and long-term
medical management of portosystemic shunts.
Portosystemic shunts are serious conditions in dogs and
require significant efforts to diagnose and treat. Specialized
surgical techniques, in association with advances in medical
and dietary management, allow the condition to be effectively
managed and provide a reasonable quality of life.
Dr. Watson qualified from the Royal (Dick) School of Veterinary Studies at the University of Edinburgh in 1987
and received his PhD from the University of Glasgow in 1992. He joined the Waltham Center in 1994, where he currently holds the position of Research Manager.

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