What is a portosystemic shunt?
In most, if not, all normal mammals, blood from
the gastrointestinal system (stomach, small intestine, majority of the
large intestine, pancreas, and spleen) flows into the portal vein. The
portal vein divides into multiple branches, which enter the liver. This
blood exits the liver by hepatic (note: hepatic refers to liver) veins,
which transport blood into the vena cava. Once blood enters the vena cava,
it is goes through the heart, lungs and out to all other organs of the
body.
The hepatic portal system is very important for two reasons. One is that
it provides a major source of blood supply to the liver. The other reason
is that blood drained from the intestines carry toxins absorbed from the
gastrointestinal tract. Many of these toxins are metabolites produced
by bacteria within the gastrointestinal system. The liver processes these
toxins in such a way to make them less harmful to other organs of the
body.
Even in normal mammals, there are small vascular connections between the
portal system and the vena cava, which allow a small amount of blood drained
from the gastrointestinal (GI) system to bypass the liver and enter directly
into the systemic circulation. However, in an animal with a portosystemic
shunt a large amount of blood draining the GI system bypasses the liver.
This results in systemic toxin levels that are harmful to many organs
of the body such as the brain.
Portosystemic shunts (PSS) are classified by their location (intrahepatic
vs. extrahepatic), time of formation (congenital vs. acquired), and number
(single vs. multiple). Intrahepatic shunts are within the liver and extrahepatic
shunts are outside of the liver. (Figure A.
A diagrammatic representation of normal portal anatomy).(Figure
B. A diagrammatic representation
of an extrahepatic shunt). (Figure
C. A
diagrammatic representation of an intrahepatic shunt.) Shunts
may be due to abnormal vascular development in the fetal stages of life,
and are present at birth. These shunts are referred to as congenital shunts.
Even though the exact genes responsible for congenital shunt development
have not been identified, shunts tend to run in certain family lines.
Therefore, genetics play a role in congenital shunt development, and therefore,
breeding of animals affected with this disease is not recommended. However,
genetics is not the only factor in shunt formation, and animals born from
families with no history of congenital shunts may be affected.
Congenital shunts are most commonly single, meaning that one large anomalous
vessel connects the systemic circulation and the portal circulation. Congenital
shunts may be extrahepatic or intrahepatic. Breeds that are predisposed
to single extrahepatic congenital shunts include the Shih Tzu, miniature
Schnauzer, Yorkshire terrier, Dachshund, Maltese, Poodle, Lhaso Apso,
Pekingese, and Pug.
Intrahepatic shunts may run from any branch of the portal vein through
the liver and into either the vena cava or a hepatic vein. This type of
shunt is most commonly seen in large breed dogs such as German Shepherds,
Golden Retrievers, Irish Setters, Samoyeds, and Irish Wolfhounds. It is
rare for this type of shunt to occur in small breed dogs or cats.
Acquired shunts are usually multiple in nature. (Figure
D. An intraoperative picture of multiple
extrahepatic shunts.) These shunts are usually small tortuous vessels
that are seen outside of the liver. Therefore, they are usually referred
to as multiple acquired extrahepatic shunts. These shunts are the result
of increased pressure in the portal system. This is usually secondary
to some type of severe liver disease. As the liver is damaged due to a
disease process, it looses the ability to process the large amounts of
blood carried to it from the portal system. Therefore, blood accumulates
in the portal system, which then leads to an increase in pressure within
the portal vessels. The small normal communications between the portal
system and the systemic circulation enlarge dramatically and allow for
a large amount of blood to bypass the liver. The result is many small
vascular connections running from portal vessels to systemic veins. This
type of shunting may be seen in any animal with severe liver disease,
and may also be seen after surgical correction of single shunts, which
is discussed under the complication section.
Portosystemic shunts are uncommonly microscopic (not visible by the naked
eye). This type of portosystemic shunting is referred to as microvascular
hepatic dysplasia. These are general microscopic communications between
portal vasculature and hepatic vasculature throughout the liver. Even
though this type of shunt cannot be seen with the naked eye, the liver
does not process the portal blood normally before it is drained into the
hepatic veins and then the vena cava.
What are clinical signs of portosytemic
shunts?
There are many clinical signs that may be seen
in animals with portosystemic shunts. All of these symptoms may be seen
with various other disease processes. Therefore, a diagnosis cannot be
made on symptoms alone! Portal blood is bypassing the liver; therefore
the clinical signs noted with portosystemic shunts are related to abnormal
liver function. In animals with single extrahepatic, single intrahepatic
shunts, and microvascular dysplasia the symptoms are usually seen within
2 years after birth, however it is possible to notice the symptoms in
middle aged animals as well. In animals with multiple acquired extrahepatic
shunts, symptoms may be noted at any age since liver disease may occur
at any age.
The liver plays an important role in the metabolism of drugs. Therefore,
animals with PSS may metabolize drugs more slowly than expected in a normal
animal. The most common manifestation of this phenomenon occurs when a
pet is taken to the veterinarian to be spayed or neutered. These animals
tend to wake up from general anesthesia very slowly.
Toxins that bypass the liver may have a drastic effect on the central
nervous system (brain). This can result in periodic abnormal mentation
such as depression, head pressing against a wall or piece of furniture,
stumbling, circling, or even seizures. Many owners will report that their
pets have an unexpectedly calm disposition. These signs are most common
after a meal when a large amount of gastrointestinal products are absorbed
into the blood stream.
The liver normally metabolizes urates and ammonia which are common compounds
composing certain urinary stones. Animals with PSS do not metabolize these
products well. Therefore, they may develop high concentrations of these
compounds in the urine, which leads to ammonium biurate stones in the
urinary system. Signs of urinary stones would include increased frequency
of urination, bloody urine, straining to urinate, abdominal pain, or vomiting.
A portosystemic shunt should be suspected in any non-Dalmation breed with
ammonium biurate stones.
The liver is involved with metabolism of proteins, which are building
blocks for tissue and also fat and glucose (sugar), which are important
energy sources. Therefore, these animals may be smaller than other animals
in the litter.
In animals in which the pressure in the portal system becomes elevated,
fluid may leak out into the abdomen giving the animal a pot-bellied appearance.
This is most common with multiple acquired extrahepatic shunts, but may
be seen with other shunts as well.
Various other signs such as vomiting, diarrhea, depression, and lack of
appetite may also be seen. Pytalism is a common sign noted in cats. It
is important to note than an animal with a portosystemic shunt may show
any combination of the previously mentioned signs. Some animals will show
only one clinical symptom consistent with portosystemic shunts.
How are portosystemic shunts diagnosed?
Step 1: Suspect a shunt. This is covered in the section, which discusses clinical signs.
Step 2: Preliminary tests
Routine blood work consists of liver enzymes (ALT, ALP), kidney parameters
(BUN, creatinine), electrolytes, blood glucose, albumin (blood protein),
and a CBC, which includes a white blood cell count and red blood cell
count. A urine test (urinalysis) should also be performed. These tests
are performed to help detect other diseases, which may cause the same
signs as a portosystemic shunt, monitor for abnormalities associated with
portosystemic shunts, and help prepare the patient for further diagnostics
and potentially surgery. Routine blood work abnormalities consistent with
portosystemic shunts include low BUN, mildly elevated liver enzymes, mildly
decreased red blood cell counts, low albumin, and low blood glucose. Urine
may contain ammonium biurate crystals. Animals with moderate to severe
abnormalities in blood glucose, albumin, or electrolytes may require intravenous
fluids, plasma, or glucose prior to further diagnostics. Normal routine
blood work and urinalysis does not completely rule out a portosystemic
shunt.
Step 3: Bile acids and blood ammonia levels
The bile acid blood level is another blood test that is a very sensitive
indicator of liver disease and portosystemic shunts. This test requires
the animal to be fasted for 12 hours before the first blood sample is
drawn. The animal is fed and another sample is drawn 2 hours after the
meal. Bile acids are normally formed by the liver and secreted into the
GI tract to aid in digestion. They are then absorbed into the portal system
where they are filtered from the blood by the liver. In animals in which
bile acids are not being filtered by the liver (because the liver is abnormal
and/or the blood is bypassing the liver) the bile acids become elevated
in the blood stream.
A blood ammonia test may also be performed to help detect a portosystemic
shunt. Blood ammonia levels are generally high in animals with a portosystemic
shunt. Elevated bile acids and/or ammonia levels are consistent with a
portosystemic shunt, but may also be seen with other liver diseases. Therefore,
bile acids and blood ammonia levels cannot be used alone to diagnose a
portosystemic shunt.
Step 4: Definitive diagnosis
A definitive diagnosis of portosystemic shunting can be made in one of
4 ways. The least invasive technique is abdominal ultrasound (sonogram).
This involves placing a probe on the skin of the abdomen. This probe produces
sound waves, which are reflected off tissues and are then detected by
a sensor. Various tissues produce different changes in the sound waves.
Therefore, the ultrasound machine may create an image of the abdominal
organs that can be used to search for a shunt vessel around the liver.
This method of detecting shunts is advantageous in that it is non-invasive
and usually requires no anesthesia. One major disadvantage of this technique
is that it is operator dependent. Detecting shunts with an ultrasound
requires a skilled operator. Failing to detect a shunt with ultrasound
examination does not completely eliminate the possibility of a shunt.
(Figure E. An ultrasound image of a portosystemic shunt)
Another method for detecting shunts is rectal scintigraphy. For this method
the animal's colon must be free of feces, which usually requires sequential
enemas. The animal must lie very still for the procedure. Thus, most animals
require heavy sedation. A short-lived radioactive substance (Tc-pertechnetate)
is introduced into the colon. The gastrointestinal system absorbs the
compound, which is then drained into the portal system. A detector measures
the uptake of the radioactive substance both in the heart and the liver.
In a normal animal, the substance is first taken up in the liver, where
it is then drained into the hepatic vessels and eventually into the heart.
In an animal with a shunt, a variable amount of the substance will bypass
the liver and therefore, the heart will take up the radioactive substance
prior to the liver. One disadvantage of this procedure is that the animal
must be kept in isolation for at least 24 hours after the test while the
radioactive material is excreted from the body. Another disadvantage is
the limited availability of the procedure.
A third way to test for a shunt is to perform a portogram. For this the
animal must be placed under general anesthesia and a small incision must
be made into the abdomen. A mesenteric vein (vein draining the small intestine)
is catheterized and contrast medium is injected. Serial radiographs (X-rays)
of the abdomen are then taken. A normal portogram will highlight the portal
vein branching into the liver prior to contrast medium entering the caudal
vena cava and heart. (Figure F. A normal portogram.)
If a shunt vessel is present, it will be highlighted as it carries blood
past the liver. This is a very reliable test to detect shunts, and it
also demonstrates the location of the shunt. (Figure
G. A portogram of an extrahepatic shunt). (Figure
H. A portogram of an intrahepatic shunt)
Finally, shunts may be detected during an abdominal exploratory surgery.
One advantage of this procedure is that surgical correction may attempted
at the same time, therefore money and time is saved avoiding another diagnostic
test. One disadvantage of this procedure is that some shunts (especially
intrahepatic) may be particularly difficult to identify. In cases where
the shunt cannot be identified at surgery, but suspicion is high, a portogram
can be performed during surgery.
The exact sequence of events used to diagnose and treat a portosystemic
shunt will depend on the individual patient, owner, and attending veterinarian.
For instance, an animal of the typical breed and history for a portosystemic
shunt with blood work consistent with a portosystemic shunt may be taken
directly to surgery for diagnosis and treatment in one step. However,
in cases where clinical signs, breed, or blood work is less consistent
with a typical extrahepatic portosystemic shunt, an intravenous portogram,
nuclear scintigraphy, or abdominal ultrasound may be recommended prior
to exploratory surgery.
How are portosystemic shunts treated?
Portosystemic shunts are initially treated by
medical management until surgery may be attempted. Medical therapy does
not cure portosystemic shunting. It helps to control clinical signs. In
addition, further deterioration of the liver due to decreased blood flow
is not prevented with medical therapy. The goal of medical therapy is
to decrease the amount of toxins absorbed by the gastrointestinal system.
Lactulose is a laxative that will help to decrease absorption of ammonia
and other GI toxins. Antibiotics such as neomycin, metronidazole, and
ampicillin are used to decrease the number of bacteria in the GI tract
resulting in less production of ammonia and other toxins. The diet should
be protein restricted since proteins are the substrates bacteria utilize
for the production of ammonia and other toxins. It is important to remember
that these animals may already have low blood protein levels. Therefore,
the ideal diet would contain a decreased quantity but increased quality
of proteins.
Some animals may require more intensive supportive care prior to surgery
if their clinical signs are severe. Dehydrated animals may need intravenous
fluid therapy. Dextrose may need to be added to fluids if the animal is
hypoglycemic (low blood glucose). Systemic antibiotics may be indicated
for animals with a bladder or other systemic infection.
Surgery is recommended if possible in an attempt to cure the underlying
problem (close the shunt vessel). (Figure I. An
intraoperative picture of a portosystemic shunt.) There are various
means for closing shunt vessels down. The easiest way to perform this
task is to completely ligate (tie off) the vessel with suture material
during one surgical procedure. However, in most animals this is not possible.
Since the liver is underdeveloped, it cannot usually adapt to a sudden
increase in blood flow. Therefore, the blood tends to pool in the portal
system, which leads to congestion of the gastrointestinal tract and pancreas.
This is termed portal hypertension and can be fatal within hours. Therefore,
most animals require gradual occlusion of the shunt vessel. This may be
performed by partial ligation of the vessel during one surgical procedure
and further ligation of the vessel 1-2 months later after the liver has
had time to adapt to additional blood flow.
Another means for gradual occlusion of the shunt vessel is placement of
an ameroid constrictor. (Figure J. An ameroid constrictor
ring.) This object is made of a dehydrated (dried) casein surrounded
by a metal ring. This is placed around the shunt vessel. As water moves
into the casein, it begins to swell. Since it is surrounded by metal it
can only swell inward, and thus the shunt vessel is gradually closed.
In addition, the ameroid constrictor is also inflammatory to the vessel,
which also contributes to vessel closure. The amount of time required
for complete closure could range from 10 days to 3 months. If it closes
too rapidly portal hypertension could occur which could be fatal. Portal
hypertension after ameroid constrictor placement is rare and if noted
would require additional surgery in an attempt to save the animal.
Other methods for gradual shunt occlusion have been used less commonly.
At a small number of veterinary hospitals, cellophane bands are placed
around the shunt vessel. Similar to ameroid constrictors, this results
in gradual closure of the shunt vessel. Transvenous thrombogenic coils
have been placed with limited success. This method involves passing a
catheter into the shunt from a vein in a limb. An object (coil) is then
introduced through the catheter and left inside the vein. Over time the
coil should lead to clot formation within the vessel. Alternatively, coils
of various sizes may be placed over a 2-3 month time period until the
vessel is completely closed.
What is the aftercare once surgery
has been performed?
The animal generally stays in the hospital for
2-4 days after surgery for pain control and to monitor for complications
(which will be covered in the next section). This is the time period when
life-threatening complications are most likely to occur. Since complete
vessel occlusion may not occur for 1-3 months in most patients, the animal
usually remains on medical management (as mentioned previously) until
the first recheck visit. The incision site should be monitored for swelling
or discharge. The animal should be confined in a clean area where no running,
jumping, or playing is allowed for 10-14 days while the incision is healing.
The animal may be taken outside on a leash to urinate and defecate during
the period of exercise restriction. The animal should be monitored for
clinical signs of shunting such as seizures or depression and should also
be monitored for complications of surgery such as portal hypertension.
Portal hypertension may be manifested by abdominal swelling, acute severe
vomiting/diarrhea or depression.
A recheck visit for suture removal may be necessary 10-14 days after surgery
if external skin sutures are present. A recheck appointment to determine
if continued shunting is present is usually scheduled 1-3 months after
surgery. At this visit, bile acids are rechecked. This requires that the
animal be fasted for 12 hours prior to the visit. Blood is drawn, the
animal is fed and blood is again drawn two hours later for post-prandial
bile acid levels. This test helps to determine if continued shunting is
present. Bile acids will decrease in animals with decreased shunting,
however, bile acids will rarely be within the normal range after surgery.
If bile acids or clinical signs indicate continued shunting, portal scintigraphy,
an intravenous portogram, or exploratory surgery may be necessary to determine
if further therapy is required. Therefore, the recheck exam may require
only a few hours, or may require hospitalization for 2-4 days.
Further appointments may be required if a second surgical procedure was
necessary. In some cases continued shunting cannot be cured even after
two surgeries and medical management is necessary for the lifespan of
the animal. These animals may require check-ups at regular intervals to
determine necessary medication changes.
Complications can be divided into early and late
categories. Early complications occur during anesthesia, surgery, or within
the first month after surgery. Late complications occur anytime from months
to years after surgery. One of the most common early complications is
portal hypertension. If the liver cannot adapt to the increased blood
flow, the pressure will increase in the portal vessels. This leads to
decreased perfusion to the digestive system, which, if severe, eventually
results in necrosis or death of the intestines and pancreas (digestive
organ). In severe cases this results in the death of the animal. There
are multiple mechanisms, which can lead to portal hypertension. The most
obvious is excessive constriction of the vessel at surgery. However, as
mentioned previously, there are current guidelines at surgery, which has
decreased this phenomenon markedly. The ameroid constrictor is designed
to place little to no constriction on the vessel initially, but gradually
close the vessel over time. This should prevent early severe portal hypertension.
When partial ligation is performed, portal pressures can be measured by
the surgeon so that ligation is stopped before dangerously high portal
pressures are reached. However, even with these safety mechanisms there
are instances when portal pressure can become high after surgery. One
is shunt vessel thrombosis (clot formation within the vessel). Dissection
around the shunt vessel causes inflammation, which can predispose the
area to clot formation. If the clot forms quickly after surgery portal
hypertension will result. Another mechanism for portal hypertension is
displacement of the vessel by the ameroid ring. The vessel is elastic
and can be completely shut off if too much weight is placed upon it. If
ameroid ring places sufficient weight on the vessel, it will be "kinked
off" and portal hypertension will result.
A less common complication is post-operative seizures. At this time, the
definitive cause for this phenomenon is unknown. However, when it occurs,
it usually happens within 3 days of surgery. These seizures are very difficult
to control and in many instances requires general anesthesia. Most animals,
which develop this problem, are eventually euthanized, since seizures
occur each time the animal is awakened from general anesthesia.
A very rare occurrence is shunt vessel rupture. The shunt vessel is fragile
and can be torn during dissection. The risk of this complication is associated
with the location of the shunt. Shunt vessels between the liver and diaphragm
are more difficult to isolate and therefore, are more likely to be ruptured.
If the vessel can be completely ligated, repaired, or if an additional
shunt can be created, there is a chance for survival.
Anesthetic risk, surgical site infection, or incision site dehiscence
(breakdown of the suture line) can occur during any surgical procedure
and is not unique to portosystemic shunt surgery.
Retrospective studies have been performed to evaluate mortality rates
due to the previously mentioned complications. The mortality rate has
been reported to be anywhere from 2.11 to 14%2. Currently, with increasing
knowledge of portosystemic shunts, the true mortality rate after surgery
is probably less than 10%.
Late complications result in recurrence of clinical signs related to portosystemic
shunting. One reason for recurrence of shunting is acquired multiple portosystemic
shunts. This is most likely due to a gradual onset portal hypertension.
In contrast to the early severe portal hypertension previously mentioned,
this phenomenon occurs over time and therefore, the body adapts to the
increasing portal pressure. Many small blood vessel connections between
the portal veins and systemic veins increase in size, allowing a large
amount of blood from the portal system to bypass the liver. The result
is recurrence of signs related to portosystemic shunting. Unfortunately,
we cannot predict the animals in which this will occur. In addition, there
is no surgical procedure that provides consistent clinical improvement
when multiple acquired portosystemic shunting occurs. Therefore, these
animals must be managed medically as previously mentioned. The long-term
prognosis for these animals is variable and has been reported to be anywhere
from weeks to 2 years3.
Another cause for continued shunting is the inability to completely ligate
the shunt vessel. Some animals have portal pressures that are excessively
high to allow for complete ligation. For instance, the vessel may be partially
ligated during one procedure and the animal then scheduled for a second
procedure 2 months later. However, if pressures are excessively high during
the second procedure, the vessel cannot be completely ligated. These animals
may show an initial improvement with partial ligation, but if complete
ligation is not eventually accomplished, signs of portosystemic shunting
will usually recur and medical management will be required. One study
revealed that 41% of animals with only partial ligation would have recurrence
of portosystemic shunting within 2 years2.
Another theoretical cause for continued portosystemic shunting is incomplete
closure of the ameroid ring. The ameroid ring results in vessel closure
by gradual constriction and thrombosis secondary to an inflammatory response.
It is possible for the ameroid ring to only result in partial occlusion
of the vessel.
One study revealed recurrent signs of portosystemic shunting due to multiple
acquired portosystemic shunts or the inability to completely ligate the
shunt in 22% of animals undergoing surgery1. Recurrent signs can be noted
months to years after surgery.
It is important to note that intrahepatic shunts are usually much more
difficult to repair, and therefore, have a higher risk for early complications.
One study revealed an 18 percent mortality rate with intrahepatic portosystemic
shunts4.
In summary, surgical correction of portosystemic shunting is associated
with a low mortality rate and a good prognosis for improvement of clinical
symptoms. However, some animals (approximately 20%) will have recurrent
symptoms within months to years after surgery.
1. Hunt GB and Hughes J: Outcomes after extrahepatic portosystemic shunt ligation in 49 dogs. Aust Vet J 77:303-307, 1999.
2. Hottinger HA, Walshaw R, Hauptman JG: Long-Term Results of Complete and Partial Ligation of Congenital Portosystemic Shunts in Dogs. Vet Surg 24:331-336, 1995.
3. Fossum TW, et al: Small Animal Surgery. St. Louis, MS, Mosby, 1997, pp 374-384.
4. White RN, Burton CA, McEvoy FJ: Surgical treatment of intrahepatic portosystemic shunts in 45 dogs. Veterinary Record 142:358-365, 1998.
 Figure
A. |
||
 Figure
B.
|
||
 Figure
C. |
||
 Figure
D. |
||
|
|
||
|
|
||
|
. |
||
|
|
||
 Figure
I. |
||
 Figure
J. |
||
Figure
E. 
Figure
F. 
Figure
G. 
Figure
H.