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Some
cats are born with an anatomic defect that affects the blood supply to
the liver. Normally, after
food is eaten and digested, the nutrients, as well as some toxins, are
absorbed through the intestine and enter the intestinal blood vessels.
From there, they flow into a large vein called the portal vein.
The blood in the portal vein travels to the liver, where the
toxins are detoxified, metabolized, and removed.
Occasionally,
the portal vein does not flow into the liver.
Instead, an abnormal vessel diverts the blood so that it bypasses
the liver and flows directly to the heart, where it is pumped to the
rest of the body. This
abnormal vessel is called a portosystemic shunt (PSS). Because the
shunted blood has not been properly detoxified by the liver, the toxins
in the bloodstream cause the cat to become ill.
The most striking clinical signs tend to be behavioral. The
circulating toxins cause many cats to become stuporous, comatose, or
lethargic, although some cats become abnormally aggressive.
Seizures are common, and some cats will walk in circles, press
their heads against the wall, tremble, appear to be suddenly blind, or
appear to be having apparent hallucinations.
Poor appetite, vomiting, diarrhea, and weight loss can accompany
the behavioral signs. Excessive drooling is a common finding, and a PSS should be
high on the list of possibilities in a young cat with excessive
drooling.
Treatment
of this condition in cats can be either medical or surgical.
The goal of medical management is to control the clinical signs
of illness. Dietary
management is the mainstay of therapy.
Protein is the main culprit, as the waste products from protein
metabolism seem to be the most toxic to the brain.
Low protein diets, such as Hill’s Prescription diet k/d, or
l/d, often dramatically reduce the clinical signs associated with PSS.
Cats with PSS frequently have stomach ulcers. Any blood from these ulcers acts as a high protein meal, and
can contribute to the clinical signs.
Ulcer medication is often part of the medical management of this
disorder. Other
medications, such as oral antibiotics are also used as part of the
medical management. Unfortunately, medical management provides only temporary
relief from clinical signs. Eventually,
clinical signs progress, and cats require surgical intervention or
euthanasia.
The
goal of surgical management is to find the abnormal vessel and constrict
it, so that blood is redirected through the liver, instead of bypassing
it. This is not a surgery
that can be performed by a general practitioner.
A board certified veterinary surgeon at a university or referral
center should be contacted for this procedure.
Ideally,
the abnormal vessel should be isolated, and blood flow through this
vessel should be impeded by tying a ligature around the vessel.
This re-directs the blood through vessels that normally course
through the liver, allowing the liver to detoxify the blood. Unfortunately, when the offending vessel is tied off, the
blood pressure in the portal vein can increase dramatically.
A manometer is used during surgery to measure the pressure, and
if the pressure is too high, the surgeon must only partially occlude the
vessel. Partial occlusion
is tricky; the surgeon has to slowly tie off the vessel while monitoring
the rise in portal blood pressure, making certain it does not rise above
established guidelines.
It
would be ideal if the offending blood vessel could be constricted
gradually over time, allowing the re-directed blood flow through the
liver to slowly become better and better established.
The development of a device called the ameroid constrictor is now
making that possible in veterinary medicine.
The
ameroid constrictor is a metal band that can be slipped around a
shunting vessel. Lining
this metal band is a compressed, dehydrated protein material.
The constrictor absorbs abdominal fluid, causing the dehydrated
protein material lining the band to expand.
Because the band is metallic, outward expansion is prevented.
Instead, the material expands inwardly, causing gradual occlusion
of the shunt. This gradual
occlusion allows for the liver to adapt to the redirected blood flow.
Most
patients treated with ameroid constrictors improve after surgery, but
long-term follow studies are still pending, and I’ve only seen a few
case reports in the veterinary literature so far.
Other methods for occluding portosystemic shunts are being
developed in an attempt to find the best method of treating this defect.
At the moment, post-surgical complications in cats undergoing PSS
surgery are common, however, cats in which complete shunt vessel
occlusion can be achieved are expected to have a relatively normal life.
Updated
2/9/06 |
