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Tricuspid
Dysplasia :
A Cardiologist's Perspective
By Lori Seimens, DVM, DACVIM
Posted with Permission of the Author. |
Tricuspid Valve Dysplasia (TVD) is a congenital, heritable
heart defect that seems to be increasing in prevalence
in Labrador Retrievers. The tricuspid valve is one of
four heart valves and allows blood to flow in one direction
from the right atrium into the right ventricle. If a puppy
inherits the gene ( or genes) responsible for the defect,
its tricuspid valve will be malformed in utero. The medical
term for malformation of a valve is "Dysplasia". Thus,
the puppy will have a dysplastic valve at birth that does
not shut tightly, allowing blood to leak (regurgitate)
back through it. The size of the leak will dictate the
severity of the disease. A puppy with mild regurgitation
will likely have a normal life span. If the regurgitation
is severe, the puppy may not live to see its first birthday.
Severe TVD causes the right side of the heart to progressively
enlarge. The blood leaking backwards into the right atrium
eventually causes the pressure inside this chamber to
rise.Since veins returning from the liver connect to the
right atrium, the pressure in the veins of the liver also
increases. The high venous pressure causes fluid to ooze
from these liver vessels which then accumulates in the
abdominal cavity. This fluid is termed "ascites" and is
a form of congestive heart failure. The onset of ascites
indicates advanced, end stage cardiac disease. Medications
can help palliate the symptoms ,but it is considered a
terminal condition.
How can you tell if your dog has TVD? Fortunately any
leakage from a dysplastic valve that is signifigant enough
to cause health problems should be detectable on a physical
exam. Most of the time, a heart murmur will be heard with
a stethoscope. Occasionally a gap between the valve leaflets
will be so large ,that rather than producing an audible
heart murmur, the leak can be felt by placing a hand on
the right side of the chest- the palpable buzzing sensation
is called a "thrill". If a heart murmur or thrill is detected
in a young dog, chest x-rays and an ultrasound of the
heart (echocardiogram) should be done to confirm the diagnosis
and stage the disease. Labradors can have other congenital
heart defects besides or in addition to TVD, so a full
cardiac work-up for these dogs is important.
The diagnosis of TVD is controversial only when attempting
to screen asymptomatic breeding animals for the disease.
This becomes problematic because TVD is expressed on a
continuum: Severe, Moderately Severe, and not affected.
It was originally believed that all dogs with TVD would
have a heart murmur, but it has since been discovered
that occasional dogs in the Mild range do not have an
audible heart murmur. The small amount of leakage combined
with the fact that tricuspid regurgitation has a low velocity
means that a soft murmur may not be able to penetrate
through the chest wall. Even if a murmur can be heard
in perfect conditions, there may be extenuating factors
,which cause it to be missed by the examining veterinarian.
These factors would include ambient noise, panting, wiggling,and
individual variation in sensitivity to different frequencies
of sound. For these reasons, I personally believe that
echocardiography (cardiac ultrasound) is the best tool
for screening for TVD. The heart should be examined with
2 -D ultrasound which gives a black and white 2 dimensional
picture and just as importantly, with color Doppler. The
latter uses Doppler technology to show blood flowing through
the heart. It highlights abnormal blood flow such as leakage
from a valve.
We cardiologists are in the process of perfecting our
ability to evaluate the canine tricuspid valve with ultrasound.
The tricuspid valve is a complicated structure. It has
several components: : the leaflets, the chordae tendinae
(string like attachments connecting to the leaflets),
and the papillary muscles (protrusions from the heart
muscle that anchor the chordae tendinae). The valve is
in constant motion, which increases the challenge of evaluating
these structures. This is why only an experienced echocardiographer
should be solicited to screen for TVD.
At this time, we are still trying to come to a consensus
as to how much leakage is normal for a dog's tricuspid
valve. With the advent of better ultrasound technology,
we can detect even minute amounts of valvular regurgitation.
two facts indicate that the presence of very mild amounts
of regurgitation is a normal variation, especially
if the valve itself appears structurally normal. The first
is that we humans (one study found as many as 50% of the
population) have small tricuspid leaks that are considered
normal. Secondly, other breeds of dogs that are not predisposed
to TVD have individuals with detectable amounts of tricuspid
regurgitation. the major difficulty occurs when attempting
to differentiate Labs with subtle TVD from those normal
variations in valve anatomy and tricuspid regurgitation.
Medicine has many "gray zones" and this issue is certainly
one of them. For dogs that fall into this gray area, we
give them a grading of "Equivocal" This describes a dog
whoh is not obviously affected with TVD but whose echocardiogram
is abnormal enough to not pass with flying colors. This
gray zone should become more narrow as greater numbers
of Labs are echoed, but will likely never be eliminated
altogether.
The equivocal grade frustrates many breeders who desire
a "yes or no" answer. Even human cardiologists experience
the same dilemnas and are always reassessing what normal
really is as newer technologies become available. The
good news is that we are still advising that dogs with
equivocal grades be bred, especially if they have cleared
their other screening tests.
The puppies born to equivocal dogs should be screened
for heart murmur, and any puppy over 6 months with a persistant
murmur should have an echocardiogram done. If we begin
to see a pattern of equivocal dogs producing TVD puppies,
then we would have to revise our current reccomendations.
A few groups are collecting pedigrees of Labradors that
are ultrasounded in order to establish the mode of inheritance
for TVD. One of those interested in this research
is Dr. Donald Patterson at the University of Pennsylvania.
A recent comment by him on his findings thus far
is that they need more echocardiograms, especially from
mildly affected dogs, before they will be able to establish
the mode of inheritance. Another large database is located
at The Institute for Genetic Disease Control in Animals
(GDC) which is located in Davis,CA. It is a non-profit
open registry that has been accumulating numerous pedigrees
of Labradors in their TVD registry. They have not yet
analyzed their pedigrees for heritable patterns. However;
for a small fee, they will issue a "Kin Report" This is
composed of the pedigrees and the results of any screening
tests done on the parents,siblings, half siblings and
progeny of a given dog. For further information, call
GDC at (530)- 756-6773.
It is impressive how diligently breeders are working to
improve the health of the Labrador Retriever.
Adknowledging that there are problems and striving to
cleanse the breed of them is the surest sign of true love
for these wonderful dogs. |
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