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Aortic stenosis is one of the most
prevalent, congenital cardiac diseases in dogs. It is characterized by an
obstruction (stenosis) or "lesion" near the aortic valve which causes
turbulence or "noise" in the blood as it passes through the valve,
manifest in most (but not all) affected dogs as a heart murmur. The obstruction
can be valvular, supravalvular, or subvalvular (below the valve itself), the
subvalvular variety being the most prevalent, hence "Sub Aortic Stenosis"
or SAS.
Diagnosis of SAS in its mildest, or
sub-clinical, form, is extremely difficult. A dog affected with the mildest form
of SAS will lead a full life of normal duration and quality, and will most
likely be completely asymptomatic. Even those with moderate SAS can lead normal
lives. However, dogs that are severely affected are at risk of sudden death.
Heart failure is very rare except in the most severe cases, and those dogs
usually have a severe mitral valve insufficiency as well as SAS.
Electrocardiograms are often normal, regardless of the degree of disease. In the
most severe cases, radiographs can possibly show some ventricular enlargement,
but they often appear normal as well. A Doppler echocardiogram will show the
presence of subvalvular lesions, and an increased velocity in the flow of blood
across the aortic valve due to the constriction of the valve by the lesion. A
Doppler will also show aortic regurgitation in a high percentage of SAS cases.
This is an unusual congenital disease in
that the definitive "lesion" is not present at birth and does not
develop until around 3-4 weeks of age, and the resulting heart murmur may not be
detected until approximately 6-8 weeks of age. In some of the mildest cases, the
murmur may remain undetectable for several years. In puppies, it is usually an
"incidental" finding – the murmur is discovered when the pups
receive their first exams or vaccinations. In an adult dog, the path of
discovery is frequently the same – a heart murmur is found during the course
of a regular physical exam. Research studies using Doppler indicate that this is
a progressive disease, with the rate of progression being greatest in the
immature dog, and slower in the mature dog. If a dog survives to full maturity
(3 years or so), it is likely that the disease is mild enough not to interfere
with the dog's normal activities and longevity.
Medication can sometimes be helpful in
managing the more severe forms of the disease. Surgical options, at least at
this time, are ineffective. Where there is some degree of SAS known to be
present, a prophylactic course of antibiotics is recommended prior to any
surgery or dental work because there is a heightened risk of endocarditis in the
SAS affected dog. The use of Beta Blockers for the reduction of heart rate and
the risk of sudden death is suggested in moderately to severely affected dogs.
Heritability:
SAS is an inherited disease. That much has
been determined. However, the exact mode of inheritance still has not been
proven. At present, there is widespread opinion that the disease is a dominant
gene trait with "imperfect" or "variable" penetrance. It is
also believed to be polygenic. These factors make the exact mode of inheritance
extremely complex, and the struggle to eliminate carriers from a breeding
program is a difficult and elusive endeavor.
In simple terms, any affected dog can
produce SAS in its offspring; there does not need to be a matching gene in the
other parent. A clinically unaffected dog may produce SAS in its offspring if it
carries the gene. There is evidence, but no proof, to suggest that the more
severe the genetic defect, the more cumulative the affects of the disease will
be.
Until there is a definitive DNA test
available, there is no way to detect those dogs with extremely mild,
sub-clinical disease (except necropsy), nor is there a way to detect those dogs
who are completely disease free but still carry the gene for SAS.
How Do We Screen For It?
It is extremely frustrating to attempt to
diagnose this disease, and virtually all tests will fail to detect the
mildest form. Definitive diagnosis is achieved only upon post-mortem
examination. While it is said that nearly 95% of cardiac defects produce a
detectable murmur, the mildest form of SAS does not always.
The first, and perhaps most important,
examination is the auscultation, or exam with a stethoscope. It cannot be
emphasized strongly enough that it is important to have this examination
(especially one involving a mature dog) conducted by a Board Certified
Cardiologist, preferably one with experience in the examination and diagnosis of
deep-chested, heavily-muscled, short-nosed breeds.
Certainly, auscultation of a puppy should
be simple enough, and many murmurs are detected at an early age. Some turn out
to be "innocent" and disappear as the puppy matures. Those that linger
on at 16 and 20 weeks can pose problems. Any persistent murmur, especially one
near or over the aortic valve, should be evaluated by an experienced
cardiologist, and followed up with a Doppler echocardiogram. A murmur detected
in an adult dog should be followed by an echocardiogram with Doppler. A high
velocity of flow across the aortic valve, coupled with a persistent heart
murmur, is indicative of SAS.
Here is where the experts differ in terms
of grading the severity of the disease, but most will agree that if the flow is
under 4 m/s at maturity, the dog will most likely live a fairly normal life. If
it is greater than 5 m/s, it will most likely succumb to the disease. These
parameters, of course, refer only to the dog's quality of life and not to
whether or not the dog should be included in a breeding program.
Ongoing Research:
The AKC Canine Health Foundation, in
conjunction with the Golden Retriever and Newfoundland breed organizations, is
presently funding an ongoing investigation into SAS.
Their first year results seem to suggest
that the gene can be present even in the absence of the disease. As stated
above, the disease can exist in such a mild form that is detectable only upon
post-mortem examination. For their study, they have used the following
parameters to determine which dogs are affected:
Affected
Murmur at rest
Doppler velocity of 2.0 or higher
Equivocal
Very soft murmur at rest or only after exercise
Doppler velocity of 1.8 or 1.9
Unaffected
No detectable murmur before or after exercise
Dr. Hogan, DVM, DACVIM-Cardiology,
Assistant Professor at Purdue University School of Veterinary Medicine, was kind
enough to go on record with his criteria and recommendations. His thoughts and
guidelines appear verbatim:
"There are many guidelines used
in screening dogs for SAS. These can be quite diverse and confusing. We do our
best not to spread the disease while not removing dogs from the breeding pool
unnecessarily. My own guidelines have changed since I started based on these
principles.
Unaffected:
No murmur at rest or with exercise. If Doppler echo-cardiography is performed,
I would like to see the aortic velocities [also known as left ventricular
outflow tract velocities (LVOT)] less than 2 m/s (from the subcostal view).
Equivocal:
This could include many scenarios. No murmur at rest, soft murmur with
exercise and a LVOT velocity of 2-2.5 m/s. Soft murmur at rest with no or mild
increase in intensity with exercise and LVOT of 2-2.5 m/s. These are all
contingent upon seeing any structural changes to the LVOT (i.e.: no ridge or
narrowing), aortic valve, ascending aorta and left ventricular walls.
I will usually tell owners that these
dogs MAY be used for breeding but there may also be a risk. The owners
have to weigh the benefits of the individual dog versus the possibility of
producing some affected puppies.
Affected:
These are going to be dogs with murmurs at rest of III/IV or louder that may
or may not increase with exercise. Structural abnormalities are always seen
and the LVOT velocities are generally over 3 m/s but could be >2.5 m/s.
THESE ANIMALS SHOULD NOT BE USED FOR
BREEDING REGARDLESS OF BENEFICIAL TRAITS THE INDIVIDUAL MAY HAVE.
I hope you find this helpful. Once
the blue-ribbon panel has made their recommendations, they will be made public
through the American College of Veterinary Internal Medicine, Specialty of
Cardiology."
Conclusions?
Well, basically there are none.
The cardiologists consulted for this
article all stated that there are widely divergent opinions in the specialty
regarding SAS. An off-the-record, very unscientific sampling of board certified
cardiologists revealed quite diverse opinions regarding the diagnosis of SAS in
the absence of clinical signs. At one end of the spectrum is the view that a
velocity of 1.7 m/s or greater is in itself an indicator of SAS. At the opposite
end is the belief that a definitive pre-mortem diagnosis of SAS requires both a
detectable heart murmur and a velocity of 3 or greater. Most opinions fell
somewhere in between.
Another complicating factor is that the
Doppler values vary greatly from machine to machine, so a 1.8 on one machine may
well be a 2.4 on another. An example of this has been forwarded to the authors:
A particular male was tested by one
cardiologist, receiving an evaluation no murmur and a velocity of 2.3.
He was placed in the equivocal category by this cardiologist since his
"clear" limit was 2.0. Upon retesting at a different facility, the
results were a velocity of 1.9 with no murmur. The second cardiologist also
placed the dog in the equivocal category, using his ceiling for
"clear" of 1.8 LVOT. Both cardiologists reached the same diagnosis
using different sets of parameters.
Therefore, a "negative" Doppler
is not necessarily a definitive "clearance", especially in the
presence of a heart murmur. There did seem to be some modest agreement that the
presence of a heart murmur, coupled with a "positive" Doppler, is
indicative of the presence of SAS, though there was widely divergent opinion as
to what exactly indicates a "positive" Doppler. Some cardiologists
opine that if there is no heart murmur, regardless of the velocity, then there
is no SAS.
There are two very obvious traps here.
One, of course, is taking the risk of passing on a serious cardiac disease, and
we must do our best to screen out those affected dogs that can be positively
identified. However, there is also great danger in eliminating too many dogs
from our already small gene pool. In selectively breeding to produce desirable
traits and eliminate undesirable ones in order to achieve the dog we want, there
is always the "Catch 22" of inadvertently eliminating the
"good" genes or characteristics and locking in the "bad"
because they are "linked". SAS is considered to be autosomal dominant
with incomplete penetration, which makes identifying the mode of inheritance
particularly difficult. Eliminating positively diagnosed dogs (meaning those
with a heart murmur, increased velocity, and a lesion) is a means to at least
curtail, if not eliminate, passing on this disease.
But, there is the problem of those mild
cases that escape detection. Will the disease continue to be passed on in such a
mild version that it has no affect on the quality or length of the dog's life?
Can the sub-clinical dog produce offspring with severe SAS? Are there
breed-specific parameters for diagnosis of this insidious malady? What is the
real risk, in percentage terms, of producing an affected dog (to any degree)
from a "carrier"? At what point do we opt to eliminate a dog from the
breeding pool? What constitutes "reasonable risk"? We don’t know the
answers to these questions at this point. However, the more information we have
to work with, the more informed our breeding decisions will be, so it is
important that any concrete data concerning this disease, or even fact-based
opinions, be freely shared in order to create a data base which may help us
solve some of the mysteries surrounding this disorder.
While cardiac concerns are certainly
serious ones, by their very nature, it doesn't seem at this time that our dogs
are dying at a rapid rate from SAS. Does it exist in our breed? Most certainly.
But to eliminate dogs from a breeding program based on what is nothing more than
pure speculation is very short-sighted and ultimately not in the best interests
of the breed. The disease is rampant in the Golden Retriever population, but
many of those showing clear and demonstrable SAS live well into their teens with
no impairment.
Screening, to at least identify, to the
best of our abilities at present, those dogs who are themselves possible
carriers of SAS is imperative. Follow up and careful, complete investigation of every
dog with a detectable heart murmur, no matter how "innocent" it may
seem via auscultation, must become part of our regular recommended health
testing. If that particular dog has any offspring (presumably produced before
detection of the heart problem), all of the offspring must be followed
and tested as well. Every dog that dies suddenly should be examined
post-mortem for evidence of SAS. In reality, the best way to screen and identify
carrier dogs would be post-mortem examination of every dog used for breeding,
regardless of the ultimate cause of death, to identify even those sub-clinical
cases of SAS. Again, though, caution must be the watchword here. While
identifying all affected dogs, even the sub-clinical ones, is important, at this
point there is simply not enough known about the mode of inheritance or the
degree to which the disease is passed on relative to the degree to which the
"carrier" dog was affected by the disease.
Careful research and gathering of
information is vitally important in our quest to eliminate this disease from our
breed. It is important, as always, to look at the whole dog, not just one piece.
It is important, as well, to have open, honest and forthright discussion of this
particular disease as well as others that are known to afflict our canine
population. Ignoring the problem or, worse, concealing it, is very, very
dangerous.
Equally dangerous is unsubstantiated rumor
and rampant speculation.
None of us fanciers (with the possible
exception of the veterinarians among us) is equipped to "track down and
root out" a complicated and insidious disease that has the entire canine
cardiology community baffled. There is widespread disagreement within that
veterinary community as to which dogs should be bred and which shouldn't.
There are wide variations in equipment. The mode of inheritance is presently
unknown. We can't even effectively screen the existing population to find the
sub-clinical cases when they're still alive and walking around. Until there is
some consensus among veterinarians and geneticists as to the best way to
approach this problem, we must continue to act conservatively and responsibly.
References:
http://home.europa.com/~dshecklr/SAS.html
www.grca.org/sas.htm
www.boxerunderground.com/bu2000/april2001/sas.htm
With thanks to Dr. Hogan, DVM, DACVIM-Cardiology,
Assistant Professor at Purdue University School of Veterinary Medicine
The authors of this article,
Andrea Kelly, Bob Spohr, and Lynn Spohr, take full responsibility for its
content. It does not reflect any policy of the ABA. It was produced and
published to give guidance and information to the many people who have inquired
about this disease.
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