Sub-Aortic Stenosis in the Rottweiler
An Overview:
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
