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BOXER CARDIOMYOPATHY
by Wendy
Wallner, DVM
Atlanta, GA
(Published on the
Boxerunderground website http://www.boxerunderground.com)
What is boxer cardiomyopathy?
Boxer
cardiomyopathy as we know it consists primarily of an electrical conduction
disorder which causes the heart to beat erratically (to have an arrhythmia)
some of the time. If the erratic beats occur infrequently and singly, the
dog will probably not have symptoms of heart disease. If the erratic beats
occur in sequence, weakness, collapse or sudden death may result. These
arrhythmias may or may not be detected by listening to the heart with a
stethoscope. Whether or not they are detected depends on the frequency of
the abnormal rhythm. If frequent, they can easily be heard with a
stethoscope. The arrhythmia usually consists of VPCs (ventricular premature
contractions) that are heard as an extra beat or a skipped beat that do not
have a corresponding pulse. To identify these, the listener must therefore
have one hand on the stethoscope holding it to the chest and one hand
feeling for a pulse (usually at the femoral artery on the inside of the hind
leg). In the normal functioning heart, there is a pulse for every beat that
is heard. When a VPC occurs, a beat is heard through the stethoscope (and it
sounds like a stutter as it is not in the normal rhythm sequence of the
sinus beats), but there is no pulse to go with it. These VPCs have a
characteristic pattern on an ECG and this is the way they are confirmed.
Often this is the first abnormality noticed in a boxer with cardiomyopathy.
Usually the dog is having no symptoms of heart disease when these are
noticed by a veterinarian during a routine exam. If the frequency of these
irregular beats increases, the animal may suffer "fainting" spells (called
syncopal episodes). This happens because these abnormal beats do not pump
the blood effectively (no corresponding pulse) to the vital organs like a
normal beat does and the brain becomes oxygen deprived while the abnormal
beats are occurring. Usually when an animal faints, they are having what is
known as a run (several in a row) of VPCs. If the heart corrects itself, the
animal regains consciousness in a matter of seconds to minutes. If the run
of VPCs continues, this is termed ventricular tachycardia and can lead to
the development of ventricular fibrillation that is fatal if the rhythm is
not converted. This ventricular fibrillation (V-fib) is the cause of sudden
death in most boxers with cardiomyopathy. There is no blood being pumped
through the body when the animal is in V-fib. Cardiomyopathy can also be
responsible for sudden death associated with anesthesia. Now, just because a
boxer has VPCs does not absolutely mean it has cardiomyopathy IF there is
another disease process at work. I have seen animals with severe infection
or cancer have VPCs that resolved completely once the infection was cleared
or the malignancy removed. If, however, VPCs are seen in an otherwise
healthy boxer, one would have a high index of suspicion for cardiomyopathy
because of the prevalence of the disease in the boxer breed.
Some boxers with cardiomyopathy will enter another phase of
disease where the ventricles of the heart start to dilate. At this time it
is unclear whether this is a progression of the electrical conduction
disorder, a separate disease more like that seen in other large breed dogs,
or a subset of boxer CM that is not necessarily a progression of the
previously arrhythmic dogs. With this condition, the walls of the heart
become thin, the heart muscle weakens and these animals can show symptoms of
heart failure such as coughing (from lung congestion) and/or fluid retention
in the abdomen (ascites) depending on which side of the heart is most
affected. In time as the heart becomes very enlarged it begins to be an
inefficient pump and dogs so affected may require numerous medications to
keep the heart functioning well enough to sustain life. Still, most boxers
affected with cardiomyopathy will ultimately die of their arrhythmia, not of
congestive heart failure. The only way to definitively make the diagnosis of
cardiomyopathy is to have a veterinary pathologist evaluate tissue samples
from the heart muscle after death.
How is Boxer Cardiomyopathy Diagnosed?
The best way to evaluate a boxer for arrhythmia is to use a 24-hour ECG
called a Holter monitor. While an ECG can pick up arrhythmias if they are
very frequent, the Holter is much better at doing so. It will tell you if
your dog has VPCs, whether they are frequent or infrequent, single or
multiple, from a single focus in the heart or from several sites. Not enough
boxers have been studied to know if a small number of VPCs may be normal,
but what is known is that most boxers that go on to die of cardiomyopathy
have many VPCs in a 24 hour period (hundreds to thousands) and that they
have runs of ventricular tachycardia. The Holter monitor allows us to
identify dogs that may have problems due to these runs of VPCs. For example,
most asymptomatic animals have single VPCs interspersed with their normal
beats throughout the 24-hour period. If a Holter shows many clusters or runs
of VPCs, this means that this animal may be at higher risk for syncope or
sudden death and can affect how the dog is treated (with anti-arrhythmic
drugs, for example). Comparing statistics for ECG and Holter, it becomes
evident why the Holter is superior in detecting subtle arrhythmias. The
average 3 minute ECG provides the cardiologist with only 240 beats compared
with 90,000 to 110,000 on the average Holter tape. Studies from human
medical literature claim that individuals with more than 3000 VPCs in 24
hours have a 29% chance of having a normal random ECG, those with 1000-3000
VPCs in 24-hours have a 50% chance of having a normal random ECG and those
with less than 300 VPCs have almost a 100% chance of having a normal random
ECG. This is why so many affected boxers have normal random ECGs. ECG data
is meaningless unless it is abnormal!
Why Not Use Echo?
Since boxer CM is a disease characterized primarily by arrhythmias, Echo is
not the method of choice to make a diagnosis of CM. An echocardiogram is
useful to determine if the heart is contracting properly. It will also help
detect and identify the source of any murmurs that may have been heard on
auscultation with a stethoscope by allowing visualization of the heart
valves and blood flow patterns through those valves. It can be used to rule
out the inherited condition of sub-aortic stenosis (SAS) which is known to
affect the boxer and can also lead to sudden death. It can also show whether
or not there is any enlargement of the heart chambers or any thinning (as
seen in dilated cardiomyopathy) or thickening (as seen in hypertrophic
cardiomyopathy) of the heart muscle walls. It is not a good tool for
detecting an arrhythmia, unless the arrhythmia is very frequent. Most boxers
with CM will have normal echocardiograms unless they also have SAS or have
the type of CM (progression, different disease or subset?) that causes
dilatation of the ventricles.
What about supplementing with L-Carnitine?
It has been shown that dogs on commercial diets have adequate amounts of L-carnitine
in their plasma and that 80% of dogs with cardiomyopathy that have a
deficiency of L-carnitine in the heart muscle have normal to increased L-carnitine
levels in their blood. Although there has been a correlation between two
sibling boxers with dilated cardiomyopathy and a response to supplementation
with the L-carnitine, many more boxers have shown no improvement with
supplemental L-carnitine. The two sibling boxers were found not to have a
deficiency of carnitine in the diet, but most likely had an inability to
utilize the carnitine present in their blood and to transport it into the
heart cells where it must be actively concentrated so that it can be used
for fatty acid metabolism, generation of energy and detoxification of
certain metabolic compounds. These dogs most likely had an inherited defect
of the membrane transport of L-carnitine. While supplementation with L-carnitine
improved the contractility of these dogs and caused a temporary improvement,
it did not decrease their arrhythmias. One of these dogs eventually died due
to ventricular arrhythmia, the other due to an apparent sudden onset of
Addison's disease. Both parents were also affected with CM, but died before
treatment with L-carnitine could be evaluated. (Keene, 1991)
Historical Perspectives
This condition was identified and defined by Dr. Neil Harpster back in the
late 60s and early 70s. The first paper was published in 1983 and was the
result of examination of 64 boxers over a 15-year period with varying
presentations of the condition. He described it as being quite different
from other large and giant breed cardiomyopathies as had been characterized
in the Doberman and Great Dane in that the hearts of the affected boxers
showed an absence of dilitation of the ventricles, the dogs rarely suffered
from atrial fibrillation and the heart muscle showed extensive changes
histologically on post mortem exam.
The disease was characterized as a cardiomyopathy based on
the human nomenclature which calls myocardial disorders for which no
specific cause can be found "primary cardiomyopathies." Because the dogs
that Harpster studied were closely related, he proposed an inherited origin
for the condition. In the original group of 64 dogs studied, Dr. Harpster
found a slight male predisposition (57.8%) and an age range of 1-15 years
with only 15.6% of the dogs less than 6 years of age and 25% over the age of
10. The average age at the time of diagnosis was 8.2 years. (In a 1991
report which added another 48 dogs to the original study, the average age at
diagnosis dropped to 6.9 years.)
Dr. Harpster divided the 64 dogs into 3 categories based on
the clinical features of their disease. The first category included dogs
that had no clinical signs of disease. The second group had occasional
episodes of fainting or weakness usually after a stressful event but were
otherwise completely normal. The third group included dogs with signs of
heart failure. The most common finding on the physical exams of all of these
dogs was the presence of a cardiac arrhythmia. ECG findings consistently
showed ventricular premature beats (VPCs) occurring singly, in pairs and in
runs and episodes of ventricular tachycardia. The portions of ECG which did
not show these abnormal beats appeared normal.
Of the 64 dogs in the original study, only 18 were presented
for necropsy. All of the hearts had extensive and diffusely distributed
changes in the myocardium (heart muscle tissue). The changes included the
presence of cells that are not normally seen in heart muscle, the
replacement of muscle tissue by fibrous tissue (scarring), and infiltration
of fat into the muscle tissue. (Harpster, 1983,1991)
The Present:
While Dr. Harpster saw equal numbers of each of the three categories of
boxer CM, we now seem to see more boxers in the first two categories and
very few with signs of congestive heart failure. Dr. Kate Meurs at The Ohio
State University, currently the most active researcher in regard to boxer CM
and the recipient of the recent ABC/AKC grant, has placed that number at
probably less than 10%. This may be due to increased awareness of the
condition on the part of boxer owners and veterinarians and an increasing
number of "normal" or at least asymptomatic boxers being critically
evaluated for the presence of arrhythmias.
A few years ago, upon my appointment to the ABC's Health and
Research Committee, I came up with a protocol for heart testing for boxers.
These guidelines were published in the ABC News Bulletin in December
of 1996. The recommendations were as follows:
Minimum
Heart Screening
for boxers involved in breeding programs and/or performance events:
Age 1
year:
auscultation by a board certified veterinary cardiologist (If arrhythmia
detected - Holter exam; if murmur detected - Echocardiogram)
Rationale: One year is the accepted time for clearance of sub-aortic
stenosis. This auscultation screening could be performed at the national
specialty and at individual breed club's specialty shows for a nominal fee.
Any dogs with murmurs would be referred to a cardiologist in their area for
further workup.
Age 2 years:
Holter monitor, auscultation (Echo if murmur detected)
Rationale: The 2-year check would occur before the animal was used
for breeding (at least extensively) and would be useful in detecting dogs
with early arrhythmias before they are bred. In some dogs, arrhythmias have
been detected as early as 12 weeks of age.
Age 5 years:
Holter monitor, auscultation (Echo if murmur detected)
Rationale: By 5 years, many animals would show signs of arrhythmia if
they were going to develop CM, since the arrhythmia often precedes clinical
disease by several years.
The main purpose of these screens was to develop a database
which could be analyzed and related to causes of death in dogs so that some
sort of standardized system of interpreting the holter results could be
determined. Not only would it help to identify and eliminate dogs with SAS
from breeding programs, it would also help identify and eliminate those
asymptomatic boxers with very large numbers of VPCs. It was never intended
to eliminate any and all dogs with VPCs from breeding programs. There simply
is not enough information available to use the results in this manner. If
large numbers of boxers are not holtered and followed over time, there never
will be a database large enough to provide meaningful holter results. If
this is the case, we are at the mercy of the only other test that will
identify boxer CM - a genetic marker for the disease. Unfortunately, this
type of test can take decades to establish. Since we know that the hallmark
of boxer CM is arrhythmia and that the Holter is the best tool to detect
arrhythmia, it is the only method we currently have to try to evaluate our
breeding stock before they have produced offspring. While there currently
are no concrete numbers to identify normal vs. abnormal dogs, the Holter is
still extremely important in identifying those grossly abnormal dogs with
hundreds or thousands of VPCs who are asymptomatic and would otherwise be
unknowingly reproduced. Until many Holtered dogs have been followed into old
age and their medical histories analyzed and causes of death determined, we
will not know the true significance of lower numbers of VPCs. We can,
however, use all information obtained through Holter testing responsibly by
slanting a breeding program toward those boxers that seem less affected.
The above guidelines were also sent to the Secretary of the
ACVIM - Specialty of Cardiology with a request to disseminate the
information to all US boarded cardiologists. This was done by Dr. Rebecca
Gompf and several responses from veterinary cardiologists ensued with offers
of assistance. The four ongoing projects, which have arisen out of that
communication, are:
a study of boxer heart specimens from deceased dogs
known to have had CM (due to abnormal holter results); a low cost means of
analyzing holter tapes; production of pamphlets detailing SAS and boxer CM
for distribution by ABC; and making low-cost echocardiograms available at
ABC.
Project number 1
is being jointly conducted by Dr. Kate Meurs (OSU) and Dr. Phil Fox (AMC, NY).
Boxer hearts are removed (according to a protocol) from affected dogs after
death and shipped to Ohio State University where they are subjected to MRI
(magnetic resonance imaging). They are then sent to Dr. Fox who is working
with a pathologist known for his interest and expertise in cardiac pathology
to determine the histologic changes, variables and similarities from dog to
dog.
Project number 2
involves Dr. Luis Braz-Ruivo's offer to analyze holter tapes from boxers for
only $30.00. Dr. Meurs will also analyze tapes for $35 (or $25 if a pedigree
is included) but can no longer ship monitors away from OSU. Both doctor's
offers necessitate clubs or individuals purchasing their own testing
equipment.
In the ABC News Bulletin of June 1997, there was a
recommendation for local boxer clubs to purchase their own Holter monitors
for member (and possibly non-member) use. Through Dr. Braz-Ruivo, contact
was established with a Holter monitor manufacturer and a price reduction in
the cost of monitors was offered to anyone affiliated with boxers. The
company was Rozinn and they offered us monitors for a limited period at a
cost of $995 (about $500 below the normal price of a monitor). While the
limited period expired long ago, Rozinn is still honoring their price quote
of $995 to anyone that orders a monitor and mentions the ABC or Dr.
Braz-Ruivo. The Georgia Boxer Club bought a monitor in 1997, and the
membership voted to charge members a $25/dog usage fee. Non-members would
pay $50/dog. The cost of supplies to attach the monitor is approximately
$15/dog, which brings the total cost of monitoring one dog to $70 for GBC
members. Unfortunately, only a handful of GBC members have used the monitor
to date. However, this small group has used it on multiple occasions.
Regarding project number 3,
Dr. Braz-Ruivo is currently working on the two pamphlets which will be
available for the ABC to use in whatever manner they wish. The pamphlets
will detail in lay terms the conditions of SAS and Boxer CM.
Project number 4
- Dr. Braz-Ruivo will also be conducting echocardiograms at the ABC in 1999
for a very reduced cost of $75 for pre-registered dogs and $100 for on-site
registered dogs. Anyone who has been referred for echo but does not have
easy access to a cardiologist or has a dog with a murmur of questionable
origin should take advantage of this great service.
References:
Harpster NK: Boxer Cardiomyopathy. In Kirk RW (ed): Current Veterinary
Therapy VIII. Philadelphia, WB Saunders, 1983, pp 329-337.
Harpster NK: Boxer Cardiomyopathy - A Review of the Long-Term
Benefits of Antiarrhythmic Therapy. In Veterinary Clinics of North America:
Small Animal Practice - Volume 21, No. 5, September 1991, pp 989-1004.
Keene BW: Myocardial L-Carnitine deficiency in a family of
dogs with dilated cardiomyopathy. JAVMA 198:647, 1991. |
