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| Boxer Cardiomyopathy Wendy Wallner, DVM, Atlanta,
Georgia |
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What is Boxer Cardiomyopathy? |
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Boxer Cardiomyopathy, as we know it, consists primarily of
an electrical conduction disorder that 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,
then 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 they occur frequently
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 does 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 normally
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 noted in a Boxer with
cardiomyopathy. Usually the dog is having no symptoms of heart
disease when a veterinarian notices these VPCs during a routine
exam. If the frequency of these irregular beats increases the animal
may suffer "fainting spells" called syncopal episodes. This happens
because the abnormal beats do not pump blood effectively (no
corresponding pulse) to the vital organs like a normal beat would
and the brain becomes oxygen deprived while the abnormal beats are
occurring. Usually when an animal faints they are having what is
know 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. Ventricular fibrillation 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 effectively 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 were 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 such as the Doberman pinscher,
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 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. |
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How is it diagnosed? |
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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 frequent, the Holter is much better at doing
so. It will determine the presence or absence of VPCs, whether they
are frequent or infrequent, single or multiple, from a single focus
in the heart or from several sites (multifocal). Not enough Boxers
have been studied to know if a small number of VPCs may be normal.
It is known that most Boxers that go on to die of
cardiomyopathy have many VPCs in a 24-hour period (hundreds to
thousands) and that they often 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 could indicate that the animal may be at higher
risk for syncope or sudden death and can affect treatment (with
antiarrhythmic 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
ECG. This is why so many affected Boxers have a normal random ECG.
ECG data is therefore meaningless unless it is
abnormal. |
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Why not use Echo? |
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Since Boxer CM is a disease characterized primarily by
arrhythmia, echocardiography is not the method of choice to make a
diagnosis of CM in this breed. An echocardiogram is useful to
determine if the heart is functioning 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 and
is noticed during the echo exam. 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
dilation of the ventricles. |
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What about supplementing with
L-Carnitine? |
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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 Boxer
siblings with dilated cardiomyopathy and a response to
supplementation with 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 their 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 hypoadrenocorticism (Addison's disease). Both
parents were also affected with CM but died before treatment with
L-Carnitine could be evaluated. (Keene, 1991) |
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Historical Perspectives |
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This condition was identified and defined by Dr Neal
Harpster back in the late 1960s and early 1970s. 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.
Dr. Harpster described the syndrome as being different from
other large and giant breed cardiomyopathies, as had been
characterized in Dobermans and Great Danes, in that the hearts of
the affected Boxers showed an absence of dilation of the ventricles,
the dogs rarely suffered from atrial fibrillation and the heart
muscles showed extensive histologic changes 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 of dogs
had occasional episodes of fainting or weakness, usually after a
stressful event, but was otherwise completely normal. The third
group included dogs with signs of heart failure. The most common
finding on the physical exams of all these dogs was the
presence of a cardiac arrhythmia. ECG findings consistently showed
ventricular premature beats (VPCs) occurring singly, in pairs and in
runs of ventricular tachycardia. The portions of ECGs that did not
show abnormal beats appeared normal.
Of the 64 dogs in the original study only 18 were
presented for necropsy. All of the dog's 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) |
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