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Asthma is a recurring respiratory compromise that
occurs when the lung airways constrict either spontaneously or in
response to stimuli that normally should not cause a reaction. Excess
mucus forms, airways swell with inflammation and can ulcerate, and the
airway muscles spasm leading to constriction. Airway constriction leads
to inability to draw a deep breath, intolerance to exercise, coughing,
and musical sighing sounds called wheezing. Not all of these signs need
be observed; sometimes only a low grade chronic cough is the only sign
but it should be remembered that an acute asthmatic crisis can arise at
any time and can represent a life-threatening event.
Is Feline Asthma the Same as Human Asthma?
The feline condition was named asthma due to the clinical features
shared with the human disease. As of this time, it certainly appears
that all the diagnostic criteria needed to make the asthma diagnosis in
humans are certainly shared by cats; still, we are still working out the
mechanics of this syndrome in cats and have a great deal to learn. For
example, in humans we know that while symptoms of asthma occur in
episodes, the airways of the lung are diseased all the time. We do not
know yet if feline airways are also diseased all the time or if airway
structural changes occur only when there are clinical signs of distress.
How Is the Diagnosis Made?
Because of the constricted airways, the actual volume of air this
patient can move in and out of the lungs each breath is reduced. There
is often a great deal of effort seen in the cat’s breathing. The abdomen
appears to be working to push air out and the breaths are shallow and
rapid. The cat may even be breathing with its mouth open in an effort to
move the largest possible amount of air.
The next step toward making a diagnosis of feline asthma is the chest
radiograph, assuming the cat is not in too much distress to hold still
in position for this procedure. Classically, this radiograph will show
what is called air-trapping. This means that the small airways have
constricted such that inhaled air cannot be exhaled. The lungs are
larger in appearance than normal as they are over-inflated. The
diaphragm may seem flattened due to this over-inflation.
Inflammation and mucus build up within the airways causing their walls
to appear thickened in the radiograph. The terms used for such airway
appearance are doughnuts (when viewing the airway end-on) or tramlines
(when viewing the airway from the side). You may hear your veterinarian
use these terms and they are classical findings in airway disease.
But Some Asthmatic Cats Have Normal Radiographs
Since visible changes are not always evident on radiographs this can
lead to the diagnosis of asthma when it is not in fact present. Indeed,
the diagnosis of asthma can be complicated and it is famously over-Eosinophils
diagnosed because it is consistent with normal radiographs.
Procedures that retrieve cells from the lower respiratory tract may be
helpful in patients with normal radiographs. Such procedures include the
tracheal wash and bronchoscopy. The cell type of allergy known as the
eosinophil is copious in the secretions of an asthmatic patient. But
even this finding is made complicated since eosinophils occur in normal
feline respiratory secretions. Further, parasitic infections such as
lungworm and heartworm also lead to eosinophil-rich respiratory
secretions, but hopefully other tests have been used to rule these
infections out.
Response to Therapy as a Diagnostic Test
One important asthma feature is that the airway constriction in
engenders is reversible. In an emergency situation, a small dose of
epinephrine (adrenalin) can reverse an asthmatic crisis in as little as
15 minutes. Response to injection with an airway dilator such as
terbutaline usually occurs within 30 minutes. Response to a long acting
corticosteroid injection (such as methylprednisolone acetate/depomedrol)
generally yields a positive response within 48 hours.
Sometimes, diagnostic tests still leave room for question and one has to
simply go with medical treatment for asthma and regard response to
therapy as evidence that the diagnosis is correct. See below for list of
medications commonly used in the long-term management of this problem.
How do we treat Asthma?
It is crucial to realize that the underlying problem in the airway is
inflammation. To resolve inflammation, corticosteroid medications such
as prednisone are necessary; doses are frequently high and continued
long-term. In asthmatic humans, inhalers of corticosteroid medications
have been especially helpful in delivering the benefits of these
hormones without the burden of side effects. Recently, there have been
some advances in using pediatric inhaler devices to aid in the
administration of these drugs to cats.
In an acute episode, it may be necessary to sedate the cat to reduce the
anxiety associated with the difficulty breathing. Administration of
oxygen and injectable drugs to bring the attack under control can then
be administered.
Other medications that might be helpful include:
Airway Dilators: Terbutaline (Brethine) and theophylline (Theo-Dur)
are airway dilators commonly used in the management of asthma. It makes
sense that if constriction is an important feature of this disease,
eliminating constriction would be therapeutically helpful. Terbutaline
is an inhalant or injectable. Some veterinarians encourage owners to
keep a bottle of injectable terbutaline at home in case of a crisis and
show them how to give it. If you are interested in this, let your
veterinarian know.
Antihistamines: Histamine is an inflammatory substance released
during allergic response. Histamine has been thought to be involved in
the airway constriction mechanism and antihistamines are widely
prescribed to asthmatic cats. Recent research, however, indicates that a
special receptor in the feline airway (called an H3) receptor leads to a
dilation response to histamine instead of a constriction response. This
could be why antihistamines have not been found to be of dramatic
assistance in the management of feline asthma. More research in this
area is currently needed.
Cyproheptadine: One of the biochemicals involved in the asthma
inflammation cascade is called serotonin. It is directly involved in
constriction of the airways in cats. Cyproheptadine is an anti-serotonin
medication with concurrent antihistamine properties. It is often used in
cats who need extra help beyond steroids or who are having problems that
preclude steroid use. Side effects of cyproheptadine include increased
appetite and tranquilization.
Zamflurkast (Accolate): This new medication has received a lot of
attention in the human medical arena. It is meant to supplement other
treatments and perhaps reduce the "requirement" of steroids needed to
manage asthma. It is still fairly experimental in cats, but many
anecdotal reports have been favorable. It is an inhibitor of chemical
mediators called leukotrienes.
Cyclosporine: The use of cyclosporine in asthma is relatively
new. Cyclosporine is an immunomodulator often used in organ transplant
patients. It is very expensive and is generally reserved for asthmatics
who have not responded to other treatments.
IT IS IMPORTANT TO REALIZE THAT ASTHMA CAN CULMINATE IN A RESPIRATORY
CRISIS THAT CAN BECOME LIFE-THREATENING IF IGNORED.
If your cat begins to breathe with an open mouth or if you see excessive
abdominal movement during respiration and the cat is not purring, you
may have an emergency situation. Contact your veterinarian immediately.
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