|
Canine Cushing's Disease (Hyperadrenalcorticism)
What is Canine Cushing's Disease?
Canine Cushing's disease,
or Hyperadrenalcorticism, is a disease in which one or
both adrenal glands produce excessive
amounts of certain hormones. The condition is one of the
most common endocrine disease of middle-aged and
older
dogs.
A very
similar, but metabolically distinct disease process commonly
affects older ferrets, but beyond that, hyperadrenalcorticism
is infrequent in other domestic animals.
The adrenal glands produce
several vital substances – hormones - which regulate a
variety of body functions and are necessary
to sustain life. The most widely known of these substances
is cortisol, commonly known as cortisone.
Overproduction of cortisol can lead to serious problems.
Cause
There are two mechanisms by
which sspontaneous hyperadrealcorticism can occur. These
are described below. Regardless of the cause,
the symptoms are essentially the same. It is important to
identify the cause, if possible, however, because different
forms of the disease are treated in different ways and typically
have different outcomes.
Pituitary gland
tumor. The most common cause of Canine Cushing's
disease (85 -90 % of all cases) is a tumor of the pituitary
gland, which is a small organ, located in the skull, just
below the brain. This form of the disease is referred to
as pituitary-dependent hyperadrenocorticism (PDH). The pituitary
tumor produces abnormal chemicals which, in turn, cause the
adrenal glands to produce excessive cortisol. The pituitary
tumor may be either benign or malignant, and it may be either
microscopic or quite large. Large tumors may produce additional
symptoms in addition to the typical Cushing’s disease symptoms.
Generally, if the activity of the adrenal gland can be controlled,
many dogs with this form of Cushing's disease can live normal
lives for many years as long as they take their medication
and stay under close medical supervision.
Adrenal gland tumor. Functional adrenal tumors are a far
less frequent cause of hyperadrenocorticism in dogs. Adrenal
tumors causing Canine Cushing’s Disease can also be either
benign or malignant. If benign, surgical removal is usually
curative. Malignant adrenal tumors can sometimes be managed
for a time, sometimes with surgery, but the long term prognosis
is much less favorable.
Iatrogenic Cushing’s disease. Many of the signs, symptoms,
and lesions of naturally occurring hyperadrenocorticism
can be induced by long-term, daily administration of large
doses of corticosteroids. This is called Iatrogenic
Hyperadrenalcorticism or Iatrogenic
Cushing's Disease. Dogs are much more susceptible
to this medicinal side effect than are cats.
What are the symptoms and clinical signs?
The symptoms and clinical signs of Canine Cushing’s Disease
are somewhat vague and variable in the beginning; late stage
changes are more obvious. Findings may include any or all
of the following:
- Polyuria – excessive urination, a very common finding
- Polydipsia - excessive thirst – a very common finding
- Polyphagia - excessive hunger – a very common finding
- Behavioral problems - lethargy, sleep-wake cycle disturbances,
panting, and decreased interaction with owners
- Muscle weakness - especially affecting the extremities and
abdomen
- Muscle atrophy - especially affecting the extremities
and abdomen
- Gradual
abdominal enlargement
- Sway back appearance (lordosis)
- Muscle
trembling
- General
lethargy and weakness
- Hepatomegaly (liver enlargement)
- Hair
thinning and hair loss (Alopecia) - symmetrical and may
involve a significant portion
of the body surface
- Thin skin – due to atrophy of the epidermis
and pilosebaceous apparatus,
combined with loss of skin’s collagen and elastin
- Cutaneous
(skin) mineralization
Diagnosis
Diagnosis of Canine Cushing’s Disease can be tricky. Typically,
several different tests over a period of days are necessary.
One of the most important tests for canine Cushing Disease
is the ACTH Stimulation Test. If it does not confirm the
diagnosis, the Low-Dose Dexamethasone Suppression Test is
performed. Other tests are needed to decide which form of
the disease is present. Our experience at the Chastain Veterinary
Medical Group has shown that an ultrasound examination can
be a valuable part of the testing process. This permits us
to visualize the adrenal gland tumor and determine its size.
Although testing can be expensive, it is necessary before
treatment can be started.
Treatment
Iatrogenic Cushing's disease: This is the easiest form of
Hyperadrenalcorticism to treat. All we do here is gradually
taper off, and then finally discontinue, the corticosteroids
that are responsible for the condition. Note that this should
be done in a careful and controlled manner so as to avoid
or minimize metabolic problems associated with corticosteroid
withdrawal. Discontinuation of corticosteroids may result
in a recurrence of the symptoms of the disease for which
the corticosteroids were originally prescribed.
Adrenal Tumor. Treatment of Canine Cushing’s Disease caused
by an adrenal tumor requires surgery. If surgery is not an
option, some of these can be well managed with medications.
Pituitary Tumor: Many dogs
with PDH can be adequately managed by administration of
the drug
mitotane (o,p"-DDD). Because
the sensitivity of individual dogs to mitotane varies considerably,
pet owners should be aware of the possibility of rebound
hypoadrenocorticism (cortisol deficiency) developing after
several months of treatment. Pet owners should remain vigilant
for suggestive signs of possible hypoadrenalcorticism (e.g.,
vomiting and diarrhea).
Most dogs treated with mitotane show rapid improvement in
their symptoms of Canine Cushing’s Disease. Initially, water
consumption, frequency of urination, and appetite are reduced.
Muscle strength and physical activity improve over the next
few weeks. Hair re-growth usually takes several months. Dogs
that respond favorably to mitotane therapy must remain on
the medicine for life to prevent recurrence of clinical signs.
In addition to mitotane, the drugs ketoconazole and l-deprenyl
have been effective in the management of PDH in dogs. In
fact, l-deprenyl has recently won FDA approval for this
use. It is effective in ~80% of cases and is associated
with fewer side effects and less stringent patient monitoring.
| 
