Cushing's Disease or Hyperadrenocorticism
Cushing's disease is probably more accurately referred to as
hyperadrenocorticism -- the production of too much adrenal hormone,
in particular corticosteroids. It can be naturally occurring or due
to over administration of corticosteroids such as prednisone
(iatrogenic Cushing's). The latter is easy to cure - just cut out
the corticosteroid administration slowly to allow the body to return
to normal function. The former is more difficult.
Hyperadrenocorticism occurs for two reasons --- a tumour of the
adrenal gland that produces adrenal hormones or stimulation of the
normal adrenal glands from the hormones that control it. The primary
reason for this to occur is a pituitary gland tumour that produces
excessive ACTH, which stimulates the adrenal gland to produce
corticosteroids. Adrenal gland tumours account for 15% of the cases
of spontaneous hyperadrenocorticism. Pituitary tumours account for
85%.
Cushing's disease causes increased drinking, increased urination,
increased appetite, panting, high blood pressure, hair loss -
usually evenly distributed on both sides of the body, pendulous
abdomen, thinning of the skin, calcified lumps in the skin,
susceptibility to skin infections and diabetes, weakening of the
heart and skeletal muscles, nervous system disease and other
symptoms. Most owners reach a point where the water consumption and
urination become bothersome to them.
The diagnosis of Cushing's can be done with several blood tests. A
general hint of Cushing's can be obtained by a blood panel. To
confirm it, a test known as a low dose dexamethasone test is done. A
baseline blood sample is drawn in the morning, an injection of
dexamethasone given and a follow-up blood test done 8 hours later.
In a normal dog, the dexamethasone should suppress cortisol levels
in the blood stream. In Cushing's disease this effect does not
occur. Once the disease is diagnosed, it is possible to
differentiate between the adrenal tumours and pituitary gland
tumours using a second test, a high dose dexamethasone suppression
test. Most dogs with pituitary tumours will have cortisol
suppression on this test. There are other tests used, including ACTH
response tests and urine cortisol/creatinine ratios to diagnose this
disease. X-rays and ultrasonography can help determine if an adrenal
gland tumour is present.
If it can be determined that there is an adrenal gland tumour, it
can be removed. Many veterinarians prefer to have a specialist
attempt this since the surgical risks can be high. Pituitary gland
tumours are not usually removed in veterinary medicine. This
situation is treated using Lysodren (o'p'-DDD, which is a relative
of DDT) or ketaconazole. Some research with Deprenyl for treatment
of this is being done, too, I think. Lysodren selectively kills the
outer layer of the adrenal gland that produces corticosteroids.
By administering it in proper amounts it is possible to kill just
enough of the gland off to keep the production of corticosteroids to
normal levels. Obviously, close regulation of this using blood
testing is necessary since overdoing it can cause severe problems
with Addison's disease - hypoadrenocorticism.
Adverse reactions to Lysodren occur at times but it is the standard
treatment at this time. Over medication with Lysodren can cause
inappetence, vomiting, diarrhoea, lethargy and weakness. If any of
these signs occur then your veterinarian should be immediately
notified.
Treatment of Cushing's disease caused by pituitary tumours is
symptomatic therapy -- it does not cure the pituitary tumour. The
average lifespan of dogs diagnosed with Cushing's, with or without
treatment is estimated at 2 years by Dr. Mark Peterson, but in a
recent conversation with another endocrinologist I came away with
the impression that this was an "educated guess" rather than the
result of extensive survey of Cushing's patients. At present,
though, I think that treatment should be viewed as a means of
providing a better quality lifestyle rather than as a method of
extending longevity.
