Cushing`s Disease in Dogs

 Cushing’s Disease in Dogs
General Inform ation for Pet Owners by Dr Sophie Haynes
Internal M edicine Specialist
Cushing’s disease (Hyperadenocorticism) is a relatively common hormonal disease of older dogs.
It occurs rarely in cats. There are two main types, pituitary-dependent (approximately 85% of
cases) and adrenal-dependent (15%). In both types, the main cause of the clinical signs is too
much cortisol production in the adrenal glands. Pituitary-dependent Cushing’s occurs because a
small benign growth in the pituitary gland at the base of the brain sends out an increased amount
of signalling protein (ACTH) to the adrenal glands, overstimulating them. Adrenal –dependent
Cushing’s occurs when one of the adrenal glands contains a mass/tumour which produces too
much cortisol.
Clinical signs
The clinical signs that may suggest your dog has Cushing’s disease are numerous and varied,
some dogs only have one or two signs, others have many. This can sometimes make it hard to
diagnose. Common signs are:
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Drinking and urinating increased amounts
Increased appetite
Panting
Pot-bellied appearance
Thin haircoat
Thinning skin, sometimes with firm areas of mineralisation
Less commonly, shaking and muscle weakness or mental dullness
The clinical signs can be very subtle in the beginning and progress over weeks to months. It is a
slow insidious disease.
There are other diseases that can cause similar signs, such as diabetes, low thyroid levels, liver
disease or heart disease and so tests are required to differentiate between these diseases.
Diagnostic Tests
The first tests that are performed are blood tests (CBC, biochemistry, total T4) and analysis of a
urine sample. These tests often help to rule out diabetes, liver disease and sometimes
hypothyroidism. Blood changes typical of Cushing’s may be seen, however they do not give a
definite diagnosis of Cushing’s.
If the blood changes are typical of Cushing’s, a series of other tests are performed, usually in
combination rather than one single test. These tests include:
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Low dose dexamethasone suppression test (Screening test, can be altered by other
disease)
ACTH stimulation test (Screening test, less altered by other disease but sometimes
misses cases)
Plasma ACTH Assay (helps differentiate between pituitary vs adrenal)
High dose dexamethasone suppression test (less common, also differentiates)
Abdominal ultrasound (Check for other abdominal disease and is very useful for
differentiating between pituitary or adrenal)
The combination of tests used does depend on the vet involved and sometimes if one test is
equivocal, another will need to be performed to gather more evidence.
Treatment
The first decision to make is whether or not to treat. Cushing’s is a slow onset insidious disease
which does generally impact on the dog’s quality of life, but is not often an acutely life-threatening
condition. However, it can lead to complications such as blood clots, hypertension, liver disease,
diabetes and kidney disease. Treatment does require commitment -medication is required every
day and there are ongoing costs involved.
Treatment of pituitary-dependent Cushing’s disease involves one of two drugs, which are
administered, in most cases for the rest of the patient’s life. We are not curing the disease with
treatment, but simply reducing the cortisol production and therefore the clinical signs.
Mitotane is an older drug and essentially acts by killing the cells that produce the cortisol in the
adrenal. It is first used every day until the clinical signs (especially appetite) are reduced, and
then a maintenance dose twice a week is commenced. In the induction phase there is a risk of
removing too many cortisol-producing cells so these patients have to be monitored very closely.
The second (and most common) drug is called trilostane. This suppresses the production of
cortisol in the adrenals, without (in most cases) destroying the cells. It is thought to be a little
safer because the effects are reversible, unlike mitotane. These tablets are given once or twice a
day long-term.
Both drugs do require on-going monitoring and blood tests, initially every 1-2 weeks, and then
every 1-3 months. It is important to get a rough idea from your vet about the ongoing costs, so
they don’t come as a surprise. Some of the clinical signs improve quickly on the right dose – such
as the drinking and urinating, but other signs, like the pot-belly and skin changes may take
months. If your dog loses their appetite, has vomiting or diarrhoea or becomes lethargic whilst on
these medications, contact your vet straight away.
Adrenal-dependent Cushing’s disease can be treated medically with the above drugs or, ideally,
the adrenal tumour is removed surgically. Screening tests to check for spreads to other organs
are performed before the surgery.
With the right treatment and monitoring, the prognosis for Cushing’s disease is good, with many
dogs living a good quality life for years.