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Although still known as Cushing’s Disease in North America, in Britain we tend to favour the term Cushing’s Syndrome since the condition produces a much wider variety of signs (symptoms) than are normally associated with any particular disease. The more accurate medical term is hyperadrenocorticism, indicating that it is caused by high circulating levels of cortisol in the blood stream. Cortisol is the naturally produced form of cortisone or corticosteroid. Cortisol is produced by the outer part of the cortex of the paired adrenal glands. These are closely associated with the kidneys. The name of the condition is derived from that of Harvey W Cushing, a Boston surgeon (1869-1939). Today, Cushing’s Syndrome is one of the most common hormonal problems found in the dog.
WHAT CAUSES HYPERADRENOCORTICISM
The most common cause is over production of adenocorticotropic hormone, (ACTH), by the pituitary gland. ACTH controls the secretion of the cortisol (cortisone) by the adrenal glands. Over production of ACTH is usually caused by a tumour and thankfully most of these pituitary tumours are benign. Approximately 85% of all cases of Cushing’s Syndrome are Pituitary derived Hyperadrenocorticism (PDH). This can occur in young middle-aged dogs.
The other main type, Adrenal Dependent Hyperadrenocorticism (ADH) is again usually caused by a benign tumour, this time centred in the adrenal glands. This type is usually found in older dogs (10-12 years of age) and only accounts for 10-15% of all dogs with Cushing’s Syndrome.
Finally, mention must be made of iatrogenic Cushing’s Disease. Here the problem has been caused due by the administration of cortisone for another medical condition, a skin problem for example. This is an area, which always presents me, as a veterinary expert witness, considerable difficulties.
The usual scenario is that a dog with a devastatingly itchy skin condition has been controlled by not cured, sometimes for years, by the administration of a very appropriate dosage of cortisone. Then, suddenly, the dog develops Cushing’s Disease. The owners, unsurprisingly, are concerned that the veterinary surgeon has ‘over done’ the drug! Usually there is no evidence of this at all. It is a risks and benefits situation just like the treatment of epilepsy with anti-epileptic drugs which allow the dog to lead a normal life but long term can have side effects on the liver.
THE CLINICAL SIGNS OF CUSHING’S SYNDROME
Usually there is an increase in thirst and urination, together with polyphagia (increase in appetite). Hair loss, lack of energy, muscle wastage and obesity with a pendulous abdomen can all follow. In addition panting, pigmentation of the areas of the body that have lost hair, lack of sex drive etc., can also occur.
Subclinical cases will often present with a minimal hair loss and a paper-thin skin. This is frequently attributed to advancing years by the owner.
CAN IT BE ACCURATELY DIAGNOSED?
Accurate diagnosis depends upon laboratory results of a series of blood tests, the most important of which is the ACTH stimulation test. Other tests will then be needed to decide whether we are dealing with PDH or ADH. Treatment obviously depends on the type of Cushing’s.
Iatrogenic Cushing’s obviously has to be treated by the discontinuation of any corticicosteroids being administered. This, unfortunately, can lead to recurrence of the original condition. ADH often responds to surgery, particularly if the adrenal tumour is benign but obviously the surgery is not without risk.
Finally, PDH, the most common type of Cushing’s, is usually treated medically. For many years this condition has been successfully treated in dogs using products intended for human use. Today there is a drug, Trilostane (Vetoryl) that is licensed in the UK for treatment of Cushing’s in dogs. However, it should be borne in mind that sometimes treatment may be for life and regular monitoring, approximately every three months, at least initially, will be necessary, therefore treatment can be costly.
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