Cushing’s Disease (hyperadrenocorticism) is a condition in which the adrenal glands produce excessive levels of steroid hormones (the main one is cortisol). It occurs in both dogs and cats but is much more common in dogs.
How does this disease occur?
There are three mechanisms by which this disease can occur. Regardless of the cause, the clinical signs are essentially the same. It is important to identify the type of Cushing’s Disease because the various forms are treated differently and have different prognoses.
Pituitary gland tumour The most common cause of Cushing’s Disease (85% of all cases) is a tumour of the pituitary gland. The tumour results in the pituitary gland to overproduce a hormone that stimulates the adrenal glands to produce excessive cortisol. The tumour is usually benign and so small its only effect is due to the hormone it produces. Occasionally (10-20% cases) the tumour grows large (macroadenoma) and causes other problems (neurological signs). 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 if they take their medication and stay under close medical supervision. Growth of the pituitary tumour would give the patient a less favourable prognosis.
Adrenal gland tumour Cushing’s Disease may be the result of a benign or malignant tumour of the adrenal gland. If benign, surgical removal cures the disease. If malignant, surgery may help for a while, but the prognosis is less favourable than for a benign tumour.
Iatrogenic Iatrogenic Cushing’s Disease means that the excess of cortisol has resulted from excessive administration of a steroid (oral or injectable) medication. Although the injections or tablets were given for a legitimate medical reason, their excess is now detrimental.
The most common clinical signs associated with Cushing’s Disease are
• Increase in appetite
• Increase thirst and hence increase in urination and sometimes incontinence
• Lethargy, lack of activity, muscle weakness
• A poor hair coat -poor hair regrowth, thin skin, black heads, pigmented skin and skin infections
• A bloated “pot-bellied” appearance to their abdomen due to an increase of fat within the abdominal organs and a stretching and weakening of the abdominal wall muscles.
Left untreated death may occur from the potential complications which include
• Kidney and bladder infections or stones
• Sugar diabetes
• High blood pressure
• Pulmonary thromboembolism (blood clots in the lungs)
How is it diagnosed?
Blood tests are necessary to diagnose and confirm Cushing’s Disease. The two most common tests to detect Cushing’s Disease are the ACTH stimulation Test and the Low-Dose Dexamethasone Suppression Test. Other tests are needed to decide which form of the disease is present as well as look for secondary complications (diabetes, urinary infections etc). An ultrasound examination can be a valuable part of the testing process. This permits visualization of the adrenal glands and determines their size and the presence of a tumour. Although some of these tests are somewhat expensive, they are necessary.
What are the treatment options?
Iatrogenic Cushing’s Disease: Treatment of this form requires a discontinuation of the steroid that is being given. This must be done in a very controlled manner so that other complications do not occur. Unfortunately, it usually results in a recurrence of the disease that was being treated by the steroid. Because there may have been adverse effects on the adrenal glands, treatment is also needed to correct that problem.
Adrenal Tumour. Treatment of an adrenal tumour requires major surgery. Although this is a high-risk surgery, if successful and the tumour is not malignant, there is a good chance that the dog will regain normal health. If surgery is not an option, some of these patients can be managed with the medication discussed next.
Pituitary Tumour: Treatment of the pituitary-induced form of Cushing’s Disease is the most complicated. There are two drugs (Trilostane and Lysodren) that can be used. Trilostane is the most common drug recommended. Trilostane inhibits an enzyme involved in the synthesis of the steroid hormones. Trilostane is given usually once a day with food in the morning. Blood tests are required when using this medication usually 10, 30 and 90 days after starting treatment then every 4-6 months to ensure life threatening over suppression of the adrenal glands does not occur.
Although a cure is not achieved with medical treatment, control is possible for many years if the tumour is small. If the tumour is large, local effects of the tumour invading surrounding tissues in the brain can be the limiting factor in survival.