The Granby Animal Clinic, Inc.
CANINE CUSHING'S DISEASE
Cushing's disease is a disease in which the adrenal glands overproduce certain hormones. Another medical term for this disease is hyperadrenocorticism.
The adrenal glands produce several vital substances 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. Either deficient production or excessive production of these substances may be life threatening.
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 cause, however, because the various forms are treated differently and each form has a different prognosis.
Pituitary gland tumor. The most common cause of Cushing's disease (85% of all cases) is a tumor of the pituitary gland. The tumor may be either benign or malignant. The tumor causes the pituitary to overproduce a hormone which stimulates the adrenal glands. Excessive cortisone secretion results. The tumor may be either microscopic or quite large. Depending on the size of the tumor, the presence of signs other than Cushing's will be variable. 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. Fast growing pituitary tumors would give the patient a less favorable prognosis.
Adrenal gland tumor. Cushing's disease may be the result of a benign or malignant tumor of the adrenal gland. If benign, surgical removal cures the disease. If malignant, surgery may help for a while, but the prognosis is less favorable than for a benign tumor. Medications are less effective for this form of the disease.
Iatrogenic. Iatrogenic Cushing's disease means that the excess of cortisone has resulted from excessive administration of corticosteroids. This may occur from oral or injectable medications. Although the injections or tablets were given for a legitimate medical reason, their use is now detrimental.
The most common clinical signs associated with Cushing's disease are a significant increase in appetite, water consumption, and urination. Lethargy, or lack of activity, panting and a poor hair coat are also common. Many of these dogs develop a bloated appearance to their abdomen due to increase fat within the abdominal organs and a stretching of the abdominal wall as the organs get heavier. The pot-bellied appearance also develops because the muscles of the abdominal wall become weaker. Recurrent infections involving the skin, eyes and urinary tract system are common in dogs with Cushing’s disease.
A number of tests are necessary to diagnose and confirm Cushing's disease. Some tests help to diagnose the disease, other tests help to determine what form of Cushing’s disease is present. The ACTH Stimulation Test and the Low-Dose Dexamethasone Suppression Test are frequently used to diagnose Cushing’s disease and the High-Dose Dexamethasone Suppression Test is used to determine the type of Cushing’s disease. An ultrasound examination can also be a valuable part of the testing process. Ultrasonic examination permits visualization of the adrenal glands and determines their size. There is also a simple urine test, unfortunately, this test is often inconclusive and it cannot distinguish between the different forms of Cushing’s. Although some of these tests are somewhat expensive, they are necessary.
The method of treatment depends on the form of the disease.
Iatrogenic Cushing's disease: Treatment of this form requires a discontinuation of the cortisone that is being given. This must be done in a very controlled manner so that other consequences do not occur. Unfortunately, it usually results in a recurrence of the disease that was being treated by the cortisone. Because there may have been adverse effects on the adrenal glands, treatment is also needed to correct that problem.
Adrenal Tumor: Treatment of an adrenal tumor requires major surgery. If the surgery is successful and the tumor is not malignant, there is a good chance that the dog will return to good health. If surgery is not an option, some of these tumors can be managed with the medication discussed next.
Pituitary Tumor: Treatment of the pituitary-induced form of Cushing's disease is the most complicated. There are three medications that are commonly used to treat Cushing’s disease.
The most common medication used is trilostane (Vetoryl®). Trilostane reduces the synthesis of cortisol. The dose of trilostane varies between dogs and can increase over time. Routine testing using the ACTH stimulation test is necessary to ensure the correct dose is being administered.
Mitotane (Lysodren® or o,p'-DDD) is an adrenal cytotoxic agent. The drug is used to destroy the abnormal adrenal tissue. If not enough drug is used, the abnormal tissue persists and the disease continues. If too much is used, most or all of the adrenal cortex will be destroyed, which can be life-threatening. Therefore, careful monitoring and testing of the dog is necessary in order to achieve good results. As with trilostane treatment is lifelong and dosage increases are common over time because the pituitary is not being affected by the treatment. The pituitary tumor continues to grow and to stimulate the adrenal gland.
Selegiline (Anipryl®) can be used in early cases of hyperadrenocorticism. It only works in a small percentage of dogs to decrease the signs of hunger and thirst and is not effective in cases with more serious complications.
Except in the case of surgical removal of a benign adrenal tumor, treatment of Cushing’s disease is aimed at decreasing clinical signs and complications of the disease. With treatment, many dogs can be successfully managed for years. Unfortunately, the disease will progress eventually causing poor quality of life.
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