Cushing's disease (hyperadrenocorticism) is the overproduction of the hormone cortisol by the adrenal glands that are located in the belly near the kidneys. Cushing's disease occurs commonly in dogs, but is rare in cats. Most dogs with Cushing’s disease are about 6 years old or older but sometimes occurs in younger dogs. Cortisol affects the function of many organs in the body, so the signs of Cushing’s disease may be varied. Some of the more common signs include hair loss, pot-bellied appearance, increased appetite, and increased drinking and urination called polydipsia and polyuria (PU/PD). Hair loss caused by Cushing’s disease occurs primarily on the body, sparing the head and legs. The skin is not usually itchy as it is with other skin diseases. If you pick up a fold of skin you may notice that the skin is thinner than normal. The pet may have fragile blood vessels and may bruise easily.
Less common signs of Cushing’s disease are weakness, panting, and an abnormal way of walking (stiff or standing or walking with the paws knuckled over). Some dogs develop a blood clot to the lungs and show a rapid onset of difficulty breathing.
Dogs that are given prednisone or similar drugs can develop signs that look like Cushing’s disease (called iatrogenic Cushing’s).
There are two types of Cushing’s disease that are treated differently. The most common form is caused by the overproduction of a hormone by the pituitary gland in the brain that in turn controls the amount of cortisol produced by the adrenal glands. This is called pituitary-dependent Cushing’s. A small percentage of dogs have a tumor of one of the adrenal glands which is called adrenal-dependent Cushing’s.
The large amount of cortisol in the body suppresses the immune system and allows the dog to get bacterial infections. The most common location for infection is the bladder. Dogs with Cushing’s disease may have a silent bladder infection meaning they don’t show signs of having the infection such as straining to urinate. A culture of the urine may be necessary to diagnose the infection.
X-rays of the belly often show a large liver. Occassionally the x-ray will show calcium in the area of one of the adrenal glands that is suggestive of an adrenal tumour. Ultrasound of the belly may show enlargement of both adrenal clands in pets with pituitary-dependant Cushing's or enlargement of just one of the adrenal clands in pets with an adrenal tumour
There is no single test to diagnose Cushing’s disease. Specific tests are performed to confirm the diagnosis and to determine the type of Cushing’s disease that is present, pituitary-dependent, or adrenal-dependent. Specific tests for Cushing’s disease have varied results. In some cases the results are clear cut and the diagnosis is made, but in other cases the test results are not clear cut and a series of tests must be performed. Some of the specific tests for Cushing’s disease include urine cortisol/creatinine ratio, low dose dexamethasone suppression test, high dose dexamethasone suppression test, and an ACTH stimulation test.
The treatment of the most common type of Cushing's disease (Pituitary-dependant) is lifelong oral medication. The initial treatment can have serious side effects, so dogs being treated for Cushing’s disease must be closely watched.
However the prognosis for pituitary-dependant Cushing's disease with treatment is usually good. Some signs disappear quickly and others gradually. Appetite and water consumption usually return to normal in a few weeks where as full return of the coat may take several months.