Cushing's syndrome, or hyperadrenocorticism, occurs when the adrenal glands release excessive levels of the hormone cortisol. The pituitary gland in the brain directs the adrenal glands to secrete the hormone, so problems with the pituitary gland or the adrenal glands can cause cortisol levels to increase. Cortisol, which is a type of natural steroid, affects many body processes. For example, it influences the body's reaction to stress and infection and has many other functions, making Cushing's syndrome a complex disorder.
Risk factors and detection
Cushing's syndrome is common in middle-aged to older dogs. Poodles, dachshunds, Boston terriers, boxers, and beagles show the highest incidence of disease.
Causes of Cushing's syndrome include:
Pituitary-dependent hyperadrenocorticism (PDH). About 85 percent of dogs with spontaneous Cushing's syndrome suffer from a pituitary abnormality that causes overstimulation of the adrenal glands. The abnormality may be hyperactivity of the pituitary gland's cells, neurotransmitter imbalances, or a tumor.
Hyperadrenocorticism caused by an adrenal tumor. About 15 percent of dogs with spontaneous Cushing's syndrome have an adrenal gland tumor that causes overproduction of cortisol.
Iatrogenic hyperadrenocorticism. "Iatrogenic" means resulting from the physician's actions. This condition results from excessive or long-term use of corticosteroid products. Dogs treated for allergies or immune-system disorders often receive high doses of steroids or are on these drugs for long periods.
Symptoms of Cushing's syndrome include:
- increased thirst
- increased urination
- increased appetite
- hair loss over the entire body
- an enlarged abdomen
- decreased activity
- weakness
- panting
- loss of muscle mass
- thin skin
- behavioral changes
- neurologic signs, such as seizures (in advanced cases of a pituitary
tumor).
Medical complications of Cushing's syndrome include blood clots, high blood pressure, congestive heart failure, kidney disease, increased susceptibility to infection, and diabetes mellitus. The longer the syndrome goes untreated, the more serious the complications.
Your veterinarian will perform a complete examination, including blood and urine tests, to detect Cushing's syndrome. He or she may recommend X-rays or ultrasound examination and perform multiple blood tests to distinguish among the different causes of the syndrome.
Prevention and treatment
Following your veterinarian's directions when administering steroids can help prevent iatrogenic Cushing's syndrome. And never give your dog any topical or oral steroid product without first consulting your veterinarian.
You can't prevent spontaneous Cushing's syndrome, but with early treatment, you may prevent potential complications. Treatment for spontaneous Cushing's syndrome includes medication to reduce blood cortisol levels (mitotane and ketoconazole) or to improve brain neurotransmitter balance (selegiline), or surgery to remove the adrenal glands, which are located near the kidneys. Your veterinarian will treat iatrogenic disease by slowly reducing your dog's steroid treatment.
Veterinarians have used mitotane for years to selectively destroy a portion of the adrenal glands to reduce cortisol production. Close monitoring of this lifelong treatment is essential, because severe side effects can occur.
Selegiline (l-deprenyl) controls the symptoms of Cushing's syndrome in dogs with PDH. Selegiline causes fewer side effects than mitotane, but improvement is slower and may be less significant.
Veterinarians often choose ketoconazole for dogs who can't tolerate mitotane. It has fewer side effects, but may not be effective in all dogs.
Surgery to remove the adrenal glands is the preferred treatment for an adrenal tumor in dogs. Because the adrenal glands secrete hormones, the dog will need lifelong hormone replacement therapy. Surgery to remove pituitary tumors is not practical because of their location in the brain. Radiation therapy is used in some dogs with moderate success.
Prognosis
Dogs successfully treated for an adrenal tumor and dogs treated for PDH live an average of two and a half years. However, the presence of other diseases, especially diabetes mellitus, can make treatment difficult. You can help by watching for treatment side effects or for symptoms to recur. Dogs with untreated Cushing's syndrome may deteriorate rapidly, depending on the stage of disease and any resulting complications.
