Cushing’s disease, or hyperadrenocorticism, is a common endocrine (glandular) disease in dogs. It is caused by the excess production, by the adrenal glands, of a hormone called cortisol. It can develop naturally or be caused by the administration of corticosteroids such as prednisone (iatrogenic Cushing's disease). More than 80 to 90 percent of dogs with the naturally acquired form of this disease –"pituitary dependent" Cushing's disease — has a tumor in the pituitary gland, which is located at the base of the brain. This tumor stimulates the adrenal gland to produce cortisol. The remaining cases of naturally occurring Cushing’s disease — adrenal tumor hyperadrenocorticism — result from a tumor of the adrenal gland that causes the production of too much cortisol. Cushing’s disease usually has a negative effect on quality of life in affected dogs, but it is very rarely life threatening.
Cushing’s disease is a syndrome comprised of a group of clinical symptoms. The most common symptoms of Cushing’s disease include excessive drinking and urination, increased appetite, hair loss, muscle weakness, a "potbellied" appearance, panting, thin skin, and lethargy. Other symptoms include susceptibility to infections and slow wound healing. Virtually all dogs with Cushing's disease have at least one of these symptoms, but it would be uncommon to have all of them. Most owners reach a point where the water consumption and urination become noticeable and a matter of concern.
Whenever your pet is showing signs of a health issue your first step is to contact your primary care veterinarian. If it is indicated that your pet may suffer from Cushing's disease or another serious condition, a veterinary specialist is available at an ExpertVet certified hospital.
The most important initial screening tests for Cushing's disease are a review of the pet’s history and a physical examination. Cushing’s disease is comprised of a constellation of clinical signs referable to the disease. Testing patients with isolated blood work abnormalities (such as an elevation in serum alkaline phosphatase) without compatible clinical signs may lead to a false diagnosis of hyperadrenocorticism. Cushing's disease is not an acute disease (having a short and relatively severe course), but one with signs symptoms that progress slowly over time. Rarely are these patients critically or seriously ill.
Both laboratory testing and imaging studies (such as ultrasound) are helpful in making the diagnosis of Cushing’s disease and differentiate between pituitary dependent Cushing’s disease and adrenal tumor Cushing’s disease. A biochemical profile can provide additional information that Cushing's disease may be present. Urine tests may be performed to determine how concentrated the urine is or to screen for infection.
To confirm the presence of Cushing’s disease in a patient with compatible clinical signs and laboratory findings, a test known as a low-dose dexamethasone suppression test may be recommended. This test can help determine a dog's ability to respond to suppression of the adrenal glands. It is important to understand that false positive adrenal function tests are common when another disease having clinical signs that mimic Cushing’s disease is present. This test should not be done in an ill patient, as false positive results will likely be present. Abdominal radiographs, ultrasonography, and CT scans can help determine if an adrenal gland tumor is present.
Treatment and Prognosis
Pituitary gland tumors are not usually removed in veterinary medicine; so surgery is rare. The two main drugs for treatment are mitotane (Lysodren) and trilostane. Lysodren selectively destroys the area of the adrenal gland that produces cortisol. By administering it in proper amounts, it is possible to destroy just enough of the gland to keep the production of cortisol at normal levels. Trilostane also decreases the amount of cortisol produced by the adrenal glands but does so by inhibiting specific steps in the glands’ production of cortisol. When used under the correct circumstances and when dogs on these medications are monitored appropriately, both mitotane and trilostane can be very effective at treating and controlling the symptoms of Cushing’s disease.
You may notice a decrease in appetite in your pet during the beginning phase of treatment (the induction phase). If your dog's eating or drinking stops, contact your veterinarian immediately and stop the medication. If vomiting or diarrhea develops you should also contact your veterinarian and stop the medication. After this induction phase of treatment, you will be asked to bring your dog to the hospital for an ACTH stimulation test. This test stimulates the adrenal gland. If the mitotane has done its job, the adrenal gland will not overreact to this stimulation. The results of this test will help your veterinarian determine a starting maintenance dose of mitotane for your dog. This test can be done on an outpatient basis.
Trilostane does not require an induction phase. Small adjustments to the dose of trilostane, however, are often needed early in treatment. Over the life of your dog, other changes may become necessary, based on routine monitoring with blood tests and how well the clinical signs of Cushing's disease are being controlled. Trilostane may also need to be administered up to twice per day for the rest of your dog's life.
Whichever medication is used, it will need to be continued for the rest of your dog's life, though the dose may be periodically adjusted. Additional ACTH stimulation tests initially are performed as often as monthly, until control of excessive cortisol production is achieved. After that, testing should be performed at regular intervals, determined by the level of control achieved with your pet. Close observation and frequent veterinary follow-up examinations are strongly recommended for the long-term management of your pet’s condition.
Cushing's disease is an expensive condition to manage due to medication costs and the need for frequent blood monitoring. With good follow-up and periodic monitoring of adrenal function, though, the prognosis is good. In pets with inadequate follow-up and monitoring, disease symptoms often return, and severe illness or death may occur because of complications resulting from poorly regulated cortisol production.