What is Cushing's disease?
Cushing's disease (also known as Cushing’s syndrome) is a rare condition that can affect middle-aged or older cats, in which the adrenal glands overproduce certain hormones. The medical term for this disease is hyperadrenocorticism (HAC). Literally translated, “hyper” means over-active, “adreno” means adrenal gland, and “corticism” refers to the outer part (cortex) of the adrenal gland.
The adrenal glands are located near the kidneys and produce several vital substances that regulate a variety of body functions and are necessary to sustain life. The most widely known of these substances is a hormone called cortisol. Over-production or under-production of these substances, especially cortisol, may be life-threatening.
What causes this disease?
There are three types of Cushing’s disease, each of which has a different cause. Identifying the cause is important because each type is treated differently, and each has a different prognosis (expected outcome).
Pituitary gland tumor
The most common cause of Cushing's disease (80%-85% of cases) is a tumor of the pituitary gland, which is located at the base of the brain. The tumor is most often benign (harmless) but some are malignant (cancerous). The tumor causes the pituitary gland to overproduce a hormone (ACTH) that stimulates the adrenal glands to produce cortisol.
Pituitary gland tumors may be microscopic or large. Depending on the size of the tumor, clinical signs other than those of Cushing's disease may be present, since a large tumor might press on or interfere with nearby structures in the area. If the pituitary tumor grows, it will affect the brain, often resulting in neurological signs (e.g., blindness, seizures) and giving the pet a less favorable prognosis. Some patients with pituitary tumors will have other hormonal effects, including abnormal bone growth of the legs, chin, and other parts of the body (acromegaly).
Adrenal gland tumor
Cushing's disease may be the result of a benign or malignant tumor of the adrenal gland itself (adenoma or carcinoma, respectively). If the tumor is benign, surgical removal will cure the disease. If the tumor is malignant, surgery may help for some time, but the prognosis is much less favorable.
Excessive cortisol from prolonged use of steroids
The third type of the disease is called iatrogenic Cushing's disease. It is caused when there is prolonged administration of an oral or injectable steroid. Although steroids are usually given for a legitimate medical reason, in this case, their prolonged or excessive use has become harmful to the patient.
What are the clinical signs of Cushing’s disease?
Regardless of the type, the clinical signs of Cushing’s disease are essentially the same. In the early stages of the disease, cats may develop weight gain, a bloated-looking or pendulous abdomen, muscle loss over the spine, and an enlarged liver. A few cats will lose weight. Lethargy (drowsiness or lack of activity), fur loss, fragile skin, and poor haircoat are also common in pets with hyperadrenocorticism. As the disease progresses, cats show an increase in water consumption and urination.
What other concerns can warrant testing for Cushing’s disease?
Many cats with Cushing’s disease often have coexisting diabetes mellitus, believed to be caused by excess cortisol hormone, which causes insulin resistance. These cats often require a large amount of insulin to control their glucose (sugar) levels.
Some affected cats also have chronic kidney disease, pancreatitis, and/or high blood pressure (hypertension) that can lead to sudden blindness due to retinal detachment (hypertensive injury to the eye).
How is Cushing’s disease diagnosed?
Initial bloodwork, consisting of a complete blood count (CBC) and a biochemistry panel, often shows hyperglycemia (high blood sugar), high cholesterol, and a high blood urea nitrogen (BUN) with or without elevations in creatinine, and a mild increase in alkaline phosphatase (ALP). The urine may have excess protein or glucose present, and it may be less concentrated than normal (dilute).
Many tests are used to diagnose and confirm Cushing’s disease. Currently, there is no “best test.” A urine cortisol:creatinine ratio (UCCR) may be recommended to rule out Cushing’s, as a negative test is very unlikely in an affected cat. The two most common tests to detect Cushing’s disease are the ACTH stimulation test and the low-dose dexamethasone suppression test (LDDST).
Other tests that may be used to help determine the type of disease include endogenous ACTH levels (the level of the hormone ACTH in the body) or a high-dose dexamethasone suppression test (HDDST).
An abdominal ultrasound examination can be a valuable part of the diagnostic process for Cushing’s disease. Ultrasound lets your veterinarian see the adrenal glands and determine their size. It also can indicate the presence of a tumor and can help identify any tumor spread (metastasis), if malignant.
Your veterinarian may also recommend X-rays to identify other conditions that may affect your cat’s response to treatment or prognosis. Adrenal masses may sometimes be identified.
"Although some of these tests can be expensive, they are necessary to determine the best treatment and prognosis for your pet."
Although some of these tests can be expensive, they are necessary to determine the best treatment and prognosis for your pet. See handout "Cushing's Disease - Testing" for further information on testing for Cushing's disease.
What are the treatment options?
Treatment depends on which type of the disease is present and whether there are other conditions present.
Pituitary tumor
There are several options for treatment of a pituitary tumor, including removal of the pituitary gland (hypophysectomy), removal of both adrenal glands (bilateral adrenalectomy), and medical therapy.
- Surgery to remove the tumour in the brain has been proven successful; however, your cat will need treatment for the loss of other hormones produced in the pituitary gland. The tumor can recur.
- Bilateral adrenalectomy can also be considered; however, complications are frequent and long-term oral medication is often needed to replace adrenal gland production. Your ability to give your cat oral medication without stress is an important consideration prior to surgery.
- Two drugs may be used to treat Cushing’s in cats: trilostane (brand name Vetoryl®) and mitotane (brand name Lysodren®). Trilostane appears more effective than mitotane, but cats have varied responses. Ketoconazole, sometimes used for Cushing’s in dogs, is not effective in cats and may be harmful.
- Radiation therapy may be recommended if neurologic signs develop secondary to an enlarging pituitary tumor. This therapy may also improve other clinical signs of Cushing’s disease.
Adrenal tumor
Treatment of an adrenal tumor requires major abdominal surgery. Additional imaging beyond ultrasound, such as a CT scan, may be recommended prior to surgery to better assess the risk to your cat and screen for tumor spread (metastasis). If the entire tumor is removed, and it is not malignant, there is a good chance your cat will regain normal health, although adrenal support medication such as prednisolone may be needed initially.
Some cats with diabetes may require less insulin or go into remission within a few months of surgery. If surgery is not an option, some of these patients can be managed with medication, as discussed above. Medical treatment may be recommended prior to surgery to improve healing.
Iatrogenic Cushing’s disease
Treatment of this form requires discontinuation of the steroid being given. The steroid must be discontinued in a controlled, gradual way so that other complications do not occur. Unfortunately, it usually results in a recurrence of the disease that was being treated by the steroid. The steroid may have caused adverse effects on the adrenal glands, so treatment is often needed to help replace the hormones that the adrenal gland normally produces.
What do I need to know if my cat’s disease is being managed with medication?
Your veterinarian will outline a treatment plan for your cat’s specific condition. Be sure to follow their guidelines closely because these treatments often depend on consistent and regular administration of the medication. Lifelong treatment may be necessary.
Some cats can be successfully treated with few medication side effects. However, they must be carefully monitored using clinical signs, physical examinations, and blood testing. Follow-up examinations and blood tests are very important to be certain your pet is receiving the proper dosage; too much or too little of the drug can cause complications.
What is the prognosis?
Left untreated, Cushing’s disease will progress, and the prognosis is poor as cats suffer from severe infection or uncontrolled diabetes mellitus. Prognosis is improved with appropriate treatment; however, not all cats respond to medication and complications after adrenalectomy are high. Cats who have successful bilateral adrenalectomy appear to live the longest.
