Cushing’s disease most commonly affects dogs but can affect cats as well (up to 80% of cats with Cushing’s disease will also develop diabetes, making control very difficult.)
The pituitary gland in the brain is responsible for telling the adrenal glands to make and store cortisol in the body. There are two forms of Cushing’s disease: pituitary dependent and adrenal dependent. It is important to find out which one your pet has to allow for the best care and outcome.
Pituitary Dependent Cushing’s Disease accounts for about 85% of the Cushing’s cases and is caused by a small tumor that is generally benign but causes the over production of a hormone (ACTH) which is responsible for telling the adrenal glands to make cortisol, leading to enlargement of both adrenal glands. Rarely, the tumor can grow large enough to cause neurological disease.
Adrenal Dependent Cushing’s Disease accounts for the additional 15% of cases and is caused by a tumor on one of the adrenal glands, resulting in 1 large and 1 small adrenal gland. About 50% of the adrenal tumors are malignant. Radiographs (x-rays) and ultrasound can be helpful in determining if adrenal dependent disease exists as the adrenals can often be visualized using these modalities.
If Cushing’s Disease is suspected, full blood work (complete blood count, biochemistry, urine and thyroid) will be performed to rule out other causes of concern such as liver disease, kidney disease, bladder disease, or diabetes. There are lab work patterns that can lead to a suspicion of Cushing’s Disease, at which point additional testing can be performed.
A urine cortisol/creatinine ratio can be performed on the first urine sample of the day. If this test is positive Cushing’s disease may be present. If this test is negative then Cushing’s is ruled out.
A Low-Dose Dexamethasone Suppression Test is the most accurate test to confirm Cushing’s disease. About 90% of dogs with Cushing’s disease will test positive. With this test, a blood sample is taken for a baseline cortisol level and then an injection of dexamethasone is given in the vein. Additional samples are taken at 4 and 8 hours after injection. This test is also used to help determine if your animal has pituitary dependent or adrenal dependent disease, which is important for treatment.
An ACTH stimulation test can be used for diagnosis, but is less accurate than the low dose dexamethasone suppression test. It is more commonly used to monitor for appropriate treatment of Cushing’s disease. With this test, a blood sample is taken and then synthetic ACTH hormone is given, and another blood sample is taken 1 hours later. This test is performed every time it is necessary to change the dose of your pet’s medication.
Once a pet has been confirmed as having Cushing’s Disease, the next step is to determine which form of Cushing’s Disease the pet has since treatment is different for each form. If an adrenal tumor is present, there is a 50% chance that it is a malignant tumor. Surgical exploration is generally warranted so that the tumor may be removed. Adrenal tumors can be treated with medications just as pituitary tumors can be but the protocols are completely different so it is imperative that classification be complete.
Trilostane is the most commonly used medication for medical treatment of Cushing’s disease. This is given 1 or 2 times a day and can have side effects such as lethargy and decreased appetite, especially when first starting the medication. The dose is modified according the results of periodic ACTH stimulation tests (at 10-14 days, 30 days, 90 days, and then every 6 months). You should call your veterinarian if any of the following medication side effects are observed:
- Diarrhea or vomiting
- Appetite loss (this may be as subtle as less enthusiasm toward eating when the food is served, not running for the bowl etc.)
- Decrease in water consumption (it may be helpful for you to measure water consumption during the induction period)
If any of these signs occur, let your veterinarian know. It may be time for an early ACTH stimulation test or possibly even for an antidote pill (prednisone).
Addison’s disease, also called hypoadrenocorticism, is the opposite of Cushing’s disease; Addison's disease results from a deficiency of cortisol. If the Trilostane dose is too high it can cause an Addisonian reaction; This is usually temporary but can be permanent. The symptoms mentioned above (vomiting, diarrhea, listlessness, appetite loss) are serious. If one suspects an Addisonian reaction is occurring, a dose of prednisone - which has hopefully been provided to keep on hand in case of emergency - should reverse the reaction within 30 minutes, and within a couple of hours at most. If no response to prednisone is seen, the dog has some other illness. If the dog is back to normal after the prednisone dose, then the veterinarian should be contacted for further instructions. The prednisone will likely have to be continued for a couple of weeks.