Cushing’s disease (CD) is also known as hyperadrenocorticism. Cortisol is the stress hormone associated with the fight or flight response, and is controlled by the adrenal cortex, which is located on the outer layer of the adrenal glands. These glands produce hormones to help the body perform essential functions.
The pituitary gland (tiny gland found in the brain) releases adrenocorticotropic hormone (ACTH), which tells the adrenal glands to release cortisol. In a pet with CD, a tumor in the pituitary gland or adrenal gland causes the cortex to release an excess amount of cortisol.
The first signs of Cushing’s disease an owner may notice is extreme thirst, and as a result frequent urination. As the disease progresses, one may notice their dog losing muscle and becoming weak. One may also notice thinning of the skin, lesions on the skin, or hair loss on the flanks, neck, and perineum. Obesity and lack of energy are also symptoms.
It takes at least a year for these symptoms to develop, and due to them often being mistaken for aging, many dogs have the advanced form of CD before the owner recognizes a problem is present.
The most common cause of CD is a tumor in the pituitary gland. A lesser common form is a tumor present in the adrenal glands.
CD may also be caused by administering excessive amounts of prednisone or dexamethasone. Chronic application of steroid-containing ear drops can also cause this condition, as the medication is absorbed through the skin. Affected dogs present with symptoms identical to those of the tumor-based disease. However, this form resolves once the steroids are stopped.
There are certain breeds that are at a higher risk of developing CD. These include poodles, dachshunds, Boston terriers, boxers, and beagles. Almost all patients are over eight years old when CD develops.
Diagnosing Cushing’s disease is complex and expensive. This condition is often under-diagnosed due to the diagnostic testing required, expense of treatment, ongoing treatment, and the requirement of consistent monitoring.
When a vet suspects CD, blood and urine tests are needed to confirm the diagnosis. Dilute urine and an elevation of the liver enzyme alkaline phosphatase in the blood indicate testing for CD may be necessary.
CD is verified with an adrenocorticotropic hormone (ACTH) stimulation test. For this test, a blood sample is drawn from the patient, then the pet is given an injection of ACTH, and a second blood sample is taken an hour later. If the dog’s cortisol level increases, his adrenal response is normal. If it starts high and climbs even higher, a diagnosis of CD is confirmed.
The vet may also use a second method, the dexamethasone suppression test. In this test, the patient receives an injection of the steroid dexamethasone. In a healthy dog, the cortisol level goes down over the following few hours because the steroid suppresses adrenal production. If the cortisol level fails to drop, it suggests there is a tumor not responding to the medication.
Ultrasound examinations will allow the vet to determine if a tumor is present, and if it is in the pituitary gland or the adrenal glands.
The location of the tumor determines which treatment should be used. More than 90% of dogs with CD have a pituitary gland tumor. This type of tumor is only millimeters in size and is often benign. However, it is problematic as it manufactures and releases uncontrolled quantities of ACTH. Though pituitary tumors are routinely removed in people, this surgical technique is still in its infancy in dogs.
Most dogs with pituitary-based CD are treated with medication. Mitotane (Lysodren) and Trilostane (Vetoryl) are oral medications that selectively destroy part of the adrenal cortex so that if the pituitary gland tumor continues to release ACTH, cortisol levels remain normal. Careful monitoring is required to ensure the drugs do not destroy all the cortex and that the cortisol stays at a defined level.
Adrenal-based CD (rare form of the disease) is more concerning and best treated with surgery. About half of these tumors are malignant, growing aggressively and metastasizing quickly. Surgical excision of a malignant adrenal tumor is unlikely to be curative. If it is a benign growth, surgery may be curative, but the surgery is a complicated procedure.
Once treatment has been initiated, the symptoms of CD should begin to dissipate. This is usually seen as a decrease in drinking within a week. It takes a bit longer (several months) for the skin lesions to resolve.
The average survival time for a dog with CD is about two years, with only 10% of patients living beyond the four-year mark. However, this statistic does not mean this disease causes death. Since CD is most often diagnosed in geriatric dogs, most die of unrelated causes brought on by aging.
American Kennel Club – https://www.akc.org/expert-advice/health/cushings-disease-in-dogs/