CANINE HYPERADRENOCORTICISM (CUSHING’S DISEASE)
Cushing’s syndrome is a disorder caused by excessive production of the hormone cortisol (cortisone) by the adrenal glands located in the abdomen in front of each kidney.
The adrenal glands normally produce cortisol according to the needs of the body. The pituitary gland located in the brain controls the amount of cortisol produced by the adrenal glands. It monitors the level of cortisol in the bloodstream and when it recognizes the levels as being too low it releases a hormone called ACTH (adrenocorticotrophic hormone), which tells the adrenal gland to produce and release more cortisol. This system is very analogous to the thermostat and heater in your home. The thermostat acts like the pituitary gland by monitoring the heat level in the air (analogous to the level of cortisol in the blood) and tells the heater (analogous to the adrenal gland) to produce more or less heat.
There are two types of Cushing’s disease based upon the cause of excessive production of cortisol. In about ninty percent of the cases, the underlying cause is excessive production of the ACTH hormone by a benign pituitary tumor in the brain. This results in excessive production of cortisol by the adrenal gland. This is analogous to having a broken thermostat in your home telling the heater to stay on all the time. This type is called Pituitary Dependent Cushing’s disease. In the remaining dogs, a tumor in one or both of the adrenal glands causes excessive production of the cortisol independent of the pituitary gland signals. These tumors may be either malignant or benign. This type of Cushing’s disease is called Adrenal Dependent Cushing’s disease.
Cushing’s disease usually occurs in middle aged to old dogs. It develops most frequently in Poodles, Dachshunds, Boston Terriers, and Boxers but can affect any breed. The most common clinical signs are excessive urination, excessive thirst, pendulous abdomen, hair loss, and weakness. Some cases however, may not show any of these systemic signs. Since this disease affects many systems, including the immune system and thyroid gland, many other signs are possible including recurrent skin infections.
Any dog with excessive thirst and urination should be suspected of having Cushing’s. Also, any dog with highly elevated Alkaline Phosphatase enzyme in the blood can have the disease as well. Cushing’s Disease diagnosis is based upon clinical signs plus results of a special set of blood tests, which analyze the blood cortisol levels after stimulation, and/or suppression of the adrenal gland with hormone injections. Once your pet has been confirmed to have Cushing’s disease, additional tests may be necessary to determine which type they have, as treatment and prognosis is different. These tests may include other hormone tests, abdominal radiographs to look for calcification of the adrenal gland, or abdominal ultrasound.
- ACTH Test: This is a one-hour, screening test where we give an injection of ACTH hormone and then collect a blood sample on hour later to measure the amount of cortisol in the blood. A dog with Cushing’s will have a blood cortisol level in excess of the normal.
- Low Dose Dexamethasone Suppression Test (LDDS): This is an eight hour test where your pet is given a low dose of cortisone (dexamethasone) and a sample of blood is drawn immediately, and again at 4, and 8 hours post injection. In dogs with Cushing’s the cortisol level will not suppress as in a normal dog. Occasionally, this test will be inconclusive and it may need to be repeated in one to two months.
- High Dose Dexamethasone Suppression Test (HDDS): This is also an eight-hour test like the LDDS except that we use a high dose of cortisone to see is the adrenal gland will suppress once a diagnosis of Cushing;s has been made. If the gland does not suppress, then it means that your pet has the pituitary dependent form of the disease.
- Endogenous ACTH: A single blood sample is collected between 8 and 9 AM. This form of testing may be less accurate that stimulation tests.
- Urine cortisol-creatinine ratio: A urine test that may be used to rule out Cushing’s disease in a patient. This test is generally used in addition to other blood testing.
Treatment and Prognosis
Pituitary Dependent Cushing’s disease can be controlled (not cured) with oral medication and periodic monitoring. The following drugs may be used to treat the illness:
Response to treatment is monitored closely using follow-up blood testing in order to avoid drug-related side effects and complications during the early part of the treatment. Once controlled, your pet will be relieved of all the signs present before treatment.
Adrenal tumors are treated with surgical removal of the tumor. Surgery is curative if the tumor is not malignant. Some of these tumors are malignant and these patients have a poor prognosis. If the tumor is malignant and non-removable, drug therapy can be tried but limited success has been obtained. Without treatment the disease is generally a slowly progressive disorder with an unfavorable prognosis.
Occasionally the small tumor in the pituitary gland will continue to grow and a rare case may cause neurological signs including blindness, seizures and dementia at the final stages of the disease. There is no way to predict when or if this will occur in your pet. Most dogs however, can lead a normal life with careful monitoring. Dogs with Pituitary Dependent Cushing’s disease usually can be successful managed for many years.