Diagnosis
Cushing's disease can present with a variety of symptoms and may also be involved with several different disease processes. Therefore, it is recommended that any dog suspected of having Cushing's disease should have a complete blood count (CBC), blood chemistry panel, and urinalysis performed as a routine part of the evaluation. Common abnormalities in these tests include increases in alkaline phosphatase, and ALT (liver enzymes), increased cholesterol, decreased BUN (a kidney function test), and dilute urine (low specific gravity).
There are several different tests that can be performed to get a definitive diagnosis of Cushing's disease. Many times the veterinarian may perform more than one test to help confirm the diagnosis or to determine which form of the disease is present. A diagnosis of Cushing's disease, however, should never be made on the basis of laboratory tests alone. The dog needs to be showing symptoms of the disease, and have a medical history consistent with the diagnosis.
The three most common "screening" tests are the urine cortisol:creatinine ratio, the low dose dexamethasone suppression test, and ultrasound.
Causes
Latrogenic Hypercorticism is caused by administration of drugs that suppress the body's own cortisol production. Clinical signs and complications are similar to the naturally occuring disease. The dog can become dependent upon the drugs with chronic use, and abrupt withdrawal can lead to signs related to lack of cortisol, such as lethargy, depression, vomiting, and diarrhea. For these reasons, chronic steroid use is not recommended except in cases of life-threatening immune-mediated disease or chemotherapy protocols.
Pituitary dependent Cushing's disease is caused by a functional pituitary tumor that stimulates the adrenal glands to produce excess cortisol. The tumor is usually microscopic and benign. Sometimes the tumor can be large enough to cause pressure on the nerves to the eyes and the part of the brain called the hypothalamus, causing behavior changes, lack of appetite, and blindness, in addition to the other signs of Hypercorticism. Pituitary-dependent Cushing's disease makes up about 85% of spontaneous Cushing's disease cases.
Adrenal tumors can be benign, known as adenomas, or malignant, known as carcinomas. They produce cortisol independent of the normal regulatory mechanisms of the body. This is the cause of approx. 15% of occurrences of Cushings.
Treatment
Treatment depends on the type of Cushing's disease, as well as on the overall health of the canine patient. As many dogs with Cushing's are elderly and may have concurrent health problems, treatment can be complicated. The comfort of the patient should be the ultimate goal. In a dog with severe arthritis, for example, it may be more humane to allow him to remain Cushingoid than to treat the disorder. In general, surgery may be indicated for adrenal tumors. Chemotherapy in the form of Lysodren or Ketaconazole may be used to treat pituitary-dependent or adrenal-based Cushing's. And Anipryl may be tried to combat pituitary-dependent Cushing's. Treatment is best viewed as a means to improve quality of life, rather than increase lifespan.
If Cushing's disease is caused by an adrenal tumor, the logical approach is to surgically remove the tumor and the affected adrenal gland. These tumors tend not to recur on the remaining adrenal gland. In theory, this can cure adrenal-based Cushing's disease, and prognosis is very good for dogs with benign adrenal tumors. Dogs may be treated with ketoconazole prior to surgery to try to minimize the symptoms of Cushing's disease, as one significant symptom of Cushing's is delayed wound healing. There are high risks associated with adrenalectomies, and given that patients are often elderly dogs, this may deter an owner from pursuing this treatment route. 50% of adrenal tumors are malignant and may have already metastasized to liver or lungs by the time they are discovered. Most owners opt for non-surgical treatment.
Pituitary tumors are not removed surgically in veterinary medicine. These tumors tend to be very small and slow-growing and cause little or no damage on their own, aside from overstimulating the adrenal glands. With these canine patients, the symptoms themselves are treated and not the root cause.
Pituitary macroadenomas may be treated with radiation in an attempt to shrink them and thus relieve the neurological symptoms caused by their presence and the pressure they place on brain tissue. Radiation involves thousands of dollars and repeated anesthesia, either of which may be difficult to justify with an elderly patient. Once the neurological symptoms abate, the dog would then be treated for hyperadrenocorticism itself.
Prognosis
Dogs usually respond very well to therapy and are able to lead normal lives. Relapses are quite common with pituitary-dependent Hypercorticism, and occur in about half of dogs with Cushings. This generally requires a change in the maintenance dose or re-induction therapy. Good owner observation and communication with the veterinarian are the most important factors that influence the success of treatment. Surgery will cure many benign adrenal tumors and small carcinomas. If there are no post-operative complications, there can be prolonged survival even with invasive tumors or with metastatic disease.
