Cushing's Disease, Canine - contd.
A definitive diagnosis of Cushing's disease can be difficult to obtain. When animals present with the classic signs of the disease, the diagnosis is initially suspected on the basis of those signs and the results of routine laboratory tests. As with many illnesses, a complete blood count, biochemistry profile, and urinalysis are very important tools in the initial evaluation of a dog suspected of having Cushing's disease. Usually there will be changes on these tests that may indicate the possibility of Cushing's disease. Routine imaging procedures, such as abdominal x-rays, may show changes including generalized liver enlargement, and, less commonly, adrenal gland enlargement or mineralization. Abdominal ultrasound is also used frequently in the evaluation of Cushing's suspects to evaluate the liver size and texture, the size and shape of the adrenal glands, and to look for evidence of other abnormalities that might be contributing to the dog's signs.
A more definitive diagnosis depends on the results of tests that look more specifically at adrenal gland function. The urine cortisol-to-creatinine ratio is used in some cases, but it is not a very specific test because dogs with almost any illness can have an abnormal result. The two most commonly used screening tests are the ACTH stimulation test, and the low-dose dexamethasone suppression test. Yet, it can be very difficult to know for sure that an individual dog has Cushing's disease, since the results of the tests can be difficult to interpret.
Once a tentative diagnosis of Cushing's disease is established, an attempt to distinguish which form of the disease is present should be made. Pituitary dependent hyperadrenocorticism, or PDH, is the more common form, accounting for as many as 80 to 85 percent of patients with Cushing's disease. The remaining dogs have adrenal gland tumors, which can affect either one or both adrenal glands. In very rare circumstances, both PDH and an adrenal tumor can be present in the same dog. The tests most commonly used to distinguish the pituitary-dependent form from an adrenal tumor are the high-dose dexamethasone suppression test, the endogenous ACTH level, and abdominal ultrasound. CAT scans can also be used to evaluate adrenal gland structure, and MRI scans can be used to look for pathology in the pituitary gland.
