Cushing's Syndrome Testing
Cushing's syndrome testing may include a petrosal sinus sampling. This test is not always required, but in many cases, it is the best way to separate pituitary from ectopic causes of Cushing's syndrome.
Samples of blood are drawn from the petrosal sinuses -- veins that drain the pituitary -- by introducing catheters through a vein in the upper thigh or groin region, with local anesthesia and mild sedation. X-rays are used to confirm the correct position of the catheters. Often, CRH -- the hormone that causes the pituitary to secrete ACTH -- is given during this test to improve diagnostic accuracy.
Levels of ACTH in the petrosal sinuses are measured and compared with ACTH levels in a forearm vein. ACTH levels higher in the petrosal sinuses than in the forearm vein indicate the presence of a pituitary adenoma; similar levels suggest ectopic ACTH syndrome.
Imaging tests reveal the size and shape of the pituitary and adrenal glands, and help determine if a tumor is present. The most common are the CT (computerized tomography) scan and MRI (magnetic resonance imaging) scan. A CT scan produces a series of x-ray pictures, giving a cross-sectional image of a body part. MRI also produces images of the internal organs of the body but without exposing the patient to ionizing radiation.
Imaging tests are used to find a tumor after a Cushing's syndrome diagnosis has been established. Imaging is not used to make the diagnosis of Cushing's syndrome, because benign tumors, sometimes called "incidentalomas," are commonly found in the pituitary and adrenal glands.
These tumors do not produce hormones detrimental to health and are not removed unless blood tests show they are a cause of symptoms or they are unusually large. Conversely, pituitary tumors are not detected by imaging in almost 50 percent of patients who ultimately require pituitary surgery for Cushing's syndrome.