Question: What is a petrosal sinus sampling test, how is it done, and when is it used?

Answer: The majority of patients with Cushing's syndrome have an ACTH-secreting tumor (pituitary or nonpituitary) causing both adrenal glands to produce excessive amounts of cortisol. Although the majority of patients have pituitary tumors, approximately 10-15% of patients have nonpituitary tumors that can be located in many places - most commonly, the lung. Obviously, it is essential to accurately distinguish where the ACTH-secreting tumor is located so that appropriate surgery can be recommended. Pituitary imaging studies, particularly Magnetic Resonance Imaging (MRI) may be helpful in locating pituitary tumors; however, 50-60% of patients with ACTH-secreting pituitary tumors have a normal pituitary MRI. Petrosal sinus sampling for ACTH is the diagnostic test of choice to distinguish pituitary from nonpituitary ACTH-secreting tumors in patients without an obvious radiographic lesion. Petrosal sinuses are actually the veins that drain the pituitary gland. The procedure is done on an outpatient basis and should be performed by an experienced invasive radiologist. The radiologist inserts small catheters in each vein that drains the pituitary and ACTH is measured from blood samples from each vein obtained simultaneously and compared to ACTH measurements from a peripheral vein. When performed properly, this study yields 100% diagnostic accuracy and, in many cases, provides accurate localization of the small pituitary tumor within the gland. The procedure is not helpful inlocating nonpituitary tumors. Approximately 10-15% of patients with Cushing's syndrome do not have an ACTH-secreting tumor, but have a cortisol producing adrenal tumor. Petrosal sinus sampling is not necessary for the correct diagnosis of these patients.