Dexamethasone infusion testing in the diagnosis of Cushing's syndrome

Cushing's syndrome and its various aetiologies is a markedly difficult diagnosis to make given its subtle signs, sometime cyclical nature, and the lack of a single definitive diagnostic test. Although a great variety of diagnostic tests have been developed to assist in the diagnosis, even with the best clinical acumen, biochemistry and medical imaging the diagnosis can remain elusive. The long low and high dose oral dexamethasone suppression test is cumbersome, costly and often requiring an extended inpatient stay. The utility of the dexamethasone suppression test would be greatly enhanced if it could be performed as a short outpatient procedure. In this study we sought to confirm and refine the clinical utility of the high dose 4 mg intravenous dexamethasone suppression test as an alternative diagnostic test for Cushing's syndrome. There were a total of 31 subjects: 8 patients with proven pituitary Cushing's disease, 3 with primary adrenal tumors, 10 with pseudo-Cushing's syndrome and 10 healthy controls. All subjects with pseudo-Cushing's syndrome suppress serum cortisol at +5 and at +24 hours. In subjects with pituitary Cushing's disease, 7 out of 8 (88%) had serum cortisol suppressed at +5 hours but rebounded at +24 hours to at least 70% of the original serum level. Primary adrenal tumors showed a pattern of non-suppression throughout. The 4 mg intravenous dexamethasone suppression test is excellent in ruling out pseudo-Cushing's syndrome. This test is much simpler and more convenient than the oral dexamethasone suppression test in confirming clinical suspicion of pituitary Cushing's disease.