PMON98 High-dose dexamethasone suppression test in ACTH-dependent Cushing's syndrome: Lessons learned on the lack of cortisol suppression

BACKGROUND: The overnight 8-mg high-dose dexamethasone suppression test (HDDST) has been used to discern Cushing's disease from ectopic ACTH production. However, because of its reported low diagnostic accuracy, its use is limited to the assessment of ACTH-dependent Cushing's disease when combined with a corticotropin releasing hormone (CRH) or a desmopressin stimulation test. We evaluated a patient with Cushing's disease in whom the HDDST failed to identify the pituitary source. We point to limitations in test interpretation and highlight the need of future studies to enhance understanding of the clinical utility of this non-invasive test. CLINICAL CASE: A 42-year-old male presented with lower extremity cellulitis and hypertensive emergency. He reported increased abdominal girth, easy bruising, severe back pain and proximal muscle weakness and noted headache and blurred vision. Exam was pertinent for obesity (body mass index 31.5 kg/m(2)), moon facies, dorso-cervical hump and prominent abdominal striae. Lumbar and thoracic compression fractures were noted on imaging. The elevated 24-hour urinary free cortisol level (198 ug/dL; reference: 10-100 ug/24hr) and lack of suppression on the overnight 1-mg dexamethasone suppression test (morning cortisol 23.7 ug/dL) confirmed Cushing's syndrome; a high ACTH level of 92 pg/dL defined it as ACTH-dependent. The 8-mg HDDST resulted in only a 9.7% reduction of his baseline morning cortisol. The patient was not on any medications that enhanced dexamethasone clearance. Serum dexamethasone levels were measured to aid in test interpretation and, although a level was provided, reference values for the single overnight 8-mg dose of dexamethasone were lacking. A CRH stimulation test was performed and resulted in a 60% increase in ACTH levels from baseline, supporting the diagnosis of Cushing's disease. Additionally, a pituitary MRI revealed a suprasellar 2.5×1.5×1.3 cm mass. Due to ambiguity posed by the HDDST result, despite tumor size, neurosurgery requested inferior petrosal sinus sampling (IPSS). Prior to this procedure, the patient underwent an overnight 16-mg HDDST. While limited information remains on the diagnostic performance of this test, we observed that the baseline morning cortisol decreased by 53.9%. IPSS confirmed the diagnosis of Cushing's disease and pathology identified a corticotroph adenoma. CONCLUSION: The diagnosis of Cushing's disease is complex and recommendations continue to evolve. Th role of the HDDST in combination with another dynamic test remains clinically relevant. It is important that when considering a HDDST, clinicians review medication history to exclude agents that may interfere with dexamethasone metabolism. Our case highlights the need of studies that determine an appropriate reference range for dexamethasone plasma levels after a single 8-mg overnight dose and that evaluate the diagnostic performance of the HDDST in this context. It also brings attention to the need of further elucidating the diagnostic utility of a higher dexamethasone dose in the evaluation of ACTH-dependent Cushing's syndrome. Presentation: Monday, June 13, 2022 12:30 p.m. - 2:30 p.m.