Cushing's syndrome caused by an ectopic pituitary adenoma

A 49-year old woman with a 5-year history of Cushing's syndrome was evaluated. Biochemical measurements revealed high cortisol and adrenocorticotropic hormone (ACTH) levels consistent with the ACTH-dependent type of Cushing's syndrome. However, the source of ACTH seemed to be autonomous as she demonstrated abnormal feedback control, with lack of response to metyrapone and high coses of dexamethasone. A search for an ectopic ACTH-secreting nonpituitary neoplasm was unsuccessful. Transsphenoidal pituitary exploration revealed a normal pituitary gland, but an ectopic pituitary adenoma was found incidentally in the mucosa of the sphenoid sinus. This adenoma stained strongly positive for ACTH on immunocytochemical studies. Resection of this lesion led to remission of the Cushing's syndrome. This variant of ACTH-dependent Cushing's syndrome can mimic the ectopic ACTH-dependent type and should be looked for in patients who fail to respond to pituitary operation.