[Cushing's syndrome: diagnostic exploration]

The diagnosis of Cushing's syndrome is one of the most perplexing and controversial problems in endocrinology. However, significant advances in the diagnosis procedures have been made in the past decade. The diagnostic studies involved in the evaluation of patients with suspected Cushing's syndrome fall into two categories: confirming the presence of true hypercortisolism and establishing the precise aetiology. Diagnosis of Cushing's syndrome: ambulatory screening relies on the overnight 1 mg dexamethasone test. Negative tests are confirmed by measuring cortisol in two 24-hour urine samples. If cortisol excretion is slightly above normal, a 48-hour low-dose dexamethasone suppression test or an intravenous infusion dexamethasone suppression test are required. Diagnosis of the aetiology of Cushing's syndrome: the first step is to establish if the hypercortisolism is ACTH-dependent or not. This step is solved by measuring plasma ACTH and cortisol in the late afternoon. Computed tomography scanning of the adrenal glands is required in ACTH-independent Cushing's syndrome. A unilateral tumour will be demonstrated in most of cases. If bilateral lesions are found, dynamic testing using cortisol releasing factor and/or metyrapone must be performed to confirm the ACTH-independency of the syndrome. In ACTH-dependent Cushing's syndrome, the major difficulty is to distinguish between a pituitary source and an ectopic source of ACTH secretion. Magnetic resonance imaging of the pituitary with gadolinium enhancement must be preferred to computed tomography scanning but its sensitivity is not better than 70-80% and false positives can occur. When no macroscopic pituitary lesion can be detected, bilateral inferior petrosal sinus sampling coupled to CRH injection for ACTH measurement will indicate the source of ACTH secretion. If this test indicates the patient has Cushing's disease, pituitary trans-sphenoidal surgery can be performed. If the test indicates the patient has ectopic ACTH-secretion, a cervico-thoraco-abdominal scanning is necessary to identify the tumour. In the case of occult tumour the hypercortisolism must be controlled by pharmacological agents and the imaging investigations must be repeated at appropriate intervals.