A 42-year-old woman is evaluated for progressive central weight gain, severe fatigue, easy bruising, and proximal muscle weakness over the past 8 months. Examination reveals facial plethora, prominent supraclavicular fat pads, and wide violaceous striae on the lower abdomen. Her blood pressure is 158/96 mm Hg. The physician suspects Cushing syndrome. Assuming the evaluation ultimately reveals an ACTH-dependent etiology but initial localization imaging is equivocal, arrange the following diagnostic steps in the most appropriate sequential order, from initial evaluation to definitive confirmation.
- 1Review of all pharmacological records for topical, inhaled, or intra-articular glucocorticoid exposure
- 2Biochemical confirmation of hypercortisolism using a 1-mg overnight dexamethasone suppression test or 24-hour urine free cortisol
- 3Measurement of early morning plasma adrenocorticotropic hormone (ACTH) concentration
- 4Magnetic resonance imaging (MRI) of the pituitary gland
- 5Inferior petrosal sinus sampling (IPSS) for central-to-peripheral ACTH gradient measurement
Answer
The correct sequential order for evaluating suspected Cushing syndrome is: 1) Review for exogenous glucocorticoids, 2) Biochemical confirmation of hypercortisolism, 3) Measurement of plasma ACTH, 4) MRI of the pituitary, and 5) Inferior petrosal sinus sampling (IPSS).
The evaluation of Cushing syndrome follows a strict chronological hierarchy to maximize diagnostic accuracy and avoid the pitfalls of incidentalomas. The most common cause of Cushingoid features is exogenous glucocorticoid use, which must be ruled out by a careful medication history before any biochemical testing. Once excluded, endogenous hypercortisolism must be definitively established using at least one first-line test (e.g., 24-hour urine free cortisol, 1-mg overnight dexamethasone suppression test, or late-night salivary cortisol). Only after hypercortisolism is confirmed should a plasma ACTH level be drawn to differentiate ACTH-dependent (pituitary or ectopic) from ACTH-independent (adrenal) causes. For ACTH-dependent disease, an MRI of the pituitary is the initial localizing step because Cushing disease (pituitary adenoma) is the most common etiology. However, because small pituitary incidentalomas are common and small corticotroph adenomas can be missed on MRI, an equivocal scan (e.g., a normal MRI or a lesion <6 mm) requires inferior petrosal sinus sampling (IPSS) to definitively differentiate a pituitary source from an occult ectopic ACTH-secreting tumor.
Step-by-Step Solution
Key Concept
The sequential diagnostic algorithm for Cushing syndrome progresses from excluding exogenous causes to confirming endogenous hypercortisolism, determining ACTH dependence, and finally localizing the source with imaging and invasive sampling.
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