A 45-year-old woman is being evaluated for progressive weight gain, proximal muscle weakness, and easy bruising. Physical examination reveals central adiposity, wide purple abdominal striae, facial plethora, and a blood pressure of 160/100 mmHg. The physician suspects Cushing syndrome. Assuming each step dictates the need for the subsequent one to identify an occult etiology, arrange the following diagnostic procedures in the correct sequential order, from initial confirmation to final localization.
- 1Establish the presence of endogenous hypercortisolism with a screening test (e.g., overnight low-dose dexamethasone suppression test).
- 2Determine the dependency of hypercortisolism by measuring a morning plasma ACTH level.
- 3Localize the source of an ACTH-dependent condition by obtaining an MRI of the pituitary gland.
- 4Differentiate an occult pituitary adenoma from an ectopic ACTH source using bilateral inferior petrosal sinus sampling (IPSS).
Answer
The correct sequence for evaluating Cushing syndrome is: 1) Confirm hypercortisolism with a screening test, 2) Measure morning ACTH, 3) Perform a pituitary MRI, and 4) Perform inferior petrosal sinus sampling (IPSS).
The evaluation of Cushing syndrome follows a strict stepwise approach to prevent misdiagnosis due to incidentalomas or physiological variations. First, hypercortisolism must be confirmed with screening tests (e.g., 24-hour urine free cortisol, low-dose dexamethasone suppression). Second, plasma ACTH is measured to classify the disease as ACTH-dependent or independent. Third, for ACTH-dependent disease, a pituitary MRI is obtained since Cushing disease is the most common etiology. Finally, if the MRI is inconclusive (e.g., lesion <6 mm or no lesion), inferior petrosal sinus sampling (IPSS) is performed to definitively differentiate between a pituitary and an ectopic ACTH source.
Step-by-Step Solution
Key Concept
The diagnostic algorithm for Cushing syndrome progresses sequentially from confirming hypercortisolism to determining ACTH dependence, followed by non-invasive imaging, and finally invasive localization if imaging is equivocal.