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Difficulty: MediumEndocrinology and Metabolism

A 38-year-old woman is referred to the endocrinology clinic due to progressive weight gain, facial plethora, and easy bruising over the past eight months. Physical examination reveals prominent supraclavicular fat pads, violaceous abdominal striae, and proximal muscle weakness. Her blood pressure is 155/95155/95 mm Hg. Suspecting Cushing syndrome, the physician decides to initiate a comprehensive diagnostic evaluation. Arrange the following diagnostic and therapeutic steps in the most appropriate chronological sequence for this patient.

  1. 1Perform a 1-mg overnight dexamethasone suppression test or measure 24-hour urine free cortisol.
  2. 2Measure a baseline plasma adrenocorticotropic hormone (ACTH) concentration.
  3. 3Obtain a magnetic resonance imaging (MRI) scan of the pituitary gland.
  4. 4Perform bilateral inferior petrosal sinus sampling (BIPSS).
  5. 5Refer the patient for transsphenoidal surgical resection.

Answer

The correct sequence is: 1) Initial screening test to confirm hypercortisolism, 2) Plasma ACTH measurement to determine dependence, 3) Pituitary MRI for anatomic localization, 4) Bilateral inferior petrosal sinus sampling (BIPSS) to confirm central source if MRI is inconclusive, 5) Transsphenoidal surgery.
The evaluation of Cushing syndrome must follow a strict, sequential algorithm to prevent misdiagnosis due to incidental imaging findings or physiologic cortisol fluctuations. First, hypercortisolism is confirmed via an initial screening test (e.g., 24-hour urine free cortisol or a 1-mg overnight dexamethasone suppression test). Once confirmed, plasma ACTH is measured. A suppressed ACTH level indicates an ACTH-independent process (e.g., adrenal adenoma), whereas a normal or elevated ACTH level indicates an ACTH-dependent process. For ACTH-dependent hypercortisolism, a pituitary MRI is ordered to search for a corticotroph adenoma (Cushing disease). However, if the MRI is negative or reveals a lesion smaller than 6 mm, bilateral inferior petrosal sinus sampling (BIPSS) is required to definitively distinguish a pituitary source from an occult ectopic ACTH-secreting tumor (e.g., small cell lung cancer or bronchial carcinoid). After BIPSS confirms a central pituitary source, the patient is referred for transsphenoidal adenomectomy.

Step-by-Step Solution

1
Confirm endogenous hypercortisolism
Perform a 1-mg overnight dexamethasone suppression test or a 24-hour urine free cortisol test.
Before attempting to localize a lesion, the biochemical diagnosis of hypercortisolism must be definitively established to prevent false-positive anatomical diagnoses.
2
Determine ACTH dependence
Measure baseline plasma ACTH.
This critically differentiates primary adrenal causes (ACTH-independent, suppressed ACTH) from pituitary or ectopic causes (ACTH-dependent, normal or high ACTH).
3
Localize the source of ACTH-dependent hypercortisolism
Obtain a pituitary MRI.
Because pituitary adenomas (Cushing disease) are the most common cause of ACTH-dependent hypercortisolism, targeted imaging of the sella turcica is the preferred initial localization step.
4
Differentiate pituitary from ectopic ACTH production
Perform BIPSS.
Many ACTH-secreting pituitary microadenomas are too small to be seen on MRI, and non-functioning incidentalomas are common. BIPSS unequivocally confirms whether the excess ACTH is of pituitary origin.
5
Provide definitive treatment
Proceed with transsphenoidal adenomectomy.
Once the pituitary source is structurally and functionally confirmed, surgical resection is the primary curative therapy.

Key Concept

The diagnostic evaluation of Cushing syndrome strictly follows a stepwise algorithmic approach: first confirm hypercortisolism, second establish ACTH dependence, third localize the tumor anatomically with imaging, fourth utilize invasive sampling if imaging is inconclusive, and finally proceed with surgical management.
Estimated Time:1m 30s
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Topics

Endocrinology and Metabolism

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