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Difficulty: HardDiagnostic Workup and Test Sequencing

An endocrinology consultation is requested for a 38-year-old woman presenting with progressive central weight gain, severe fatigue, and easy bruising over the past year. Her blood pressure is 162/98 mmHg. Physical examination reveals significant central adiposity, violaceous striae on her abdomen, and pronounced weakness when rising from a chair. Initial investigations confirm hypercortisolism, demonstrating an elevated 24-hour urine free cortisol level and a failure to suppress serum cortisol during a low-dose dexamethasone suppression test. Subsequent biochemical evaluation reveals a plasma adrenocorticotropic hormone (ACTH) concentration of 95 pg/mL (normal: 10 to 60 pg/mL). To identify the source of her condition, a dedicated gadolinium-enhanced magnetic resonance imaging (MRI) scan of the pituitary gland is performed, which shows a symmetrically normal-appearing gland with no evidence of a focal adenoma. Which of the following is the most appropriate next step in the diagnostic workup for this patient?

  1. A
    Computed tomography of the chest, abdomen, and pelvis
  2. B
    High-dose dexamethasone suppression testing
  3. C
    Somatostatin receptor scintigraphy (Octreoscan)
  4. D
    Empiric transsphenoidal pituitary exploration
  5. Bilateral inferior petrosal sinus samplingAnswer

Answer

Bilateral inferior petrosal sinus sampling
This patient presents with ACTH-dependent Cushing syndrome, indicated by clinical manifestations of hypercortisolism alongside an elevated plasma ACTH level. The differential diagnosis for ACTH-dependent hypercortisolism primarily involves a pituitary adenoma (Cushing disease) or an ectopic ACTH-secreting tumor (such as a bronchial carcinoid or small cell lung cancer). When the pituitary MRI is completely normal or shows an equivocal lesion smaller than 6 mm, the anatomic source of the excess ACTH remains ambiguous. Bilateral inferior petrosal sinus sampling (IPSS), usually performed with corticotropin-releasing hormone (CRH) stimulation, is the most accurate and appropriate next diagnostic step. A significant central-to-peripheral ACTH gradient confirms Cushing disease, whereas the absence of a gradient indicates an ectopic source, accurately guiding subsequent imaging or surgical management.

Step-by-Step Solution

1
Evaluate the initial diagnostic laboratory findings.
The patient has confirmed hypercortisolism based on the elevated 24-hour urine free cortisol and abnormal low-dose dexamethasone suppression test.
Establishing the presence of hypercortisolism is the mandatory first step before determining its etiology.
2
Determine the ACTH dependence of the hypercortisolism.
The elevated plasma ACTH level (95 pg/mL) indicates ACTH-dependent Cushing syndrome.
An elevated or inappropriately normal ACTH points to either a pituitary adenoma (Cushing disease) or an ectopic ACTH-secreting tumor, ruling out primary adrenal disease.
3
Interpret the imaging findings in the context of the biochemical results.
The pituitary MRI is normal, meaning the source of ACTH secretion remains radiologically occult.
Pituitary microadenomas are often too small to be detected on MRI. A negative MRI does not rule out Cushing disease, nor does it confirm an ectopic source.
4
Select the appropriate definitive localizing test.
Bilateral inferior petrosal sinus sampling (IPSS) is chosen as the next step.
IPSS directly measures ACTH concentrations draining from the pituitary gland compared to the periphery, definitively distinguishing a central (pituitary) source from a peripheral (ectopic) source.

Key Concept

In patients with ACTH-dependent Cushing syndrome and a normal or equivocal pituitary MRI, bilateral inferior petrosal sinus sampling (IPSS) is the gold standard diagnostic step to differentiate Cushing disease from ectopic ACTH syndrome.
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Diagnostic Workup and Test Sequencing

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