A 52-year-old man with confirmed Cushing syndrome (elevated 24-hour UFC, abnormal LDDST) has ACTH level of 45 pg/mL (normal 10–50 pg/mL, upper limit). HDDST shows cortisol suppression from 32 µg/dL to 8 µg/dL (>50% suppression). Pituitary MRI is normal. What is the most appropriate next step in management?

Explanation

## Clinical Context The patient has: - Confirmed Cushing syndrome (elevated UFC, abnormal LDDST) - ACTH-dependent Cushing (ACTH 45 pg/mL) - **Positive HDDST** (>50% suppression: 32 → 8 µg/dL) - **Normal pituitary MRI** This constellation indicates **pituitary-dependent Cushing syndrome (Cushing disease)** with a non-visualized microadenoma. ## Diagnostic Approach to ACTH-Dependent Cushing ```mermaid flowchart TD A[ACTH-dependent Cushing confirmed]:::outcome --> B[Perform HDDST]:::action B --> C{Cortisol suppresses >50%?}:::decision C -->|Yes| D[Pituitary source likely]:::outcome C -->|No| E[Ectopic ACTH source likely]:::outcome D --> F{Pituitary MRI shows adenoma?}:::decision F -->|Yes| G[Proceed to pituitary surgery]:::action F -->|No| H[IPSS with CRH stimulation]:::action E --> I[CT chest/abdomen for ectopic tumor]:::action H --> J{IPSS gradient >3:1 peripheral?}:::decision J -->|Yes| K[Pituitary adenoma confirmed]:::outcome J -->|No| L[Ectopic ACTH source confirmed]:::outcome ``` **Key Point:** When HDDST is positive (suppression >50%) but pituitary MRI is normal, **IPSS with CRH stimulation** is the gold standard to confirm pituitary source and localize the side of the adenoma. IPSS has >95% sensitivity and specificity for pituitary Cushing disease [cite:Endocrine Society Guidelines]. ## IPSS Interpretation | Finding | Interpretation | |---------|----------------| | Central-to-peripheral ACTH ratio >3:1 (basal) or >2:1 (post-CRH) | Pituitary source confirmed | | Ratio <3:1 and <2:1 | Ectopic ACTH source | | Unilateral gradient (one side >2× other) | Adenoma side localized | **High-Yield:** IPSS is indicated when: 1. HDDST positive (pituitary source likely) BUT 2. Pituitary MRI normal (microadenoma not visualized) IPSS guides surgical approach (selective adenomectomy vs. hemihypophysectomy). **Clinical Pearl:** Microadenomas (<10 mm) are present in ~50% of Cushing disease cases but are not visible on standard MRI. IPSS is the bridge between biochemistry and surgery, allowing safe localization without additional imaging.