In a 50‑year‑old man with Cushingoid appearance, proximal muscle weakness, sustained upward gaze for one minute, and elevated serum ACTH and cortisol, what is the most likely diagnosis and what diagnostic and treatment steps should be taken?

ACTH-Dependent Cushing's Disease (Pituitary Adenoma)

This patient has Cushing's disease (pituitary adenoma) based on the combination of Cushingoid features, proximal myopathy, and elevated ACTH with elevated cortisol—the next steps are to confirm hypercortisolism with 2-3 measurements of 24-hour urinary free cortisol, late-night salivary cortisol, or 1-mg dexamethasone suppression test, followed by pituitary MRI and bilateral inferior petrosal sinus sampling (BIPSS) to localize the source 1, 2, 3.

Diagnostic Reasoning

The elevated ACTH with elevated cortisol immediately narrows this to ACTH-dependent Cushing's syndrome (80-85% of endogenous cases), which includes either:

• Cushing's disease (pituitary adenoma) - most common (~70% of ACTH-dependent cases)
• Ectopic ACTH syndrome (from neuroendocrine tumors, small cell lung cancer, etc.)

The proximal muscle weakness is a highly discriminatory clinical feature for Cushing's syndrome and should never be dismissed 4, 5. The case report 4 demonstrates how this can be the sole presenting manifestation and was misdiagnosed as dermatomyositis—emphasizing that proximal myopathy in the context of other Cushingoid features demands endocrine evaluation.

The ability to maintain upward gaze for 60 seconds is clinically significant because it argues against a large pituitary macroadenoma causing mass effect and ophthalmoplegia. This suggests either a microadenoma or an ectopic source.

Diagnostic Algorithm

Step 1: Confirm Hypercortisolism

Even with high clinical suspicion, biochemical confirmation is mandatory 1, 2, 3:

• Perform 2-3 measurements of any first-line test to account for variability:
  • 24-hour urinary free cortisol (UFC) - accounts for 50% intra-patient variability
  • Late-night salivary cortisol (LNSC) - 2-3 samples
  • 1-mg overnight dexamethasone suppression test (DST)

Critical pitfall: Single abnormal values are insufficient. The guideline explicitly states multiple collections are needed 1. UFC values in pseudo-Cushing's are typically <3-fold normal elevation, whereas true Cushing's syndrome shows more marked elevation.

Step 2: Differentiate ACTH-Dependent from ACTH-Independent

This patient already has elevated ACTH, confirming ACTH-dependent disease 2, 3.

Step 3: Distinguish Pituitary from Ectopic ACTH Source

Pituitary MRI with gadolinium contrast is the initial imaging study:

• Identifies pituitary adenomas in 50-70% of Cushing's disease cases
• Microadenomas (<10mm) are most common
• Negative MRI does NOT exclude Cushing's disease

Bilateral inferior petrosal sinus sampling (BIPSS) is the gold standard for distinguishing pituitary from ectopic sources 2, 3:

• Central-to-peripheral ACTH gradient >2 (baseline) or >3 (after CRH stimulation) confirms pituitary source
• Required when MRI is negative or equivocal
• Should be performed at experienced centers

If ectopic ACTH is suspected (no pituitary gradient on BIPSS), pursue imaging for neuroendocrine tumors 6:

• Chest/abdomen/pelvis CT with contrast
• Octreotide scan or Ga-68 DOTATATE PET/CT
• Consider sinonasal imaging if epistaxis or nasal symptoms present (rare ONB cases) 6

Treatment Approach

First-Line: Surgical Resection

Trans-sphenoidal surgery for pituitary adenoma is first-line treatment 3, 7:

• Remission rates: 65-90% for microadenomas, 50-65% for macroadenomas
• The case report 4 demonstrates dramatic improvement in proximal weakness post-surgery—patient went from wheelchair-dependent to walking independently after two surgeries

Preoperative Medical Management

If hypercortisolism is severe (hypokalemia, psychosis, infections, uncontrolled diabetes/hypertension), initiate medical therapy before surgery 7, 8, 9:

Steroidogenesis inhibitors (block cortisol production):

• Ketoconazole 200-400mg twice daily, titrate to 600-1200mg/day
• Metyrapone 250-500mg 3-4 times daily
• Osilodrostat 2mg twice daily, titrate up (newest agent, potent)

Use titration approach: adjust dose to normalize cortisol levels with frequent monitoring (every 1-2 weeks initially) 8.

Alternative block-and-replace approach: Use higher steroidogenesis inhibitor doses to fully suppress cortisol, then add hydrocortisone 15-20mg daily in divided doses 8.

Second-Line Options After Surgical Failure

If surgery fails or recurrence occurs 7, 9:

• Repeat trans-sphenoidal surgery if feasible
• Medical therapy (steroidogenesis inhibitors long-term)
• Pituitary radiation therapy (takes 6-24 months for effect, requires medical therapy bridge)
• Bilateral adrenalectomy (definitive but requires lifelong glucocorticoid/mineralocorticoid replacement)

For Cushing's disease specifically, pasireotide (somatostatin analog targeting pituitary tumor) can be considered, though efficacy is modest (15-25% normalization) 9.

Critical Monitoring Points

Postoperative:

• Expect transient adrenal insufficiency (suppressed normal corticotrophs)
• Requires glucocorticoid replacement until HPA axis recovers (weeks to months)
• Monitor morning cortisol to assess recovery

Long-term:

• Lifelong surveillance for recurrence (10-20% over 10 years)
• Address persistent comorbidities: hypertension, diabetes, osteoporosis, psychiatric disorders
• These often persist despite biochemical cure and require ongoing management 5, 3

Common pitfall: The proximal myopathy can be so prominent that it mimics primary neuromuscular disease (as in the dermatomyositis misdiagnosis 4). Always screen for Cushing's syndrome when proximal weakness occurs with weight gain, hypertension, or other suggestive features.