📚 Lecture Overview
This lecture covers Cushing Syndrome, an uncommon disorder characterized by prolonged and inappropriately high tissue exposure to glucocorticoids. The material outlines clinical presentation, systematic diagnostic approaches (screening vs. localization), and primary management strategies.
🎯 Key Concepts & Definitions
- Cushing Syndrome: A group of signs and symptoms resulting from chronic, excessive glucocorticoid exposure; most commonly caused iatrogenically by prescribed corticosteroids.
- Cushing Disease: Specifically refers to Cushing syndrome caused by an ACTH-secreting pituitary adenoma.
- Pseudo-Cushing Syndrome: A condition involving chronic activation of the Hypothalamic-Pituitary-Adrenal (HPA) axis that mimics clinical and biochemical features of Cushing syndrome.
- Iatrogenic Cushing: Excess glucocorticoids caused by exogenous medication intake; identified by high basal cortisol at 0900h.
- BIPSS (Bilateral Inferior Petrosal Sinus Sampling): The gold standard invasive test used to differentiate pituitary-dependent Cushing disease from ectopic ACTH secretion.
📖 Main Content
Clinical Presentation & History
- History Findings: Patients present with fatigue, depression, sleep disturbances, weight gain, menstrual irregularities, hypertension, glucose intolerance, easy bruising, and fractures from minimal trauma.
- Physical Examination: Key signs include central obesity, supraclavicular fat accumulation, dorsocervical fat pads ("buffalo hump"), facial plethora, and thinned skin.
- Discriminatory Features: Highly suspicious features include reddish-purple striae (>1 cm wide), plethora, proximal muscle weakness, bruising without obvious trauma, and unexplained osteoporosis.
Diagnostic Approach: Step 1 (Is it Cushing?)
The biochemical workup follows a two-step process starting with screening tests based on three pathophysiologic derangements:
1. Loss of normal diurnal pattern with abnormally high late-night cortisol secretion.
2. Failure to suppress cortisol despite the absence of ACTH stimulation.
3. Excess production of cortisol.
Primary Screening Tests:
- Late-Night Salivary Cortisol (11pm-midnight): 92–100% sensitivity and 93–100% specificity. Interfered with by liquorice or tobacco.
- 24-Hour Urinary Free Cortisol (UFC): Values >50 µg/24h suggest Cushing; values >3 times the Upper Limit of Normal (ULN) are diagnostic. Requires adequate renal function (CrCl >60 ml/min).
- Overnight Low-Dose Dexamethasone Suppression Test (LDDST): Administer 1 mg between 11 PM and midnight; measure serum cortisol at 8 AM–9 AM. Normal suppression is <1.8 µg/dL (50 nmol/L).
- 2-Day Low-Dose Dexamethasone Suppression Test (Liddle Test): 0.5 mg every 6 hours for 2 days (8 doses). Optimal when pseudo-Cushing is suspected; a 30% fall from baseline suggests Cushing Disease.
Diagnostic Approach: Step 2 (Localizing the Cause)
Once Cushing syndrome is confirmed biochemically, determine the cause using ACTH levels and imaging:
- Plasma ACTH Level: Morning sample (8-10 AM) separated rapidly and frozen to differentiate ACTH-dependent from ACTH-independent causes.
- High-Dose Dexamethasone Suppression Test (HDDST): 2 mg every 6 hours for 48 hours. >50% suppression of basal cortisol points to Cushing Disease; <50% suppression suggests ectopic ACTH or an adrenal source.
- Radiological Imaging:
- Adrenal CT for adrenal adenomas, carcinomas, or macronodular hyperplasia.
- Pituitary MRI with gadolinium for Cushing Disease (note: ~40% are normal, and up to 10% of healthy individuals have incidental pituitary masses).
- BIPSS: Measures the central-to-peripheral ACTH gradient; basal ratio >2 or post-CRH ratio >3 confirms pituitary-dependent Cushing disease.
Treatment & Management
- Surgery: Surgical removal is the primary therapy for all causes of Cushing syndrome.
- Perioperative Preparation: Control diabetes mellitus (DM) and hypertension (HTN), provide peptic ulcer prophylaxis, and use broad-spectrum antibiotics due to severe immunosuppression.
📊 Visual Learning
💡 Important Points to Remember
- Iatrogenic Cushing is the most common overall cause and must be excluded using a basal 0900h cortisol test before extensive workups.
- Normal morning basal cortisol does not screen for Cushing syndrome; dynamic suppression tests are required.
- Post-dexamethasone cutoff: Serum cortisol <1.8 µg/dL rules out Cushing syndrome.
- UFC threshold: Values >3x ULN are diagnostic for Cushing syndrome.
- Liddle test rule: A 30% drop from baseline during the 2-day low-dose test strongly points toward Cushing Disease.
- HDDST interpretation: Suppression greater than 50% indicates a pituitary source (Cushing Disease); less than 50% points to ectopic or adrenal causes.
- BIPSS is the ultimate gold standard for differentiating pituitary sources from ectopic ACTH secretion.
- Pituitary MRIs can be normal in up to 40% of patients with surgically proven Cushing Disease.
- Always check that CrCl >60 ml/min before interpreting a 24-hour UFC test.
- Perioperative management requires rigorous control of blood pressure, blood glucose, and infection risks.
⚠️ Common Exam Questions
- MCQ Trick: Examiners will present a patient with classic signs of Cushing syndrome who is taking topical or inhaled corticosteroid creams for skin conditions. Students incorrectly jump to pituitary workups instead of recognizing iatrogenic Cushing caused by exogenous steroids.
- Diagnostic Threshold Confusion: Test questions often mix up the normal suppression cutoff (1.8 µg/dL) with baseline values or fail to apply the >3x ULN rule for UFC.
- Localization Pitfall: Questions may trick students by stating a pituitary MRI is completely normal in a patient with confirmed ACTH-dependent Cushing, leading them to rule out a pituitary tumor (remember: 40% of pituitary adenomas are MRI-occult, requiring BIPSS).
📝 Quick Review Checklist
I can explain the difference between Cushing Syndrome, Cushing Disease, and Pseudo-Cushing
I understand the 3 pathophysiological derangements of Cushing syndrome
I can define normal cutoff values for the overnight dexamethasone suppression test (<1.8 µg/dL)
I know how to interpret High-Dose Dexamethasone Suppression Test results (>50% vs <50% suppression)
I can explain the role of BIPSS in localizing ACTH-dependent Cushing syndrome