Cushing Syndrome

Cushing Syndrome – High‑Yield Study Guide for Medical Students

Definition

Cushing syndrome is the clinical state resulting from chronic exposure to excess glucocorticoids, most commonly cortisol, from any source. It includes both exogenous (iatrogenic glucocorticoid therapy) and endogenous causes (ACTH-dependent or ACTH-independent hypercortisolism). Cushing disease refers specifically to Cushing syndrome caused by an ACTH-secreting pituitary corticotroph adenoma.

Epidemiology

Cushing syndrome is relatively rare but important to recognize due to its morbidity.

• Incidence (endogenous): Approximately 1–3 new cases per million population per year.
• Sex: Endogenous Cushing syndrome (especially Cushing disease and adrenal adenomas) is more common in females.
• Age: Most cases present between 20–50 years; adrenal carcinomas may present at younger or older extremes.
• Most common overall cause: Chronic iatrogenic glucocorticoid therapy (e.g., for autoimmune disease, transplantation).
• Most common endogenous cause: ACTH-secreting pituitary adenoma (Cushing disease).

Pathophysiology

Cushing syndrome results from prolonged exposure to supraphysiologic cortisol levels, leading to widespread metabolic, cardiovascular, musculoskeletal, and immunologic effects.

Classification by Etiology

• Exogenous (iatrogenic)
  • Most common overall cause.
  • Due to chronic use of glucocorticoids (e.g., prednisone, dexamethasone, hydrocortisone), including oral, IV, high-dose inhaled, or potent topical preparations.
  • Suppresses hypothalamic CRH and pituitary ACTH via negative feedback, leading to bilateral adrenal atrophy.
• Endogenous ACTH-dependent
  • Pituitary Cushing disease: ACTH-secreting pituitary corticotroph microadenoma is the most common endogenous cause.
  • Ectopic ACTH secretion: From nonpituitary tumors, classically small cell lung carcinoma and bronchial carcinoid; also thymic, pancreatic neuroendocrine tumors.
  • High ACTH stimulates bilateral adrenal hyperplasia and excess cortisol production.
• Endogenous ACTH-independent
  • Adrenal adenoma: Benign cortisol-producing tumor.
  • Adrenal carcinoma: Malignant cortisol-secreting tumor, often with higher cortisol levels and rapid progression.
  • Primary bilateral macronodular or micronodular adrenal hyperplasia.
  • High cortisol suppresses ACTH; the contralateral adrenal may be atrophic if unilateral tumor.

Hormonal and Systemic Effects of Cortisol Excess

• Metabolic
  • Increased gluconeogenesis and insulin resistance → hyperglycemia, secondary diabetes mellitus.
  • Proteolysis and collagen breakdown → muscle wasting and thin skin.
  • Lipolysis with redistribution of fat (central obesity, dorsocervical fat pad).
• Cardiovascular and renal
  • Mineralocorticoid receptor activation and RAAS sensitization → sodium retention, expansion of extracellular volume, hypertension.
  • Increased vascular reactivity to catecholamines.
• Bone and connective tissue
  • Inhibition of osteoblasts and increased bone resorption → osteoporosis, fractures.
  • Impaired collagen synthesis → easy bruising, poor wound healing, striae.
• Immune system
  • Anti-inflammatory and immunosuppressive effects → increased susceptibility to infection, reactivation of latent infections.
• Neuropsychiatric
  • Mood disorders (depression, irritability, anxiety), cognitive impairment, sleep disturbance, in severe cases psychosis.
• Gonadal axis
  • Inhibition of GnRH and gonadotropins → menstrual irregularities, decreased fertility, decreased libido, erectile dysfunction.

Clinical Presentation

The clinical picture of Cushing syndrome evolves gradually and may be subtle in early stages. Not every patient has all features; careful pattern recognition is important.

Key Clinical Features

• Body habitus and fat distribution
  • Central (truncal) obesity with relatively thin extremities.
  • Moon facies: rounded, plethoric face.
  • Dorsocervical fat pad ("buffalo hump").
  • Supraclavicular fat accumulation.
• Skin and connective tissue
  • Wide (>1 cm), violaceous striae on abdomen, breasts, thighs, buttocks.
  • Thin, fragile skin with easy bruising and ecchymoses.
  • Slow wound healing, increased skin infections.
  • Facial plethora and acne; hirsutism in women due to adrenal androgens (particularly in ACTH-dependent cases).
• Musculoskeletal
  • Proximal muscle weakness (difficulty climbing stairs, rising from chair).
  • Myopathy and muscle wasting of limbs.
  • Osteopenia/osteoporosis with increased fragility fractures (vertebral compression fractures are classic).
• Cardiometabolic
  • Hypertension (very common).
  • Impaired glucose tolerance or diabetes mellitus.
  • Dyslipidemia.
  • Weight gain, particularly truncal.
• Neuropsychiatric
  • Fatigue, sleep disturbance.
  • Depression, emotional lability, irritability.
  • Cognitive difficulties (poor concentration, memory issues).
  • Severe cases: mania or psychosis.
• Reproductive and endocrine
  • Oligomenorrhea or amenorrhea in women.
  • Decreased libido and erectile dysfunction in men.
  • Decreased fertility in both sexes.
• Infections and other features
  • Increased frequency and severity of infections (e.g., skin infections, pneumonia).
  • Glucocorticoid-induced myopathy and steroid myopathy.
  • In children: growth retardation with weight gain is a key clue.

Red Flags Suggesting Ectopic ACTH or Malignancy

• Very rapid onset and progression of Cushingoid features.
• Severe muscle weakness and profound hypokalemic alkalosis.
• Hyperpigmentation (from very high ACTH levels).
• Evidence of underlying malignancy (e.g., lung mass, systemic symptoms).

Diagnostic Approach

Diagnosis of Cushing syndrome involves two major steps: (1) confirming endogenous hypercortisolism and (2) determining the etiology (ACTH-dependent vs ACTH-independent, pituitary vs ectopic vs adrenal).

Step 1: Confirming Hypercortisolism (Screening Tests)

Initial testing should be performed in patients with appropriate clinical features, especially those progressive and multiple. At least one, and often two, of the following first-line tests are used:

• 1 mg overnight dexamethasone suppression test (ONDST)
  • Administer 1 mg oral dexamethasone at 11 PM; measure serum cortisol at 8 AM next morning.
  • Normal response: suppression of morning serum cortisol below a low threshold (e.g., <1.8 µg/dL or <50 nmol/L; lab-specific cutoffs).
  • Lack of suppression indicates loss of normal feedback regulation consistent with Cushing syndrome.
• Late-night salivary cortisol
  • Reflects loss of normal circadian rhythm; cortisol normally low at midnight.
  • Elevated late-night salivary cortisol on two separate samples supports Cushing syndrome.
• 24-hour urinary free cortisol (UFC)
  • Measures unbound cortisol excreted in urine over 24 hours.
  • At least two elevated UFC collections > upper limit of normal suggest hypercortisolism.
  • Useful to quantify cortisol burden; markedly high levels often seen in ectopic ACTH or adrenal carcinoma.

Exogenous glucocorticoid use must be excluded by careful history; if present, endogenous testing is not indicated until exogenous steroids are tapered and stopped when clinically feasible.

Step 2: Determining ACTH Dependence

Once Cushing syndrome is biochemically confirmed, next determine whether the cause is ACTH-dependent or ACTH-independent.

• Morning plasma ACTH level
  • Low or undetectable ACTH (<5 pg/mL, assay dependent) → suggests ACTH-independent adrenal cause (adenoma, carcinoma, adrenal hyperplasia).
  • Normal or high ACTH → indicates ACTH-dependent Cushing (pituitary Cushing disease or ectopic ACTH secretion).

Step 3: Localizing the Source

ACTH-Independent Cushing Syndrome

• Imaging
  • Adrenal CT or MRI to identify unilateral or bilateral adrenal lesions.
  • Unilateral adrenal mass with suppressed ACTH suggests functioning adenoma or carcinoma.
  • Bilateral thickened adrenals may indicate macronodular or micronodular hyperplasia.

ACTH-Dependent Cushing Syndrome

• High-dose dexamethasone suppression test (HDDST; now less used but important conceptually)
  • Higher dose dexamethasone (e.g., 8 mg overnight or 2 mg q6h for 48 h).
  • Pituitary Cushing disease often shows partial suppression of cortisol (e.g., >50% reduction), due to retained feedback sensitivity.
  • Ectopic ACTH usually fails to suppress because ectopic tumors lack normal feedback.
• CRH stimulation test
  • Administer exogenous CRH and measure ACTH/cortisol response.
  • Pituitary adenomas typically show a rise in ACTH and cortisol.
  • Ectopic ACTH tumors generally show minimal or no response.
• Pituitary MRI
  • Used to detect pituitary microadenoma or macroadenoma.
  • Note: up to 10% of normal population has incidental pituitary microadenomas (<1 cm), so imaging findings must be correlated with biochemical tests.
• Inferior Petrosal Sinus Sampling (IPSS)
  • Gold standard to distinguish pituitary ACTH source from ectopic ACTH when imaging and dynamic tests are equivocal.
  • Involves sampling ACTH from inferior petrosal sinuses and peripheral blood, at baseline and after CRH.
  • Central-to-peripheral ACTH gradient supports pituitary source; absent gradient suggests ectopic ACTH.
• Search for ectopic source
  • If ACTH-dependent and not pituitary, perform targeted imaging (e.g., chest CT for small cell lung carcinoma, CT/MRI of thorax/abdomen, PET/functional imaging).

Additional Laboratory and Workup

• Fasting glucose, HbA1c (assess for diabetes).
• Lipid profile (dyslipidemia).
• Electrolytes (hypokalemic metabolic alkalosis in ectopic ACTH or severe disease).
• Bone mineral density (osteopenia/osteoporosis).

Management of Cushing Syndrome

The primary goal is to remove or control the source of excess cortisol, reverse clinical features, and minimize long-term complications. Management depends on etiology and patient factors.

General Principles

• Treat underlying cause definitively whenever feasible (surgery, radiotherapy, or oncologic therapy).
• Use medical therapy to control hypercortisolism when surgery is not possible, as a bridge to surgery, or while awaiting radiotherapy effect.
• Manage comorbidities: hypertension, diabetes, osteoporosis, infection risk, psychiatric symptoms.
• Gradual withdrawal of exogenous glucocorticoids when iatrogenic; monitor for adrenal suppression.

Iatrogenic (Exogenous) Cushing Syndrome

• Strategy
  • Review all glucocorticoid exposures (oral, IV, inhaled, topical, intra-articular).
  • If clinically safe, taper to the minimum effective dose or discontinue.
  • Switch to steroid-sparing agents where possible (e.g., DMARDs, biologics for rheumatologic disease).
  • Because prolonged exogenous steroids suppress HPA axis, tapering should be gradual and tailored to duration and dose of therapy.

ACTH-Dependent – Pituitary Cushing Disease

• First-line: Transsphenoidal pituitary surgery
  • Preferred initial treatment for most patients with Cushing disease.
  • Remission rates are highest in microadenomas with experienced surgeons.
  • Postoperative evaluation: measure morning cortisol early after surgery; low levels suggest remission but require temporary glucocorticoid replacement until HPA recovery.
• Adjunctive or alternative therapies
  • Radiotherapy (conventional or stereotactic radiosurgery) for persistent or recurrent disease not cured by surgery or inoperable cases.
  • Medical therapy to control hypercortisolism while awaiting radiotherapy effect or if surgery/radiation not feasible.

ACTH-Dependent – Ectopic ACTH Syndrome

• Treat underlying tumor
  • Oncologic management such as surgical resection of localized tumor, chemotherapy, radiotherapy, or targeted therapies depending on tumor type (e.g., small cell lung carcinoma).
• Control hypercortisolism
  • Use steroidogenesis inhibitors (e.g., ketoconazole, metyrapone, osilodrostat, etomidate [IV in critical illness]) to rapidly reduce cortisol.
  • In refractory cases or when tumor cannot be controlled, bilateral adrenalectomy is an option to definitively eliminate cortisol production (requires lifelong glucocorticoid and mineralocorticoid replacement).

ACTH-Independent – Adrenal Causes

• Adrenal adenoma
  • Preferred treatment is laparoscopic unilateral adrenalectomy for functioning adenomas.
  • Postoperative glucocorticoid replacement is typically required due to contralateral adrenal suppression; taper as HPA axis recovers.
• Adrenal carcinoma
  • Open adrenalectomy with en bloc resection when possible.
  • Consider adjuvant mitotane and/or systemic therapy; prognosis often poor.
• Primary bilateral adrenal hyperplasia
  • Options include bilateral adrenalectomy (with lifelong replacement) or medical adrenal blockade.

Medical Therapy for Hypercortisolism (Key Drug Classes)

• Steroidogenesis inhibitors
  • Ketoconazole: Antifungal that inhibits adrenal steroid synthesis enzymes; used orally; monitor liver function.
  • Metyrapone: Inhibits 11β-hydroxylase; can cause accumulation of androgens and 11-deoxycortisol → hirsutism, hypertension.
  • Osilodrostat: Potent 11β-hydroxylase inhibitor for endogenous Cushing syndrome.
  • Etomidate: IV anesthetic used at low doses in ICU to acutely control severe hypercortisolism.
• Adrenolytic agent
  • Mitotane: Cytotoxic to adrenal cortex; used particularly in adrenal carcinoma.
• Pituitary-directed agents
  • Cabergoline: Dopamine agonist; can reduce ACTH in some pituitary adenomas.
  • Pasireotide: Somatostatin analog with activity at SST5 receptor; can reduce ACTH secretion in Cushing disease but may worsen hyperglycemia.
• Glucocorticoid receptor antagonist
  • Mifepristone: Blocks glucocorticoid receptors; used especially in Cushing patients with type 2 diabetes or glucose intolerance; cortisol levels remain high, so clinical response and side effects guide dosing.

Management of Comorbidities and Complications

• Hypertension: Standard antihypertensives (ACE inhibitors, ARBs, calcium channel blockers, etc.), address mineralocorticoid excess.
• Diabetes mellitus: Lifestyle modification, oral agents, often insulin during active hypercortisolism.
• Osteoporosis: Calcium, vitamin D, bisphosphonates; fall prevention; screening for vertebral fractures.
• Infection risk: Low threshold for evaluation and treatment; consider vaccinations where appropriate.
• Psychiatric symptoms: Evaluate and manage depression, anxiety, insomnia; symptoms often improve after cortisol normalization.

Prognosis and Follow-Up

• Even after biochemical cure, some features (e.g., cardiovascular risk, bone loss) may persist and need ongoing follow-up.
• Regular clinical and biochemical monitoring is required to detect recurrence, especially after pituitary surgery.
• Longstanding uncontrolled Cushing syndrome increases risk of cardiovascular disease, infections, thromboembolism, and mortality.

Key Clinical Pearls for Exams and Practice

• Cushing syndrome vs Cushing disease
  • Cushing syndrome = clinical state of hypercortisolism from any cause.
  • Cushing disease = Cushing syndrome due to ACTH-secreting pituitary adenoma.
• Most common causes
  • Overall: Exogenous glucocorticoids.
  • Endogenous: Pituitary ACTH adenoma (Cushing disease).
• Classic phenotype
  • Truncal obesity, moon face, buffalo hump, purple striae, easy bruising, proximal muscle weakness, hypertension, glucose intolerance.
• Initial screening tests
  • 1 mg overnight dexamethasone suppression test.
  • Late-night salivary cortisol.
  • 24-hour urinary free cortisol.
  • At least one, and usually two, abnormal tests are required for diagnosis.
• ACTH level is the key branching point
  • Low ACTH → adrenal cause (ACTH-independent) → adrenal imaging.
  • Normal/high ACTH → ACTH-dependent → pituitary vs ectopic workup (MRI, CRH test, IPSS).
• Physical exam clues to differentiate from simple obesity
  • Violaceous, wide striae and proximal muscle weakness make Cushing syndrome more likely than uncomplicated obesity.
  • Children with obesity plus growth failure should raise concern for Cushing syndrome.
• Ectopic ACTH syndrome clues
  • Very rapid onset, severe hypercortisolism, weight loss rather than gain.
  • Marked muscle weakness, severe hypokalemia, metabolic alkalosis, hyperpigmentation.
  • Often associated with small cell lung cancer or other neuroendocrine tumors.
• Post-treatment adrenal insufficiency
  • After curative surgery (pituitary or adrenal), patients often develop transient or chronic adrenal insufficiency due to suppressed HPA axis.
  • They require glucocorticoid replacement and stress dosing until axis recovers.
• High-yield imaging pattern
  • Exogenous steroids → bilateral adrenal atrophy.
  • ACTH-dependent (pituitary or ectopic) → bilateral adrenal hyperplasia.
  • Adrenal adenoma/carcinoma → unilateral mass with contralateral atrophy.

Summary

Cushing syndrome is a multi-system disorder caused by chronic glucocorticoid excess. For medical students and clinicians, mastering the pattern recognition of clinical features, the stepwise diagnostic approach (screening → ACTH level → localization), and the etiology-specific treatments (surgery for pituitary or adrenal tumors, oncologic therapy for ectopic sources, careful management of exogenous steroids) is essential. Early diagnosis and effective treatment significantly reduce long-term complications and improve outcomes.