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Cushing Syndrome

(Cushing's Syndrome)

by Ashley B. Grossman, MD, FRCP, FMedSci

Cushing syndrome is a constellation of clinical abnormalities caused by chronic high blood levels of cortisol or related corticosteroids. Cushing disease is Cushing syndrome that results from excess pituitary production of ACTH, usually secondary to a pituitary adenoma. Typical symptoms and signs include moon facies and truncal obesity, easy bruising, and thin arms and legs. Diagnosis is by history of receiving corticosteroids or by finding elevated serum cortisol. Treatment depends on the cause.

Etiology

Hyperfunction of the adrenal cortex can be ACTH dependent or ACTH independent.

ACTH-dependent hyperfunction may result from

  • Hypersecretion of ACTH by the pituitary gland

  • Secretion of ACTH by a nonpituitary tumor, such as small cell carcinoma of the lung or a carcinoid tumor (ectopic ACTH syndrome)

  • Administration of exogenous ACTH

ACTH-independent hyperfunction usually results from therapeutic administration of corticosteroids or from adrenal adenomas or carcinomas. Rare causes include primary pigmented nodular adrenal dysplasia (usually in adolescents) and macronodular dysplasia (in older patients).

Whereas the term Cushing syndrome denotes the clinical picture resulting from cortisol excess from any cause, Cushing disease refers to hyperfunction of the adrenal cortex from pituitary ACTH excess. Patients with Cushing disease usually have a small adenoma of the pituitary gland.

Symptoms and Signs

Clinical manifestations include moon facies with a plethoric appearance, truncal obesity with prominent supraclavicular and dorsal cervical fat pads (buffalo hump), and, usually, very slender distal extremities and fingers. Muscle wasting and weakness are present. The skin is thin and atrophic, with poor wound healing and easy bruising. Purple striae may appear on the abdomen. Hypertension, renal calculi, osteoporosis, glucose intolerance, reduced resistance to infection, and mental disturbances are common. Cessation of linear growth is characteristic in children. Females usually have menstrual irregularities. In females with adrenal tumors, increased production of androgens may lead to hypertrichosis, temporal balding, and other signs of virilism.

Diagnosis

  • Dexamethasone suppression test

  • Midnight serum or salivary cortisol levels

    Urinary free cortisol (UFC) level

  • ACTH levels; if detectable, provocative testing

Diagnosis is usually suspected based on the characteristic symptoms and signs. Confirmation (and identification of the cause) generally requires hormonal and imaging tests.

In some centers, testing begins with measurement of UFC, the best assay for urinary excretion (normal, 20 to 100 μg/24 h [55.2 to 276 nmol/24 h]). UFC is elevated > 120 μg/24 h (> 331 nmol/24 h) in almost all patients with Cushing syndrome. However, many patients with UFC elevations between 100 and 150 μg/24 h (276 and 414 nmol/24 h) have obesity, depression, or polycystic ovaries but not Cushing syndrome. A patient with suspected Cushing syndrome with grossly elevated UFC (> 4 times the upper limit of normal) almost certainly has Cushing syndrome. Two to 3 normal collections usually excludes the diagnosis. Slightly elevated levels generally necessitate further investigation, as do normal levels when clinical suspicion is high. A baseline morning (eg, 09:00 am ) serum cortisol measurement should also be done.

An alternative approach to investigation uses the dexamethasone suppression test, in which 1, 1.5, or 2 mg of dexamethasone is given po at 11 to 12 pm and serum cortisol is measured at 8 to 9 am the next morning. In most normal patients, this drug suppresses morning serum cortisol to 1.8 μg/mL ( 50 nmol/L), whereas patients with Cushing syndrome virtually always have a higher level. A more specific but equally sensitive test is to give dexamethasone 0.5 mg po q 6 h for 2 days (low dose). In general, a clear failure to suppress cortisol levels in response to low-dose dexamethasone establishes the diagnosis.

If results of these tests are indeterminate, the patient is hospitalized for measurement of serum cortisol at midnight, which is more likely to be conclusive. Alternatively, salivary cortisol samples may be collected and stored in the refrigerator at home. Cortisol normally ranges from 5 to 25 μg/dL (138 to 690 nmol/L) in the early morning (6 to 8 am ) and declines gradually to < 1.8 μg/dL (< 50 nmol/L) at midnight. Patients with Cushing syndrome occasionally have a normal morning cortisol level but lack normal diurnal decline in cortisol production, such that the midnight serum cortisol levels are above normal and the total 24-h cortisol production may be elevated. Serum cortisol may be spuriously elevated in patients with congenital increases of corticosteroid-binding globulin or in those receiving estrogen therapy, but diurnal variation is normal in these patients.

ACTH levels are measured to determine the cause of Cushing syndrome. Undetectable levels, both basally and particularly in response to corticotropin-releasing hormone (CRH), suggest a primary adrenal cause. High levels suggest a pituitary cause. If ACTH is detectable (ACTH-dependent Cushing syndrome), provocative tests help differentiate Cushing disease from ectopic ACTH syndrome, which is rarer. In response to high-dose dexamethasone (2 mg po q 6 h for 48 h), the 9 am serum cortisol falls by > 50% in most patients with Cushing disease but infrequently in those with ectopic ACTH syndrome. Conversely, ACTH rises by > 50% and cortisol rises by 20% in response to human or ovine-sequence CRH (100 mcg IV or 1 mcg/kg IV) in most patients with Cushing disease but very rarely in those with ectopic ACTH syndrome (see Table: Diagnostic Tests in Cushing Syndrome). An alternative approach to localization, which is more accurate but more invasive, is to catheterize both petrosal veins (which drain the pituitary) and measure ACTH from these veins 5 min after a bolus of CRH (human or ovine) 100 mcg or 1 mcg/kg. A central-to-peripheral ACTH ratio > 3 virtually excludes ectopic ACTH syndrome, whereas a ratio < 3 suggests a need to seek such a source.

Diagnostic Tests in Cushing Syndrome

Diagnosis

Serum Cortisol, 9 am

Salivary or Serum Cortisol, Midnight

Urinary Free Cortisol

Low-Dose or Overnight Dexamethasone

High-Dose Dexamethasone

Corticotropin-Releasing Hormone

ACTH Level

Normal

N

N

N*

S

S

N

N

Cushing disease

N or

NS

S

N or

N or

Ectopic ACTH

N or

NS

NS

Flat

↑↑

Adrenal tumor

N or

NS

NS

Flat

*May be elevated in non-Cushing conditions.

Flat = no significant rise in ACTH or cortisol; N = normal; NS = nonsuppression; S = suppression; ↑↑= greatly increased; = increased; = decreased.

Pituitary imaging is done if ACTH levels and provocative tests suggest a pituitary cause; gadolinium-enhanced MRI is most accurate, but some microadenomas are visible on CT. If testing suggests a nonpituitary cause, imaging includes high-resolution CT of the chest, pancreas, and adrenals; scintiscanning with radiolabeled octreotide; and occasionally fludeoxyglucose (FDG)-PET scanning.

In children with Cushing disease, pituitary tumors are very small and usually cannot be detected with MRI. Petrosal sinus sampling is particularly useful in this situation. MRI is preferred to CT in pregnant women to avoid fetal exposure to radiation.

Treatment

  • Metyrapone or ketoconazole

  • Surgery or radiation therapy to remove tumors

Initially, the patient’s general condition should be supported by high protein intake and appropriate administration of K. If clinical manifestations are severe, it may be reasonable to block corticosteroid secretion with metyrapone 250 mg to 1 g po tid or ketoconazole 400 mg po once/day, increasing to a maximum of 400 mg tid. Ketoconazole is slower in onset, sometimes hepatotoxic, and its current availability is uncertain.

Pituitary tumors that produce excessive ACTH are removed surgically or extirpated with radiation therapy. If no tumor is shown on imaging but a pituitary source is likely, total hypophysectomy may be attempted, particularly in older patients. Younger patients usually receive supervoltage irradiation of the pituitary, delivering 45 Gy. Improvement usually occurs in < 1 yr. However, in children, irradiation may reduce secretion of growth hormone and occasionally cause precocious puberty. In special centers, heavy particle beam irradiation, providing about 100 Gy, is often successful, as is a single focused beam of radiation therapy given as a single dose (radiosurgery). Response to irradiation occasionally requires several years, but response is more rapid in children.

Studies suggest that mild cases of persistent or recurrent disease may benefit from the somatostatin analog pasireotide. However, hyperglycemia is a significant adverse effect. The dopamine agonist cabergoline may also occasionally be useful.

Bilateral adrenalectomy is reserved for patients with pituitary hyperadrenocorticism who do not respond to both pituitary exploration (with possible adenomectomy) and irradiation. Adrenalectomy requires life-long corticosteroid replacement.

Adrenocortical tumors are removed surgically. Patients must receive cortisol during the surgical and postoperative periods because their nontumorous adrenal cortex will be atrophic and suppressed. Benign adenomas can be removed laparoscopically. With multinodular adrenal hyperplasia, bilateral adrenalectomy may be necessary. Even after a presumed total adrenalectomy, functional regrowth occurs in a few patients.

Ectopic ACTH syndrome is treated by removing the nonpituitary tumor that is producing the ACTH. However, in some cases, the tumor is disseminated and cannot be excised. Adrenal inhibitors, such as metyrapone 500 mg po tid (and up to a total of 6 g/day) or mitotane 0.5 g po once/day, increasing to a maximum of 3 to 4 g/day, usually control severe metabolic disturbances (eg, hypokalemia). When mitotane is used, large doses of hydrocortisone or dexamethasone may be needed. Measures of cortisol production may be unreliable, and severe hypercholesterolemia may develop. Ketoconazole 400 to 1200 mg po once/day also blocks corticosteroid synthesis, although it may cause liver toxicity and can cause addisonian symptoms. Alternatively, the corticosteroid receptors can be blocked with mifepristone. Mifepristone increases serum cortisol but blocks effects of the corticosteroid. Sometimes ACTH-secreting tumors respond to long-acting somatostatin analogs, although administration for >2 yr requires close follow-up, because mild gastritis, gallstones, cholangitis, and malabsorption may develop.

Nelson syndrome

Nelson syndrome occurs when the pituitary gland continues to expand after bilateral adrenalectomy, causing a marked increase in the secretion of ACTH and its precursors, resulting in severe hyperpigmentation. It occurs in 50% of patients who undergo adrenalectomy. The risk is probably reduced if the patient undergoes pituitary radiation therapy. Although irradiation may arrest continued pituitary growth, many patients also require hypophysectomy. The indications for hypophysectomy are the same as for any pituitary tumor: an increase in size such that the tumor encroaches on surrounding structures, causing visual field defects, pressure on the hypothalamus, or other complications. Routine irradiation is often done after hypophysectomy if it was not done previously, especially when a tumor is clearly present. Radiation therapy may be delayed if there is no obvious lesion. Radiosurgery, or focused radiation therapy, can be given in a single fraction when standard external beam radiation therapy has already been done, as long as the lesion is at a reasonable distance from the optic nerve and chiasm.

Key Points

  • Diagnosis is usually made by elevated nocturnal serum or salivary cortisol levels, or 24-h urinary free cortisol, and a dexamethasone suppression test.

  • Pituitary causes are distinguished from nonpituitary causes by ACTH levels.

  • Imaging is then done to identify any causative tumor.

  • Tumors are usually treated surgically or with radiation therapy.

  • Metyrapone or ketoconazole may be given to suppress cortisol secretion prior to definitive treatment.

Resources In This Article

Drugs Mentioned In This Article

  • Drug Name
    Select Trade
  • H.P. ACTHAR GEL
  • OZURDEX
  • SANDOSTATIN
  • No US brand name
  • NIZORAL
  • METOPIRONE
  • MIFEPREX
  • SIGNIFOR
  • LYSODREN

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