 |
Cushing's
syndrome is a constellation of clinical abnormalities caused by
chronic high blood levels of cortisol or related corticosteroids.
Cushing's disease is Cushing's syndrome that results from excess
pituitary production of ACTH, usually secondary to a pituitary adenoma.
Typical symptoms include moon facies and truncal obesity with 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
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's syndrome denotes the clinical picture resulting from cortisol excess from any cause, Cushing's disease refers to hyperfunction of the adrenal cortex from pituitary ACTH excess. Patients with Cushing's 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
Diagnosis is usually suspected based on the characteristic symptoms and signs. Confirmation (and identification of the underlying cause) generally requires hormonal and imaging tests.
In some centers, testing begins with measurement of urinary free cortisol (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's 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's syndrome. A patient with suspected Cushing's syndrome with grossly elevated UFC (> 4 times the upper limit of normal) almost certainly has Cushing's syndrome. Two to 3 normal collections virtually exclude the diagnosis. Slightly elevated levels generally necessitate further investigation.
An alternative approach to investigation uses the dexamethasone suppression test, in which 1, 1.5, or 2 mg of dexamethasone is administered po at 11 to 12 pm and plasma cortisol is measured at 8 to 9 am the next morning. In most normal patients, this drug suppresses morning plasma cortisol to ≤ 1.8 μg/mL (≤ 50 nmol/L), whereas patients with Cushing's 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 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. 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's syndrome occasionally have a normal morning cortisol level but lack normal diurnal decline in cortisol production, such that the midnight plasma cortisol levels are above normal and the total 24-h cortisol production is elevated. Alternatively, salivary cortisol samples may be collected and stored in the refrigerator at home. Plasma 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's 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's syndrome), provocative tests help differentiate Cushing's 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's disease but infrequently in those with ectopic ACTH syndrome. Conversely, ACTH and cortisol rise by > 50% and 20%, respectively, in response to human or ovine-sequence CRH (100 μg IV or 1 μg/kg IV) in most patients with Cushing's disease but very rarely in those with ectopic ACTH syndrome (see
Table 2: Adrenal Disorders: Diagnostic Tests in Cushing's 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 100 μg or 1 μg/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.
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 PET scanning.
In children with Cushing's 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
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 more readily available but slower in onset and sometimes hepatotoxic.
Pituitary tumors that produce excessive ACTH are removed surgically or extirpated with radiation. If no tumor is demonstrated 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.
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.
Nelson syndrome occurs when the pituitary gland continues to expand after 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. 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, producing 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. 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.
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 (RU 486). Mifepristone increases plasma 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.
Last full review/revision November 2007 by Ashley B. Grossman, MD
Content last modified November 2007
| |
