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Hypercortisolism (Cushing Syndrome)

Cushing syndrome can be caused by exogenous administration of glucocorticoids or by endogenous overproduction of ACTH or cortisol.
Cushing syndrome can be caused by exogenous administration of glucocorticoids or by endogenous overproduction of ACTH or cortisol.

Elevated levels of glucocorticoids for a prolonged period result in a characteristic combination of clinical features known as Cushing syndrome.

These features include weight gain, central obesity, hypertension, muscle weakness and wasting, thin skin with easy bruising, abdominal striae, acne, menstrual irregularity, osteoporosis, glucose intolerance, and psychiatric disturbance.

A buffalo hump and moon facies from centripetal fat redistribution on the upper back and face are pathognomonic. One of the most important signs of Cushing syndrome in children is impairment of linear growth. These children continue to gain weight but do not grow longitudinally, unless there is also an associated excess production of androgens.

Cushing syndrome can be caused by exogenous administration of glucocorticoids or by endogenous overproduction of ACTH or cortisol. Patients with pituitary overproduction of ACTH are said to have Cushing disease. This is most often the result of an anterior pituitary microadenoma. Rarely children have excessive ACTH production from an ectopic source, such as a pulmonary or pancreatic neoplasm, thymoma, or carcinoid.

Cortisol hypersecretion can be caused by adrenal tumors, either adenomas or carcinomas. Before 6 years of age, excess cortisol production is usually the result of an adrenal tumor. In older children, the cause is more likely to be excessive pituitary secretion of ACTH, resulting in bilateral adrenal hyperplasia.

Testing for Cushing syndrome entails demonstrating that excessive secretion of cortisol is present and that normal feedback regulation is absent (10). The simplest test is to obtain a fasting serum cortisol level at 8:00 a.m. and 6:00 p.m. to coincide with the normal diurnal variation of plasma cortisol.

Serum cortisol levels of 15 to 20 Вµg per dL are normal. Cushing syndrome can be detected in 80% of patients by measurement of high fasting serum cortisol levels lacking diurnal variation. However, the most sensitive screening method for hypercortisolism is the 24-hour urine measurement of 17-hydroxycorticosteroid or free cortisol. In patients with Cushing syndrome, the urinary-free cortisol level is greater than 100 Вµg per day.

An additional screening method, the overnight low-dose dexamethasone suppression test, consists of administering 1 mg of dexamethasone (synthetic glucocorticoid) at 11:00 p.m. and measuring the plasma cortisol level at 8:00 a.m. With an intact pituitary-adrenal axis, the cortisol level is suppressed by 50% or more of baseline, to less than 5 Вµg per dL.

In Cushing syndrome, this dose of dexamethasone is inadequate to suppress corticosteroid production and the levels remain unchanged. False-positive results can occur in patients receiving medications that accelerate dexamethasone metabolism, such as phenytoin and phenobarbital.

After demonstration of cortisol excess, other testing is commonly used to distinguish pituitary from nonpituitary causes. Plasma ACTH levels are usually low (less than 5 pg per mL) with adrenal causes of hypercortisolism, moderately elevated with pituitary neoplasms, and markedly elevated with tumors producing ectopic ACTH.

The high-dose dexamethasone suppression test can also be used. Traditionally, this test involves administering 2 mg of oral dexamethasone every 6 hours for 48 hours. A 24-hour urine collection is then obtained to measure 17-hydroxycorticosteroid and free cortisol levels. If hypercortisolism is due to overproduction of ACTH by the pituitary gland, causing adrenal hyperplasia, suppression of steroid excretion occurs with the high-dose regimen. In patients with autonomous adrenal tumors, however, suppression does not occur. An overnight 8-mg dexamethasone suppression test is reported to achieve similar results.

Another test that is helpful in defining the cause of Cushing syndrome involves the administration of CRH . One Вµg per kg of CRH is given intravenously, and serial blood samples are obtained for 3 hours after the drug is given. Normally, there is a moderate increase in ACTH and cortisol levels. With Cushing disease, the increase in ACTH and cortisol in the bloodstream is more marked. When high levels of cortisol are due to autonomous adrenal tumors or ectopic ACTH production, there is no response to CRH because the pituitary gland is chronically suppressed.

The diagnostic tests for Cushing syndrome are summarized in patients with Cushing disease, magnetic resonance imaging of the sella turcica localizes a pituitary adenoma over one-half of the time. When this fails, bilateral simultaneous petrosal sinus ACTH sampling with CRH stimulation is the diagnostic test of choice. Surgical treatment of a pituitary adenoma is by transsphenoidal hypophysectomy, with an 85% rate of remission.


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