Cushing's syndrome is a hormonal disorder caused by prolonged exposure of the body's tissues to high levels of the hormone cortisol. Sometimes called "hypercortisolism," it is relatively rare and most commonly affects adults aged 20 to 50. An estimated 10 to 15 of every million people are affected each year.
What are the symptoms?
Symptoms vary, but most people have upper body obesity, rounded face, increased fat around the neck, and thinning arms and legs. Children tend to be obese with slowed growth rates.
Other symptoms appear in the skin, which becomes fragile and thin. It bruises easily and heals poorly. Purplish pink stretch marks may appear on the abdomen, thighs, buttocks, arms and breasts. The bones are weakened, and routine activities such as bending, lifting or rising from a chair may lead to backaches, rib and spinal column fractures.
Most people have severe fatigue, weak muscles, high blood pressure and high blood sugar. Irritability, anxiety and depression are common.
Women usually have excess hair growth on their faces, necks, chests, abdomens, and thighs. Their menstrual periods may become irregular or stop. Men have decreased fertility with diminished or absent desire for sex.
What causes Cushing's syndrome?
Cushing's syndrome occurs when the body's tissues are exposed to excessive levels of cortisol for long periods of time. Many people suffer the symptoms of Cushing's syndrome because they take glucocorticoid hormones such as prednisone for asthma, rheumatoid arthritis, lupus and other inflammatory diseases, or for immunosuppression after transplantation.
Others develop Cushing's syndrome because of overproduction of cortisol by the body. Normally, the production of cortisol follows a precise chain of events. First, the hypothalamus, a part of the brain which is about the size of a small sugar cube, sends corticotropin releasing hormone (CRH) to the pituitary gland. CRH causes the pituitary to secrete ACTH (adrenocorticotropin), a hormone that stimulates the adrenal glands. When the adrenals, which are located just above the kidneys, receive the ACTH, they respond by releasing cortisol into the bloodstream.
Cortisol performs vital tasks in the body. It helps maintain blood pressure and cardiovascular function, reduces the immune system's inflammatory response, balances the effects of insulin in breaking down sugar for energy, and regulates the metabolism of proteins, carbohydrates, and fats. One of cortisol's most important jobs is to help the body respond to stress. For this reason, women in their last 3 months of pregnancy and highly trained athletes normally have high levels of the hormone. People suffering from depression, alcoholism, malnutrition and panic disorders also have increased cortisol levels.
When the amount of cortisol in the blood is adequate, the hypothalamus and pituitary release less CRH and ACTH. This ensures that the amount of cortisol released by the adrenal glands is precisely balanced to meet the body's daily needs. However, if something goes wrong with the adrenals or their regulating switches in the pituitary gland or the hypothalamus, cortisol production can go awry.
Pituitary adenomas cause most cases of Cushing's syndrome. They are benign, or non-cancerous, tumours of the pituitary gland which secrete increased amounts of ACTH. Most patients have a single adenoma. This form of the syndrome, known as "Cushing's disease," affects women five times more frequently than men.
Ectopic ACTH Syndrome
Some benign or malignant (cancerous) tumours that arise outside the pituitary can produce ACTH. This condition is known as ectopic ACTH syndrome. Lung tumours cause over 50 percent of these cases. Men are affected 3 times more frequently than women. The most common forms of ACTH-producing tumours are oat cell, or small cell lung cancer, which accounts for about 25 percent of all lung cancer cases, and carcinoid tumours. Other less common types of tumours that can produce ACTH are thymomas, pancreatic islet cell tumours, and medullary carcinomas of the thyroid.
Sometimes, an abnormality of the adrenal glands, most often an adrenal tumour, causes Cushing's syndrome. The average age of onset is about 40 years. Most of these cases involve non-cancerous tumours of adrenal tissue, called adrenal adenomas, which release excess cortisol into the blood.
Adrenocortical carcinomas, or adrenal cancers, are the least common cause of Cushing's syndrome. Cancer cells secrete excess levels of several adrenal cortical hormones, including cortisol and adrenal androgens. Adrenocortical carcinomas usually cause very high hormone levels and rapid development of symptoms.
Familial Cushing's Syndrome
Most cases of Cushing's syndrome are not inherited. Rarely, however, some individuals have special causes of Cushing's syndrome due to an inherited tendency to develop tumours of one or more endocrine glands. In Primary Pigmented Micronodular Adrenal Disease, children or young adults develop small cortisol-producing tumours of the adrenal glands. In Multiple Endocrine Neoplasia Type I (MEN I), hormone secreting tumours of the parathyroid glands, pancreas and pituitary occur. Cushing's syndrome in MEN I may be due to pituitary, ectopic or adrenal tumours.
How is Cushing's syndrome diagnosed?
Diagnosis is based on a review of the patient's medical history, physical examination and laboratory tests. Often x-ray exams of the adrenal or pituitary glands are useful for locating tumours. These tests help to determine if excess levels of cortisol are present and why.
24-Hour Urinary Free Cortisol Level
This is the most specific diagnostic test. The patient's urine is collected over a 24-hour period and tested for the amount of cortisol. Levels higher than 50–100 micrograms a day for an adult suggest Cushing's syndrome. The normal upper limit varies in different laboratories, depending on which measurement technique is used.
Once Cushing's syndrome has been diagnosed, other tests are used to find the exact location of the abnormality that leads to excess cortisol production. The choice of test depends, in part, on the preference of the endocrinologist or the center where the test is performed.
Dexamethasone Suppression Test
This test helps to distinguish patients with excess production of ACTH due to pituitary adenomas from those with ectopic ACTH-producing tumours. Patients are given dexamethasone, a synthetic glucocorticoid, by mouth every 6 hours for 4 days. For the first 2 days, low doses of dexamethasone are given, and for the last 2 days, higher doses are given. Twenty-four hour urine collections are made before dexamethasone is administered and on each day of the test. Since cortisol and other glucocorticoids signal the pituitary to lower secretion of ACTH, the normal response after taking dexamethasone is a drop in blood and urine cortisol levels. Different responses of cortisol to dexamethasone are obtained depending on whether the cause of Cushing's syndrome is a pituitary adenoma or an ectopic ACTH-producing tumour.
The dexamethasone suppression test can produce false-positive results in patients with depression, alcohol abuse, high oestrogen levels, acute illness, and stress. Conversely, drugs such as phenytoin and phenobarbital may cause false-negative results in response to dexamethasone suppression. For this reason, patients are usually advised by their physicians to stop taking these drugs at least one week before the test.
CRH Stimulation Test
This test helps to distinguish between patients with pituitary adenomas and those with ectopic ACTH syndrome or cortisol-secreting adrenal tumours. Patients are given an injection of CRH, the corticotropin-releasing hormone which causes the pituitary to secrete ACTH. Patients with pituitary adenomas usually experience a rise in blood levels of ACTH and cortisol. This response is rarely seen in patients with ectopic ACTH syndrome and practically never in patients with cortisol-secreting adrenal tumours.
Direct Visualization of the Endocrine Glands (Radiologic Imaging)
Imaging tests reveal the size and shape of the pituitary and adrenal glands and help determine if a tumour is present. The most common are the CT (computerised tomography) scan and MRI (magnetic resonance imaging). A CT scan produces a series of x-ray pictures giving a cross-sectional image of a body part. MRI also produces images of the internal organs of the body but without exposing the patient to ionizing radiation.
Imaging procedures are used to find a tumour after a diagnosis has been established. Imaging is not used to make the diagnosis of Cushing's syndrome because benign tumours, sometimes called "incidentalomas," are commonly found in the pituitary and adrenal glands. These tumours do not produce hormones detrimental to health and are not removed unless blood tests show they are a cause of symptoms or they are unusually large. Conversely, pituitary tumours are not detected by imaging in almost 50 percent of patients who ultimately require pituitary surgery for Cushing's syndrome.
Petrosal Sinus Sampling
This test is not always required, but in many cases, it is the best way to separate pituitary from ectopic causes of Cushing's syndrome. Samples of blood are drawn from the petrosal sinuses, veins which drain the pituitary, by introducing catheters through a vein in the upper thigh/groin region, with local anaesthesia and mild sedation. X-rays are used to confirm the correct position of the catheters. Often CRH, the hormone which causes the pituitary to secrete ACTH, is given during this test to improve diagnostic accuracy. Levels of ACTH in the petrosal sinuses are measured and compared with ACTH levels in a forearm vein. ACTH levels higher in the petrosal sinuses than in the forearm vein indicate the presence of a pituitary adenoma; similar levels suggest ectopic ACTH syndrome.
The Dexamethasone-CRH Test
Some individuals have high cortisol levels, but do not develop the progressive effects of Cushing's syndrome, such as muscle weakness, fractures and thinning of the skin. These individuals may have Pseudo Cushing's syndrome, which was originally described in people who were depressed or drank excess alcohol, but is now known to be more common. Pseudo Cushing's does not have the same long-term effects on health as Cushing's syndrome and does not require treatment directed at the endocrine glands. Although observation over months to years will distinguish Pseudo Cushing's from Cushing's, the dexamethasone-CRH test was developed to distinguish between the conditions rapidly, so that Cushing's patients can receive prompt treatment. This test combines the dexamethasone suppression and the CRH stimulation tests. Elevations of cortisol during this test suggest Cushing's syndrome.
Some patients may have sustained high cortisol levels without the effects of Cushing's syndrome. These high cortisol levels may be compensating for the body's resistance to cortisol's effects. This rare syndrome of cortisol resistance is a genetic condition that causes hypertension and chronic androgen excess.
Sometimes other conditions may be associated with many of the symptoms of Cushing's syndrome. These include polycystic ovarian syndrome, which may cause menstrual disturbances, weight gain from adolescence, excess hair growth and sometimes impaired insulin action and diabetes. Commonly, weight gain, high blood pressure and abnormal levels of cholesterol and triglycerides in the blood are associated with resistance to insulin action and diabetes; this has been described as the "Metabolic Syndrome-X." Patients with these disorders do not have abnormally elevated cortisol levels.
How is Cushing's syndrome treated?
Treatment depends on the specific reason for cortisol excess and may include surgery, radiation, chemotherapy or the use of cortisol-inhibiting drugs. If the cause is long-term use of glucocorticoid hormones to treat another disorder, the doctor will gradually reduce the dosage to the lowest dose adequate for control of that disorder. Once control is established, the daily dose of glucocorticoid hormones may be doubled and given on alternate days to lessen side effects.
Several therapies are available to treat the ACTH-secreting pituitary adenomas of Cushing's disease. The most widely used treatment is surgical removal of the tumour, known as transsphenoidal adenomectomy. Using a special microscope and very fine instruments, the surgeon approaches the pituitary gland through a nostril or an opening made below the upper lip. Because this is an extremely delicate procedure, patients are often referred to centers specializing in this type of surgery. The success, or cure, rate of this procedure is over 80 percent when performed by a surgeon with extensive experience. If surgery fails, or only produces a temporary cure, surgery can be repeated, often with good results. After curative pituitary surgery, the production of ACTH drops two levels below normal. This is a natural, but temporary, drop in ACTH production, and patients are given a synthetic form of cortisol (such as hydrocortisone or prednisone). Most patients can stop this replacement therapy in less than a year.
For patients in whom transsphenoidal surgery has failed or who are not suitable candidates for surgery, radiotherapy is another possible treatment. Radiation to the pituitary gland is given over a 6-week period, with improvement occurring in 40 to 50 percent of adults and up to 80 percent of children. It may take several months or years before patients feel better from radiation treatment alone. However, the combination of radiation and the drug mitotane (Lysodren®) can help speed recovery. Mitotane suppresses cortisol production and lowers plasma and urine hormone levels. Treatment with mitotane alone can be successful in 30 to 40 percent of patients. Other drugs used alone or in combination to control the production of excess cortisol are aminoglutethimide, metyrapone, trilostane and ketoconazole. Each has its own side effects that doctors consider when prescribing therapy for individual patients.
Ectopic ACTH Syndrome
To cure the overproduction of cortisol caused by ectopic ACTH syndrome, it is necessary to eliminate all of the cancerous tissue that is secreting ACTH. The choice of cancer treatment—surgery, radiotherapy, chemotherapy, immunotherapy, or a combination of these treatments—depends on the type of cancer and how far it has spread. Since ACTH-secreting tumours (for example, small cell lung cancer) may be very small or widespread at the time of diagnosis, cortisol-inhibiting drugs, like mitotane, are an important part of treatment. In some cases, if pituitary surgery is not successful, surgical removal of the adrenal glands (bilateral adrenalectomy) may take the place of drug therapy.
Surgery is the mainstay of treatment for benign as well as cancerous tumours of the adrenal glands. In Primary Pigmented Micronodular Adrenal Disease and the familial Carney's complex, surgical removal of the adrenal glands is required.