Dexamethasone inhibition test
The so-called dexamethasone inhibition test is a provocation test. In a healthy organism, the production rate of the adrenal cortex and thus the concentration of glucocorticoids (e.g. cortisol) is controlled by a regulatory circuit between the pituitary gland and the adrenal cortex. At high cortisol concentrations, the production of a hormone of the pituitary gland (adrenocorticotropin; short: ACTH) is reduced.
This in turn causes the adrenal cortex to reduce its synthesis capacity. At low cortisol concentrations, the pituitary gland produces increased amounts of adrenocorticotropin, which is transported via the bloodstream to the adrenal cortex and stimulates its synthesis rate. More cortisol is produced.
Dexamethasone as a synthetic glucocorticoid is now able to simulate an increased cortisol level in the organism and thus reduce the production of adrenocorticotropin and finally the synthesis of cortisol. In a healthy person, the cortisol concentration should therefore drop rapidly after taking the preparation. In patients suffering from Cushing’s disease, however, pituitary and adrenal communication gets out of control.
In spite of high cortisol levels, adrenocorticotropin is produced, which ultimately leads to an uncontrolled increase in cortisol concentration. After taking dexamethasone there is therefore no inhibition of the cortisol synthesis as expected. In general, a distinction is made between a low dose and a high dose dexamethasone inhibition test.
In the low dose procedure, a single oral administration of one to a maximum of 2 mg dexamethasone is administered. The high dose test, on the other hand, requires an intake of approximately 8 mg of the active substance. As a rule, two blood samples are taken on consecutive days.
From the first sample, the cortisol concentration is determined BEFORE taking the dexamethasone, the second sample is taken about 12 hours AFTER the administration of the preparation. The cortisol concentration is determined from this sample after taking dexamethasone. A positive dexamethasone test (i.e. no reduction in synthesis after administration of the preparation) alone is sufficient to prove the presence of Cushing’s disease.
The dexamethasone inhibition test only gives a first clue. For further diagnostics a so-called CRH test, an insulin hypoglycaemia test and a 24-hour cortisol determination in the urine should be carried out. For further diagnostics, a so-called CRH test, an insulin hypoglycaemia test and a 24-hour cortisol determination in the urine should be carried out.
Side effect
The side effects of a therapy with dexamethasone are limited if the therapy is given in low doses for a short time. However, as this cannot always be adhered to, numerous side effects can occur, especially in the context of a longer-term therapy. The blood and immune system may be more susceptible to infections, changes in the blood count and a weakening of the immune system.
In the area of metabolic bodily functions, dexamethasone therapy can lead to the development of diabetes mellitus (diabetes), increased appetite and weight gain, fat storage in certain areas (trunk obesity, bull neck, bloated face) and lipid metabolism disorders. Possible psychological symptoms are irritability, increased drive, restlessness, depression, insomnia, psychosis and manic states, so that increased caution is recommended especially for patients with pre-existing mental illness. Furthermore, there may be an increased risk of seizures in patients with known epilepsy.
Possible side effects on the eye are the development of a green or grey cataract (glaucoma/cataract). In the heart, there may be cardiac insufficiency, high blood pressure and cardiac arrhythmia due to changes in blood salts. In the area of the digestive organs, possible side effects include stomach ulcers with an increased risk of bleeding, inflammation of the pancreas (pancreatitis), inflammation of the oesophagus (esophagitis), nausea, vomiting and flatulence.
The skin and hair may show increased hair growth, thinning of the skin with parchment skin, spotty or streaky changes in the skin and allergic reactions of the skin. In the area of the skeletal system, bone loss (osteoporosis), increased bone fragility, torn tendons, muscle weakness and growth inhibition in children can occur. Cycle disorders in women or impotence in men are also possible side effects.