Cushing's disease (also known as Itsenko-Cushing disease) is a cause of Cushing's Syndrome characterised by increased secretion of adrenocorticotropic hormone (ACTH) from the anterior pituitary. This is most often as a result of a pituitary adenoma. Pituitary adenomas are responsible for 70% of endogenous Cushing's syndrome, that is, when excluding Cushing's syndrome from exogenously administered corticosteroids, which is the major cause of Cushing's syndrome.
Signs and Symptoms
The condition is characterised by mainly the same features as Cushing's Syndrome with the added condition that ACTH levels are also elevated, despite the elevated levels of glucocorticoids such as cortisol.
Diagnosis is made first by diagnosis of Cushing's Syndrome which involves the 1 mg dexamethasone suppression test. If the patient has Cushing's Syndrome, the 8 mg dexamethasone suppression test is carried out which will confirm the etiology of the disease.
Following the 8 mg dexamethasone suppression test plasma cortisol should be low; while plasma ACTH should be at the higher end of normal or should exceed the reference range. This will distinguish whether the cause is an ectopic ACTH secreting tumour, an adrenal carcinoma or a pituitary adenoma - ranges differ between different labs.
A CT or MRI of the pituitary may also show the ACTH secreting tumour if present.
Administration of corticotropin releasing hormone (CRH) can differentiate this condition from ectopic ACTH secretion. If ectopic, the plasma ACTH and cortisol levels should remain unchanged; if this is pituitary related, levels of both would rise.
Treatment of a pituitary adenoma usually involves removal of the tumor via transphenoidal surgery. Access to the sphenoidal sinus can either be transnasal (through the nostril) or sublabial (through an incision under the top lip).