Cushing's disease Flashcards
What is Cushing’s syndrome?
Cushing’s Syndrome is used to refer to the signs and symptoms that develop after prolonged abnormal elevation of cortisol. Cushing’s Disease is used to refer to the specific condition where a pituitary adenoma (tumour) secretes excessive ACTH. Cushing’s Disease causes a Cushing’s syndrome, but Cushing’s Syndrome is not always caused by Cushing’s Disease.
Cushing’s syndrome features?
Round in the middle with thin limbs:
Round “moon” face
Central Obesity
Abdominal striae
Buffalo Hump (fat pad on upper back)
Proximal limb muscle wasting
High levels of stress hormone:
Hypertension
Cardiac hypertrophy
Hyperglycaemia (Type 2 Diabetes)
Depression
Insomnia
Extra effects:
Osteoporosis
Easy bruising and poor skin healing
Causes of cushing’s
Exogenous steroids (in patients on long term high dose steroid medications)
Cushing’s Disease (a pituitary adenoma releasing excessive ACTH)
Adrenal Adenoma (a hormone secreting adrenal tumour)
Paraneoplastic Cushing’s
Paraneoplastic Cushing’s is when excess ACTH is released from a cancer (not of the pituitary) and stimulates excessive cortisol release. ACTH from somewhere other than the pituitary is called “ectopic ACTH”. Small Cell Lung Cancer is the most common cause of paraneoplastic Cushing’s.
How does a dexamethasone suppression test work?
The dexamethasone suppression test is the test of choice for diagnosing Cushing’s Syndrome. This involves initially giving the patient the “low dose” test. If the low dose test is normal, Cushing’s can be excluded. If the low dose test is abnormal, then a high dose test is performed to differentiate between the underlying causes.
To perform the test the patient takes a dose of dexamethasone (a synthetic glucocorticoid steroid) at night (i.e. 10pm) and their cortisol and ACTH is measured in the morning (i.e. 9am). The intention is to find out whether the dexamethasone suppresses their normal morning spike of cortisol.
Low Dose Dexamethasone Suppression Test (1mg dexamethasone)
A normal response is for the dexamethasone to suppress the release of cortisol by effecting negative feedback on the hypothalamus and pituitary. The hypothalamus responds by reducing the CRH output. The pituitary responds by reducing the ACTH output. The lower CRH and ACTH levels result in a low cortisol level. When the cortisol level is not suppressed, this is the abnormal result seen in Cushing’s Syndrome.
High Dose Dexamethasone Suppression Test (8mg dexamethasone)
The high dose dexamethasone suppression test is performed after an abnormal result on the low dose test.
In Cushing’s Disease (pituitary adenoma) the pituitary still shows some response to negative feedback and 8mg of dexamethasone is enough to suppress cortisol.
Where there is an adrenal adenoma, cortisol production is independent from the pituitary. Therefore, cortisone is not suppressed however ACTH is suppressed due to negative feedback on the hypothalamus and pituitary gland.
Where there is ectopic ACTH (e.g. from a small cell lung cancer), neither cortisol or ACTH will be suppressed because the ACTH production is independent of the hypothalamus or pituitary gland.
Other investigations apart from a dexamethasone test?
24-hour urinary free cortisol can be used as an alternative to the dexamethasone suppression test to diagnose Cushing’s syndrome but does not indicate the underlying cause and is cumbersome to carry out.
FBC (raised white cells) and electrolytes (potassium may be low if aldosterone is also secreted by an adrenal adenoma)
MRI brain for pituitary adenoma
Chest CT for small cell lung cancer
Abdominal CT for adrenal tumours
Treatment?
The main treatment is to remove the underlying cause (surgically remove the tumour)
Trans-sphenoidal (through the nose) removal of pituitary adenoma
Surgical removal of adrenal tumour
Surgical removal of tumour producing ectopic ACTH
If surgical removal of the cause is not possible another option is to remove both adrenal glands and give the patient replacement steroid hormones for life.
