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Diagnosis

When Cushing's is suspected, a dexamethasone suppression test (administration of dexamethasone and frequent determination of cortisol and ACTH levels) and 24-hour urinary measurement for cortisol have equal detection rates (Raff & Findling 2003). Dexamethasone is a glucocorticoid and simulates the effects of cortisol, including negative feedback on the pituitary gland. When dexamethasone is administered and a blood sample is tested, high cortisol would be indicative of Cushing's syndrome because there is an ectopic source of cortisol or ACTH (eg: adrenal adenoma) that is not inhibited by the dexamethasone. A low cortisol reading would be indicative of Cushing's disease because the dexamethasone inhibited the pituitary adenoma so that its' output of ACTH decreased, resulting in decreased cortisol levels. A novel approach, recently cleared by the US FDA, is sampling cortisol in saliva over 24 hours, which may be equally sensitive, as late night levels of salivary cortisol are high in Cushingoid patients. Other pituitary hormones may need to be determined, and performing physical examination directed for any visual field defect may be necessary if a pituitary lesion is suspected (which may compress the optic chiasm causing typical bitemporal hemianopia).

When these tests are positive, CT scanning of the adrenal gland and MRI of the pituitary gland are performed to detect the presence of an adrenal or pituitary adenoma. These should be performed when other tests are positive, to decrease likelihood of incidentalomas (incidental discovery of harmless lesions in both organs). Scintigraphy of the adrenal gland with iodocholesterol scan is occasionally necessary. Very rarely, determining the cortisol levels in various veins in the body by venous catheterisation working towards the pituitary (petrosal sinus sampling) is necessary.

 


Date: 2015-01-12; view: 696


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Clinical signs | Diagnosis
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