Le défi du diagnostic du syndrome de Cushing

The diagnosis of the Cushing's syndrome must be divided into two distinct stages, the clinical and biological diagnosis of the hypercortisolism, follow-up of the differential diagnosis in order to specify the etiology. The initial biochemical investigations must confirm that the Cushing's syndrome is really present. The first diagnostic test is the increase in the cortisol secretion, and the absence of circadian rhythm, the relative insensitivity of the patient to the negative feedback of glucocorticoids (explored by dexamethasone feedback). The evaluation of urinary free cortisol is relevant. It is specific of an organic hypercorticism when the urinary cortisol is above 3 N. In addition, the study of the nychthemeral cycle of plasmatic cortisol with several samples carried out at various times of the day and the night is a good examination since the abolition of the nycthemeral variation of the secretion of cortisol makes possible to affirm the existence of a hypersecretion, the essential point with the diagnosis being the loss of the physiological fall of cortisol at midnight. The measurement of cortisol in the saliva, carried out by the patient himself, avoids the stress of a blood test, which is particularly appreciable in children. Indeed, only the latter can diffuse through the acinous cells of salivary glands. In addition, the variations of salivary cortisol follow a nychthemeral cycle completely parallel with that of plasmatic cortisol. In case of hypercortisolism, this cycle is missing in saliva; thus the proportioning of salivary cortisol at midnight can be proposed for the tracking of a hypercorticism. After having established the diagnosis of hypercortisolism, a dramatic ACTH decrease (ACTH < 2 pmol/L), is evidence for a syndrome of ACTH independent Cushing and consequently a primary adenocortical disease. An imagery with CT and/or IRM makes it possible to delimit with precision the lesion, whose treatment is usually surgical. In the presence of detectable rates of ACTH, the patients with Cushing's disease (syndrome of hypophyseal Cushing), must be differentiated from those with syndrome of ACTH ectopic secretion, which can be occult and difficult to locate.