General Screening...Provocative Testing...Non-Provocative Tests

It is suggested that the reader be familiar with the general features of canine hyperadrenocorticism and the normal regulation of adrenal corticosteroid levels before proceding with the following discussion

The diagnosis of Canine Hyperadrenocorticism is not straight forward. General bloodwork and urinalysis results, though they may be suggestive, are not specific for the diagnosis of hyperadrenocorticism (aka "Cushing's Syndrome").The plasma or urine cortisol (or related corticosteroid) measurements are the most non-specific tests that, at best , are only suggestive for the presence of canine hyperadrenocorticism. Initially, therefore, more direct modalities for making the initial diagnosis should employ provocative testing and imaging studies.HOWEVER, having said that, It should be noted that many other illnesses...non-adrenal in nature... and even stress will interfere with the interpretation of most of the commonly used methods.

Finally, not all dogs with Cushing's Syndrome read the text books...so it is often the case that dogs with this disease will test (falsely) negative. In these instances, it is necessary to re-interpret the results of several tests and imaging methods as a unit, combine this with the patient's clinical signs and then rely upon the clinical judgement of the veterinarian in making the defintive diagnosis.

Once an unequivocal diagnosis is obtained via initial provocative testing, other assays and imaging methods are used to ascertain the type of hyperadrenocorticism (i.e. PDH or ADH...see introduction for explanation of these and other basic information about adrenal gland activity) . This becomes important because the recommended treatments for each differ.

General Screening Tests:

Provocative Tests for Hyperadrenocorticism:

Anyway, the principles of provocative testing involves a little understanding about the stimulation of normal adrenal activity by the pituitary and hypothalamus and the inhibition of these centers by elevated serum corticosteroid levels. For review of these phenomena, please refer to an earlier discussion. The goals of these initial tests are to determine with certainty 1) whether the animal has canine hyperadrenocorticism and 2) if it does, whether it is PDH (Pituitary-Dependent Hyperadrenocorticism) or ADH (Adrenal-Dependent Hyperadrenocorticism). The preferred provocative tests to diagnose hyperadrenocorticism are the: ACTH Stimulation Test and the Low Dose Dexamethasone Suppression Test (LDDST). Only the former provocative test is useful to diagnose Iatrogenic Cushing's Disease. Other tests (e.g High Dose Dexamethasone Suppression Test are help in distinguishing whether Cushing's disease is due to PDH or ADH. In addition to the ACTH Stimulation Test, measuring the concentration of endogenous blood ACTH may be helpful in the diagnosis of iatrogenic disease. The most common tests are described below.

Note: In some animals with signs of hyperadrenocorticism, post-stimulation cortisol levels are not increased but one or more sex hormones (which can cause similar signs as cortisol when present in excess) may be increased. The hormones most likely to increase are: progesterone and/or 17-hydroxyprogesterone and/or androstenedione. Some are now advocating the measurement of all of these sex steroid hormones levels before and after stimulation with ACTH (if results of standard stimulation of cortisol levels are inconclusive or negative).
(Reference: JAVMA 226 (February 2005 ): pp556)

Non Provocative Tests

 

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