What is Canine Cushing’s syndrome?

Ideally, when testing for any disease, it is best to choose a test which is both highly specific and highly sensitive. In practical terms, a test with high sensitivity is good at finding animals with the disease and therefore will produce few false negative results. A test with high specificity gives a positive result in animals that really do have the disease, therefore will produce few false positives.

However, no test for Cushing’s has both high sensitivity and high specificity – all are a compromise between the two. This means the diagnostic tests we have for canine hypercortisolism often have to be used in combination, in order to be confident in the diagnosis.

(Please note - the percentage values provided for sensitivity and specificity below should be considered approximate and are representative of multiple investigative studies on this topic. The author refers the reader to the 2019 Bennaim et al review for further information)

ACTH Stimulation Test (ACTHST):

High specificity (90%), moderate sensitivity (85% PDH and 50% ADH)
False positives are less common. False negatives are fairly common.

Low-Dose Dexamethasone Suppression Test (LDDST):

Moderate specificity (70%), high sensitivity (95%).
False positives are fairly common. False negatives are less common.

Urine Cortisol: Creatinine Ratio (UCCR):

Poor specificity (20%), high sensitivity (97%).
False positives are common. False negatives are less common.

When interpreting any of these tests, it is worth considering the positive and negative predictive value. Positive predictive value (PPV) is the fraction of dogs with a positive test that truly have Cushing’s. Negative predictive value (NPV) is the fraction of dogs with a negative test that are truly free of the condition.

The positive and negative predictive values take both the test and the population being tested into account. If you use a test in two populations with different disease prevalence, the predictive values will be different. Therefore the PPV and the NPV vary depend on how frequently you test.

When interpreting any of these tests, it is worth considering the positive and negative predictive value. Positive predictive value (PPV) is the fraction of dogs with a positive test that truly have Cushing’s. Negative predictive value (NPV) is the fraction of dogs with a negative test that are truly free of the condition.

The positive and negative predictive values take both the test and the population being tested into account. If you use a test in two populations with different disease prevalence, the predictive values will be different. Therefore the PPV and the NPV vary depend on how frequently you test.

• If we were to test almost every patient we see (a population with a low prevalence - 5%)

In this scenario, if we use the LDDST we can be 100% sure a negative result is truly negative. However only 16% of positive results will actually have Cushing’s.

• If we were more critical about the patients we test (a population with a high prevelance - 90%)

In this scenario, if we use the LDDST, 62% of negative results will truly be negative and 97% of positive results will have Cushing’s.

Conclusion

The 2012 ACVIM Consensus Statement panel (Behrend et al. 2013) considers the low-dose dexamethasone suppression test (LDDST) as the screening test of choice unless iatrogenic Cushing’s is suspected. Because of the ACTH stimulation test’s lower sensitivity its diagnostic usefulness as a screening test for natural occurring Cushing’s syndrome is inferior to the LDDST'.

However, it is also important to take other factors into account when selecting a diagnostic test, including test availability, cost and concurrent disease present in the patient. It is equally important to improve the potential outcome of the test by ensuring there is a high index of suspicion of disease before confirmatory testing is undertaken.

The low-dose dexamethasone suppression test measures the resistance of the pituitary-adrenal axis to suppression by dexamethasone. The following protocol has been prepared by Dechra in combination with specialist laboratories, however, if you have any queries, we would recommend checking this protocol with your regular laboratory before carrying out the test.

1. Collect a basal fasted blood sample (1-2 ml) and label this tube as ‘pre Dex’. Suitable sample types at most laboratories include separated heparinised plasma or serum, or centrifuged serum gel tubes


2. Immediately inject 0.01 mg/kg to 0.015 mg/kg dexamethasone I.V. It has been suggested that using 0.015 mg/kg dexamethasone may help reduce the chance of false positive results.
   
   The volume of dexamethasone to administer in ml = (Bodyweight (kg) x Dose (mg/kg)) / Concentration of Dexamethasone solution (mg/ml). Volumes for injection are small for this test and, in some cases, it can be helpful to make a 1:10 dilution of dexamethasone before administration.


3. Collect two further blood samples, 3 hours and 8 hours post Dexamethasone injection. Label the sample times clearly on the tubes (e.g. ‘3hrs post’)


4. Submit tubes and request form to the laboratory

In a normal healthy dog, the dexamethasone inhibits pituitary ACTH production via negative feedback, resulting in reduced cortisol production by the adrenal glands. Cortisol secretion is inhibited within 2 - 3 hrs and suppression lasts as long as 24 - 48 hr.

The low-dose dexamethasone suppression test measures the resistance of the pituitary-adrenal axis to suppression by dexamethasone and is interpreted in 2 stages:

• Firstly, the presence or absence of Cushing’s syndrome is determined by examining the 8 hour result. An 8 hour cortisol value greater than 40 nmol/l is generally considered to represent a ‘positive’ result
  
  Typically in a dog with ADH, the adrenal tumour secretes cortisol autonomously and ACTH production is already suppressed, thus cortisol production is not suppressed in response to dexamethasone administration.

• The second step only applies in the positive cases and checks for evidence of cortisol suppression. In up to 60% of PDH cases, there will be marked suppression of cortisol (to <50% of the baseline value) at either 3 hours or 8 hours providing a way to differentiate between the two types of hypercortisolism.

The ACTH stimulation test is a measure of adrenocortical reserve. The following protocol has been prepared by Dechra in combination with specialist laboratories and using the 2012 ACVIM consensus statement on diagnosis of spontaneous canine Cushing’s (Behrend et al, 2013). However, if you have any queries, we would recommend checking this protocol with your regular laboratory before carrying out the test.

1. Collect a basal fasted blood sample (1-2 ml) and label this tube as ‘pre ACTH’ Suitable sample types at most laboratories include separated heparinised plasma or serum, or centrifuged serum gel tubes


2. Immediately inject 5 µg/kg synthetic ACTH IV.


3. Collect a second blood sample (1-2 ml) one-hour post injection of synthetic ACTH. Label tube as ‘post ACTH’


4. Submit tubes and request form to the laboratory
    

The following video illustrates how a normal healthy dog should respond to an ACTHST. As you can see, the synthetic ACTH stimulates the adrenal glands to produce more cortisol, and subsequently levels should increase in the circulation. A normal increase is thought to be to around 300 – 400 nmol/l. A positive test result is normally defined as a 1 hour cortisol  >600 nmol/l, in dogs with compatible clinical signs and no evidence of concurrent non-adrenal illness.

The ACTH stimulation test will identify approximately 85% of cases of pituitary-dependent hypercortisolism.

Dogs with PDH have bilaterally enlarged adrenal glands. With the greater adrenocortical mass, an exaggerated response to ACTH can be expected

The ACTHST will also identify >50% of cases of adrenal-dependent hypercortisolism, as most dogs with ADH will also demonstrate an exaggerated response to ACTH.

However, in some dogs, especially with ADH, there may be atrophy of the normal adrenal tissue and/or the tumour may be insensitive to ACTH. In these cases, as is demonstrated by this video, we can see a ‘flat-line, mid-range’ cortisol response. This explains why the ACTHST is less sensitive for ADH than PDH.

As the sensitivity of the test is less than ideal, especially for dogs with ADH, a diagnosis of Cushing’s should not be ruled out on the basis of a normal ACTH stimulation test result if there is sufficient clinical suspicion.

The ACTH stimulation test will identify approximately 85% of cases of PDH and >50% of cases of ADH.

Therefore the sensitivity of the test is less than ideal, especially in ADH, and some dogs that actually do have hypercortisolism will have normal ACTH stimulation test results. Therefore a diagnosis of Cushing’s should not be ruled out on the basis of a normal ACTH stimulation test result if there is sufficient clinical suspicion.

In these circumstances it is recommended that a LDDST be undertaken in order to try to determine a correct diagnosis. Alternatively, if the clinical signs allow, the patient can be monitored clinically and the ACTHST repeated in 4-6 weeks’ time.

The ACTHST is the only diagnostic test which allows us to differentiate between iatrogenic and spontaneously occurring Cushing’s syndrome. Chronic administration of exogenous glucocorticoid containing treatments, including oral and injectable glucocorticoid medications, topical ear drops and skin preparations, will cause suppression of the hypothalamic, pituitary adrenal axis. This, in turn, will produce a subnormal response to the ACTHST.

The following video illustrates this:

If left untreated, symptoms are likely to progress over time. Dogs will continue to eat, drink and urinate more often. Lethargy can worsen, as can skin and coat changes associated with the syndrome. There is likely to be abdominal distention and eventually the quality of life of the affected dog can be seriously negatively impacted.

When treated with trilostane (the main medical therapy for Cushing’s syndrome) it is anticipated that life expectancy of the patient is maintained. Importantly, however, it is expected that treatment with trilostane will improve quality of life for both the patient and owner alike.

The biggest behavioural changes that are seen with canine Cushing’s are related to eating, drinking and urination. Dogs may become ravenous and drink in excess. Equally, an increase in urination may often lead to accidents occurring within the house. Lethargy is also commonly noted - Cushingoid dogs may slow down on walks, may be unwilling to exercise and may spend more time sleeping.

There are two main forms of treatment for Cushing’s. The first is medical management, of which the licensed veterinary medication is trilostane. The second is surgical management which involves removal of tumour tissue from the adrenal gland or the from pituitary at the base of the brain. The option which is best will naturally vary from patient to patient, however Cushing’s is most commonly managed using medical therapy.