Hyperadrenocorticism Part 1: Pathophysiology and Diagnosis

Presenter: Dr Christopher Byers DVM,DA CVECC,DACVIM (SAIM), CVJ from criticalcaredvm.com

I relish getting stuck into a decent medical work up and this includes diagnosing hyperadrenocorticism (HAC) which rarely disappoints. We all know these cases don’t always follow the textbook, but in my opinion, this is exactly what makes them so interesting, all be it challenging. There are many potential pitfalls in diagnosing HAC, and in order to avoid these, a strong knowledge base of the subject is necessary, and Dr Christopher Byers did an excellent job in delivering this at last week’s veterinary webinar organised by ‘The Webinar Vet’.

This was part one of a two part webinar series discussing HAC and covered the pathophysiology and diagnosis of HAC with a change in nomenclature featuring as one of the many essential facts delivered. The terms pituitary dependant and adrenal dependant disease are no longer used to categorise HAC and have been replaced by ACTH dependant and ACTH independent respectively.

ACTH dependant disease is seen in animals with micro/macroadenomas of the pituitary gland and affects 80-85% of dogs with HAC. Dr Byers emphasised that we must ensure owners understand, in the majority of cases, that we are dealing with a benign tumour of the pituitary gland. Dr Byers explained how some of his referred cases come with owners assuming their dog has a ‘brain tumour’ which is of course technically correct but sounds terrifying to them and can sometimes sway the owner away from treatment. Dr Byers also warned us that some adenomas of the pituitary gland can cause neurological signs but often these signs only present after treatment has been initiated. This is due to the anti-inflammatory effect of the previously hyper cortisol state on the pituitary mass which is lost once treatment is started.

ACTH independent disease is seen in 15-20% of cases caused by either a benign or malignant adrenal tumour which causes unilateral adrenomegaly with contralateral adrenal atrophy. This is in contrast to ACTH dependant disease which causes bilateral adrenal enlargement. This can be very useful information when an HAC diagnosis has been made as ultrasound of the adrenal glands in the hands of an experienced ultrasonographer can be used to differentiate between ACTH dependant and ACTH independent disease.

Of course an actual diagnosis of HAC has to be made prior to reaching for ultrasound, and this is where life can be tricky. There are essentially three tests used to screen for HAC, urine cortisol:creatinine ratio, the low dose dexamethasone supression test and ACTH stimulation test. Dr Byers discussed the pros and cons of each of these tests within the webinar. I was intrigued by his advice on how the physiological effects of steroids could affect the test results. For example when performing a urine cortisol:creatinine ratio, the urine must be collected at home and at least 3 days after a visit to the surgery in order to minimise the effects of stress on the test. The use of exogenous steroids must also be considered even with the use of topical steroids such as eye drops where a wash out period of at least 4 weeks is recommended.

What is clear from this veterinary webinar is that there is no perfect screening test for HAC. A choice has to be made based on each specific case and arming ourselves with the information delivered by Dr Byers gives us the best opportunity to get our diagnosis right and move onto treatment. This brings us onto ‘Part 2 – The Treatment of HAC’ for which I await in anticipation.

The Stethoscope (MRCVS)