Feline Hyperadrenocorticism (HAC – Cushing’s Disease)
Hyperadrenocorticism is a rare condition in the cat, much more so than in the dog. The scarcity of true clinical cases means that there are few studies of diagnostic accuracy for the different screening tests. Unlike the situation in the dog, we do not have good information on diagnostic sensitivity and specificity. In its absence, recommendations have been derived from studies in healthy cats and some extrapolated from the dog.
The clinical appearance of reported cases has been dramatic. Alopecia has not been a feature but potbelly and dramatic skin thinning have been. The skin may become so thin that it tears spontaneously or on handling.
The cat does not have a steroid induced iso-enzyme of alkaline phosphatase like that in the dog.
If feline hyperadrenocorticism is being considered as a differential diagnosis for insulin resistance in a diabetic, an investigation for acromegally (by IGF-1) may prove more fruitful.
Feline ACTH Stimulation Test
As with any test for hyperadrenocorticism (see canine HAC) this test may be affected by a variety of chronic illnesses causing a significant false positive rate.
- Collect basal blood sample.
- Inject 0.125 mg* of synthetic ACTH (Synacthen*) i/v.
- Collect two further blood samples at one and three hours later.
- Label sample tubes clearly and request cortisol.
* 0.25mg may be used in cats over 5kg
* If Synacthen is unavailable Dechra Veterinary Products Ltd have an alternative drug to Synacthen available on special prescription, contact technical@dechra.com for details.
Interpretation
Normal cats will increase up to about 400 nmol/L.
Feline Combined High-Dose Dexamethasone/ACTH Test
Because more than one diagnostic endpoint is assessed, this test may be more accurate than an ACTH stimulation test alone.
- Collect basal blood sample.
- Inject 0.1mg/kg Dexamethasone i/v.
- Collect a second blood sample at 2 hours.
- Immediately inject 0.125 mg of synthetic ACTH (Synacthen*) i/v.
- Collect a third blood sample at 3 hours (1 hour after ACTH).
- Label sample tubes clearly and request cortisol.
* If Synacthen is unavailable Dechra Veterinary Products Ltd have an alternative drug to Synacthen available on special prescription, contact technical@dechra.com for details.
Interpretation
Normal cats show at least 50% suppression to a value <40nmol/L after dexamethasone and normal cortisol response after ACTH stimulation (up to 400 nmol/L). Cats with HAC show little suppression after dexamethasone and an exaggerated response after ACTH.
ACTH – Endogenous ACTH may be used to assist in the diagnosis of feline HAC and is useful in differentiating PDH and ADH but it should not be used as a diagnostic test in the first instance. Special sample handling procedures are required (see canine section, page 24).
Cortisol/Creatinine Ratio (CCR) – This is a very sensitive test to exclude HAC but must not be used to diagnose HAC, as it is very poorly specific because non-adrenal illness will commonly cause a positive result.
A morning urine sample is collected and this reflects the cortisol release over several hours. Samples taken from a litter tray in the home environment will have a lower false positive rate than those obtained from animals in the clinic. Non-absorptive material such as gravel, glass beads etc should be used as litter.
Normal cats have a CCR less than 10 x 10-6.
Cortisol/Creatinine Ratio (CCR) with Dexamethasone Suppression Test
When Cushing’s disease is strongly suspected, this is a test which is especially useful in cats, as they usually get very stressed when visiting the vet. The owner does all the sampling at home and samples are sent to the lab together.
The procedure is the same as for the dog CCR/Dex. test, except that the litter in the litter tray is replaced with washed and dried aquarium gravel and the urine samples collected with a plastic pipette. Wash the tray and gravel thoroughly between samples.