Cushing’s Disease, or Hyperadrenocorticism, is a condition caused by excessive production of natural steroid hormones (mainly cortisol) in the adrenal glands. It is most common in dogs, but does also occur (rarely) in cats (it’s also fairly common in horses and ferrets, but those are separate blog articles!).
What causes it?
There are three forms of the disease:
- Pituitary Dependent Cushing’s – 85% of true canine Cushing’s cases are due to a malfunction in the pituitary gland. This is usually a benign tumour (adenoma) that sends the signal (a chemical messenger called ACTH) to the adrenal glands instructing them to keep producing cortisol, even if there is plenty already in circulation.
- Adrenal Dependent Cushing’s is due to a tumour in the adrenal gland that constantly produces more and more cortisol. It is less common than the pituitary form.
- Iatrogenic Cushing’s (sometimes called “False Cushing’s”) occurs when excessive amounts of artificial steroids (e.g. prednisolone) is given to a dog or cat, causing steroid overdose and the symptoms of Cushing’s. It is probably the most common, but is very easily cured by lowering the dose of medication!
In both dogs and cats, true Cushing’s is more common in middle-aged to elderly animals.
What are the symptoms?
Steroids have a wide range of different effects, and therefore Cushing’s has a lot of potential symptoms. The more common ones include:
- Increased urination and thirst (steroids lead to more dilute urine being produced, so the animal must drink more to compensate, meaning they urinate more – a vicious cycle).
- Increased hunger.
- Panting and dislike of hot weather.
- Obesity and/or muscle wasting and a pot-bellied appearance.
- Muscle weakness
- Recurring urinary tract infections.
- Secondary respiratory infections, especially in cats.
- Increased risk of diabetes.
- low healing of minor wounds.
- Skin changes:
- Hair loss, typically along the flanks.
- Thinning of the skin.
- Blackheads (comedones) in the skin.
- Abnormal whitish calcium deposits in the skin (calcinosis cutis).
- Skin infections.
- Increased risk of Demodex mite infestation.
- Increased skin frailty, resulting in easily bruised or cut skin.
- Other possible symptoms (although they are rarer) include:
- Increased blood pressure.
- Abnormal clotting.
- Heart failure.
- Seizures or blindness.
- Increased risk of cruciate ligament injury (especially in small dogs).
- Shrinkage of the testicles.
- Overgrowth of genitalia in the bitch.
- Prostate overgrowth in neutered dogs.
How is it diagnosed?
The clinical signs are often very suggestive, but unfortunately there is no one test that is perfect for diagnosing Cushing’s – this is because there is a lot of overlap between normal dogs with naturally high cortisol levels, and dogs with Cushing’s with relatively low cortisol levels.
The three tests that are most commonly used are:
- Urine Cortisol:Creatinine Ratio is a good screening test and can be performed on a urine sample collected at home, but although it will pick up most dogs with Cushing’s, it also has a high false-positive rate (so the test comes back positive in a non-Cushingoid dog).
- ACTH Stimulation Test – a blood sample is taken to measure the blood cortisol, then synthetic ACTH is injected and another sample taken a little while later. The degree of response to the drug can be used to distinguish most Cushing’s patients from those with other illnesses (although there are quite a lot of false negative results as it doesn’t pick out all the Cushing’s patients).
- Dexamethasone Suppression Test – the same basic protocol as the ACTH test, but an artificial steroid (dexamethasone) is injected instead. In a normal animal, the artificial steroid suppresses natural steroid production, but not in Cushingoid animals. Although this test also has a lot of false positives, it has the great advantage that some varieties can be used to distinguish between Pituitary and Adrenal Dependent Cushing’s.
Most of the time, we’ll use a combination of clinical signs and one or more of these tests to make a firm diagnosis of Cushing’s.
Can it be treated?
Yes, there are a number of different treatment options. In Adrenal Cushing’s where there is a single adrenal tumour, it can sometimes be surgically removed. There is also a drug called trilostane that permanently destroys adrenal gland tissue – however, there is a severe risk of causing Addison’s Disease (where the dog cannot make enough steroids), so this drug is rarely used nowadays.
However, by far the most common treatment option is a drug called mitotane, which suppresses cortisol production in the adrenal glands. This is given as a capsule by mouth once or twice daily and is very effective in the vast majority of affected dogs.