What Is Cushing's Disease?
The endocrine system is the collective system of glands that produce and secrete hormones in the body, one of which is cortisol. At regular levels, cortisol performs many functions, including helping pets respond to stress, but too much cortisol in the pets body can do damage.
The condition associated with an excess of cortisol is medically referred to as hyperadrenocorticism, or Cushing’s disease, and it is one of the most common endocrine disorders that affects dogs.
Cushing’s disease in dogs develops when their own body overproduces cortisol.
Symptoms of Cushing's Disease
To determine whether or not a dog has Cushing’s disease, the vet will need to look not just at the dog’s symptoms, but also at the results of several different diagnostic tests.
Some common symptoms associated with Cushing’s disease in dogs are:
Urinating at night or having accidents
Fat pads on the neck and shoulders
Loss of hair
Lack of energy
Darkening of the skin
Hard, white scaly patches on the skin and elbows
What Causes Cushing's Disease?
The most common cause of hyperadrenocorticism in dogs is a benign pituitary tumour.
When the dog’s Cushing’s disease develops because of problems within the pituitary gland, the condition is called pituitary-dependent hyperadrenocorticism (PDH). PDH is responsible for approximately 80 to 85 percent of cases of naturally occurring hyperadrenocorticism in dogs.
Tumours within the adrenal gland (adrenal-dependent hyperadrenocorticism or ADH) are responsible for the other 15 to 20 percent of cases of naturally occurring Cushing’s disease in dogs. Adrenal tumours have about an equal chance of being benign or malignant.
Excessive administration of corticosteroid medications can also cause hyperadrenocorticism in dogs. These drugs are commonly used to treat allergies, immune disorders and some types of cancer; to reduce inflammation; or as replacement therapy for low, naturally occurring cortisone levels.
To diagnose Cushing’s disease in dogs, the vet will first take a health history of the dog and then perform a complete physical exam. Lab work like a blood chemistry profile, complete blood cell count, feacal examination and a urinalysis will follow.
The first test is often a urine cortisol: creatinine ratio. If the test results are normal, then the dog probably does not have Cushing’s disease. If the dog has a high urine cortisol: creatinine ratio, then further testing is needed.
The most common test used to diagnose Cushing’s disease in dogs is the low-dose dexamethasone suppression test (LDDST). A blood sample is taken to measure the dog’s baseline cortisol level, and then a small amount of dexamethasone is administered by injection. Blood cortisol levels are measured four and eight hours after the dexamethasone is given.
In a normal dog, the dexamethasone injection inhibits secretion of a hormone that stimulates cortisol secretion, which leads to a decrease in circulating cortisol levels. In a dog with Cushing’s disease, cortisol is not suppressed.
Veterinarians may need to also run an ACTH stimulation test and/or perform an abdominal ultrasound to determine whether or not the dog has Cushing’s, and if so, whether PDH or ADH is to blame.
Treatment for Cushing's Disease
Treatment for Cushing’s disease that develops due to corticosteroid medication overuse is straightforward. Dogs should be slowly weaned off of these medications while under a veterinarian’s care. Removing these medications too quickly can lead to a life-threatening condition called an Addisonian crisis.
Dogs with mild symptoms associated with pituitary-dependent Cushing’s disease may not need immediate treatment but should be closely monitored to determine when it would be beneficial. In general, treatment should start when a dog develops symptoms that are potentially dangerous and/or troublesome to the pet or owner.
Once the decision to treat a dog’s pituitary-dependent Cushing’s disease has been reached, a veterinarian will likely prescribe trilostane (Vetoryl). This drug can have serious side effects, so dogs taking it should be closely monitored.
When a patient is diagnosed with an adrenal tumour, chest radiographs and possibly a CT scan or MRI should be taken to examine the body for any possible metastatic spread of the disease. If no metastases are seen, the dog is often given a medication (trilostane) for a few months to shrink the tumor, followed by surgery to remove it.
If the dog is being treated with trilostane for Cushing’s disease, the owner needs to be prepared to continue treatment for the life of their pet. They will need to be observant for any adverse reactions to these powerful medications.
Typical signs of an adverse reaction are lack of energy, weakness, lack of appetite, vomiting, diarrhea and sometimes difficulty walking.
Because of the cost and risks associated with use of trilostane, it is often recommended to under-treat rather than over-treat the dog.
The vet will schedule regular follow-up visits to monitor for the adverse effects of trilostane.
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