What is canine Cushing’s disease?
Cushing’s disease is another name for hyperadrenocorticismーmeaning inappropriately high levels of cortisol, a steroid hormone produced by the body.
Where does cortisol come from?
Cortisol is secreted by the adrenal glands in response to stress. Its production is regulated by the pituitary gland, an organ located at the base of the brain. If the body needs more cortisol, the pituitary gland releases another hormone called ACTH. This acts on the adrenal gland to increase cortisol production and release. Lower levels of ACTH result in lower levels of cortisol.
What does cortisol do?
Cortisol’s natural role is multi-facetedー it acts on almost every system in the body! It is involved in blood sugar management, regulation of the immune system, kidney function, and even secretion of acid into the stomach. It’s a vital hormone, and its absence can cause major health issues. However, too much of a good thing is equally harmful!
Which dogs tend to get Cushing’s disease?
Cushing’s is more common in older dogs, with the median age of onset being eleven years old. It can occur in any breed, although poodles, dachshunds, miniature Schnauzers, terriers, and boxers are overrepresented.
Why does it happen?
There are two major types of hyperadrenocorticismーpituitary dependent and adrenal dependent. Pituitary dependent hyperadrenocorticism (PDH) is by far the most common cause of disease, involving about 85% of dogs with Cushing’s. In these cases, the pituitary gland is making too much ACTH, which tells the adrenal glands to keep making cortisol, even when there is too much in the body. Usually this is caused by a benign tumor in the pituitary gland. This tumor typically grows very slowly, and rarely spreads to other tissues. Its main effects are to play havoc with the regulation of hormones. PDH occurs equally in both sexes, and is more common in small breed dogs.
The less common type of Cushing’s disease is adrenal dependent hyperadrenocorticism (ADH). In these cases, a benign or malignant tumor in the adrenal gland itself is secreting excessive amounts of cortisol. It’s more common in female and large breed dogs.
There is another type of disease which should also be mentioned, known as iatrogenic Cushing’s. Iatrogenic disease is an illness caused by medical care. Chronic administration of steroids can cause identical clinical signs as naturally occurring Cushing’s disease. Steroids are powerful medications, and there are many times when they are medically necessary. However, they should be used cautiously, and at the lowest effective dose, for the shortest amount of time possible. Talk to your veterinarian if you have concerns about using steroids in your pet, and never stop steroid administration abruptly, as this can cause other problems.
What are the symptoms of Cushing’s disease?
Cushing’s often has a subtle, insidious onset. Its classic signs include increased thirst and urination, increased appetite, excessive panting, abdominal distension, a thinning haircoat, poor muscle condition, and weight gain. Affected dogs may be less energetic than normal. Many of these changes may be chalked up to old age, but if an older dog seems lethargic, potbellied, and balding, it may be wise to check for Cushing’s. Some dogs will down bowl after bowl of water, and begin having accidents in the house, even if they were previously housebroken.
Oftentimes, dogs with Cushing’s will have skin issues. These can include thin skin, comedones (blackheads), and alopecia (hair loss). Skin infections are also common. A rare but distinctive sign is called calcinosis cutis, and refers to hard beads of calcium deposited in the skin itself.
Some of the signs of Cushing’s seem more like inconveniences than major medical issues. However, untreated Cushing’s disease can increase the risk of some very serious problems. Chronic high cortisol levels cause slowed wound healing and susceptibility to other infections. Cushingoid dogs are very likely to have skin or urinary tract infections. They are also predisposed to high blood pressure, which can cause damage to the kidneys and eyes, and increase the risk of dangerous blood clots. Cushing’s disease also increases the risk of developing diabetes mellitus due to its effects on glucose regulation. Dogs with Cushing’s also develop pancreatitis at a higher rate than those without it.
Less commonly, dogs with pituitary dependent disease may develop neurologic signs from the tumor itself, including behavior changes, circling, or seizures.
How is Cushing’s disease diagnosed?
This disease can be tricky to diagnosis. Due to the risk of false positives, only dogs with clinical signs of disease should be tested for Cushing’s. However, “sick” dogs should not be tested. This means that dogs with vomiting or diarrhea, or significant illness caused by something else, shouldn’t be tested for Cushing’s disease until they have recovered from that issue. Cortisol levels will be higher in a sick dog, so testing for Cushing’s would not be accurate.
Oftentimes, there are suggestive changes on screening labwork, including a complete blood count, serum chemistry, and urinalysis. These changes may prompt your veterinarian to suggest more targeted testing for Cushing’s.
Most tests for Cushing’s disease measure cortisol levels in the body. One main type is called an ACTH stimulation test. In this test, cortisol is measured before and after administering an injection of ACTH. In dogs with Cushing’s disease, the spike in cortisol levels after giving ACTH will be exaggerated.
Another type of test, a low or high dose dexamethasone suppression test, works on the opposite principle. Cortisol is measured before and after giving an injection of a synthetic steroid. In a normal dog, cortisol levels will decrease after the injection, but not in a dog with Cushing’s disease.
Both of these tests can be appropriate, and finer subtypes may be necessary to differentiate pituitary dependent versus adrenal dependent disease. Other diagnostics may also be suggested, such as a urine culture to screen for concurrent urinary tract infection, or an abdominal ultrasound to assess the size and shape of the adrenal glands.
How is it treated?
Pituitary dependent disease is typically managed medically. There are two common medications used to treat it, called trilostane and mitotane. Although they work in slightly different ways, both medications have been shown to be equally effective in treatment, and are given as oral tablets. Dogs on either of these medications will require lifelong treatment, and regularly scheduled labwork to monitor their response. In a few select locations, surgery to remove the pituitary tumor itself is possible. This is usually completely curative, but it is not economically feasible for most owners.
For adrenal dependent disease, surgical removal of the affected adrenal gland is the treatment of choice. This is a complicated procedure, and should ideally be performed by a boarded surgical specialist. Before surgery, it is important to screen for metastatic disease in other parts of the body, as this can affect the pet’s outcome. If surgery is not possible, medical treatment with trilostane or mitotane can be used, but is often not as effective as treatment of PDH.
For iatrogenic Cushing’s disease, the treatment is simple — stop giving steroids! Most dogs’ clinical signs will resolve if the external source of steroids is removed.
It is important to recognize that for dogs with naturally occurring Cushing’s disease, treatment must be carefully monitored. There are possible side effects with any medication. For both of the commonly used drugs for Cushing’s, the most common side effect is self-limiting gastrointestinal signs. However, one risk with treatment is temporary or permanent HYPOadrenocorticism, or dangerously low levels of cortisol. This can cause vomiting, diarrhea, weakness, tremors, and even collapse. In some cases, it can be fatal. Your vet may even send home a dose of a steroid which can be given in an emergency at home, if low cortisol levels are suspected. If you think that your pet is displaying unusual signs while being treated for Cushing’s disease, be sure to contact us promptly.
What is the prognosis?
For dogs with pituitary dependent disease, the prognosis is excellent with treatment. Most dogs will go on to live several years, and may pass away from unrelated causes. Concurrent diseases and issues such as diabetes and hypertension may shorten life expectancy.
With adrenal dependent disease, surgery may completely resolve the issue. However, surgery to remove an adrenal gland has a relatively high risk of complications, with a post-operative mortality rate as high as thirty percent. If surgery and recovery go smoothly, there is a median survival time of two years. With medical management, the median survival time is eleven months. However, this varies substantially from dog to dog.
Rothrock, K., & Shell, L. (2015, December 11). Hyperadrenocorticism, Adrenal-Dependent. Retrieved June 1, 2019, from https://www.vin.com/Members/Associate/Associate.plx?DiseaseId=3060.
Rothrock, K., & Shell, L. (2015, November 24). Hyperadrenocorticism, Pituitary-Dependent. Retrieved June 1, 2019, from https://www.vin.com/Members/Associate/Associate.plx?DiseaseId=1324.
Wilson, S. (2017, July 6). Diagnosing Hyperadrenocorticism FAQ. Retrieved June 1, 2019, from https://www.vin.com/doc/?id=7549494.