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Hyperadrenocorticism, known as Cushing's disease, is a hormone imbalance that results from excessive cortisol in the bloodstream over a long period of time. Cortisol is produced and stored in the adrenal glands, which are two little glands that “sit” on top of the kidneys and is what is released in times of stress, preparing for a “flight or fight” response. However, if this system goes awry, and a dog's body is exposed to this hormone for a majority of the time instead of just in times of stress, it can become chronically debilitating.
There are 3 main ways that a dog can get Cushing's Disease. The first way is from a tumor forming on the pituitary gland, and is known as Pituitary-dependent hyperadrenocorticism (PDH). These tumors are generally small and non-cancerous, although rarely a cancerous tumor can occur. The pituitary gland is located in the brain and is the “master gland” of the body. One of its jobs is to detect when cortisol levels are declining, and in response, secrete a stimulating substance, called ACTH, that kicks the adrenal gland into gear causing it to secrete more cortisol. When the body reaches normal levels of cortisol, the pituitary gland stops this message from being sent, and production is halted. In the case of PDH, a tumor causes the pituitary gland to go into overdrive, telling the adrenals to produce excessive amounts of cortisol, despite there already being too much in the body. This is the most common form of Cushing's and accounts for about 85% of dogs with disease.
Adrenal gland tumors are the next common cause, account for approximately 15% of dogs affected. This is a situation where a tumor is on the adrenal gland instead of the pituitary, and puts the gland into overdrive. The adrenals begin making excessive steroids all on their own, no longer “listening” to the pituitary when it tells the adrenals to shut off. These tumors are generally larger in size (usually detectable on ultrasound) and both cancerous and non-cancerous tumors are possible. Another problem that can happen with this type of tumor is that the pituitary “sees” that there is enough cortisol in the body, and subsequently stops producing ACTH. As a result, the other adrenal gland (the one without the tumor) becomes shrunken due to nonuse.
The last major cause is what is known as Iatrogenic Cushing's and is the result of the long-term use of steroids, or medications containing steroids. This is not from any inherent disease in the pet's system, but from the effects of the hormones given over the long term. Over time, the pituitary gland perceives that the body is getting enough steroids (thinking it is being produced from the adrenal glands-not knowing it is coming from medications) and quits sending its signal to produce more. In turn, the adrenal glands stop doing their job and also begin to shrink, temporarily loosing their ability to release cortisone on their own should their body require it to do so. This inability to produce steroids naturally in the body can last for several months following the stopping of medication. This is why your veterinarian will instruct tapering doses of steroids instead of an abrupt stop: this gives these important glands time to recover and begin working on their own again.
There are a multitude of clinical signs that can be seen, and as always, can mimic many other disease processes. Signs are generally gradual, and because of this, they are often attributed to “normal aging” and disregarded. The most common signs that are easily observed in pets by their owners are increased drinking and urination, increased appetite (a good reminder that eating well is not always a sign of normal health), a “pot-bellied” appearance, thin skin and sparse hair coat, blackheads and/or darkening of the skin, and loss of muscle mass or muscle weakness. Aside from these symptoms described, advanced or untreated Cushing's disease can also put a dog at risk for the development of bladder stones, diabetes and blood clots to the lungs.
If you notice any of these signs in your pet, a veterinary exam is in order. Your veterinarian can begin the process of testing for the disease, ensuring your pet is appropriately treated. If your veterinarian has reason to suspect Cushing's (based on history, physical exam and initial blood work), it will then be necessary to perform confirming blood tests and ideally, an ultrasound. This is not an easy diagnosis to make, and it requires several specific tests to positively identify not only the presence of Cushing's, but whether the problem is in the pet's pituitary or adrenal glands, as there is a different treatment for each form of disease.
The expected course of disease depends on which type of Cushing's is present, as well as response to treatment. PDH generally carries a good prognosis, and survival time for a dog treated with appropriate therapy is 2 years, with at least 10% of dogs surviving 4 years (this is better than it sounds, as dogs are generally diagnosed at an older age, usually 10-12 years old). Dogs with non-cancerous adrenal gland tumors usually have a good to excellent prognosis; those with cancerous tumors that have not spread can have a fair to good prognosis, making early detection important.
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