Cushing's Syndrome in Dogs
Cushing's Syndrome is a disorder whereby the hormone cortisol is produced to excess. Its proper name is "Hyperadrenocorticism," but that's too much of a mouth-full so we'll refer to it as Cushing's Syndrome, after the man who first discovered it.
Cortisol is made by the adrenal gland, found next to the kidney. Before we consider what's going wrong in Cushing's Syndrome, you need to know a bit about how the normal adrenal gland works. The adrenal gland produces lots of different hormones, and is divided into a number of sections. It's the outermost section, or "cortex", that produces cortisol. However, cortisol can only be produced by the normal adrenal gland when stimulated to do so by a second hormone called adreno-corticotrophic hormone, or ACTH for short. ACTH is produced by the pituitary gland, which is found at the base of the brain. The pituitary gland is in turn stimulated to produce ACTH by the hypothalamus, which is a nearby region of the brain. This complex arrangement of hypothalamus-pituitary-adrenal gland is called the HPA axis.
In the dog, two forms of Cushing's Syndrome are recognised. The first occurs due to the pituitary gland producing too much ACTH, usually because of a tumour within the gland. This causes an overstimulation of an otherwise normal adrenal gland, which then produces too much cortisol. This is called pituitary dependant. The second form is when the adrenal gland itself becomes diseased, again due to a tumour forming within it, and starts to produce cortisol independently of the pituitary gland. This is called adrenal dependant.
The majority of the symptoms of Cushing's Syndrome are due to the excess of cortisol production. It's important to remember that no dog will show all of the following symptoms, and some are much more common than others. The duration of the disease will also determine which symptoms the dog will have.
There are many other abnormalities, especially with the composition of the blood and its chemistry.
No disease has a consistent set of symptoms which are seen in all cases, that proves without doubt what it is. Many different diseases have very similar symptoms. For example, in many Cushing's Syndrome cases, the only symptom is increased thirst. Increased thirst is often the only symptom of Addison's disease, which is the opposite of Cushing's Syndrome (i.e. not enough cortisol!). So, we have to use tests which will definitively confirm the disease as Cushing's Syndrome, and nothing else.
On the basis of the history, physical examination, and possibly blood and urine tests, Cushing's Syndrome may be suspected. Before embarking on an expensive course of medication to stop the adrenal gland working, we have to be sure it is Cushing's Syndrome, because the adrenal gland produces hormones which are essential for life. That means, without a functional adrenal gland, the animal will die.
Testing for Cushing's Syndrome
At Vetrica, we use the currently accepted most reliable test, which is the Synacthen® stimulation test. A blood sample is taken to determine the amount of cortisol naturally present. This is followed by an injection of synthetic ACTH (Synacthen®), and any rise in cortisol determined by taking a second blood test one hour later. This is a quick, easy, cheap test to perform.
Unfortunately, no test for Cushing's Syndrome is perfect. Some dogs with Cushing's Syndrome will not test positive. In cases where the symptoms are very suggestive of Cushing's Syndrome, after eliminating all other possible causes for the dog's symptoms, we usually repeat the test a month or so later.
Cushing's Syndrome can be treated both medically and surgically, depending on its exact origin in any particular animal. The vast majority of dogs are treated medically.
Medical Management of Cushing's Syndrome
The treatment of choice for Cushing's Syndrome is a drug called trilostane, marketed in the UK as Vetoryl®. This is a new drug, long term studies are still in progress. However, it appears to be safer, and more effective than the previously used drug, mitotane, which will not be discussed further here.
Trilostane directly inhibits the production of cortisol by the adrenal gland. This effect is reversible, which means in the event of overdosage simply withdrawing the drug should be sufficient treatment; but it also means that the drug will have to be given for the remainder of the dog's life.
The dose is variable, and has to be established for each patient. A starting dose is determined based on the dog's weight, followed by regular monitoring of the effect of the drug by performing further Synacthen® stimulation tests. Most dogs only require the drug to be given once daily, but occasionally it must be given twice.
Surgical Management of Cushing's Syndrome
There are three types of surgery which can be employed to treat Cushing's Syndrome, and once more, it depends to a large extent which type the dog has.
All of these techniques are high risk, and are associated with high mortality rates. The reason for this is probably that a major operation is being performed on a very sick animal. The standard of aftercare has to be impeccable, and the patient cooperative. In cases of confirmed adrenal gland tumour, where it's thought not to have spread to any other organ, then removal can be contemplated. Discussion of these procedures is beyond the scope of this article.
Cushing's Syndrome is a very serious disease, which if untreated leads to great suffering in the affected animal. Long term studies using trilostane are not yet available. This is primarily a disease of old dogs, and once treated successfully, it's quite possible that the dog will live out its natural lifespan.
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