Cushing's syndrome is a group of symptoms produced by excess steroids circulating in the body. The steroids in this case are glucocorticoids or glucose stimulating, not anabolic, which are testosterone-like, muscle building steroids. Cushing syndrome is found in all species. In the horse, equine Cushing's syndrome or ECD, occurs as a result of a tumor-like growth in the pituitary gland, which is a gland at the base of the brain that produces a variety of hormones.
In affected horses, the pituitary gland tumor secretes an excessive quantity of several hormones, particularly one that is called adrenocorticotropic hormone (ACTH). Excessive ACTH acts directly on the adrenal glands producing sustained and elevated levels of cortisol, the important glucocorticoid hormone of horses. Other names for ECD include pars intermedia adenoma, which refers to the pituitary gland tumor, and hyperadrenocorticism, which refers to the excess cortisol released from the adrenal glands. A cause for the development of the pituitary tumor has not been identified, but it is believed to be a result of abnormal neurotransmitter production in the brain itself.
What to Watch For
ECD is typically seen in older horses. It is rare in horses less than 18 years of age. The clinical signs of ECD are attributable to the effects of excessive cortisol and include:
Not all of these symptoms are found in every horse with ECD. The most frequently recognized symptom is hirsutism, but more subtle signs are seen in most cases earlier in the stage of disease. They just go unrecognized. Veterinarians are suspicious of underlying ECD in older horses that show any of these symptoms.
At the same time, ECD can worsen other conditions that typically affect old horses. An important example is the development of tooth root abscesses that occur because the teeth are worn down and high levels of steroids from ECD that depress the immune system, allowing infections to take hold.
Diagnosis of ECD is based, first of all, on recognition of the appropriate clinical symptoms in the older horse. Specific diagnosis entails the use of various blood tests. Blood ACTH and/or insulin levels are unusually high in ECD. Blood glucose and urine glucose are also increased, and the white blood count reflects a picture of stress (high white blood cell count which is mostly neutrophils).
If a diagnosis of ECD is confirmed, treatment is often very effective. The two most effective drugs are pergolide and cyproheptadine. These drugs are used to attempt to restore the brain's neurotransmitter imbalance and suppress ACTH release from the pituitary gland. If ACTH release can be inhibited, cortisol release from the adrenal glands will also be reduced and the severity of the clinical symptoms will be lessened.
It might require several weeks for a beneficial response to treatment to be noticed, because, under on-going stimulation by ACTH, the adrenal glands become quite enlarged. In some cases, one or another symptom will improve to the greatest extent. Laminitis is sometimes difficult to reverse, but in the horse with low-grade laminitis, it can usually be controlled. Excessive water drinking and sweating are the easiest to control.
Sometimes these drugs are not effective at reducing ACTH release, so obvious changes will not be seen in the blood work.