Cushing's syndrome refers to the signs and changes that occur secondary to excessive cortisol production by the adrenal glands. Cortisol is a hormone that is necessary for many normal body functions. Its secretion from the adrenal glands is regulated by input from two regions in the brain, the hypothalamus and the pituitary gland. The hypothalamus, a small structure in the brain, secretes corticotropin-releasing hormone, or CRH, which in turn acts on the pituitary gland, also located in the brain. The pituitary gland releases adrenocorticotropic hormone, or ACTH, which then stimulates the adrenal gland to make cortisol. The cortisol produced by the adrenal gland feeds back to both the hypothalamus and the pituitary gland to regulate production of CRH and ACTH.
Although the exact mechanism that causes Cushing's disease to develop is not completely understood, there are many possible causes. In general, there are two forms -- PDH and adrenal gland tumors. The most common is the pituitary-dependent Cushing's disease. Some dogs with pituitary-dependent disease have microscopic tumors that cause the excessive ACTH production, leading to excessive cortisol secretion from the adrenal glands, while others have larger tumors that cause the same result. Some dogs with PDH do not have discrete tumors in their pituitary glands, but the cells there still produce excessive ACTH.
Dogs with the less common form of the disease, adrenal gland tumors, may have either benign or malignant tumors that produce excessive amounts of cortisol from one or both adrenal glands. The excessive cortisol concentrations in the dog's blood then lead to the clinical signs, and the changes in the bloodwork and other diagnostic findings discovered by the veterinarian. Although most dogs with Cushing's disease are not in critical condition at the time of diagnosis, serious complications of Cushing's disease are possible. These include high blood pressure; urinary tract infection and stone formation; changes in the portion of the kidney where blood is filtered; congestive heart failure; pancreatitis; diabetes; and blood clot formation, especially in the vessels supplying the lungs.
There are both surgical and medical treatments for Cushing's disease. Surgery is the preferred way to manage dogs with adrenal tumors, while medical treatment is used for dogs with PDH. The goals with treatment are to eliminate or minimize signs due to excessive cortisol production, and to prevent potentially life-threatening complications from the disease. Dogs treated successfully for Cushing's disease live for an average of two to three years.
