Hyperadrenocorticism (Cushing's Disease)
Introduction
Hyperadrenocorticism, or Cushing's disease, is a very common endocrine disorder of dogs. The disease occurs when there is an excessive amount of cortisol present in the body. This can occur because of an increased production of cortisol within the body, or by excessive administration of corticosteroids. There are three types of hyperadrenocorticism: iatrogenic, pituitary-dependent, and adrenal tumor.
Iatrogenic Hyperadrenocorticism: This occurs as a result of the excess administration of corticosteroids (such as prednisone). This usually occurs when prednisone or other steroids have been prescribed for therapeutic purposes. This can be easily treated by slowly discontinuing the steroid administration.
Pituitary-Dependent Hyperadrenocorticism (PDH): In this case there is a tumor in the pituitary gland that results in an excessive secretion of ACTH from the pituitary. This, in turn, results in an excessive secretion of cortisol by the adrenal gland. In the normal dog, cortisol secretion by the adrenal gland is controlled by ACTH secretion from the pituitary. When there is a lot of cortisol present in the body, ACTH production is down-regulated, leading to a down-regulation of cortisol production. This is a negative feedback system. In the case of PDH, the pituitary does not suppress its production of ACTH based on circulating cortisol levels, so this results in excess circulating cortisol.
This is the most common type of Cushing's disease, accounting for approximately 80% of cases, and is usually treated medically.
Adrenal Tumor: In this case there is a tumor of the adrenal cortex itself that is producing excessive amounts of cortisol. In this case the adrenal tumor can be removed surgically to treat the disease.
Clinical Signs:
Polyuria and polydipsia (PU/PD): This is the most common sign associated with hyperadrenocorticism. It means that the dog is drinking excessively and urinating excessively.
Pendulous abdomen: These dogs may have a "potbellied appearance" due to an enlarged liver, abdominal muscle weakness, a full urinary bladder and increased fat stores in the abdomen.
Polyphagia: These dogs are always hungry and eating more then usual.
Muscle weakness
Bilaterally symmetric alopecia (hair loss): There is usually hair loss over most of the body is a symmetrical pattern. The skin also appears thin with very prominent blood vessels.
Diagnostics
When you bring your dog to the veterinarian for these clinical signs, especially PU/PD, there are many diseases that must be ruled-out to reach a diagnosis. Typically, a veterinarian will want to perform blood work on your dog and a urinalysis. X-rays or an ultrasound may also be used. There are several screening tests used to diagnose hyperadrenocorticism. Some of these specific test include urinary cortisol:creatinine ratio, low-dose dexamethasone suppression test, high-dose dexamethasone suppression test, and ACTH stimulation test.
Secondary Complications
There are many secondary complications that can occur due to untreated Cushing's disease. Chronic urinary tract infections and infections throughout the body are common due to the immunosuppressive nature of corticosteroids. High blood pressure and diabetes mellitus are also possible secondary complications. A pituitary tumor can cause central nervous system signs if it is compressing other areas of the brain.
Treatment
Pituitary-dependent hyperadrenocorticism: In this case the treatment of choice is Lysadren (mitodane, o.p'-DDD). This is a medical therapy that will reduce the amount of cortisol being produced by the adrenal gland. There are two phases to treatment with this therapy: an induction phase and a maintenance phase. During the induction phase the dog must be closely monitored. The maintenance phase is a lifelong therapy; usually administered twice a week. Dogs can do very well on this therapy and continue to live long, happy lives.
Adrenal tumor: The treatment of choice in this case is surgical removal of the tumor. This is a very effective therapy and is usually curative. However, if the tumor is inoperable or has already undergone metastasis, another treatment must be considered.
Iatrogenic hyperadrenocorticism: Gradually taper the steroid dose. This should only be done under the direction of your veterinarian because severe side effects may occur if the steroids are removed too rapidly.