Vetiology

The inconspicuous adrenal glands are small but important organs which serve many functions in maintaining hormone levels. Diseases that affect the adrenal glands may have deleterious effects on the body. Addison’s disease, also known as hypoadrenocorticism, is a disease of the adrenal glands that results in a deficiency of either glucocorticoids or mineralocorticoids or both.

Glucocorticoids, such as cortisol, are involved with protein, lipid and carbohydrate metabolism and therefore affect blood glucose levels. Mineralocorticoids are adrenal steroids which have an effect on ion transport by epithelial cells, resulting in a loss of potassium and retention of sodium. In the case of a dog with Addison’s disease, the decreased production of glucocorticoids and/or mineralocorticoids will be reflected in blood biochemical values.

Addison’s disease is often referred to as “the great pretender” because the signs it causes are nonspecific. Clinical signs include lethargy, loss of appetite, weight loss, diarrhea, increased drinking, and vomiting. All of those signs can be attributed to a wide range of other diseases, so it may sometimes be difficult to diagnose. However, when a dog has repeated episodes of the above signs, Addison’s should be a suspect. This is especially true if the episodes were precipitated by a stressful event, such as a busy party or loud thunderstorm. Most cases of Addison’s disease are related to an immune-mediated process which destroys the adrenal tissue.

Occasionally, a dog may suffer from an acute episode of Addison’s disease which presents as generalized weakness, a low heart rate, heart arrhythmia (abnormal heart beat), collapse and shock. The dog will find him/herself in the emergency room and will require immediate aggressive fluid therapy, steroids and electrolytes to recover.

Although Addison’s disease can occur at any age and affect any breed or sex, it is most common in young adult female dogs. Addison’s disease has been reported in dogs as young as eight weeks of age, but the mean age of dogs at the time of diagnosis is 4 years. Breeds that may be genetically predisposed to developing Addison’s disease include the Great Dane, Portuguese Water Dog, Rottweiler, Standard Poodle, and the West Highland White Terrier.

The first step in diagnosing the disease is using a blood panel to analyze blood biochemistry levels. Addisonian dogs have unbalanced sodium and potassium levels due to the kidney’s inability to regulate those electrolytes. Liver enzymes and kidney values are also commonly elevated. To reach a definitive diagnosis, blood cortisol levels must be measured. An adrenocorticotrophic hormone (ACTH) test is performed. This test involves two blood samples. The first blood sample is drawn and then the patient is given an injection of ACTH. A second blood sample is taken a few hours later. Cortisol is measured in both samples and the two results are compared. A patient with Addison’s has a low baseline cortisol level that fails to rise after stimulation with ACTH. ACTH normally stimulates the adrenal cortex to produce glucocorticoids and mineralocorticoids. Failure of the adrenals to respond to ACTH results in an inadequate production of the hormones, which leads to the clinical signs associated with the disease.

Treatment of Addison’s disease involves supplementing the dog with both types of adrenal steroids. One steroid, called prednisone, is given at a very low dose to boost glucocorticoid levels. Another drug called fludrocortisone acetate helps replenish mineralocorticoid levels. Both drugs are given orally. An alternative to oral medications is an injection of desoxycorticosterone pivalate (DOCP), which is given about once per month. The clinical signs of Addison’s disease usually disappear after fine-tuning the treatment protocol, and protocols may vary from dog to dog. Close monitoring of blood levels to ensure that the medications are working as they should is important.

For an acute Addisonian crisis, the dog is treated as an emergency case. Aggressive treatment with IV fluids, steroids and glucose are indicated. Once stabilized, Addisonian dogs have a good prognosis and can continue to live long, happy lives.