Affects: Cats, Dogs
Hypoadrenocorticism in dogs, or, as it is known in people, Addison's disease, is an endocrine system disorder that occurs when the adrenal glands fail to produce enough hormones for normal function. The adrenal glands secrete glucocorticoids such as cortisol and mineralocorticoids such as aldosterone; when proper amounts of these are not produced, the metabolic and electrolyte balance is upset. Mineralocorticoids control the amount of potassium, sodium, and water in the body. Hypoadrenocorticism is fatal if left untreated.
The most common cause of inadequate adrenal production is idiopathic adrenocortical atrophy. All causes for hypoadrenocorticism are not yet known. The usual causes are genetic, often related to autoimmune disorders, where the body attacks and kill its own tissue ("immune mediated destruction"). Other cases are caused by various disease processes, including failure of the pituitary gland to secrete ACTH, the hormone which stimulates the adrenal production of cortisol.
Hypoadrenocorticism is more frequent in dogs than in humans; in fact, it may occur one hundred times more often in the canine population. It mostly affects young to middle-aged female dogs, as the average age at diagnosis being four years old (although it has been found in puppies and dogs up to twelve years old). About seventy percent of dogs that are diagnosed with hypoadrenocorticism are female. Hypoadrenocorticism is still relatively uncommon or underdiagnosed in dogs. Statistics gathered from a large veterinary hospital placed the number at 0.36 dogs per 1000. For an average veterinary practice with two veterinarians and 1500 canine patients, this would mean an average of one diagnosis of the disease each year.
Signs And Symptoms: The most common clinical manifestations are related to mental status and gastrointestinal function; they include lethargy, anorexia, vomiting, weight loss, and weakness. Additional findings may include dehydration, bradycardia, weak femoral pulses, abdominal pain, lack of appetite, tremors or shaking, muscle weakness, low body temperature, collapse, and pain in the hindquarters. Polyuria and polydipsia, diarrhea, and shivering are occasionally reported.
Hypoglycemia can also be present, and initially may be confused with a seizure disorder or an insulin-secreting pancreatic tumor (insulinoma). Hypoadrenocorticism may also be misdiagnosed as food poisoning, parvovirus enteritis, gastric volvulus, or spinal/joint problems, earning this disease nicknames like "the Great Mimic" and "the Great Imitator". It is possible not to see any signs of the disease until 90% of the adrenal cortex is no longer functioning.
Causes: The adrenal glands are located above the kidneys. The adrenal outer layer, or cortex, has three layers; each produces a specific type of steroid.
Diagnosis: Hypoadrenocorticism is often tentatively diagnosed on the basis of history, physical findings, clinical pathology, and, for primary adrenal insufficiency, characteristic electrolyte abnormalities.
Clinical pathology - Abnormalities may be identified on hematology, biochemistry and urinalysis. Elevated concentrations of potassium (hyperkalemia), and low sodium and chloride values (hyponatremia and hypochloremia) are the classic electrolyte alterations. The sodium/potassium ratio often is <27 (normal is between 27:1 and 40:1) and maybe <20 in animals with primary adrenal insufficiency. However, not all dogs have an abnormal electrolyte ratio during an Addisonian episode.
ECG - The severity of the ECG abnormalities correlates with the severity of the hyperkalemia. Therefore, the ECG can be used to identify and estimate the severity of hyperkalemia and to monitor changes in serum potassium during therapy.
Treatment: Aggressiveness of therapy depends on the clinical status of the patient and the nature of the insufficiency (glucocorticoid, mineralocorticoid, or both). Many dogs and cats with primary adrenal insufficiency are presented in Addisonian crisis and require immediate, aggressive therapy. In contrast, secondary insufficiency often has a chronic course.
Hypoadrenocorticism is treated with oral daily administration of fludrocortisone (trade name Florinef) or a monthly injection of desoxycorticosterone pivalate, DOCP (Percorten-V or Zycortal) and daily prednisone or prednisolone. One drug is needed to supplement mineralcortidoids and the other to supplement corticosteroids. This effectively replaces what the adrenal cortex is failing to produce. Routine blood work is necessary in the initial stages until a maintenance dose is established. Most of the medications used in the therapy of hypoadrenocorticism cause excessive thirst and urination. It is absolutely vital to provide fresh drinking water for a canine with this disorder.
If the owner knows about an upcoming stressful situation (shows, traveling etc.), the animals generally need an increased dose of prednisone (2-4 times maintenance) to help deal with the added stress. Avoidance of stress is important for dogs with hypoadrenocorticism. Physical illness also stresses the body and may mean that the medication(s) need to be adjusted during this time. Most dogs with hypoadrenocorticism have an excellent prognosis after proper stabilization and treatment.