Hypoadrenocorticism (Addison's disease) has been referred to as 'the great pretender,' due to its ability to mimic other common diseases in the dog and thereby represent a diagnostic challenge.
Naturally occurring hypoadrenocorticism is an uncommon canine disease, with a predisposition in young to middle-aged female dogs.
The disease is usually caused by an immune-mediated destruction of all adrenocortical layers of the adrenal glands (adrenalitis), resulting in deficiencies of mineralocorticoids (aldosterone) and glucocorticoids (cortisol). A small number of dogs suffer from glucocorticoid deficiency only.
Rare causes of primary adrenal cortex destruction include infiltration by fungus (Histoplasma spp, Blastomyces spp, Coccidioides spp, Cryptococcus spp), neoplasia, amyloidosis, trauma, or coagulopathy.
Iatrogenic primary hypoadrenocorticism is relatively common cause due to treatment of hyperadrenocorticism with drugs such as mitotane. Somer dogs may develop permanent hypocortisolism from mitotane and require lifelong glucocorticoid supplementation.
Clinical signs of hypoadrenocorticism are associated with deficiency in aldosterone and cortisol. Dogs suffering from hypoadrenocorticism may present in a variety of conditions, from a mild gastroenteritis to acute shock.
Diagnosis is based on clinical signs supported by evidence of hyponatremia and hyperkalemia. A reduced sodium: potassium ratio (<25:1) raises the index of suspicion for hypoadrenocorticism. . Other features of hematology include a stress leukogram, anemia, hypercalcemia, azotemia, hypoglycemia, hypoalbuminemia and hypocholesterolemia. Mild elevations in liver enzymes (ALT, AST and GGT) are common observed.
A definitive diagnosis is based on a subnormal ACTH stimulation test (pre- and post-ACTH cortisols) and adrenal ultrasonography. Approximately 85% of dogs with hypoadrenocorticism have basal and post-ACTH cortisol concentrations of <1.0ug/dL and more than 90% have cortisol concentrations <2.0ug/dL. Very rarely, ACTH stimulation cortisol levels are not abnormally low, but serum aldosterone levels are.
The prognosis for hypoadrenocorticism is usually good with appropriate maintenance therapy and appropriate owner education.
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