Zycortal Symposium Proceedings

Redefining ‘atypical’ Addison’s disease

Claudia E. Reusch

History and currently used terms

The term “atypical” hypoadrenocorticism was first introduced into the literature 36 years ago 1 . The authors described three dogs with unspecific clinical signs such as lethargy, anorexia, weight loss, episodic haematochezia and episodic vomiting. The most consistent laboratory findings were non-regenerative anaemia, eosinophilia and azotaemia, the latter was considered to be prerenal. Sodium and potassium concentrations were within normal limits. In all three dogs basal cortisol was low and failed to increase after ACTH leading to the diagnosis of hypoadrenocorticism. Two dogs were treated with daily prednisone/prednisolone and fludrocortisone from the beginning, one dog only received daily prednisolone. The latter dog initially recovered but was returned to the hospital after 15 weeks because of anorexia, lethargy, ataxia and vomiting. At that time the “typical” electrolyte abnormalities (hyponatremia, hyperkalaemia) were present and fludrocortisone was added to the treatment regimen. Another dog revealed hyponatremia during a re-check 14 months after diagnosis which resolved by increasing the fludrocortisone dose. The authors stated that the dogs most likely suffered from primary (and not secondary) hypoadrenocorticism, in particular those two dogs which developed electrolyte abnormalities later in the course of the disease. Although aldosterone was not measured they hypothesised that the initial clinical signs had been caused by glucocorticoid deficiency and that the later development of electrolyte abnormalities indicated the onset of additional aldosterone deficiency. Thereafter, various case reports and small case series describing dogs with hypoadrenocorticism and normal electrolytes were published. Since the study by Rogers et al 1 the terms “atypical” hypoadrenocorticism and “glucocorticoid-deficient hypoadrenocorticism” have been used by various authors interchangeably. A clear definition is still lacking (see also later). Most investigators would certainly agree that the term flags dogs with primary hypoadrenocorticism without hyponatremia and hyperkalaemia. Some authors have used the term “atypical” hypoadrenocorticism for dogs which had normal potassium but low sodium 2 , this, however, is not common practice today. Unfortunately, however, most studies lack thorough work-up. Usually, measurement of endogenous ACTH was not performed and it is, therefore, possible that some of the “atypical” cases in fact suffered from secondary hypoadrenocorticism. Additionally, most of the time only pre- and post-ACTH cortisol concentrations were evaluated and as the electrolytes were normal it was assumed that the zona glomerulosa and the aldosterone secretory capacity were intact. This assumption, however, most likely is incorrect for many cases. Dogs with so called “atypical” hypoadrenocorticism usually have low-undetectable aldosterone concentrations 3,4 (see later). The long-used term “glucocorticoid-deficient hypoadrenocorticism” is therefore misleading.

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