Zycortal Symposium Proceedings

The reason why the dogs are able to maintain normal electrolyte concentrations is currently unknown. A few dogs with primary hypoadrenocorticism and normal electrolytes have normal (usually low normal) aldosterone concentrations with no or very little increase after ACTH. It is possible, that in those dogs the zona glomerulosa is only partially destroyed at that particular point in time. As described above, segmental sparing of the zona glomerulosa has been reported. It may be that with time the destructive, immune-mediated process will finally involve all three zones equally, although this assumption is nearly impossible to prove. Successive loss of the different zones of the adrenal gland is certainly a rare event. Interestingly, one case report documented a dog with initial hypoaldosteronism, followed by hypocortisolaemia six weeks later 36 . In a third – most likely very small – subgroup of dogs with primary hypoadrenocorticism and normal electrolytes the immune-mediated destruction only involves the zona fasciculata/reticularis and the zona glomerulosa is normal. It is likely, but currently unknown, that those dogs have normal aldosterone concentrations. In principle only those dogs would ‘deserve’ the term “atypical” hypoadrenocorticism.

Concluding remarks

During the work-up of dogs with suspected hypoadrenocorticism and normal sodium and potassium several measures are important to consider:

• Most important is to exclude previous steroid application.

• An EDTA sample for measurement of endogenous ACTH should be taken prior to the administration of synthetic ACTH, centrifuged immediately and stored at -20°C.

• If the ACTH stimulation test confirms hypoadrenocorticism endogenous ACTH should be measured to differentiate between primary and secondary hypoadrenocorticism.

• Increased ACTH confirms primary hypoadrenocorticism, low ACTH is suspicious for secondary hypoadrenocorticism. In the latter case, imaging of the pituitary gland should be considered. • In case of primary hypoadrenocorticism, measurement of aldosterone additional to measurement of cortisol (in the pre- and post-ACTH samples) would be interesting, however, will not have implications for treatment. It will be low in most of the cases.

• Glucocorticoid treatment should be initiated and tapered to the lowest possible dose.

• Regular re-evaluations (initially every 2-3 weeks) should be performed, to ensure that electrolytes are still normal.

25