Hyponatremia: terminology and more

Karen Yeates and associates,1 in their article on the management of hyponatremia, use the terms “serum osmolality” and “tonicity” interchangeably, a common practice. Although there is not a major difference in meaning, it is important to differentiate these terms in this context. Tonicity is effective serum osmolality and is equal to serum osmolality minus the concentration of ineffective osmoles (mainly urea), since urea can diffuse in and out of the cell and is not an effective osmole.

In the algorithm for the management of hyponatremia (Fig. 1 of the paper), Yeates and associates1 advise assessing extracellular fluid (ECF) volume status after initial treatment of symptomatic acute or chronic hyponatremia, but this should be done before treatment is started. In cases of acute hyponatremia, treatment would not have any ill effects, but if the hyponatremia is chronic and is treated aggressively, the consequences could be fatal, especially in women.2 In addition, aggressive treatment of chronic hyponatremia secondary to syndrome of inappropriate secretion of antidiuretic hormone (SIADH) might lead to a worsening of the hyponatremia,3 as alluded to by Yeates and associates in the text of their article.1 If urine electrolyte levels are determined after treatment (i.e., after volume repletion), the results are often equivocal and thus may not be helpful in patient management.

In the section “The case revisited,” the authors recommend an alternative medication to treat the patient's systolic hypertension. However, the patient is described as having taken a thiazide diuretic for 5 years with no previous history of hyponatremia. The acute episode of hyponatremia had a clear cause: volume depletion secondary to gastroenteritis and volume replacement with free water. It would be more appropriate to withhold the diuretic until the acute illness had resolved and to reintroduce it with caution, rather than changing the drug entirely.

Malvinder S. Parmar Medical Director Medical Program (Internal Medicine) Timmins and District Hospital Timmins, Ont.