Classifying Hyponatremias According to Tonicity Disorder: Hypotonic Hyponatremia, Hypertonic Hyponatremia, and Pseudohyponatremia as Distinct Entities.

Tonicity of a solution refers to its ability to cause cells in it to either swell or shrink through osmotic fluid flux across the cell membranes due to a difference in the total concentrations of osmoles in the solution and in the intracellular space. For serum, the sodium concentration ([Na]S) constitutes the major determinant of tonicity. Three categories of tonicity can be found in cases of hyponatremia: (a) hypotonic hyponatremia created by a relative excess of body water over the sum of exchangeable body sodium plus exchangeable body potassium and causing osmotic fluid entry into cells; (b) hypertonic hyponatremia created by gain of an extracellular solute other than a sodium salt causing osmotic fluid exit from cells; and (c) pseudohyponatremia (isotonic hyponatremia), which is the result of measurement of [Na]S by a method requiring pre-measurement dilution of the serum sample in patients with a high serum solid content. In this latter case, the low value of the measured [Na]S falsely suggests hypotonic hyponatremia. The principles of treatment vary among these three categories of hyponatremia. Identifying the pathophysiological category of a case of hyponatremia constitutes the first step in its management. This review presents important clinical characteristics of the three pathophysiological categories of hyponatremia and a method for identifying the category of any hyponatremia.