YPONATREMIA is defined as a decrease in the serum sodium concentration to a level below 136 mmol per liter. Whereas hypernatremia always denotes hypertonicity, hyponatremia can be associated with low, normal, or high tonicity. 1,2 Effective osmolality or tonicity refers to the contribution to osmolality of solutes, such as sodium and glucose, that cannot move freely across cell membranes, thereby inducing transcellular shifts in water. 3 Dilutional hyponatremia, by far the most common form of the disorder, is caused by water retention. If water intake exceeds the capacity of the kidneys to excrete water, dilution of body solutes results, causing hypo-osmolality and hypotonicity (Fig. 1B, 1E, 1F, and 1G). Hypotonicity, in turn, can lead to cerebral edema, a potentially life-threatening complication. 4 Hypotonic hyponatremia can be associated, however, with normal or even high serum osmolality if sufficient amounts of solutes that can permeate cell membranes (e.g., urea and ethanol) have been retained (Fig. 1C). Importantly, patients who have hypotonic hyponatremia but normal or high serum osmolality are as subject to the risks of hypotonicity as are patients with hypo-osmolar hyponatremia. The nonhypotonic hyponatremias are hypertonic (or translocational) hyponatremia, isotonic hyponatremia, and pseudohyponatremia. 1,2 Translocational hyponatremia results from a shift of water from cells to the extracellular fluid that is driven by solutes confined in the extracellular compartment (as occurs with hyperglycemia or retention of hypertonic mannitol); serum osmolality is increased, as is tonicity, the latter causing dehydration of cells (Fig. 1D). Re- H
