Hyponatremia

Hyponatremia 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 . . .