Hyponatremia

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Hyponatremia, defined as a serum sodium concentration less than 135 mEq/L (135 mmol/L), is the most common electrolyte disturbance seen in children, both in inpatient and outpatient settings. Sodium-water balance hinges on the principles that water follows sodium, but maintenance of intravascular volume takes precedence over sodium concentration.

Normal distribution of body water depends on osmotic forces, with faulty distribution leading to dysnatremias (hypo- or hypernatremia). Body water is distributed between the intracellular (ICF) and extracellular fluid (ECF) spaces, the latter comprised of the interstitial and intravascular spaces. Intravascular volume is essential for tissue perfusion and depends largely on sodium balance. By contrast, the intracellular volume, essential for cell function, is regulated primarily through the effects of water balance on plasma osmolality.

Plasma osmolality (P_osm) is the sum of the osmolalities of individual plasma solutes. Sodium salts are the principal extracellular osmoles, with lesser contributions from other ions, glucose, and urea. Therefore, the P_osm is approximately 2×plasma (Na⁺).

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