Chapter 81

#### High-Yield Facts

• Sodium concentration reflects both the sodium and water contents of the body.

• Antidiuretic hormone (ADH) is one of the primary regulatory hormones for sodium concentration. It responds to the changes in osmolality and intravascular volume to restore homeostasis to the body.

• Hypernatremia may result from:

• Primary sodium excess

• Hypovolemic hypernatremia

• Hypernatremia should generally be corrected gradually to avoid the complication of cerebral edema.

• Aggressive treatment of hyponatremia may lead to the osmotic demyelination syndrome.

#### Sodium

The concentration of sodium reflects the total body store of sodium and its relation to total body water (TBW). In both hypo- and hypernatremia, the total body store of sodium may be high, low, or normal. It is the amount of TBW relative to total body sodium that determines sodium concentration. Sodium is found in highest concentration in the extracellular compartment and is normally maintained between 135 and 145 mEq/L.

One of the primary regulatory mechanisms is antidiuretic hormone (ADH). ADH is secreted by the posterior pituitary gland in response to stimulation of osmoreceptors residing in the anterior hypothalamus, baroreceptors in the great vessels, and volume receptors in the left atrium. The release of ADH results in increased water absorption by the renal tubule. Osmoreceptors detect increasing osmolarity, with sodium being the primary ion responsible for extracellular osmolarity. Baroreceptors and volume receptors regulate the intravascular volume. In cases of decreasing intravascular volume and diminishing osmolarity (due to body water in excess of body sodium), the volume receptors will override the osmoreceptors, resulting in ADH secretion and water retention, despite decreasing concentrations of sodium.1

##### Hypernatremia

Hypernatremia is defined as serum sodium >150 mEq/L. It may result from intake of sodium in excess of water or, more commonly, from loss of water in excess of sodium (Table 81-1). Primary sodium excess is usually associated with iatrogenic causes, such as inadequately diluted infant formula, excessive sodium bicarbonate administration, or intravenous hypertonic saline administration. More frequently encountered is hypovolemic hypernatremia, in which water loss exceeds sodium loss. The most common cause in pediatrics is gastroenteritis with diarrhea and vomiting. Other causes include increased insensible water loss (i.e., fever, use of radiant warming devices, burns), diabetes mellitus (solute diuresis), or inadequate access to free water.

TABLE 81-1   Causes of Hypernatremia

Diabetes insipidus (DI) is a less common cause of hypovolemic hypernatremia. The essential feature is a functional lack of ADH, resulting in urinary ...

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