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  • Thorough history and volume status examination are essential to determine etiology.

  • Hyponatremia reflects excess water relative to sodium; total body sodium may be appropriate, low, or high.

  • Hypotonic fluids commonly contribute to hyponatremia in hospitalized patients.


Defined as a serum sodium concentration less than 135 mEq/L (135 mmol/L), hyponatremia is the most common electrolyte abnormality in hospitalized patients. Hyponatremia can be subclassified as mild (130–134 mEq/L), moderate (125–129 mEq/L), or severe (below 125 mEq/L).

A common misconception is that the sodium concentration is a reflection of total body sodium. In fact, total body sodium can be low, normal, or high in hyponatremia since the kidney independently regulates sodium and water homeostasis. Hyponatremia reflects water imbalance relative to sodium, which frequently, but not always, is secondary to elevated levels of antidiuretic hormone (ADH), causing the kidney to retain water. A diagnostic algorithm using serum osmolality and volume status separates the causes of hyponatremia into therapeutically useful categories (Figure 21–1).

Figure 21–1.

Evaluation of hyponatremia using serum osmolality and extracellular fluid volume status. GI, gastrointestinal; SIADH, syndrome of inappropriate antidiuretic hormone; Uosm, urine osmolality. (Modified, with permission, from Narins RG et al. Diagnostic strategies in disorders of fluid, electrolyte and acid-base homeostasis. Am J Med. 1982 Mar;72(3):496–520. Copyright © Elsevier.)


A. Isotonic and Hypertonic Hyponatremia

Serum osmolality identifies isotonic and hypertonic hyponatremia, although these cases can often be identified by careful history or previous laboratory tests.

Isotonic hyponatremia, also called pseudohyponatremia, is a laboratory artifact that can cause the sodium concentration to be underestimated in the setting of an abnormally elevated percentage of serum that is solid rather than liquid, such as with hyperlipidemia or hyperproteinemia. Pseudohyponatremia does not occur if the serum sodium is measured with an ion-specific electrode in a direct assay of an undiluted serum specimen.

Hypertonic hyponatremia, also called translocational hyponatremia, occurs when a large amount of a substance that cannot easily cross the cell membrane is added to the ECF compartment. This change in tonicity “pulls” intracellular water into the ECF and dilutes the sodium concentration. Classic examples of this phenomenon are hyperglycemia and iatrogenic mannitol administration (most often as a temporizing measure for increased intracranial pressure). Hypertonic hyponatremia is not a disorder of sodium or water; rather, it is water redistributing in response to a change in tonicity. When the offending substance is cleared (for example when the hyperglycemia is treated), the ECF tonicity decreases and water shifts back into the intracellular space. Occasionally, hyponatremia is misidentified as hypertonic hyponatremia because the serum osmolality is elevated. However, the substance raising the serum osmolality is an ineffective osmole. For example, in kidney disease, the calculated osmolality may be elevated because of high urea levels. ...

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