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Key Features

Essentials of Diagnosis

Thorough history and volume status examination are essential to determining 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

General Considerations

  • Most common electrolyte abnormality in hospitalized patients

  • Defined as a serum sodium concentration < 135 mEq/L (135 mmol/L)

  • Can be subclassified as

    • Mild (130–134 mEq/L)

    • Moderate (125–129 mEq/L)

    • Severe (< 125 mEq/L)

  • 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.)

Clinical Findings

History and physical examination

  • Obtain careful history, including

    • New medications

    • Changes in fluid and solute intake (polydipsia, anorexia, intravenous fluid rates, and composition)

    • Fluid output (vomiting, diarrhea, ostomy output, polyuria, oliguria, insensible losses)

  • The physical examination should help categorize the patient’s volume status into

    • Hypovolemia

    • Euvolemia

    • Hypervolemia

Symptoms and Signs

  • Mild hyponatremia (sodium concentrations of 130–135 mEq/L)

    • Usually asymptomatic

    • Gait disturbances, falls, and fractures may occur

  • Nausea and malaise may progress to headache, lethargy, and disorientation as the sodium concentration drops

  • Most serious symptoms are seizure and, very rarely, coma, brainstem herniation, and death


  • Laboratory assessment should include

    • Serum electrolytes

    • Creatinine

    • Osmolality

    • Urine sodium, potassium, and osmolality

  • Additional tests of thyroid and adrenal function may be necessary


  • Hypovolemic hyponatremia

    • Patients require adequate fluid resuscitation, usually with isotonic fluid

    • Diuretics should be withheld in hypovolemic patients

    • Underlying cause of volume depletion should be addressed

    • Patients believed to have cerebral salt wasting may benefit from hypertonic saline (with close monitoring)

  • Hypotonic hypervolemic hyponatremia

    • Loop diuretics are generally required

    • Dialysis may be needed in severe acute or chronic kidney injury

    • V2 receptor antagonists (such as tolvaptan) may be helpful for severe hypervolemic hyponatremia from heart failure but are contraindicated in cirrhosis

  • Hypotonic euvolemic hyponatremia

    • Should be treated based on the underlying etiology

    • Psychogenic polydipsia can correct quickly with water restriction alone

    • Mild to moderate hypotonic euvolemic hyponatremia

      • Often responds to fluid restriction and withholding offending medications or treating pain and nausea when due to secondary to SIADH (see SIADH)

      • Increased solute intake and loop diuretics can also be used to increase water clearance in refractory cases that are not severe

    • Acute severe symptomatic hyponatremia ...

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