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For further information, see CMDT Part 21–02: Hyponatremia

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

  • Hyponatremia in hospitalized patients commonly caused by administration of hypotonic fluids

General Considerations

  • Most common electrolyte abnormality in hospitalized patients

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

  • Whether hyponatremia is symptomatic depends on both severity and acuity

  • Severity can be subclassified as

    • Mild (130–134 mEq/L)

    • Moderate (125–129 mEq/L)

    • Severe (< 125 mEq/L)

  • Acuity

    • Acute hyponatremia defined as lasting < 48 hours

    • Chronic hyponatremia defined as lasting > 48 hours (often diagnosed on routine electrolyte panels)

  • A diagnostic algorithm separates the causes of hyponatremia using serum osmolality and volume status (Figure 21–1)

  • SIADH is a clinical diagnosis characterized by

    • Hyponatremia

    • Hypoosmolality (< 280 mOsm/kg)

    • Absence of heart, lung, or kidney disease

    • Normal thyroid and adrenal function

    • Urine Na usually > 20 mEq/L

    • BUN may be < 10 mg/dL [3.6 mmol/L] and serum uric acid < 4 mg/dL [238 mcmol/L]

    • Low BUN and low serum uric acid are not only due to dilution but to increased urea and uric acid clearances in response to volume expansion

Figure 21–1.

A diagnostic algorithm for the causes of hyponatremia using serum osmolality, urine osmolality, and urine sodium. ADH, antidiuretic hormone; GFR, glomerular filtration rate; SIADH, syndrome of inappropriate antidiuretic hormone.

Clinical Findings

History and physical examination

  • Obtain 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


  • Documented acute hyponatremia < 48 hours

    • Sodium can be corrected at the rate at which it fell

  • Most hyponatremia is chronic > 48 hours

    • In general, sodium needs to be corrected more slowly to minimize risk of osmotic demyelination

  • Pseudohyponatremia from hypertriglyceridemia or hyperproteinemia

    • No therapy required except confirmation with the clinical laboratory

  • Translocational hyponatremia

    • From hyperglycemia or mannitol

    • Can be managed with glucose correction or mannitol discontinuation ...

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