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Key Features

Essentials of Diagnosis

  • Serum sodium > 145 mEq/L (> 145 mmol/L)

  • Increased thirst and water intake are the main defense against hypernatremia

  • Urine osmolality helps differentiate renal from nonrenal water loss

General Considerations

  • An intact thirst mechanism usually prevents hypernatremia

  • Excess water loss can cause hypernatremia only when water intake is inadequate

  • Rarely, excessive sodium intake may cause hypernatremia

  • The hypernatremic patient is typically hypovolemic due to free water losses

  • However, hypervolemia is frequently seen in hospitalized patients with impaired access to free water

  • Hypernatremia in primary aldosteronism is mild and usually does not cause symptoms

Etiology

  • Urine osmolality > 400 mOsm/kg

    • Nonrenal losses

      • Excessive sweating, burns

      • Insensible respiratory tract losses

      • Diarrhea, vomiting, nasogastric suctioning, osmotic cathartics (eg, lactulose)

    • Renal losses

      • Diuretics

      • Osmotic diuresis (eg, hyperglycemia, mannitol, urea)

      • Postobstructive diuresis

      • Diuretic phase of acute tubular necrosis

    • Hypertonic sodium gain

      • Salt intoxication (rare)

      • Hypertonic intravenous fluids, tube feeds, enema

      • Primary hyperaldosteronism (hypernatremia usually mild and asymptomatic)

  • Urine osmolality < 250 mOsm/kg

    • Central diabetes insipidus: idiopathic, head trauma, CNS mass

    • Nephrogenic diabetes insipidus: lithium, demeclocycline, prolonged urinary tract infections, interstitial nephritis, hypercalcemia, hypokalemia, congenital

Clinical Findings

Symptoms and Signs

  • With dehydration, orthostatic hypotension and oliguria are typical findings

  • Lethargy, irritability, and weakness are early signs

  • With severe hyperosmolality, hyperthermia, delirium, seizures, and coma may be seen

  • Symptoms in elderly may not be specific; recent change in consciousness is associated with poor prognosis

Diagnosis

Laboratory Tests

  • Urine osmolality > 400 mOsm/kg when renal water-conserving ability is functioning

  • Urine osmolality < 250 mOsm/kg when renal water-conserving ability is impaired

  • Serum osmolality invariably increased in the dehydrated state

Treatment

Medications

Type of fluid for replacement

  • Hypernatremia with hypovolemia

    • Severe hypovolemia: give 0.9% normal saline (osmolality 308 mOsm/kg) to restore volume deficit and treat hyperosmolality, followed by 0.45% saline to replace any remaining free water deficit

    • Milder hypovolemia: give 0.45% saline and 5% dextrose in water

  • Hypernatremia with euvolemia

    • Encourage water drinking or give 5% dextrose and water to cause excretion of excess sodium in urine

    • If GFR is decreased, give diuretics to increase urinary sodium excretion; however, diuretics may impair renal concentrating ability, increasing quantity of water that needs to be replaced

  • Hypernatremia with hypervolemia

    • Give 5% dextrose in water to reduce hyperosmolality, though this will expand vascular volume

    • Administer loop diuretic (eg, furosemide, 0.5–1.0 mg/kg) intravenously to remove excess sodium

    • In severe renal injury, consider hemodialysis

Calculation of water deficit

  • Fluid replacement should include the free water deficit as well as the maintenance fluid to replace ongoing and anticipated fluid losses

  • Acute hypernatremia

    • In acute dehydration without much solute loss, free water loss is similar to weight ...

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