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Hypernatremia

Last updated: February 22, 2021

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Hypernatremia is defined as a serum sodium concentration exceeding 145 mEq/L. Sodium is the most important osmotically active particle in the extracellular space and closely linked to the body's fluid balance. An increase in the serum sodium concentration is most often due to a free water deficit caused by excessive fluid loss (e.g., diarrhea/vomiting, sweating, increased diuresis) or insufficient water intake (e.g., altered mental status, impaired thirst mechanism). In some cases, hypernatremia is due to a real sodium overload caused by high sodium intake (e.g., hypertonic infusion, drinking sea water) or inadequately high sodium reabsorption by the kidneys (e.g., primary hyperaldosteronism, Cushing syndrome). Symptoms are predominantly neurological and nonspecific (e.g., lethargy, confusion, focal neurological deficits, seizures, coma), and they are often accompanied by signs of cellular dehydration (e.g., dry mucous membranes, decreased salivation). Assessing the patient's volume status (during physical examination) and their renal ability to reabsorb free water (urine osmolality) can help determine the etiology of hypernatremia. Acuity of onset, volume status, and etiology should all be considered in order to determine the appropriate therapeutic approach and avoid complications. In children, a rapid decrease in the serum sodium concentration can cause cerebral edema, which carries the risk of brain herniation. In adults, a relationship between rapid correction and cerebral edema has not been observed; however, guidelines still recommend cautious correction.

Hypernatremia: elevated serum sodium concentration (> 145 mEq/L) [1][2]

In hypernatremia, serum osmolality is always increased, resulting in a hypertonic state. This is either due to a free water deficit (due to low intake or loss) or increased sodium (due to high intake or retention).

Hypovolemic hypernatremia

Euvolemic hypernatremia

  • Description: high serum Na+ levels with normal or minimal changes in extracellular volume as a result of pure water deficit
  • Extrarenal causes (manifests with oliguria due to decreased water intake)
  • Renal cause (causes increased thirst due to polyuria)
    • Diabetes insipidus
      • Central: complete or partial lack of ADH secretion
      • Nephrogenic: complete or partial resistance to the action of ADH

Hypervolemic hypernatremia

Overview of fluid compartment changes

Volume status Fluid compartment changes
Total body water Total body sodium
Hypovolemic hypernatremia ↓↓
Euvolemic hypernatremia Normal
Hypervolemic hypernatremia ↑↑

Hypernatremia is always a hyperosmolar state!

References:[3]

Acute hypernatremia (onset < 48 hours) [4][5]

Symptoms are primarily neurological and depend on the severity of hypernatremia.

Chronic hypernatremia (onset > 48 hours ago)

Approach [3]

Laboratory studies [3]

In a state of free water deficit, highly concentrated urine (urine osmolality) is a sign of normal kidney function.

Hypernatremia is always a hypertonic state!

Evaluation of hypernatremia [2][3][4]
Etiology History and clinical features Supportive diagnostic findings
Urine osmolality > 600 mOsm/kg
(extrarenal mechanisms)
GI losses
Skin losses
Insufficient access to water
Impaired thirst
Excessive salt intake
  • UNa > 100 mEq/L
Urine osmolality < 600 mOsm/kg
(intrarenal mechanisms)
Nephrogenic diabetes insipidus
Central diabetes insipidus [6]
Osmotic diuresis
Diuretics
Primary hyperaldosteronism
Cushing syndrome

Treatment of hypernatremia requires replacing the free water deficit with sterile water enterally (oral, nasogastric tube, PEG tube) or 5% dextrose in water (D5W) intravenously. All patients should be carefully monitored with serial labs and some may need additional therapies to restore volume status.

Approach [2][4]

The cornerstone of hypernatremia management is replacing the free water deficit.

Calculation of free water deficit in hypernatremia [4]

Calculations of the free water deficit and ongoing water losses are more accurate than rough estimates. Electrolytes must be carefully monitored during treatment and regimens should be individually tailored.

Correction of free water deficit [2][4]

  • Acute hypernatremia (onset within < 48 hours)
    • Decrease Na+ concentration by 1–2 mEq/L/hour (i.e., replace entire free water deficit in < 24 hours). [4]
    • Estimated fluid replacement regimen: ∼ 3 mL/kg/hour of IV D5W or enteral water [4]
  • Chronic hypernatremia (onset within > 48 hours)
    • Gradually restore a normal Na+ level by decreasing Na+ concentration by 0.5 mEq/L/hour (max. 10–12 mEq/L per 24 hours). [4][13][14]
    • Oral rehydration with free access to water may be sufficient in stable and alert patients.
    • Estimated fluid replacement regimen: 1.35 mL/kg/hour of IV D5W or enteral water [15]
  • Adjustment of infusion rate
    • Fluids other than D5W or sterile water require a different flow rate to provide the same amount of free water.
    • Depending on serial Na+ concentration

Additional fluid management strategies

Alleviate Acute hypernatremia Aggressively and Correct Chronic hypernatremia Carefully!

Monitoring and disposition

  • Serial BMPs every 1–2 hours for acute hypernatremia; every 4–6 hours for chronic hypernatremia
  • Adjust free water infusion rate according to target Na+ correction rates.
  • Monitor for hyperglycemia. [16]
  • Replace electrolyte deficiencies (see “Electrolyte repletion").
  • Consider ICU admission for patients with severe hypernatremia (Na+ > 160 mEq/L) and high-risk symptoms (e.g., seizures).
  • Confirm hypernatremia (repeat BMP).
  • Assess volume status.
  • If the patient is hypovolemic, volume resuscitate first with isotonic fluids (e.g., normal saline).
  • Determine whether hypernatremia is acute or chronic in nature.
  • Free water replacement (enterally or D5W IV): Correct aggressively for acute hypernatremia and gradually for chronic hypernatremia.
  • Order serial BMPs, monitor closely, and adjust fluid rate as needed.
  • Treat the underlying cause and prevent further fluid losses.
  • Consider ICU admission in patients with severe hypernatremia (i.e., > 160 mEq/L).

Acute hypernatremia

Chronic hypernatremia

We list the most important complications. The selection is not exhaustive.

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