Hypernatremia

Causes

Free water deficit

• Decreased access to free water
  • Young children
  • Altered mental status
    • Sedation
    • Coma
  • Mechanical ventilation (e.g., intubation)
  • Immobilization, restraints
  • Impaired thirst
  • Impaired intake
    • Breastfeeding infants
• Hypotonic fluid losses
  • Kidney losses
    • Diuretics (loop, thiazide)
    • Osmotic diuresis
      • Hyperglycemia (e.g., uncontrolled diabetes)
      • Mannitol
    • Post-AKI diuresis (recovery from acute tubular necrosis)
    • Postobstructive diuresis
      • Obstructive uropathy
  • GI
    • Diarrhea
      • Laxative/cathartic use/abuse (e.g., lactulose)
    • Nasogastric suction
    • Drains/fistulas/ostomies
  • Respiratory tract
  • Skin (burns)
  • Fever
    • Impaired urinary concentrating ability
      • Kidney dysplasia
      • Medullary cystic kidney disease
      • Polycystic kidney disease (PKD)
      • Tubulointerstitial disease

      • Diabetes insipidus (DI)

        • Central: ↑ ADH production (arginine vasopressin deficiency, AVP-D)
        • Nephrogenic: ↓ responsiveness to ADH (arginine vasopressin resistance, AVP-R)
  • Transient water shifts
    • Occurs in setting of seizures, intense exercise
    • Resolves within 15 minutes

Sodium gain

• Salt poisoning (very rare)
  • Incorrect formula preparation
• Iatrogenic causes
  • Hypertonic electrolyte solutions
    • Sodium bicarbonate (e.g., during resuscitation)
    • Hypertonic saline
  • Sodium ingestion
    • NaCl tablets
  • Sodium modeling in
    hemodialysis (HD)

Signs and symptoms

• Acute (<48 hours) hypernatremia may be symptomatic, particularly when severe

• Chronic (≥48 hours) hypernatremia is more often asymptomatic

  • More likely to occur in children with neurological abnormalities, making it difficult to distinguish which symptoms are attributable to hypernatremia

• Increased thirst

• Increased muscle tone

• Muscle twitching

• Nystagmus

• Irritability

• Restlessness

• Seizures

• Fever

• Tachypnea

• Rhabdomyolysis

• Most often due to hypovolemia

  • Delayed capillary refill
  • Dry mucous membranes
  • Hypotension
  • Oliguria/anuria
  • Orthostasis
  • Tachycardia

• Altered mental status, lethargy that may progress to coma, seizures

  • Seen in acute, severe hypernatremia

• Vascular rupture cerebral/subarachnoid hemorrhage, irreversible neuronal injury

  • Only in extreme cases (e.g., salt poisoning)

Evaluation

• Careful assessment of volume status
  • Weight loss more suggestive of Hypovolemia
• Weight gain may be seen in salt poisoning
• Laboratory evaluation
  • BUN, creatinine
  • Plasma and urine osmolality
    • Confirms true hypertonic hypernatremia
      • Spurious hypernatremia is rare, but has been documented in hypoalbuminemic neonates when using indirect ion-selective electrodes (as is typically used in main laboratory analyzers)
    • Comparison of plasma vs urine osmolality is helpful in establishing diagnosis
      • If uOsm < pOsm, urinary concentrating defect is present
        • Water deprivation test to establish etiology
      • if uOsm > pOsm, urinary concentrating ability is intact and non-kidney causes should be considered
  • FeNa
    • >2% in hydrated patient suggests salt poisoning
    • <1% suggestive of dehydration from water loss
    • Note that premature neonatal kidneys may only absorb 85-95% of filtered sodium, so a FeNa of ≤5% would suggest sodium retention in that setting
      • Premature infants tend to remain in negative sodium balance for the first 2-3 weeks of life, and FeNa is higher in those with sepsis or respiratory disease
  • Consider aldosterone, cortisol, ADH (copeptin proAVP), ACTH level
• Distinguish polyuria from solute diuresis

Treatment

• Depends upon the cause, chronicity, and clinical status of the patient

Restore intravascular volume if necessary

• Hypernatremia in children is typically associated with hypovolemia, in which cause there is a need to restore both water and electrolyte status
  • If clinically significant hypovolemia present, fluid resuscitation is the priority
    • Bolus with 20-60 mL/kg of 0.9% (normal) saline

Determine free water deficit

• Calculator the free water deficit using one of the following equations:
  • Free water deficit (in mL) = Current total body water (TBW, in mL) x ([current plasma Na/140] - 1)
    • See: kidney.wiki TBW calculator
    • Alternatively, crude estimate of TBW = 600 mL/kg (0.6 L/kg)
  • Free water deficit (in mL) = 4 mL/kg x (weight in kg) x (desired change in plasma Na)
    • Based on the rule that 4 mL/kg of free water should lower sodium by 1 mEq/L
    • Desired change in plasma Na = (current Na) - (desired Na)

Choosing a strategy to restore free water

• Rate: usually 8-12 mEq/L per 24 hours
  • Based on retrospective data showing cerebral edema was associated with a faster rate of correction
  • Emerging evidence suggests more rapid correction of hypernatremia is safe