What are the causes, pathophysiology, and treatment protocol for hypernatremia in patients?

Causes, Pathophysiology, and Treatment Protocol for Hypernatremia

Hypernatremia (serum sodium >145 mmol/L) is primarily caused by water deficit relative to sodium content and requires controlled correction at a rate of 10-15 mmol/L/24h to prevent neurological complications. 1

Causes of Hypernatremia

Hypernatremia develops through three main mechanisms:

1. Water Loss (Most Common)

   • Inadequate water intake
     • Limited access to water
     • Impaired thirst mechanism
   • Excessive water loss
     • Transepidermal water loss (especially in premature infants) 1
     • Diabetes insipidus (central or nephrogenic) 2
     • High fever
     • Diarrhea or vomiting
     • Excessive diuretic use
     • Polyuria
     • Insensible losses (respiratory, skin)

2. Iatrogenic Causes

   • Incorrect fluid replacement
   • Excessive sodium administration
   • Hypertonic saline infusion
   • Sodium bicarbonate administration 1, 3

3. Sodium Gain (Less Common)

   • Salt intoxication
   • Administration of sodium-rich solutions 4

Pathophysiology

Hypernatremia represents a hyperosmolar state that causes:

1. Cellular Dehydration: Water shifts from intracellular to extracellular space following osmotic gradient
2. Brain Cell Adaptation:
   • Acute phase: Brain cell shrinkage
   • Chronic phase (>48h): Brain cells produce idiogenic osmoles to retain water
3. Neurological Dysfunction:
   • Brain dehydration
   • Potential cerebrovascular hemorrhages 5
   • Risk of osmotic demyelination with rapid correction 2

Diagnostic Approach

1. Exclude pseudohypernatremia
2. Confirm glucose-corrected sodium (for hyperglycemia)
3. Determine volume status (hypovolemic, euvolemic, hypervolemic)
4. Measure urine sodium and osmolality
5. Calculate free water clearance
6. Check other electrolyte disorders 3

Treatment Protocol

Step 1: Assessment

• Determine severity: Mild (145-150 mmol/L), Moderate (151-160 mmol/L), Severe (>160 mmol/L)
• Determine acuity: Acute (<48h) vs. Chronic (>48h)
• Assess volume status
• Evaluate neurological symptoms

Step 2: Calculate Water Deficit

Water deficit (L) = Total body water × [(Current Na⁺/140) - 1]

• Total body water ≈ 0.6 × body weight (kg) for adults
• Adjust for age and sex as needed

Step 3: Correction Rate

• Maximum correction rate: 10-15 mmol/L/24h 1
• For severe hypernatremia with neurological symptoms, initial correction can be faster but not exceeding the daily maximum
• For chronic hypernatremia (>48h), slower correction is safer to prevent cerebral edema

Step 4: Fluid Selection

• Hypovolemic hypernatremia:

  • Initial resuscitation with isotonic saline if hemodynamically unstable
  • Then switch to hypotonic solutions (0.45% NaCl or 5% dextrose)

• Euvolemic hypernatremia:

  • Hypotonic solutions (0.45% NaCl or 5% dextrose in water)
  • For diabetes insipidus: Add desmopressin 2

• Hypervolemic hypernatremia:

  • Loop diuretics to remove excess sodium
  • Hypotonic fluids

Step 5: Monitoring

• Check serum sodium every 2-4 hours during active correction
• Monitor neurological status
• Adjust fluid rate based on sodium levels
• Monitor urine output and specific gravity

Step 6: Address Underlying Cause

• For diabetes insipidus: Desmopressin 2
• For excessive losses: Replace ongoing losses
• For inadequate intake: Ensure adequate water access
• For iatrogenic causes: Adjust medication regimen

Special Considerations

1. Pediatric Patients:

   • Higher risk in premature infants due to transepidermal water loss
   • Initial fluid therapy should be isotonic saline (0.9% NaCl) at 10-20 ml/kg/h for the first hour 1
   • Subsequent fluid therapy with 0.45-0.9% NaCl at 1.5 times maintenance requirements 1

2. Diabetes Insipidus Management:

   • Replace water deficit
   • Desmopressin for central diabetes insipidus
   • Address underlying cause 2

3. Complications of Treatment:

   • Too rapid correction: Cerebral edema, seizures, neurological injury 1
   • Too slow correction: Continued neurological symptoms

Pitfalls to Avoid

1. Correcting sodium too rapidly (>15 mmol/L/24h)
2. Failing to identify and treat the underlying cause
3. Using hypotonic fluids in hemodynamically unstable patients before volume resuscitation
4. Not accounting for ongoing losses when calculating replacement needs
5. Inadequate monitoring during correction

By following this structured approach to hypernatremia management, clinicians can effectively correct sodium imbalances while minimizing the risk of treatment-related complications.