What are the causes and management of hypernatremia?

Hypernatremia: Causes and Management

Definition and Classification

Hypernatremia is defined as plasma sodium concentration >145 mEq/L, reflecting a decrease in total body water relative to sodium content 1, 2. The disorder can be classified by duration (acute <24-48 hours vs. chronic >48 hours), severity (mild 145-150 mEq/L, moderate 150-160 mEq/L, severe >160 mEq/L), and volume status (hypervolemic, euvolemic, or hypovolemic) 2, 3.

Causes of Hypernatremia

Hypervolemic Hypernatremia

• Acute: Excessive sodium intake from hypertonic NaCl or NaHCO3 solutions 3
• Chronic: Primary hyperaldosteronism 3

Euvolemic Hypernatremia

• Central (neurogenic) diabetes insipidus: Traumatic brain injury, vascular events, infections, or neurosurgical procedures 3
• Nephrogenic diabetes insipidus: Lithium therapy, hypokalemia, hypercalcemia, or chronic kidney disease 3

Hypovolemic Hypernatremia

• Renal losses: Osmotic diuresis, loop diuretics 2
• Extrarenal losses: Gastrointestinal losses (diarrhea, vomiting), excessive sweating, burns 2, 4
• Impaired thirst mechanism or lack of access to water (most common in critically ill, elderly, or intubated patients) 1, 4

ICU-Specific Risk Factors

Critically ill patients are at particularly high risk due to sedation, intubation, altered mental status, fluid restriction, and treatment with sodium-containing fluids 4.

Clinical Presentation

Symptoms result from osmotic water movement causing intracellular dehydration, primarily affecting the central nervous system 4. Clinical features include:

• Mild to moderate: Confusion, lethargy, pronounced thirst (in awake patients), weakness 5, 3
• Severe: Coma, seizures, abnormal neuromuscular function 5, 4
• Advanced cases: Risk of hemorrhagic complications or death from vascular stretching and rupture 4

Diagnostic Approach

Eight-Step Diagnostic Algorithm 2:

1. Exclude pseudohypernatremia (check for hyperlipidemia or hyperproteinemia)
2. Confirm glucose-corrected sodium concentration (add 1.6 mEq/L to sodium for each 100 mg/dL glucose >100 mg/dL)
3. Determine extracellular volume status (assess for edema, orthostatic hypotension, skin turgor, mucous membranes) 2
4. Measure urine sodium levels (helps differentiate renal vs. extrarenal losses) 2
5. Measure urine volume and osmolality (urine osmolality <300 mOsm/kg suggests diabetes insipidus) 2
6. Check ongoing urinary electrolyte-free water clearance 2
7. Determine arginine vasopressin/copeptin levels (if diabetes insipidus suspected) 2
8. Assess other electrolyte disorders (hypokalemia, hypercalcemia can cause nephrogenic DI) 2

Management of Hypernatremia

Six-Step Management Algorithm 2:

1. Identify and Address Underlying Causes

• Discontinue sodium-containing fluids or medications causing hypernatremia 2
• For diabetes insipidus, consider desmopressin (Minirin) 5
• Restore access to water for patients with impaired thirst 1

2. Distinguish Between Acute and Chronic Hypernatremia

• Acute hypernatremia (<24-48 hours): Can be corrected rapidly without risk of cerebral edema; hemodialysis is an effective option for rapid normalization 5, 3
• Chronic hypernatremia (>48 hours): Requires slow correction to prevent cerebral edema 5, 3

3. Determine Amount and Rate of Water Administration

For chronic hypernatremia, the correction rate should not exceed 8-10 mmol/L per day (approximately 0.4 mmol/L/hour) to prevent cerebral edema from rapid correction 5, 3.

Calculate free water deficit using the formula:

• Free water deficit = 0.6 × body weight (kg) × [(current Na/140) - 1] 2, 4

4. Select Type of Replacement Solution

• Hypotonic fluids are necessary for severe hypernatremia or when intravenous replacement is required 1
• Options include: 0.45% NaCl (half-normal saline), 0.18% NaCl (quarter-normal saline), or D5W 1
• Oral free water replacement guided by thirst is ideal when feasible 4
• Avoid isotonic fluids in patients with renal concentrating defects (e.g., nephrogenic diabetes insipidus), as this will worsen hypernatremia 1

5. Adjust Treatment Schedule with Frequent Monitoring

• Monitor plasma sodium levels every 2-4 hours initially during active correction 5, 4
• Adjust fluid replacement rate based on response to prevent overcorrection 4
• Account for ongoing water losses (insensible losses typically 500-1000 mL/day) 2

6. Consider Additional Therapy for Diabetes Insipidus

• Central diabetes insipidus: Desmopressin (DDAVP) 1-4 mcg subcutaneously or intravenously 5
• Nephrogenic diabetes insipidus: Thiazide diuretics plus amiloride (for lithium-induced), or address underlying cause (correct hypokalemia, hypercalcemia) 2

Critical Pitfalls to Avoid

• Never correct chronic hypernatremia faster than 8-10 mmol/L per day to prevent cerebral edema 5, 3
• Exercise extreme caution when initiating renal replacement therapy in patients with chronic hypernatremia, as rapid sodium drops can occur 5
• Do not use isotonic saline in patients with impaired renal concentrating ability, as this exacerbates hypernatremia 1
• Inadequate monitoring during correction can lead to overcorrection or undercorrection 4
• Failing to account for ongoing losses (insensible, urinary, gastrointestinal) leads to inadequate replacement 2

Special Considerations for ICU Patients

Critically ill patients require routine assessment of free water requirements and judicious electrolyte-free water replacement 4. Parenteral fluid replacement is usually necessary due to altered mental status, intubation, or sedation 4. Close monitoring every 2-4 hours is essential to ensure appropriate response and adjust therapy 4.