How should I manage an adult patient with hypernatremia (serum sodium ≥145 mEq/L), including assessment of volume status, choice of fluids, safe correction rate, and treatment of underlying causes?

Management of Hypernatremia in Adults

For adult patients with hypernatremia (serum sodium ≥145 mEq/L), begin by assessing volume status through clinical examination, then replace free water deficit with hypotonic fluids (0.45% NaCl or D5W) at a maximum correction rate of 10-15 mmol/L per 24 hours to prevent cerebral edema, while simultaneously treating the underlying cause. 1

Initial Assessment and Volume Status Determination

Assess the patient's volume status through physical examination, looking specifically for orthostatic hypotension, dry mucous membranes, decreased skin turgor (hypovolemia), or peripheral edema and jugular venous distention (hypervolemia). 1, 2 Measure serum electrolytes, blood urea nitrogen, creatinine, glucose, and calculate serum osmolality using the formula: 2 × Na (mEq/L) + BUN (mg/dL)/2.8 + glucose (mg/dL)/18. 2 Check urine osmolality and urine sodium concentration—a urine osmolality <300 mOsm/kg with hypernatremia indicates impaired renal concentrating ability or diabetes insipidus. 1

Key Diagnostic Findings

• Hypovolemic hypernatremia: Urine sodium typically <20 mEq/L suggests extrarenal losses (diarrhea, vomiting, burns); urine sodium >20 mEq/L suggests renal losses (diuretics, osmotic diuresis). 2
• Euvolemic hypernatremia: Usually indicates diabetes insipidus (central or nephrogenic) or inadequate water intake with impaired thirst mechanism. 3
• Hypervolemic hypernatremia: Rare, typically iatrogenic from excessive sodium administration or mineralocorticoid excess. 3

Fluid Selection Based on Volume Status

Hypovolemic Hypernatremia

Administer isotonic saline (0.9% NaCl) initially at 15-20 mL/kg/h for the first hour to restore intravascular volume and hemodynamic stability. 2 Once hemodynamically stable, switch to hypotonic fluids (0.45% NaCl or D5W) to correct the free water deficit. 1, 2 Never use isotonic saline as the sole therapy for hypernatremia correction, as it delivers excessive osmotic load—requiring approximately 3 liters of urine to excrete the osmotic load from just 1 liter of isotonic fluid, which risks worsening hypernatremia. 1

Euvolemic Hypernatremia

Use hypotonic fluids as first-line therapy: 0.45% NaCl (77 mEq/L sodium, 154 mOsm/L) for moderate hypernatremia or 0.18% NaCl (31 mEq/L sodium) for more aggressive free water replacement. 1 D5W (5% dextrose in water) is the preferred fluid when pure free water replacement is required because it delivers no renal osmotic load and allows controlled decrease in plasma osmolality. 1

Hypervolemic Hypernatremia

Focus on achieving negative water balance rather than aggressive fluid administration. 1 Discontinue intravenous fluid therapy and implement free water restriction. 1 In cirrhotic patients with hypervolemic hypernatremia, sodium and fluid restriction (limiting fluid intake to 1.5-2 L/day) is recommended. 1

Safe Correction Rate Guidelines

The maximum safe correction rate for chronic hypernatremia (>48 hours duration) is 10-15 mmol/L per 24 hours. 1, 2, 3 Correcting chronic hypernatremia more rapidly can cause cerebral edema, seizures, and permanent neurological injury because brain cells synthesize intracellular osmolytes over 48 hours to adapt to hyperosmolar conditions. 1, 4

Calculating Free Water Deficit

Use the formula: Free water deficit = Total body water × [(measured Na⁺/desired Na⁺) - 1], where total body water is approximately 60% of body weight in men and 50% in women. 2 Administer hypotonic fluids at an initial rate of 25-30 mL/kg per 24 hours in adults, then titrate based on serial sodium measurements. 1

Acute vs. Chronic Hypernatremia

Acute hypernatremia (<24-48 hours) can be corrected more rapidly, up to 1 mmol/L/hour if severely symptomatic, because brain cells have not yet adapted by synthesizing intracellular osmolytes. 1, 4 However, most cases in adults are chronic and require slower correction. 3

Special Clinical Scenarios

Nephrogenic Diabetes Insipidus

Patients with nephrogenic diabetes insipidus require ongoing hypotonic fluid administration to match excessive free water losses. 1 Isotonic saline is absolutely contraindicated in these patients as it will worsen hypernatremia. 1 Consider a low-salt diet (<6 g/day) and protein restriction (<1 g/kg/day) for long-term management. 1

Heart Failure Patients

Combine sodium and fluid restriction, limiting fluid intake to 1.5-2 L/day for most hospitalized patients. 1 Diuretics remain essential for volume management but must be carefully balanced with hypernatremia correction. 1 For persistent severe hypernatremia with cognitive symptoms, vasopressin antagonists (tolvaptan, conivaptan) may be considered for short-term use. 1

Cirrhotic Patients

Evaluate for hypovolemic versus hypervolemic state. 1 For hypovolemic hypernatremia, provide fluid resuscitation with hypotonic solutions. 1 For hypervolemic hypernatremia, discontinue IV fluids and implement free water restriction while focusing on negative water balance. 1

Severe Burns or Voluminous Diarrhea

Hypotonic fluids are required to keep up with ongoing free water losses, with fluid composition matched to losses while providing adequate free water. 1 Total fluid volume often exceeds the calculated free water deficit because losses continue during treatment. 1

Monitoring Protocol

Check serum sodium every 2-4 hours initially during active correction, then every 6-12 hours once stable. 1 Monitor daily weight, supine and standing vital signs, fluid input and output, urine specific gravity/osmolarity, and urine electrolyte concentrations. 1, 2 Assess renal function (BUN, creatinine) to evaluate for worsening azotemia. 1 Track neurological status closely, watching for confusion, seizures, or altered mental status that may indicate cerebral edema from overly rapid correction. 3

Treatment of Underlying Causes

Identify and address the root cause of hypernatremia:

• Inadequate water intake: Ensure access to free water, especially in elderly or cognitively impaired patients. 3
• Excessive water loss: Treat diarrhea, vomiting, or fever; adjust diuretic therapy. 3
• Central diabetes insipidus: Administer desmopressin (DDAVP). 4
• Nephrogenic diabetes insipidus: Discontinue offending medications (lithium, demeclocycline); consider thiazide diuretics and amiloride. 1
• Iatrogenic causes: Review all IV fluids, medications, and nutritional support for excessive sodium content. 2

Critical Pitfalls to Avoid

Never correct chronic hypernatremia faster than 10-15 mmol/L per 24 hours—rapid correction can precipitate cerebral edema, seizures, and potentially fatal brain herniation. 1, 2, 4 Never use isotonic saline as the primary fluid for hypernatremia correction, especially in patients with nephrogenic diabetes insipidus or renal concentrating defects. 1 Avoid inadequate monitoring during correction, which can result in overcorrection or undercorrection. 2 Do not fail to identify and treat the underlying cause, which is often iatrogenic in vulnerable populations. 2

High-Risk Populations

Elderly patients are at higher risk for both hypernatremia and complications from correction due to reduced renal function, cognitive impairment preventing recognition of thirst, and inability to access fluids. 1 Pediatric patients, especially neonates, are more susceptible to rapid fluid shifts and cerebral edema, requiring more cautious correction. 2 Patients with chronic kidney disease require more conservative fluid rates and close monitoring for worsening azotemia. 1