How should I evaluate and manage a patient with a serum sodium of 162 mmol/L?

Management of Hypernatremia (Serum Sodium 162 mmol/L)

A serum sodium of 162 mmol/L represents severe hypernatremia requiring immediate intervention with hypotonic fluids and careful correction to prevent cerebral edema, with the primary goal of reducing sodium by no more than 10 mmol/L per 24 hours while addressing the underlying cause.

Initial Assessment

Determine the volume status and underlying mechanism:

• Assess for hypovolemia: Look for orthostatic hypotension, tachycardia, dry mucous membranes, decreased skin turgor, and flat neck veins 1
• Assess for hypervolemia: Look for peripheral edema, jugular venous distention, and pulmonary congestion 1
• Measure urine osmolality: High urine osmolality (>600-800 mOsm/kg) suggests extrarenal water loss or inadequate water intake; low urine osmolality (<300 mOsm/kg) suggests diabetes insipidus 2, 3
• Review fluid intake and output: Calculate daily fluid balance to identify whether the problem is excessive water loss, inadequate water intake, or excessive sodium administration 1

Identify the Underlying Cause

Common mechanisms in critically ill patients include:

• Renal water loss: Polyuria (>40 mL/kg/day) is common in hypernatremic patients, often due to osmotic diuresis from hyperglycemia, mannitol use, or post-obstructive diuresis 1
• Extrarenal water loss: Gastrointestinal losses (diarrhea, vomiting), insensible losses (fever, burns), or inadequate water intake in elderly/mentally impaired patients 2, 4
• Excessive sodium administration: Sodium bicarbonate infusions, hypertonic saline, or relatively hypertonic maintenance fluids can cause hypernatremia even with positive fluid balance 1
• Diabetes insipidus: Central (inadequate ADH secretion) or nephrogenic (renal resistance to ADH) causes low urine osmolality despite hypernatremia 3

Correction Strategy

The rate of correction depends on the chronicity and severity:

• For chronic hypernatremia (>48 hours): Reduce serum sodium by no more than 8-10 mmol/L per 24 hours to prevent cerebral edema from rapid osmotic shifts 2, 5
• For acute hypernatremia (<24 hours): More rapid correction is safer, but still monitor closely and avoid exceeding 1 mmol/L per hour 5
• Target serum sodium: Aim for 145-150 mmol/L initially, not immediate normalization 2

Fluid Selection and Administration

Choose hypotonic fluids based on volume status:

• For hypovolemic hypernatremia: Start with 0.9% normal saline to restore intravascular volume, then switch to 0.45% saline or 5% dextrose in water (D5W) once hemodynamically stable 2
• For euvolemic or hypervolemic hypernatremia: Use 5% dextrose in water (D5W) as the primary fluid because it delivers no renal osmotic load and allows controlled decrease in plasma osmolality 2
• Avoid isotonic saline (0.9% NaCl) in established hypernatremia: It delivers excessive osmotic load, requiring 3 liters of urine to excrete the osmotic load from just 1 liter of isotonic fluid, which risks worsening hypernatremia 1

Calculate the free water deficit:

Free water deficit (L) = 0.6 × body weight (kg) × [(current Na ÷ 140) - 1]

Replace half the deficit over the first 24 hours, and the remainder over the next 24-48 hours 2

Specific Management Based on Etiology

For diabetes insipidus:

• Central DI: Administer desmopressin (DDAVP) 1-2 mcg IV or 10-20 mcg intranasally, along with hypotonic fluid replacement 5, 3
• Nephrogenic DI: Requires ongoing hypotonic fluid administration to match excessive free water losses; avoid isotonic fluids as they worsen hypernatremia 1

For excessive sodium administration:

• Stop all sources of excess sodium: Discontinue sodium bicarbonate, hypertonic saline, and switch to hypotonic maintenance fluids 1
• Create a negative sodium balance: Use diuretics if the patient is hypervolemic to promote sodium excretion while replacing with hypotonic fluids 1

For osmotic diuresis:

• Address the underlying cause: Control hyperglycemia, discontinue mannitol if possible 1
• Replace ongoing urinary losses: Match urine output with hypotonic fluid replacement 1

Monitoring Requirements

Intensive monitoring is essential to prevent complications:

• Check serum sodium every 2-4 hours initially during active correction, then every 6-8 hours once stable 2, 5
• Monitor neurologic status closely: Watch for confusion, seizures, or altered mental status indicating cerebral edema from overcorrection 2, 4
• Track fluid balance meticulously: Record all intake and output, including insensible losses 1
• Monitor urine osmolality and output: Helps guide fluid replacement and identify diabetes insipidus 2, 3

Critical Pitfalls to Avoid

• Overcorrection: Reducing sodium faster than 10 mmol/L per 24 hours in chronic hypernatremia risks cerebral edema, seizures, and permanent neurologic damage 2, 5
• Using isotonic saline in established hypernatremia: This worsens the sodium imbalance by delivering more sodium than water 1
• Inadequate free water replacement: Hypernatremia reflects absolute or relative water deficit; sodium restriction alone is insufficient 1, 2
• Starting renal replacement therapy without adjusting dialysate: Rapid sodium correction during dialysis can cause catastrophic cerebral edema in chronic hypernatremia 5
• Ignoring ongoing losses: Failure to replace ongoing urinary or insensible losses leads to persistent hypernatremia despite treatment 1

Special Considerations

Elderly and critically ill patients:

• Hypernatremia carries significantly increased mortality in these populations, with rates approaching 40-60% in severe cases 1, 4
• These patients often have impaired thirst mechanisms and limited access to free water, making prevention crucial 4, 3
• Underlying sepsis, renal dysfunction, and hypoalbuminemia are independent risk factors for developing hypernatremia 1

Severe hypernatremia (>200 mmol/L):

• Although extremely rare, survival is possible with careful management 4
• One case report describes successful treatment using isotonic solution initially for hemodynamic stabilization, followed by gradual correction 4
• The key is balancing the need for volume resuscitation with the risk of overcorrection 4