What is the approach to treating a patient with hypernatremia?

Approach to Hypernatremia

For hypernatremia (serum sodium >145 mmol/L), correct at a rate not exceeding 0.4 mmol/L per hour (approximately 10 mmol/L per 24 hours) using hypotonic fluids, with the specific rate depending on whether the hypernatremia developed acutely or chronically.

Initial Assessment and Classification

• Determine the severity: mild (146-149 mmol/L), moderate (150-159 mmol/L), or severe (≥160 mmol/L) hypernatremia 1
• Assess the timeline: acute hypernatremia (developing over hours) versus chronic (developing over days) is critical, as this determines correction speed 1, 2
• Evaluate volume status through physical examination looking for signs of hypovolemia (orthostatic hypotension, dry mucous membranes, decreased skin turgor), hypervolemia (edema, jugular venous distention), or euvolemia 1, 3
• Check urine osmolality and urine sodium to differentiate between renal and extrarenal water losses 1, 3

Pathophysiologic Classification

Hypovolemic Hypernatremia (Most Common)

• Results from renal losses (osmotic diuresis, loop diuretics) or extrarenal losses (diarrhea, vomiting, burns, excessive sweating) 1, 4
• Urine sodium <20 mmol/L suggests extrarenal losses; >20 mmol/L suggests renal losses 1

Euvolemic Hypernatremia

• Central diabetes insipidus: caused by traumatic, vascular, or infectious CNS events with inappropriately dilute urine (osmolality <300 mOsm/kg) 1
• Nephrogenic diabetes insipidus: caused by lithium, hypokalemia, hypercalcemia, or chronic kidney disease 1, 3
• Distinguish by desmopressin challenge: central DI responds with concentrated urine, nephrogenic does not 1

Hypervolemic Hypernatremia (Rare)

• Acute form: iatrogenic from hypertonic saline or sodium bicarbonate administration 1
• Chronic form: primary hyperaldosteronism with sodium retention 1

Treatment Algorithm

Acute Hypernatremia (<24-48 hours duration)

Rapid correction is safe and improves prognosis by preventing cellular dehydration effects 5, 1

• Correct at 0.5-1.0 mmol/L per hour until symptoms resolve 5, 1
• Use 5% dextrose (D5W) as the primary fluid because it delivers no renal osmotic load and allows controlled decrease in plasma osmolality 6
• Alternative: 0.45% NaCl (half-normal saline) for moderate hypernatremia, providing both free water and some sodium replacement 6
• Critical evidence: A large study of 449 critically ill patients found no cases of cerebral edema attributable to rapid correction, and no increased mortality with correction rates >0.5 mmol/L per hour 5

Chronic Hypernatremia (>48 hours duration)

Slow correction is mandatory to prevent cerebral edema from rapid osmotic shifts 1, 2

• Maximum correction rate: 0.4 mmol/L per hour or 10 mmol/L per 24 hours 1, 3
• Use hypotonic fluids (D5W or 0.45% NaCl) based on volume status 6, 1
• For patients with renal concentrating defects (nephrogenic DI), avoid isotonic saline as it will worsen hypernatremia 6

Volume-Specific Management

Hypovolemic Hypernatremia:

• Initial resuscitation: isotonic saline (0.9% NaCl) at 15-20 mL/kg/h to restore hemodynamic stability 6
• Once hemodynamically stable: switch to hypotonic fluids (D5W or 0.45% NaCl) for free water replacement 6, 4
• Replace ongoing losses with fluids matching the composition of losses 6

Euvolemic Hypernatremia (Diabetes Insipidus):

• Central DI: desmopressin (DDAVP) 1-2 mcg IV/SC or 10-20 mcg intranasally plus free water replacement 1
• Nephrogenic DI: discontinue offending medications (lithium), correct electrolyte abnormalities (hypokalemia, hypercalcemia), and provide ongoing hypotonic fluid administration to match excessive losses 6, 1

Hypervolemic Hypernatremia:

• Loop diuretics (furosemide) to promote renal sodium excretion plus D5W for free water replacement 2, 3
• Avoid further sodium administration 1

Fluid Selection and Calculations

Preferred Hypotonic Fluids

• D5W (5% dextrose in water): 0 mEq/L sodium, ~252 mOsm/L - first choice for pure free water replacement 6
• 0.45% NaCl: 77 mEq/L sodium, ~154 mOsm/L - for moderate hypernatremia with some sodium needs 6
• 0.18% NaCl: 31 mEq/L sodium - for more aggressive free water replacement 6

Water Deficit Calculation

• Free water deficit (L) = 0.6 × body weight (kg) × [(current Na/140) - 1] 3
• This estimates total deficit but does not account for ongoing losses 3

Initial Fluid Administration Rates

• Adults: 25-30 mL/kg/24 hours as baseline maintenance 6
• Adjust based on: ongoing losses, urine output, and sodium correction rate 2, 3

Critical Monitoring Parameters

• Check serum sodium every 2-4 hours during active correction 2, 3
• Monitor for signs of cerebral edema (headache, altered mental status, seizures) if correcting chronic hypernatremia too rapidly 1, 2
• Track urine output, urine osmolality, and fluid balance to guide ongoing management 2, 3
• Assess for resolution of symptoms (improved mental status, decreased lethargy) 4

Common Pitfalls to Avoid

• Undercorrection: leaving hypernatremia untreated is associated with high mortality, particularly in elderly and critically ill patients 2, 3
• Using isotonic saline in nephrogenic DI: this worsens hypernatremia as patients cannot excrete the osmotic load, requiring 3 liters of urine to excrete the osmotic load from just 1 liter of isotonic fluid 6
• Overcorrecting chronic hypernatremia: rapid correction >0.4 mmol/L per hour risks cerebral edema from osmotic water shift into brain cells 1, 2
• Inadequate water prescription in hospitalized patients: hospital-acquired hypernatremia is usually iatrogenic and preventable with proper fluid management 4
• Ignoring ongoing losses: failing to replace ongoing water losses (from diarrhea, polyuria, fever) leads to persistent hypernatremia 4, 3

Special Populations

Critically Ill Patients:

• Hypernatremia is an independent risk factor for mortality in ICU patients 2
• Many have impaired consciousness and cannot regulate water balance through thirst, making physician-managed fluid balance critical 2
• The evidence shows rapid correction (>0.5 mmol/L per hour) is safe in this population without increased mortality or cerebral edema 5

Elderly Patients:

• Higher morbidity and mortality from hypernatremia 3
• Often have impaired thirst mechanism and limited access to free water 3
• Require careful attention to correction rates and monitoring 3

Patients with Renal Concentrating Defects:

• Require ongoing hypotonic fluid administration to match excessive free water losses 6
• Never use isotonic fluids as maintenance therapy 6

6, 5, 1, 4, 2, 3