How do I manage hypernatremia in an adult, especially an elderly patient with impaired thirst, including assessment, fluid replacement, correction rate, and treatment of underlying causes?

Hypernatremia Management in Adults

Immediate Assessment and Diagnostic Approach

For an elderly patient with hypernatremia and impaired thirst, immediately assess volume status by checking for postural pulse changes (≥30 beats per minute from lying to standing), severe postural dizziness, and signs of volume depletion including confusion, non-fluent speech, extremity weakness, dry mucous membranes, dry tongue, furrowed tongue, and sunken eyes—if at least four of these seven signs are present, moderate to severe volume depletion is likely. 1

Key Diagnostic Steps

• Confirm true hypernatremia by excluding pseudohypernatremia and calculating glucose-corrected sodium (add 1.6 mEq/L for each 100 mg/dL glucose above 100 mg/dL). 2

• Determine volume status through clinical examination: hypovolemic (orthostatic changes, dry mucous membranes, decreased skin turgor), euvolemic (normal volume status), or hypervolemic (edema, ascites). 2, 3

• Measure urine osmolality and sodium: urine osmolality <300 mOsm/kg suggests diabetes insipidus; urine sodium <20 mEq/L indicates extrarenal losses (GI, skin), while >20 mEq/L suggests renal losses or osmotic diuresis. 2, 4

• Check for ongoing losses: measure urine volume and calculate electrolyte-free water clearance to assess ongoing water losses. 2

Fluid Replacement Strategy

The cornerstone of treatment is replacing the water deficit with hypotonic fluids (0.45% NaCl or D5W) while avoiding correction faster than 8-10 mmol/L per 24 hours in chronic hypernatremia (>48 hours duration) to prevent cerebral edema and osmotic demyelination. 4, 5

Fluid Selection Algorithm

• For hypovolemic hypernatremia: Start with isotonic saline (0.9% NaCl) at 15-20 mL/kg/h for the first hour to restore intravascular volume, then switch to hypotonic fluids (0.45% NaCl or D5W) once hemodynamically stable. 1, 5

• For euvolemic or hypervolemic hypernatremia: Use hypotonic fluids (0.45% NaCl containing 77 mEq/L sodium, or 0.18% NaCl containing 31 mEq/L sodium) from the start. 5

• Avoid isotonic saline in patients with renal concentrating defects (e.g., nephrogenic diabetes insipidus) as this will worsen hypernatremia. 5

Calculating Water Deficit

• Water deficit (L) = 0.5 × body weight (kg) × [(current Na/140) - 1] for women; use 0.6 for men. 5, 2

• Replace half the calculated deficit over the first 24 hours, then the remainder over the next 24-48 hours. 2

• Add ongoing losses (insensible losses ~500-1000 mL/day plus measured urine output) to the replacement calculation. 2

Correction Rate Guidelines

For chronic hypernatremia (>48 hours), reduce sodium by no more than 8-10 mmol/L per 24 hours; for acute hypernatremia (<24 hours), faster correction is safe but still monitor closely. 4, 5

Monitoring Protocol

• Check serum sodium every 2-4 hours during active correction, then every 4-6 hours once stable. 2, 4

• Target correction rate: 0.5 mmol/L per hour maximum, or 10-12 mmol/L per 24 hours. 2

• Adjust fluid rate based on serial sodium measurements—if correcting too rapidly, slow the infusion rate or switch to a less hypotonic solution. 2

Treatment of Underlying Causes

Diabetes Insipidus

• Central diabetes insipidus: Administer desmopressin (Minirin) 1-2 mcg subcutaneously or 10-20 mcg intranasally, along with free water replacement. 4

• Nephrogenic diabetes insipidus: Requires ongoing hypotonic fluid administration to match excessive free water losses; thiazide diuretics and NSAIDs may reduce urine output. 2

Impaired Thirst Mechanism

• Elderly patients with impaired thirst require scheduled water intake of 1500-2000 mL/day, as they cannot rely on thirst to maintain hydration. 3

• Hospitalized or nursing home patients need prescribed water administration (not just "as desired") because they depend on caregivers for water access. 3

• Ensure adequate water is given with tube feedings—standard formulas require additional free water supplementation. 3

Special Considerations for Elderly Patients

Elderly patients have decreased thirst sensation, reduced renal concentrating ability, and often limited access to water, making them particularly vulnerable to hypernatremia. 3

• Frail nursing home residents are at highest risk and require proactive hydration protocols with scheduled water administration. 3

• Medications that impair water retention (diuretics, lithium) or increase water loss should be reviewed and adjusted. 3

• Monitor for complications including altered mental status, confusion, seizures, and coma—these indicate severe hypernatremia requiring urgent treatment. 4, 3

Critical Pitfalls to Avoid

• Never correct chronic hypernatremia faster than 10 mmol/L per 24 hours—rapid correction causes cerebral edema due to osmotic shifts. 4

• Do not use isotonic saline in patients with nephrogenic diabetes insipidus or other renal concentrating defects, as this delivers excessive osmotic load requiring 3 liters of urine to excrete the osmotic load from just 1 liter of fluid. 5

• Avoid relying on thirst in elderly patients—prescribe and administer adequate water proactively. 3

• Do not delay treatment while pursuing extensive diagnostic workup—begin fluid replacement immediately while investigating the cause. 5

• Monitor closely when starting dialysis in patients with chronic hypernatremia, as rapid sodium drops can occur. 4