What is the appropriate workup and treatment for a patient, particularly elderly individuals or those with underlying neurological disorders, presenting with hypernatremia?

Hypernatremia Workup

Initial Diagnostic Assessment

Begin by confirming true hypernatremia (serum sodium >145 mmol/L) and exclude pseudohypernatremia, particularly in patients with hyperglycemia—correct sodium by adding 1.6 mEq/L for each 100 mg/dL glucose above 100 mg/dL. 1, 2

Essential Laboratory Tests

• Serum sodium concentration (>145 mmol/L confirms hypernatremia) 3, 2
• Serum osmolality to assess plasma tonicity 4, 2
• Urine osmolality and urine sodium to differentiate causes 4, 3, 2
• Urine volume to assess renal water handling 2
• Serum glucose to correct for pseudohypernatremia 1, 2
• Blood urea nitrogen and creatinine to evaluate renal function 2
• Serum potassium as hypokalemia can cause nephrogenic diabetes insipidus 3

Volume Status Assessment

Determine extracellular volume status through physical examination, looking specifically for:

• Hypovolemic signs: orthostatic hypotension, dry mucous membranes, decreased skin turgor, sunken eyes, confusion, non-fluent speech, extremity weakness 1, 3, 2
• Hypervolemic signs: peripheral edema, jugular venous distention, pulmonary congestion 1, 3
• Euvolemic appearance: normal volume status with intact thirst mechanism 3, 2

Differential Diagnosis Based on Volume Status

Hypervolemic Hypernatremia

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

Euvolemic Hypernatremia

• Central (neurogenic) diabetes insipidus: Traumatic, vascular, or infectious CNS events 3, 2
• Nephrogenic diabetes insipidus: Lithium therapy, hypokalemia, hypercalcemia 3, 2
• Urine osmolality <300 mOsm/kg suggests diabetes insipidus 4, 2

Hypovolemic Hypernatremia

• Renal losses: Osmotic diuresis, post-obstructive diuresis 3, 2
• Extrarenal losses: Diarrhea, vomiting, burns, excessive sweating 3, 2, 5
• Urine sodium <20 mmol/L suggests extrarenal losses 2
• Urine sodium >20 mmol/L suggests renal losses 2

Treatment Approach

Determine Acuity

Distinguish between acute (<24-48 hours) and chronic (>48 hours) hypernatremia, as this determines correction rate. 4, 3, 2

Correction Rate Guidelines

• Acute hypernatremia (<24 hours): Rapid correction improves prognosis and prevents cellular dehydration; hemodialysis is an effective option for rapid normalization 4, 3
• Chronic hypernatremia (>48 hours): Correct slowly at no more than 0.4 mmol/L/hour or 8-10 mmol/L per day to prevent osmotic demyelination syndrome and cerebral edema 4, 3, 2

Fluid Replacement Strategy

For hypovolemic or euvolemic hypernatremia, use hypotonic fluids:

• 5% dextrose in water (D5W) as primary rehydration fluid—delivers no renal osmotic load and allows controlled decrease in plasma osmolality 1
• 0.45% NaCl (half-normal saline) for moderate hypernatremia—provides 77 mEq/L sodium with osmolarity ~154 mOsm/L 1
• 0.18% NaCl (quarter-normal saline) for more aggressive free water replacement—provides ~31 mEq/L sodium 1
• Avoid isotonic saline (0.9% NaCl) in hypernatremia as it delivers excessive osmotic load requiring 3 liters of urine to excrete the load from just 1 liter of fluid, risking worsening hypernatremia 1

Calculate Water Deficit

Water deficit = 0.6 × body weight (kg) × [(current Na/140) - 1] 2

Replace this deficit plus ongoing losses and insensible losses (typically 500-1000 mL/day) 2

Specific Treatment by Etiology

For central diabetes insipidus:

• Desmopressin (DDAVP) 1-4 mcg subcutaneously or IV, or 10-20 mcg intranasally 4, 3

For nephrogenic diabetes insipidus:

• Address underlying cause (discontinue lithium, correct hypokalemia) 3, 2
• Thiazide diuretics may paradoxically reduce urine output 2

For hypervolemic hypernatremia:

• Diuretics to promote renal sodium excretion 6
• Free water replacement 6

Monitoring During Treatment

• Check serum sodium every 2-4 hours initially during active correction 4, 2
• Adjust infusion rate to maintain correction rate <0.4 mmol/L/hour for chronic hypernatremia 4, 3
• Monitor for signs of cerebral edema: headache, nausea, seizures, altered mental status 4, 5
• Assess ongoing losses and adjust replacement accordingly 2

Critical Pitfalls to Avoid

• Never correct chronic hypernatremia faster than 8-10 mmol/L per day—this causes cerebral edema and osmotic demyelination syndrome 4, 3, 2
• Never use isotonic fluids in patients with renal concentrating defects—this worsens hypernatremia 1
• Never start renal replacement therapy without considering the rapid sodium drop in patients with chronic hypernatremia 4
• Never ignore ongoing losses—failure to replace insensible and ongoing losses leads to inadequate correction 2