What is the recommended approach to evaluate and manage hypernatremia, including confirming serum sodium >145 mEq/L, determining volume status (hypovolemic, euvolemic, or hypervolemic), and selecting appropriate fluid and medication therapy?

Approach to Hypernatremia

For hypernatremia (serum sodium >145 mEq/L), immediately determine volume status (hypovolemic, euvolemic, or hypervolemic) and correct with hypotonic fluids at a maximum rate of 10-15 mmol/L per 24 hours for chronic cases, while addressing the underlying cause. 1

Initial Assessment and Confirmation

• Confirm hypernatremia with serum sodium >145 mmol/L and assess chronicity (acute <48 hours vs. chronic >48 hours), as this determines correction speed 2
• Evaluate volume status through physical examination: look for orthostatic hypotension, dry mucous membranes, decreased skin turgor (hypovolemic); peripheral edema, ascites, jugular venous distention (hypervolemic); or normal volume status (euvolemic) 1
• Measure urine osmolality and urine sodium: inappropriately dilute urine (<300 mOsm/kg) with hypernatremia suggests diabetes insipidus, while concentrated urine (>600 mOsm/kg) suggests extrarenal water losses 1
• Check vital signs, neurological status, body weight, hematocrit, and blood urea nitrogen to assess severity and hydration status 1

Classification by Volume Status

Hypovolemic Hypernatremia (Most Common)

• Renal losses: urine sodium >20 mEq/L suggests osmotic diuresis, diuretic use, or post-obstructive diuresis 2
• Extrarenal losses: urine sodium <20 mEq/L indicates gastrointestinal losses (diarrhea, vomiting), excessive sweating, or burns 2, 3
• Treatment: administer hypotonic fluids (0.45% NaCl or D5W) at 4-14 mL/kg/h initially to replace free water deficit 1
• Never use isotonic saline as initial therapy, as it delivers excessive osmotic load and worsens hypernatremia 1

Euvolemic Hypernatremia

• Central diabetes insipidus: caused by traumatic brain injury, neurosurgery, pituitary tumors, or infections; responds to desmopressin 2
• Nephrogenic diabetes insipidus: caused by lithium, hypokalemia, hypercalcemia, or chronic kidney disease; does not respond to desmopressin 2
• Diagnostic test: water deprivation test followed by desmopressin challenge differentiates central from nephrogenic DI 2
• Treatment: for central DI, give desmopressin 1-2 mcg IV/SC or 10-20 mcg intranasally; for nephrogenic DI, provide ongoing hypotonic fluid replacement and treat underlying cause 1

Hypervolemic Hypernatremia (Rare)

• Acute form: excessive hypertonic saline or sodium bicarbonate administration 2
• Chronic form: primary hyperaldosteronism or Cushing syndrome 2
• Treatment: discontinue sodium-containing fluids, implement free water restriction, and consider diuretics to achieve negative sodium balance 1

Fluid Selection and Correction Rate

Fluid Choice

• 0.45% NaCl (half-normal saline): contains 77 mEq/L sodium; appropriate for moderate hypernatremia 1
• 0.18% NaCl (quarter-normal saline): contains 31 mEq/L sodium; provides more aggressive free water replacement for severe cases 1
• D5W (5% dextrose in water): delivers no renal osmotic load; preferred for controlled correction in severe hypernatremia 1
• Avoid isotonic saline (0.9% NaCl) in patients with renal concentrating defects, as it requires 3 liters of urine to excrete the osmotic load from just 1 liter of fluid 1

Correction Rate Guidelines

• Chronic hypernatremia (>48 hours): maximum correction of 10-15 mmol/L per 24 hours (0.4 mmol/L/h) to prevent cerebral edema 1, 2, 3
• Acute hypernatremia (<48 hours): can be corrected more rapidly, up to 1 mmol/L/h if severely symptomatic 1
• Calculate free water deficit: use formula 0.6 × body weight (kg) × [(current Na/140) - 1] to estimate total deficit 1
• Slower correction is critical for chronic cases because brain cells synthesize intracellular osmolytes over 48 hours; rapid correction causes cerebral edema, seizures, and permanent neurological injury 1

Monitoring During Treatment

• Check serum sodium every 2-4 hours initially during active correction, then every 6-12 hours once stable 1
• Monitor daily weight, fluid input/output, urine specific gravity, and urine osmolality to track response 1
• Assess neurological status frequently: watch for confusion, seizures, or altered mental status indicating cerebral edema from overly rapid correction 1
• Track renal function (BUN, creatinine) to evaluate for worsening azotemia, especially in elderly or CKD patients 1

Special Populations

Nephrogenic Diabetes Insipidus

• Requires ongoing hypotonic fluid administration to match excessive free water losses (often 5-10 L/day) 1
• Isotonic fluids will worsen hypernatremia in these patients and must be avoided 1
• Treat underlying cause: discontinue lithium if possible, correct hypokalemia (<3.5 mEq/L), or hypercalcemia (>10.5 mg/dL) 2

Heart Failure Patients

• Fluid restriction (1.5-2 L/day) is essential after initial correction to prevent volume overload 1
• Diuretics remain necessary for volume management but must be carefully balanced with hypernatremia correction 1
• Combine IV hypotonic fluids with free water via nasogastric tube if needed, targeting 10-15 mmol/L correction per 24 hours 1

Cirrhotic Patients

• Evaluate for hypovolemic vs. hypervolemic state: ascites indicates hypervolemia despite effective hypovolemia 1
• For hypovolemic hypernatremia: provide fluid resuscitation with hypotonic solutions 1
• For hypervolemic hypernatremia: focus on negative water balance rather than aggressive fluid administration; discontinue IV fluids and implement free water restriction 1

Elderly and Neonates

• Elderly patients have impaired thirst mechanism and reduced renal concentrating ability, increasing risk for both hypernatremia and complications from correction 1
• Neonates and preterm infants (<34 weeks) have immature tubular sodium reabsorption; corrections faster than 48-72 hours increase risk of pontine myelinolysis 1
• Use more conservative fluid rates and monitor closely for worsening azotemia during correction 1

Common Pitfalls to Avoid

• Correcting chronic hypernatremia too rapidly (>10-15 mmol/L per 24 hours) leads to cerebral edema, seizures, and permanent neurological injury 1, 2
• Using isotonic saline in nephrogenic diabetes insipidus exacerbates hypernatremia because patients cannot excrete the osmotic load 1
• Inadequate monitoring during correction results in overcorrection or undercorrection; check sodium every 2-4 hours initially 1
• Failing to identify and treat the underlying cause (often iatrogenic in vulnerable populations) leads to recurrent hypernatremia 1
• Applying prolonged induced hypernatremia to control ICP in traumatic brain injury is not recommended, as it requires an intact blood-brain barrier and may worsen cerebral contusions 1