What are the serum sodium cut‑offs for hypernatremia and how should hypernatremia be managed based on the patient’s volume status?

Hypernatremia: Definition and Management Based on Volume Status

Hypernatremia is defined as serum sodium >145 mmol/L, and management depends critically on distinguishing between hypovolemic, euvolemic, and hypervolemic states, with correction rates never exceeding 10-12 mmol/L per 24 hours to prevent cerebral edema. 1, 2

Diagnostic Cut-offs

• Mild hypernatremia: 145-150 mmol/L 2
• Moderate hypernatremia: 150-160 mmol/L 2
• Severe hypernatremia: >160 mmol/L 2
• Life-threatening hypernatremia: >180-200 mmol/L (associated with extremely high mortality) 3

The initial serum sodium concentration and patient age are the most important prognostic indicators, with very young patients and those with lesser degrees of hypernatremia having better survival rates. 3

Initial Assessment Algorithm

Step 1: Determine Volume Status

Physical examination findings guide the differential diagnosis: 4, 5

• Hypovolemic signs: Orthostatic hypotension, dry mucous membranes, decreased skin turgor, tachycardia, flat neck veins 5
• Euvolemic signs: Normal blood pressure, normal skin turgor, no edema 5
• Hypervolemic signs: Peripheral edema, jugular venous distention, pulmonary congestion, ascites 5

Step 2: Check Urine Osmolality

• Urine osmolality >600-800 mOsm/kg: Indicates appropriate renal water conservation; suggests extrarenal water losses (GI losses, insensible losses, burns) 4, 5
• Urine osmolality <300 mOsm/kg: Indicates impaired renal concentrating ability; suggests diabetes insipidus (central or nephrogenic) 4, 5

Management Based on Volume Status

Hypovolemic Hypernatremia (Most Common)

Replace both volume and free water deficit using hypotonic fluids: 2, 4

• First-line fluid: 0.45% NaCl (half-normal saline) for moderate hypernatremia 2
• Alternative: 0.18% NaCl or D5W for more severe hypernatremia requiring greater free water replacement 2
• Initial rate: Calculate free water deficit using the formula: Free water deficit (L) = 0.6 × body weight (kg) × [(serum Na/140) - 1] 2
• Avoid isotonic saline (0.9% NaCl): This will worsen hypernatremia in patients with renal concentrating defects 6

Euvolemic Hypernatremia (Diabetes Insipidus)

Central diabetes insipidus: 4, 5

• Desmopressin (DDAVP): 1-4 mcg subcutaneously or IV every 12-24 hours 1, 5
• Plus hypotonic fluid replacement: 0.45% NaCl or D5W to replace ongoing losses 1

Nephrogenic diabetes insipidus: 4, 5

• Hypotonic fluids: Continuous administration to match excessive free water losses 6, 4
• Thiazide diuretics: Paradoxically reduce urine output by inducing mild volume depletion 5
• Amiloride: If lithium-induced nephrogenic DI 5

Hypervolemic Hypernatremia (Sodium Excess)

Remove excess sodium while providing free water: 2, 4

• Loop diuretics (furosemide): To promote sodium excretion 2
• Plus hypotonic fluid replacement: D5W or 0.45% NaCl 2
• Hemodialysis: For severe cases or renal failure 1, 2

Critical Correction Rate Guidelines

For chronic hypernatremia (>48 hours duration): 1, 2

• Maximum correction rate: 8-10 mmol/L per 24 hours 1
• Preferred rate: 0.5 mmol/L per hour or 10-12 mmol/L per day 2
• Rationale: Rapid correction causes cerebral edema due to osmotic shifts 1, 2

For acute hypernatremia (<24 hours duration): 1

• Hemodialysis is an effective option to rapidly normalize serum sodium levels 1
• However, even in acute cases, avoid drops >12 mmol/L per 24 hours when starting renal replacement therapy 1

Monitoring Protocol

• Serum sodium: Every 2-4 hours during active correction 2
• Urine output and osmolality: Every 4-6 hours 2
• Neurological status: Continuous assessment for signs of cerebral edema (headache, confusion, seizures) 2
• Volume status: Serial physical examinations 2

Special Clinical Scenarios

Patients with Renal Concentrating Defects

Nephrogenic diabetes insipidus patients require ongoing hypotonic fluid administration to match their excessive free water losses, and isotonic fluids will worsen hypernatremia. 6

Salt Intoxication

Surprisingly small amounts of salt can cause severe hypernatremia: 3

• Ingestion of approximately 70-90 g of table salt (1,200-1,500 mEq sodium) can produce serum sodium >200 mmol/L 3
• Mortality is extremely high with initial sodium >180-200 mmol/L 3
• Treatment: Hypotonic fluid therapy to provide free water, but prognosis depends primarily on initial sodium level and patient age 3

Common Pitfalls to Avoid

• Using isotonic saline in patients with impaired renal concentrating ability will exacerbate hypernatremia 6
• Correcting chronic hypernatremia too rapidly (>10-12 mmol/L per day) causes cerebral edema 1, 2
• Undercorrection or overcorrection are both associated with poor prognosis 2
• Failing to address the underlying cause (stopping causative medications, treating diabetes insipidus, replacing ongoing losses) 4, 5
• Inadequate monitoring during correction can lead to complications 2