How is hypernatremia classified and what is the initial workup?

Hypernatremia Classification and Initial Workup

Hypernatremia is defined as serum sodium concentration >145 mmol/L and should be classified based on volume status (hypovolemic, euvolemic, or hypervolemic) and severity (mild, moderate, or severe) to guide appropriate management. The initial workup of hypernatremia should include assessment of volume status, measurement of urine osmolality and sodium, and evaluation of potential underlying causes to determine the appropriate treatment approach. 1

Classification of Hypernatremia

• Based on serum sodium level:

  • Mild: 146-150 mmol/L
  • Moderate: 151-159 mmol/L
  • Severe: ≥160 mmol/L 2

• Based on volume status:

  • Hypovolemic hypernatremia: Decreased total body water and sodium, with greater water than sodium loss
    • Causes: Renal losses (diuretics, osmotic diuresis) or extrarenal losses (excessive sweating, diarrhea, burns) 2, 3
  • Euvolemic hypernatremia: Normal total body sodium with water deficit
    • Causes: Diabetes insipidus (central or nephrogenic), inadequate water intake, insensible losses 2, 4
  • Hypervolemic hypernatremia: Increased total body sodium exceeding increase in total body water
    • Causes: Iatrogenic administration of hypertonic fluids, primary hyperaldosteronism 2, 5

• Based on duration:

  • Acute: <48 hours
  • Chronic: >48 hours 2

Initial Diagnostic Workup

• History and physical examination focused on:

  • Duration of symptoms
  • Fluid intake and output
  • Medication review (diuretics, lithium, etc.)
  • Signs of volume status (skin turgor, mucous membrane moisture, orthostatic changes) 6, 4

• Laboratory assessment:

  • Serum sodium, potassium, chloride, bicarbonate
  • Blood urea nitrogen (BUN) and creatinine
  • Serum and urine osmolality
  • Urine sodium concentration 1, 2

• Diagnostic algorithm:

  1. Measure urine osmolality:

     • Low (<300 mOsm/kg): Suggests diabetes insipidus
     • High (>600 mOsm/kg): Suggests water loss or sodium gain 1, 2

  2. Measure urine sodium:

     • Low (<20 mmol/L): Suggests extrarenal water loss
     • High (>40 mmol/L): Suggests renal water loss or sodium gain 1, 2

  3. Assess volume status:

     • Hypovolemic: Orthostatic hypotension, tachycardia, dry mucous membranes
     • Euvolemic: Normal vital signs, no edema or signs of dehydration
     • Hypervolemic: Edema, ascites, elevated blood pressure 6, 3

Special Considerations

• Patients with impaired thirst mechanism (elderly, neurological disorders) are at higher risk for developing hypernatremia due to inadequate water intake 4, 3

• Chronic kidney disease and acute kidney injury increase the risk of developing severe hospital-acquired hypernatremia (odds ratio 2.38) 5

• Rapid correction of chronic hypernatremia (>48h) can lead to cerebral edema, seizures, and permanent neurological damage 1, 4

• The correction rate for chronic hypernatremia should not exceed 10-15 mmol/L/24h to prevent neurological complications 1, 2

Common Pitfalls to Avoid

• Failing to distinguish between acute and chronic hypernatremia, which affects the appropriate correction rate 2, 4

• Overly rapid correction of chronic hypernatremia, which can lead to cerebral edema and neurological injury 1, 4

• Inadequate monitoring during correction of hypernatremia, especially in high-risk patients 4, 5

• Failing to identify and address the underlying cause of hypernatremia 6, 3

• Using normal saline for fluid replacement in patients with hypernatremia, which can worsen the condition 5