What are the categories of hyponatremia?

Categories of Hyponatremia

Hyponatremia should be categorized based on volume status (hypovolemic, euvolemic, or hypervolemic) and serum osmolality, which is the most clinically useful approach to determining the underlying cause and appropriate treatment. 1, 2

Classification Based on Serum Osmolality

• Normal or High Osmolality (Pseudohyponatremia): Serum sodium is low due to laboratory error, hyperglycemia, hyperlipidemia, or hyperproteinemia 2, 3
• Low Osmolality (True Hyponatremia): Further classified based on volume status 1, 4

Classification Based on Volume Status

1. Hypovolemic Hyponatremia

• Characterized by decreased extracellular fluid volume and signs of dehydration 5, 1
• Urinary sodium <30 mmol/L suggests extrarenal losses (e.g., vomiting, diarrhea, burns) 4, 6
• Urinary sodium >20 mmol/L suggests renal losses (e.g., diuretics, cerebral salt wasting, adrenal insufficiency) 1, 4
• Physical findings include orthostatic hypotension, dry mucous membranes, decreased skin turgor 4, 6

2. Euvolemic Hyponatremia

• Normal extracellular fluid volume without edema or signs of dehydration 1, 3
• Most commonly caused by Syndrome of Inappropriate Antidiuretic Hormone (SIADH) 1, 7
• Diagnostic criteria for SIADH include:
  • Serum sodium <134 mEq/L
  • Plasma osmolality <275 mOsm/kg
  • Urine osmolality >500 mOsm/kg (inappropriately concentrated)
  • Urinary sodium >20-40 mEq/L
  • Absence of hypothyroidism, adrenal insufficiency, or volume depletion
  • Euvolemic status 4
• Other causes include hypothyroidism, hypocortisolism, and polydipsia 4, 8

3. Hypervolemic Hyponatremia

• Characterized by increased extracellular fluid volume and edema 1, 9
• Common causes include congestive heart failure, liver cirrhosis, and renal failure 2, 8
• Urinary sodium is typically <20 mEq/L in heart failure and cirrhosis (except with diuretic use) 1, 8
• Urinary sodium is >20 mEq/L in advanced renal failure 4, 8

Diagnostic Approach

• Serum and urine osmolality measurements are essential to distinguish between different types of hyponatremia 1, 4
• Urinary sodium concentration helps differentiate between renal and extrarenal causes 5, 4
• Serum uric acid <4 mg/dL has a positive predictive value of 73-100% for SIADH (though may also be seen in cerebral salt wasting) 5, 4
• Physical examination alone has limited accuracy for determining volume status (sensitivity 41.1%, specificity 80%) 5, 4

Special Considerations in Neurological Patients

• In neurosurgical patients, distinguishing between SIADH and cerebral salt wasting (CSW) is critical as treatment approaches differ significantly 5, 6
• Both SIADH and CSW present with elevated urinary sodium, making assessment of volume status crucial 5, 6
• Central venous pressure (CVP) can help determine intravascular volume status to distinguish between SIADH and CSW 5
• CSW is characterized by hypovolemia with CVP <6 cm H₂O, while SIADH typically has normal volume with CVP 6-10 cm H₂O 5

Common Pitfalls to Avoid

• Relying solely on physical examination to determine volume status, as this has poor sensitivity 5, 4
• Misdiagnosing CSW as SIADH, which could lead to inappropriate fluid restriction and worsen outcomes 1, 6
• Failing to recognize pseudohyponatremia, which requires different management 2, 3
• Ignoring mild hyponatremia (Na 131-135 mmol/L) as clinically insignificant, when it may still be associated with cognitive impairment, gait disturbances, and increased fall risk 1, 3