Causes of Hyponatremia
Hyponatremia is classified based on volume status as hypovolemic, euvolemic, or hypervolemic, with common causes including medications, excessive alcohol consumption, very low-salt diets, and excessive free water intake. 1
Classification of Hyponatremia
Hyponatremia is defined as serum sodium level <135 mEq/L and can be categorized by severity:
- Mild: 130-135 mEq/L
- Moderate: 125-129 mEq/L
- Severe: <125 mEq/L 2
Causes by Volume Status
1. Hypovolemic Hyponatremia
- Diuretic-induced sodium depletion (most frequent cause in cirrhotic patients) 3
- Gastrointestinal losses (vomiting, diarrhea)
- Severe burns
- Third-space losses
- Renal losses
2. Euvolemic Hyponatremia
- Syndrome of Inappropriate Antidiuretic Hormone (SIADH)
- Medications (antidepressants, antipsychotics, anticonvulsants)
- Endocrine deficiencies (hypothyroidism, adrenal insufficiency)
- Reset osmostat syndrome
- Excessive water intake during exercise 1
- Post-operative state, particularly after transsphenoidal surgery for pituitary macroadenoma 2
3. Hypervolemic Hyponatremia
- Cirrhosis (occurs in approximately 21.6% of patients) 2
- Congestive heart failure
- Renal disease/nephrotic syndrome 4
Clinical Assessment
Volume Status Evaluation
- Hypovolemic: Orthostatic hypotension, tachycardia, decreased skin turgor, dry mucous membranes
- Euvolemic: No signs of volume depletion or excess
- Hypervolemic: Edema, ascites, jugular venous distension 2
Laboratory Assessment
Plasma osmolality measurement is crucial:
- High osmolality: Consider hyperglycemia
- Normal osmolality: Consider pseudohyponatremia
- Low osmolality: True hyponatremia 4
Urinary sodium concentration:
- High urinary sodium (>20 mEq/L) with low plasma osmolality: Renal disorders, SIADH, endocrine deficiencies
- Low urinary sodium (<20 mEq/L): Gastrointestinal losses, burns, acute water overload 4
Treatment Approaches
Hypovolemic Hyponatremia
- Fluid resuscitation with isotonic saline or 5% albumin
- Discontinue diuretics if applicable 2
Euvolemic Hyponatremia
- Fluid restriction (<1 L/day)
- Ensure adequate solute intake
- Consider vasopressin receptor antagonists (vaptans) in appropriate cases 2
- Tolvaptan has shown efficacy in improving serum sodium in 45-82% of patients 2, 5
Hypervolemic Hyponatremia
- For mild to moderate hyponatremia (Na 125-135 mmol/L): Continue sodium restriction (5-6.5 g/day), adjust diuretics as needed
- For severe hyponatremia (Na <125 mmol/L): Implement fluid restriction of 1-1.5 L/day, continue sodium restriction, consider temporary discontinuation of diuretics 2
Severe Symptomatic Hyponatremia
- Medical emergency requiring rapid correction with hypertonic saline
- Aim to increase serum sodium by 4-6 mEq/L within 1-2 hours
- Limit correction to 8-10 mmol/L in 24 hours and 18 mmol/L in 48 hours to prevent osmotic demyelination syndrome 2, 6
Special Considerations
Cirrhosis
- Patients with cirrhosis using tolvaptan have increased risk of gastrointestinal bleeding (10% vs 2% with placebo) 5
- Temporarily discontinue diuretics if serum sodium <125 mmol/L, worsening hypokalemia or hyperkalemia, rising serum creatinine, hepatic encephalopathy, or muscle cramps 2
High-Risk Populations
- Women and elderly patients are more sensitive to hyponatremic injury 2
- Patients with advanced liver disease, alcoholism, severe hyponatremia, malnutrition, hypokalemia, and hypophosphatemia are at increased risk for complications 2
- For these high-risk groups, limit correction rate to 4-6 mEq/L per 24 hours 2
Monitoring
- Regular monitoring of serum sodium every 2-4 hours during active correction
- Extended monitoring for at least two weeks post-correction 2
- Regular follow-up within 24-48 hours for outpatients 2
Hyponatremia is associated with increased hospital stay, mortality, cognitive impairment, gait disturbances, and increased rates of falls and fractures, making proper diagnosis and management essential for optimal patient outcomes 6.