What causes hyponatremia?

Causes of Hyponatremia (Sodium 130 mmol/L)

A sodium level of 130 mmol/L indicates hyponatremia, which develops through three primary mechanisms based on volume status: hypovolemic (too little fluid), euvolemic (normal fluid but excess ADH), or hypervolemic (too much fluid). 1

Primary Mechanisms and Common Causes

Hypovolemic Hyponatremia (Volume Depleted)

• Excessive diuretic use, particularly thiazides and loop diuretics, is a leading cause of hypovolemic hyponatremia 1, 2
• Gastrointestinal losses from diarrhea, vomiting, or other fluid losses deplete both sodium and water 3, 4
• Cerebral salt wasting syndrome in neurosurgical patients causes excessive renal sodium loss 1, 2
• Severe burns can lead to significant sodium and fluid losses 1

Euvolemic Hyponatremia (Normal Volume Status)

• Syndrome of Inappropriate Antidiuretic Hormone (SIADH) is the primary mechanism for euvolemic hyponatremia 1, 2, 3
• Malignancies including small cell lung cancer, pancreatic cancer, and lymphomas trigger SIADH 1, 2
• Medications are extremely common culprits, including:
  • SSRIs (selective serotonin reuptake inhibitors) 1, 2
  • Carbamazepine and oxcarbazepine 1, 2
  • Trazodone 1, 2
  • Chemotherapy agents (vincristine, cyclophosphamide) 1, 2
  • Desmopressin 1, 2
• Hypothyroidism and adrenal insufficiency can present as euvolemic hyponatremia 2
• Excessive alcohol consumption and very low-salt diets combined with excessive free water intake 5

Hypervolemic Hyponatremia (Volume Overloaded)

• Cirrhosis with portal hypertension causes hypervolemic hyponatremia in approximately 60% of cirrhotic patients due to non-osmotic ADH release and impaired free water clearance 1, 2
• Congestive heart failure triggers neurohormonal activation with increased ADH release despite total body fluid overload 1, 2, 4
• Nephrotic syndrome and chronic kidney disease impair free water excretion 2
• Renal disease with impaired sodium handling 4

Pathophysiology

All three types share a common final pathway: increased ADH levels combined with continued fluid intake lead to water retention and dilutional hyponatremia. 3 The key distinction is the total body volume status, which determines appropriate treatment 5, 6.

Clinical Significance

• Even mild hyponatremia at 130 mmol/L is associated with increased hospital stay, mortality, cognitive impairment, gait disturbances, and increased falls and fractures 6
• Hyponatremia affects approximately 5% of adults and 35% of hospitalized patients, making it the most common electrolyte disorder 6, 3
• A sodium level of 130 mmol/L represents mild hyponatremia (defined as 130-134 mmol/L) but should not be ignored 5, 6

High-Risk Populations

• Patients on high-risk medications (desmopressin, antiepileptics, chemotherapy, SSRIs, trazodone) require immediate attention 2
• Pediatric patients receiving perioperative medications or with congenital heart disease, liver disease, or renal dysfunction 2
• Postoperative patients are at increased risk due to elevated AVP hormone levels 7