What are the causes of hyponatremia?

Causes of Hyponatremia

Hyponatremia (serum sodium <135 mmol/L) should be evaluated based on volume status and serum osmolality to determine the underlying cause. 1

Classification by Volume Status

Hypovolemic Hyponatremia

• Excessive diuretic use, particularly in patients with liver cirrhosis 1
• Gastrointestinal losses (vomiting, diarrhea) 2
• Severe burns 1
• Blood loss 3
• Third-space fluid sequestration 2

Euvolemic Hyponatremia

• Syndrome of Inappropriate ADH secretion (SIADH) 1
• Medications (antidepressants, antipsychotics, anticonvulsants) 2
• Malignancies (particularly lung cancer) 1
• Post-operative state 2
• Hypothyroidism 1
• Adrenal insufficiency 1
• Reset osmostat syndrome 1
• Primary polydipsia (excessive water intake) 1, 2

Hypervolemic Hyponatremia

• Congestive heart failure 1, 2
• Liver cirrhosis with portal hypertension 1, 2
• Nephrotic syndrome 3
• Advanced renal failure 3
• Systemic vasodilation due to portal hypertension 1
• Activation of renin-angiotensin-aldosterone system causing excessive sodium and water reabsorption 1

Classification by Serum Osmolality

Hypotonic Hyponatremia (Low Plasma Osmolality)

• Most common form of hyponatremia 4
• Due to defective water excretion (usually elevated vasopressin) or excessive fluid intake 4
• Includes most cases of hypovolemic, euvolemic, and hypervolemic hyponatremia 2

Isotonic Hyponatremia (Normal Plasma Osmolality)

• Pseudohyponatremia (laboratory artifact) 2
• Post-transurethral prostatic resection syndrome 2

Hypertonic Hyponatremia (High Plasma Osmolality)

• Hyperglycemia 2
• Administration of mannitol or other osmotically active substances 4

Special Clinical Scenarios

Cerebral Salt Wasting (CSW)

• More common than SIADH in neurosurgical patients 1
• Caused by excessive secretion of natriuretic peptides 1
• Features include hypovolemia, excessive natriuresis, and hyponatremia 1
• More common in patients with poor clinical grade, ruptured anterior communicating artery aneurysms, and hydrocephalus 1

Beer Potomania

• Due to excessive beer consumption with low solute intake 1
• Improvement occurs with discontinuation of alcohol and normal dietary sodium intake 1

Diagnostic Approach

• Initial workup should include serum and urine osmolarity, urine electrolytes, uric acid, and assessment of extracellular fluid volume status 1
• Urinary sodium <30 mmol/L suggests hypovolemic hyponatremia 1
• Urinary sodium >20 mEq/L with high urine osmolality (>500 mosm/kg) suggests SIADH 1
• Serum uric acid <4 mg/dL has a positive predictive value of 73-100% for SIADH 1

Clinical Significance

• Hyponatremia is the most common electrolyte disorder encountered in clinical medicine 1
• Associated with increased mortality, with sodium levels <130 mmol/L linked to a 60-fold increase in fatality (11.2% versus 0.19%) 1
• Increases fall risk - 21% of hyponatremic patients present with falls compared to 5% of normonatremic patients 1
• In cirrhotic patients, sodium <130 mmol/L increases risk of hepatic encephalopathy (OR 2.36), spontaneous bacterial peritonitis (OR 3.40), and hepatorenal syndrome (OR 3.45) 1

Common Pitfalls in Diagnosis

• Failing to assess volume status accurately, which is essential for differentiating between causes of hyponatremia 1
• Misdiagnosing SIADH vs. cerebral salt wasting in neurosurgical patients, as treatment approaches differ significantly 1
• Ignoring mild hyponatremia (135 mmol/L) as clinically insignificant 1
• Failing to recognize and treat the underlying cause 1