What are the causes and treatment options for hyponatremia (low sodium levels in the blood) in patients with heart failure, liver disease, or kidney disease?

Causes of Hyponatremia

Hyponatremia (serum sodium <135 mmol/L) results from three primary mechanisms based on volume status: hypovolemic (sodium and water loss), euvolemic (water retention with normal sodium), and hypervolemic (water retention exceeding sodium retention), with the underlying pathophysiology determining the appropriate treatment approach. 1

Classification by Volume Status

Hypovolemic Hyponatremia (Sodium and Water Depletion)

Extrarenal losses (urine sodium <30 mmol/L):

• Gastrointestinal losses from vomiting, diarrhea, or nasogastric suction 1
• Burns with significant fluid loss 1
• Third-space fluid sequestration 1

Renal losses (urine sodium >20 mmol/L):

• Excessive diuretic use, particularly thiazides and loop diuretics 1, 2
• Salt-wasting nephropathy 1
• Cerebral salt wasting (CSW) in neurosurgical patients, especially with subarachnoid hemorrhage 1
• Mineralocorticoid deficiency 1

Euvolemic Hyponatremia (Water Retention with Normal Sodium)

Syndrome of Inappropriate Antidiuretic Hormone (SIADH) - the most common cause:

• Malignancies (particularly small cell lung cancer affecting 1-5% of lung cancer patients) 1
• CNS disorders including meningitis, encephalitis, stroke, and head trauma 1
• Pulmonary diseases such as pneumonia and tuberculosis 1
• Medications: SSRIs, carbamazepine, cyclophosphamide, NSAIDs 1, 3
• Postoperative states with pain, nausea, and stress triggering nonosmotic AVP release 1

Hospital-acquired hyponatremia:

• Hypotonic IV fluids in the setting of elevated ADH, affecting 15-30% of hospitalized patients 1
• Entirely preventable by using isotonic maintenance fluids 1

Endocrine disorders:

• Hypothyroidism (check TSH) 1
• Adrenal insufficiency 1

Other causes:

• Psychogenic polydipsia 4
• Beer potomania (low solute intake with excessive water consumption) 1
• Exercise-induced hyponatremia from excessive free water intake 4, 5

Hypervolemic Hyponatremia (Total Body Sodium and Water Excess)

Advanced liver cirrhosis with portal hypertension:

• Occurs in approximately 60% of cirrhotic patients with ascites 1, 2
• Systemic vasodilation causes decreased effective plasma volume 2
• Non-osmotic hypersecretion of vasopressin due to perceived arterial underfilling 1, 2
• Activation of renin-angiotensin-aldosterone system causes excessive sodium and water reabsorption 2

Congestive heart failure:

• Neurohormonal activation leading to water retention exceeding sodium retention 6
• Signs include jugular venous distention, orthopnea, dyspnea, and peripheral edema 1

Renal failure:

• Impaired free water excretion 4
• Inability to handle sodium and water appropriately 1

Nephrotic syndrome:

• Severe proteinuria with hypoalbuminemia 4

Pseudohyponatremia (Artifactual)

• Hyperproteinemia (>10 g/dL) 4
• Severe hyperlipidemia (triglycerides >1500 mg/dL) 4
• Hyperglycemia: add 1.6 mEq/L to measured sodium for each 100 mg/dL glucose >100 mg/dL 1

Clinical Significance by Severity

Mild hyponatremia (130-135 mmol/L):

• Associated with increased fall risk (21% vs 5% in normonatremic patients) 1
• Cognitive impairment and gait disturbances 3
• Should not be ignored despite minimal symptoms 1

Moderate hyponatremia (120-125 mmol/L):

• Nausea, vomiting, weakness, headache, mild neurocognitive deficits 5

Severe hyponatremia (<120 mmol/L):

• Delirium, confusion, impaired consciousness, ataxia, seizures 5
• 60-fold increase in hospital mortality (11.2% vs 0.19%) 1
• Brain herniation and death in extreme cases 5

Special Population Considerations

Cirrhotic patients with hyponatremia (sodium <130 mmol/L):

• Increased risk of spontaneous bacterial peritonitis (OR 3.40) 1, 2
• Increased risk of hepatorenal syndrome (OR 3.45) 1, 2
• Increased risk of hepatic encephalopathy (OR 2.36) 1, 2

Neurosurgical patients:

• Cerebral salt wasting is more common than SIADH 1
• Particularly in poor clinical grade, ruptured anterior communicating artery aneurysms, and hydrocephalus 1

Acute illness states:

• Acute pancreatitis causes nonosmotic AVP release 1
• Pain, nausea, and stress are potent stimuli for vasopressin secretion 1