How do you diagnose and manage hyponatremia?

Diagnosis and Management of Hyponatremia

Initial Diagnostic Approach

Hyponatremia is defined as serum sodium <135 mEq/L and requires systematic evaluation based on volume status, symptom severity, and onset timing to guide appropriate treatment. 1

Essential Diagnostic Workup

• Measure serum and urine osmolality, urine sodium, and uric acid to determine the underlying etiology 1
• Assess extracellular fluid (ECF) volume status through physical examination looking for orthostatic hypotension, dry mucous membranes, skin turgor, jugular venous distention, peripheral edema, and ascites 1
• Check serum electrolytes, renal function, thyroid-stimulating hormone (TSH), and cortisol to rule out secondary causes 1
• Urinary sodium <30 mmol/L has 71-100% positive predictive value for response to normal saline, suggesting hypovolemic hyponatremia 1
• Serum uric acid <4 mg/dL has 73-100% positive predictive value for SIADH (though may include cerebral salt wasting) 1

Volume Status Classification

Hypovolemic hyponatremia: ECF contraction with urine sodium <20 mmol/L suggests sodium depletion from gastrointestinal losses, burns, or dehydration 2

Euvolemic hyponatremia: No edema, normal blood pressure, normal skin turgor with urine sodium >20-40 mmol/L and urine osmolality >300 mOsm/kg suggests SIADH 1

Hypervolemic hyponatremia: Presence of edema, ascites, or jugular venous distention indicates heart failure, cirrhosis, or renal disease 2, 1

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Management Based on Symptom Severity

Severe Symptomatic Hyponatremia (Medical Emergency)

For patients with seizures, coma, confusion, or cardiorespiratory distress, immediately administer 3% hypertonic saline with a target correction of 6 mmol/L over 6 hours or until severe symptoms resolve. 1

• Administer 100 mL boluses of 3% saline over 10 minutes, repeating up to three times at 10-minute intervals until symptoms improve 1
• Maximum correction must not exceed 8 mmol/L in 24 hours to prevent osmotic demyelination syndrome 2, 1
• Monitor serum sodium every 2 hours during initial correction, then every 4 hours after symptom resolution 1
• Calculate sodium deficit: Desired increase in Na (mEq/L) × (0.5 × ideal body weight in kg) 1

Moderate Hyponatremia (120-125 mEq/L)

• Implement fluid restriction to 1000 mL/day for euvolemic or hypervolemic patients 2, 1
• Discontinue diuretics temporarily if contributing to hyponatremia 2, 1
• For hypovolemic patients, administer isotonic (0.9%) saline to restore intravascular volume 1

Mild Hyponatremia (126-135 mEq/L)

• Continue monitoring without specific treatment for asymptomatic patients, though even mild hyponatremia increases fall risk and mortality 1, 3
• Water restriction is not recommended at sodium levels 126-135 mEq/L 1
• Continue diuretics with close electrolyte monitoring if clinically indicated 1

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Management Based on Volume Status

Hypovolemic Hyponatremia

• Discontinue diuretics and laxatives immediately 2, 1
• Administer isotonic (0.9%) saline or 5% albumin for volume repletion 2, 1
• Preferentially use lactated Ringer's solution over normal saline when appropriate 2
• Once euvolemic, reassess to determine if additional sodium correction is needed 1

Euvolemic Hyponatremia (SIADH)

• Fluid restriction to 1 L/day is the cornerstone of treatment for mild to moderate cases 1
• Add oral sodium chloride 100 mEq three times daily if no response to fluid restriction alone 1
• Consider vasopressin receptor antagonists (vaptans) for resistant cases, starting with tolvaptan 15 mg once daily 1, 4
• Alternative pharmacological options include urea, demeclocycline, or lithium for refractory cases 1

Hypervolemic Hyponatremia (Heart Failure, Cirrhosis)

• Implement fluid restriction to 1-1.5 L/day for sodium <125 mEq/L 2, 1
• For cirrhotic patients, administer albumin infusion alongside fluid restriction 2, 1
• Avoid hypertonic saline unless life-threatening symptoms present, as it may worsen ascites and edema 1
• Temporarily discontinue diuretics if sodium <125 mEq/L 1
• Sodium restriction (2-2.5 g/day) is more important than fluid restriction for weight loss, as fluid passively follows sodium 1

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Special Populations and Considerations

Cirrhotic Patients

• Hyponatremia in cirrhosis increases risk of spontaneous bacterial peritonitis (OR 3.40), hepatorenal syndrome (OR 3.45), and hepatic encephalopathy (OR 2.36) 1
• Use more cautious correction rates (4-6 mEq/L per day) due to higher risk of osmotic demyelination syndrome 2, 1
• Tolvaptan carries higher risk of gastrointestinal bleeding in cirrhosis (10% vs 2% placebo) 1, 4
• Albumin infusion improves hyponatremia in hospitalized cirrhotic patients 2

Neurosurgical Patients

• Distinguish between SIADH and cerebral salt wasting (CSW), as treatment approaches differ fundamentally 1
• CSW requires volume and sodium replacement, not fluid restriction 1
• Consider fludrocortisone for hyponatremia in subarachnoid hemorrhage patients at risk of vasospasm 1
• Avoid fluid restriction in subarachnoid hemorrhage patients at risk of vasospasm 1

Pediatric Patients (Neonates on Parenteral Nutrition)

• Diagnostic measures rely on clinical and ECF assessment plus urinary sodium measurement 2
• ECF excess with inadequate weight loss suggests water overload; consider acute renal failure if oliguria with urine Na >20 mmol/L 2
• Primary sodium depletion is frequent in preterm infants <34 weeks gestation due to deficient tubular reabsorption 2
• Corrections more rapid than 48-72 hours increase risk of pontine myelinolysis 2

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Critical Correction Rate Guidelines

The single most important safety principle is limiting correction to prevent osmotic demyelination syndrome.

• Standard patients: 4-8 mEq/L per day, not exceeding 10-12 mEq/L in 24 hours 2, 1
• High-risk patients (advanced liver disease, alcoholism, malnutrition, prior encephalopathy): 4-6 mEq/L per day, not exceeding 8 mEq/L in 24 hours 2, 1
• Acute hyponatremia (<48 hours onset) can be corrected more rapidly without risk of osmotic demyelination syndrome 2
• Chronic hyponatremia (>48 hours) requires gradual correction to avoid osmotic demyelination syndrome 2, 1

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Management of Overcorrection

If sodium correction exceeds 8 mmol/L in 24 hours, immediate intervention is required to prevent osmotic demyelination syndrome. 1

• Immediately discontinue current fluids and switch to D5W (5% dextrose in water) to relower sodium levels 1
• Consider administering desmopressin to slow or reverse the rapid rise in serum sodium 2, 1
• Target relowering to bring total 24-hour correction to no more than 8 mEq/L from starting point 1
• Watch for osmotic demyelination syndrome signs (dysarthria, dysphagia, oculomotor dysfunction, quadriparesis) typically occurring 2-7 days after rapid correction 2, 1

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Pharmacological Interventions

Vasopressin Receptor Antagonists (Vaptans)

• Tolvaptan starting dose: 15 mg once daily, titrate to 30 mg then 60 mg based on response 1, 4
• Indicated for euvolemic or hypervolemic hyponatremia resistant to fluid restriction 1, 4
• Contraindicated with strong CYP3A inhibitors (ketoconazole increases tolvaptan AUC 5.4-fold) 4
• Avoid with moderate CYP3A inhibitors and grapefruit juice 4
• Monitor closely to avoid overly rapid correction (>8 mmol/L/day) 1, 4
• Common adverse effects: thirst (12%), dry mouth (7%), polyuria (11%) 4
• Use with extreme caution in cirrhosis due to gastrointestinal bleeding risk 1, 4

Alternative Agents

• Urea 40 g in 100-150 mL normal saline every 8 hours for neurosurgical patients with hyponatremia 1
• Fludrocortisone for cerebral salt wasting in subarachnoid hemorrhage patients 1
• Demeclocycline or lithium for refractory SIADH (less commonly used due to side effects) 1

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Common Pitfalls to Avoid

• Overly rapid correction exceeding 8 mmol/L in 24 hours leads to osmotic demyelination syndrome 2, 1
• Using fluid restriction in cerebral salt wasting worsens outcomes 1
• Inadequate monitoring during active correction increases complication risk 1
• Failing to recognize and treat the underlying cause 1
• Using hypertonic saline in hypervolemic hyponatremia without life-threatening symptoms worsens edema and ascites 1
• Ignoring mild hyponatremia (130-135 mmol/L) as clinically insignificant when it increases fall risk and mortality 1, 3
• Administering normal saline in SIADH may worsen hyponatremia 1