How to manage a patient with hyponatremia and suspected SIADH (Syndrome of Inappropriate Antidiuretic Hormone Secretion)?

Management of Hyponatremia with Suspected SIADH

Diagnostic Confirmation

Your laboratory values confirm SIADH: serum sodium 127 mEq/L (hyponatremia), serum osmolality 283 mOsm/kg (hypoosmolar), urine osmolality 555 mOsm/kg (inappropriately concentrated >500 mOsm/kg), and urine sodium 76.6 mEq/L (inappropriately elevated >20 mEq/L). 1

• The random cortisol of 291 nmol/L (assuming units) effectively rules out adrenal insufficiency, which is a required exclusion criterion for SIADH diagnosis 1
• Thyroid function should be checked to complete the diagnostic workup and exclude hypothyroidism 2
• The euvolemic state must be confirmed clinically—look specifically for absence of orthostatic hypotension, dry mucous membranes, peripheral edema, ascites, or jugular venous distention 1, 2

Treatment Algorithm Based on Symptom Severity

For Asymptomatic or Mild Symptoms (Your Patient at Na 127 mEq/L)

Implement fluid restriction to 1 L/day as first-line therapy. 1, 2 This is the cornerstone of SIADH management and should be initiated immediately.

• Avoid fluid restriction during the first 24 hours only if initiating tolvaptan, otherwise begin immediately 3
• If no response to fluid restriction after 24-48 hours, add oral sodium chloride 100 mEq three times daily 1, 2
• Monitor serum sodium every 24 hours initially, then adjust frequency based on response 1

For Severe Symptomatic Hyponatremia (Seizures, Altered Mental Status, Coma)

Administer 3% hypertonic saline immediately with a target correction of 6 mmol/L over 6 hours or until severe symptoms resolve. 1, 2

• Give as 100-150 mL IV boluses over 10 minutes, repeatable up to three times at 10-minute intervals 2, 4
• Transfer to ICU for close monitoring 1
• Check serum sodium every 2 hours during initial correction 1, 2

Critical Correction Rate Guidelines

Never exceed 8 mmol/L correction in 24 hours to prevent osmotic demyelination syndrome. 1, 2, 3 This is the single most important safety consideration.

• The FDA warns that correction >12 mEq/L/24 hours can cause osmotic demyelination resulting in dysarthria, mutism, dysphagia, lethargy, affective changes, spastic quadriparesis, seizures, coma, and death 3
• For high-risk patients (advanced liver disease, alcoholism, malnutrition), limit correction to 4-6 mmol/L per day 1, 2
• If overcorrection occurs, immediately discontinue current fluids, switch to D5W, and consider desmopressin to relower sodium 2

Second-Line Pharmacological Options

Tolvaptan (Vasopressin V2 Receptor Antagonist)

Consider tolvaptan 15 mg once daily if fluid restriction fails or is poorly tolerated. 1, 3

• Must be initiated in hospital with close serum sodium monitoring at 0,6,24, and 48 hours 3, 5
• Can titrate to 30 mg after 24 hours, maximum 60 mg daily 3
• Do not use for more than 30 days due to hepatotoxicity risk 3
• Contraindicated if unable to sense thirst, hypovolemic, or taking strong CYP3A inhibitors 3
• Side effects include thirst, polydipsia, and urinary frequency 5

Alternative Second-Line Agents

• Demeclocycline: Induces nephrogenic diabetes insipidus, long history of use in persistent SIADH 1, 2
• Urea: Very effective and safe, 30-60 g/day in divided doses 1, 4
• Loop diuretics with salt supplementation: Less commonly used 1

Common Pitfalls to Avoid

• Never use normal saline (0.9% NaCl) in SIADH—it acts as hypotonic fluid in these patients and can paradoxically worsen hyponatremia due to the high urine osmolality causing net free water retention 6
• Do not confuse SIADH with cerebral salt wasting (CSW)—CSW requires volume replacement, not fluid restriction, and is characterized by true hypovolemia with CVP <6 cm H₂O 1, 2
• Avoid ignoring mild hyponatremia (127 mEq/L)—even this level increases fall risk (21% vs 5%) and mortality (60-fold increase) 2
• Do not use hypotonic fluids (lactated Ringer's, D5W) as they will worsen hyponatremia 1, 6

Monitoring Protocol

• Check serum sodium every 24 hours for asymptomatic patients on fluid restriction 1
• For symptomatic patients receiving hypertonic saline: every 2 hours initially, then every 4 hours after symptom resolution 1, 2
• Watch for osmotic demyelination syndrome signs 2-7 days after correction: dysarthria, dysphagia, oculomotor dysfunction, quadriparesis 2

Treatment of Underlying Cause

Identify and treat the underlying etiology of SIADH concurrently. 1, 2

• Review medications (SSRIs, carbamazepine, cyclophosphamide, vincristine, NSAIDs, opioids) and discontinue offending agents 1
• Screen for malignancy (especially small cell lung cancer), CNS disorders, and pulmonary disease 1, 6
• Hyponatremia typically improves after successful treatment of the underlying cause 1