Fluid Restriction for SIADH
For patients with SIADH, fluid restriction to 1 L/day (1000 mL/day) is the cornerstone first-line treatment for mild to moderate asymptomatic hyponatremia. 1, 2, 3
Treatment Algorithm Based on Severity
Severe Symptomatic Hyponatremia (seizures, coma, altered mental status)
- Administer 3% hypertonic saline immediately with a target correction of 6 mmol/L over 6 hours or until severe symptoms resolve 1, 2, 3
- Monitor serum sodium every 2 hours during initial correction 1, 2
- Do NOT exceed 8 mmol/L total correction in 24 hours to prevent osmotic demyelination syndrome 1, 2, 3, 4
- ICU admission is recommended for close monitoring 2
- Fluid restriction should be avoided during the first 24 hours of hypertonic saline therapy to prevent overly rapid correction 4
Mild to Moderate Asymptomatic Hyponatremia
- Implement strict fluid restriction to 1 L/day (1000 mL/day) as primary therapy 1, 2, 3, 5, 6, 7, 8
- If no response to fluid restriction after several days, add oral sodium chloride 100 mEq (approximately 6 grams) three times daily 1
- Monitor serum sodium daily initially, then every 4 hours if actively correcting 1
Chronic SIADH Resistant to Fluid Restriction
- Urea (30-60 grams/day) can be used as second-line therapy when fluid restriction is ineffective or poorly tolerated 8
- Demeclocycline (600-1200 mg/day) is an alternative option 2, 9
- Vaptans (tolvaptan 15 mg/day, titrated to 30-60 mg/day) may be considered for euvolemic hyponatremia, though significantly more expensive than urea 4, 5, 8
Critical Safety Considerations
Maximum Correction Rates
- Never exceed 8 mmol/L correction in 24 hours for most patients 1, 2, 3, 4, 5
- For high-risk patients (advanced liver disease, alcoholism, malnutrition), limit correction to 4-6 mmol/L per day 1, 2, 3
- Chronic hyponatremia (>48 hours duration) requires slower correction than acute hyponatremia 1, 7
Monitoring During Vaptan Therapy
If using tolvaptan, the first 24 hours are critical for preventing overly rapid correction 5
- Check serum sodium at 0,6,24, and 48 hours after initiating therapy 5
- Avoid fluid restriction during the first 24 hours of vaptan therapy 4
- After 5-6 days of vaptan therapy, monitor for hyponatremic relapse upon discontinuation 5
Common Pitfalls to Avoid
Do NOT use fluid restriction in cerebral salt wasting (CSW) - this is a critical distinction from SIADH, as CSW requires volume and sodium replacement, not restriction 1, 2, 3, 7
The key differentiating feature is volume status: SIADH presents with euvolemia (normal skin turgor, moist mucous membranes, no orthostatic hypotension, no edema), while CSW presents with hypovolemia (dry mucous membranes, orthostatic hypotension, poor skin turgor) 2, 3
In neurosurgical patients with subarachnoid hemorrhage at risk for vasospasm, avoid fluid restriction even if SIADH is present, as this can worsen cerebral ischemia 2, 3
Overly rapid correction exceeding 8 mmol/L in 24 hours can cause osmotic demyelination syndrome, manifesting 2-7 days later with dysarthria, dysphagia, oculomotor dysfunction, and quadriparesis 1, 2
Treatment of Underlying Cause
Always identify and treat the underlying cause of SIADH when possible 2, 9, 7: