Initial Management of SIADH
For SIADH, the cornerstone of initial management is fluid restriction to 1 liter per day in patients with mild-to-moderate asymptomatic hyponatremia, while severe symptomatic cases require immediate 3% hypertonic saline with a target correction of 6 mmol/L over 6 hours. 1
Diagnostic Confirmation Before Treatment
Before initiating therapy, confirm SIADH diagnosis with the following criteria 1:
- Hyponatremia: Serum sodium <134 mEq/L 1
- Hypoosmolality: Plasma osmolality <275 mosm/kg 1
- Inappropriately concentrated urine: Urine osmolality >500 mosm/kg 1
- Elevated urinary sodium: >20 mEq/L 1
- Euvolemic state: No edema, no orthostatic hypotension, normal skin turgor, moist mucous membranes 2
- Exclusion criteria: Rule out hypothyroidism, adrenal insufficiency, and volume depletion 1
Critical pitfall: Distinguish SIADH from cerebral salt wasting (CSW), as they require opposite treatments—SIADH needs fluid restriction while CSW requires volume replacement 1, 2. Central venous pressure can help differentiate: SIADH shows CVP 6-10 cm H₂O versus CSW with CVP <6 cm H₂O 1.
Treatment Algorithm Based on Severity
Severe Symptomatic Hyponatremia (Seizures, Coma, Altered Mental Status)
Immediate action required 1:
- Transfer to ICU for close monitoring 1
- Administer 3% hypertonic saline with goal to correct 6 mmol/L over 6 hours or until severe symptoms resolve 1
- Monitor serum sodium every 2 hours initially 1
- Maximum correction limit: Never exceed 8 mmol/L in 24 hours to prevent osmotic demyelination syndrome 1, 3
- High-risk patients (malnutrition, alcoholism, advanced liver disease): Limit correction to 4-6 mmol/L per day 1, 3
Mild Symptomatic or Asymptomatic Hyponatremia (Sodium <120 mEq/L)
First-line treatment 1:
- Fluid restriction to 1 L/day is the cornerstone of management 1, 2
- Discontinue hypotonic fluids (such as D5W) immediately, as they worsen hyponatremia 1
- Avoid overly strict fluid restriction during the first 24 hours—patients can drink in response to thirst 3
- Monitor serum sodium daily initially, then adjust frequency based on response 1
Second-Line Pharmacological Options (If Fluid Restriction Fails)
When fluid restriction is ineffective or poorly tolerated 1:
- Oral sodium chloride: 100 mEq three times daily 1
- Demeclocycline: Induces nephrogenic diabetes insipidus, reducing kidney response to ADH 1
- Tolvaptan: FDA-approved vasopressin receptor antagonist 3
Alternative options include urea, lithium, and loop diuretics, though these are less commonly used 1.
Critical Safety Considerations
Osmotic demyelination syndrome prevention 1, 3:
- Never exceed 8 mmol/L correction in 24 hours for most patients 1
- Never exceed 12 mEq/L correction in 24 hours per FDA guidelines 3
- For high-risk patients: Limit to 4-6 mmol/L per day 1
- If overcorrection occurs: Administer free water or desmopressin to relower sodium 1
- Watch for symptoms 2-7 days post-correction: dysarthria, dysphagia, oculomotor dysfunction, quadriparesis 1
Special Population Considerations
Neurosurgical patients 1:
- Avoid fluid restriction in subarachnoid hemorrhage patients at risk for vasospasm 1
- Consider fludrocortisone for hyponatremia in these patients 1
Cancer patients with paraneoplastic SIADH 1:
- Treat underlying malignancy alongside hyponatremia management 1
- SIADH commonly occurs with small cell lung cancer 1
- Chemotherapy agents (cisplatin, vincristine) can exacerbate hyponatremia 1
Common Pitfalls to Avoid
- Using fluid restriction in cerebral salt wasting instead of SIADH worsens outcomes 1, 2
- Inadequate monitoring during active correction risks overcorrection 1
- Failing to identify and treat underlying cause (medications, malignancy, CNS disorders) 1
- Administering normal saline to euvolemic SIADH patients can paradoxically worsen hyponatremia 2
Post-Acute Management
After initial correction 1: