Management of SIADH
For SIADH, fluid restriction to 1 L/day is the cornerstone of first-line treatment, with 3% hypertonic saline reserved exclusively for severe symptomatic hyponatremia (seizures, altered mental status, coma), targeting correction of 6 mmol/L over 6 hours with a maximum of 8 mmol/L in 24 hours to prevent osmotic demyelination syndrome. 1
Diagnostic Confirmation
Before initiating treatment, confirm SIADH diagnosis with the following criteria:
- Hypotonic hyponatremia: Serum sodium <134 mEq/L with plasma osmolality <275 mosm/kg 1
- Inappropriately concentrated urine: Urine osmolality >500 mosm/kg despite low plasma osmolality 1
- Elevated urinary sodium: Urine sodium >20 mEq/L 1
- Clinical euvolemia: No edema, orthostatic hypotension, or signs of volume depletion 1
- Exclusion criteria: Normal thyroid function (TSH), normal adrenal function (cortisol), and absence of diuretic use 1, 2
A serum uric acid <4 mg/dL has 73-100% positive predictive value for SIADH, though this may include cerebral salt wasting in neurosurgical patients 1
Treatment Algorithm Based on Severity
Severe Symptomatic Hyponatremia (Seizures, Coma, Altered Mental Status)
Immediate ICU transfer with the following protocol:
- Administer 3% hypertonic saline as 100 mL IV bolus over 10 minutes, repeatable up to 3 times at 10-minute intervals until symptoms resolve 1, 2
- Target correction: 6 mmol/L over first 6 hours or until severe symptoms improve 1
- Absolute maximum: 8 mmol/L correction in 24 hours to prevent osmotic demyelination syndrome 1
- Monitoring: Check serum sodium every 2 hours during initial correction 1
Critical safety consideration: If 6 mmol/L is corrected in the first 6 hours, only 2 mmol/L additional correction is permitted in the remaining 18 hours 1
Mild Symptomatic or Asymptomatic Hyponatremia (Sodium <120 mEq/L)
First-line therapy:
- Fluid restriction to 1 L/day (500 mL/day initially, adjusted based on sodium response) 1, 3
- Adequate solute intake: Encourage salt and protein intake 3
- Monitor serum sodium every 24 hours initially, then adjust frequency based on response 1
If no response to fluid restriction after 48-72 hours:
Chronic SIADH Refractory to Fluid Restriction
Second-line pharmacological options (in order of preference):
Oral urea: Considered very effective and safe, though specific dosing not provided in guidelines 1, 3
Demeclocycline: Induces nephrogenic diabetes insipidus, reducing kidney response to ADH 1
Tolvaptan (vasopressin receptor antagonist):
- Starting dose: 15 mg once daily 1, 4
- Titration: Can increase to 30 mg after 24 hours, maximum 60 mg daily 4
- Avoid fluid restriction in first 24 hours to prevent overly rapid correction 4
- Efficacy: Increases serum sodium by 3.7 mEq/L at Day 4 and 4.6 mEq/L at Day 30 compared to placebo 4
- Monitoring: Check sodium at 8 hours, then daily for first 72 hours 4
Note: Almost half of SIADH patients do not respond to fluid restriction as first-line therapy, necessitating second-line treatment 3
Correction Rate Guidelines by Risk Category
Standard Risk Patients
High-Risk Patients (Cirrhosis, Alcoholism, Malnutrition, Advanced Liver Disease)
- Target: 4-6 mmol/L per day 1
- Maximum: 6-8 mmol/L in 24 hours 1
- These patients have significantly higher risk of osmotic demyelination syndrome 1
Management of Overcorrection
If sodium correction exceeds 8 mmol/L in 24 hours:
- Immediately discontinue current fluids and switch to D5W (5% dextrose in water) 1, 2
- Consider desmopressin administration to slow or reverse rapid sodium rise 1
- Monitor for osmotic demyelination syndrome: Dysarthria, dysphagia, oculomotor dysfunction, quadriparesis typically occurring 2-7 days after rapid correction 1
Treatment of Underlying Cause
Address the specific etiology:
- Malignancy: Treat underlying cancer (particularly SCLC, which causes SIADH in 1-5% of cases) 1
- Medications: Discontinue offending drugs (SSRIs, carbamazepine, chlorpropamide, cyclophosphamide, vincristine, cisplatin, NSAIDs, opioids) 1
- CNS disorders: Manage meningitis, encephalitis, or other neurological conditions 1, 5
- Pulmonary disease: Treat pneumonia or other lung pathology 1
Hyponatremia typically improves after successful treatment of the underlying cause 1
Special Populations
Neurosurgical Patients with Subarachnoid Hemorrhage
Critical distinction: Differentiate SIADH from cerebral salt wasting (CSW), as treatments are opposite:
- SIADH: Euvolemic with CVP 6-10 cm H₂O → Treat with fluid restriction 1
- CSW: Hypovolemic with CVP <6 cm H₂O → Treat with volume and sodium replacement 1
In SAH patients at risk for vasospasm:
- Avoid fluid restriction as it worsens outcomes 1
- Consider fludrocortisone 0.1-0.2 mg daily for hyponatremia management 1
Cirrhotic Patients
- Fluid restriction to 1-1.5 L/day for sodium <125 mmol/L 2
- Consider albumin infusion alongside fluid restriction 2
- Avoid hypertonic saline unless life-threatening symptoms present, as it worsens ascites and edema 2
- Tolvaptan carries higher risk: 10% gastrointestinal bleeding vs. 2% placebo 1
Common Pitfalls to Avoid
- Overly rapid correction (>8 mmol/L in 24 hours) leading to osmotic demyelination syndrome 1
- Inadequate monitoring during active correction 1
- Using fluid restriction in cerebral salt wasting instead of SIADH 1
- Failing to recognize and treat underlying cause 1
- Using hypotonic fluids (D5W, lactated Ringer's) which worsen hyponatremia 1
- Ignoring mild hyponatremia (130-135 mmol/L), which increases fall risk (21% vs. 5%) and mortality (60-fold increase at <130 mmol/L) 1, 2
Monitoring Protocol
During active correction:
- Severe symptoms: Every 2 hours 1
- Mild symptoms: Every 4 hours 1
- After symptom resolution: Daily until stable 1
Long-term management: