Diagnostic Criteria and Treatment Options for SIADH
SIADH should be diagnosed when a patient has hypotonic hyponatremia (serum sodium <135 mEq/L), inappropriately high urine osmolality relative to serum osmolality, urine sodium >20 mEq/L, clinical euvolemia, and normal renal and adrenal function. 1, 2
Diagnostic Criteria
Essential Criteria for SIADH Diagnosis
- Hypotonic hyponatremia (serum sodium <135 mEq/L) 1
- Urine osmolality inappropriately high (>100 mOsm/kg) relative to plasma osmolality 3, 4
- Urine sodium concentration typically >20-30 mEq/L 2, 5
- Clinical euvolemia (absence of edema and volume depletion) 2, 1
- Normal renal and adrenal function 2
Additional Laboratory Findings Supporting SIADH
- Serum uric acid <4 mg/dL (has a positive predictive value of 73-100% for SIADH) 1, 5
- Low blood urea nitrogen (BUN) 5
- Lower anion gap with nearly normal total CO2 and serum potassium 5
- Central venous pressure (CVP) measurements typically 6-10 cm H₂O (helps differentiate from cerebral salt wasting) 6
Differential Diagnosis
- Cerebral Salt Wasting (CSW): characterized by hypovolemia rather than euvolemia, with CVP <6 cm H₂O 6, 1
- Polydipsia: marked by excessive fluid intake causing dilutional hyponatremia 6
- Adrenal insufficiency: typically presents with lower total CO2 despite low urea and uric acid levels 5
- Hypothyroidism: should be ruled out before making a diagnosis of SIADH 4
Treatment Options
Treatment Based on Symptom Severity
For Severe Symptomatic Hyponatremia
- Administer 3% hypertonic saline with an initial goal to correct 6 mmol/L over 6 hours or until severe symptoms resolve 1, 7
- Total correction should not exceed 8 mmol/L in 24 hours to prevent osmotic demyelination syndrome 1, 7
- Consider ICU admission for close monitoring during treatment 1
- Hypertonic saline should be used only in severely symptomatic patients 8
For Mild to Moderate Hyponatremia
- Fluid restriction to 1 L/day is the cornerstone of treatment for SIADH 1, 6
- Discontinue medications that may be contributing to SIADH 7
- Ensure adequate oral salt intake 7
Pharmacological Options
Vasopressin receptor antagonists (vaptans):
- Tolvaptan can be considered for clinically significant hyponatremia resistant to fluid restriction 1, 9
- Initial oral dose of 15 mg once daily, which can be increased at 24-hour intervals to 30 mg once daily, then to 60 mg once daily 9
- Tolvaptan should be initiated only in a hospital setting where serum sodium can be monitored closely 9
- Fluid restriction should be avoided during the first 24 hours of tolvaptan therapy 9
- Not recommended for use beyond 30 days due to risk of liver injury 9
Other pharmacological options:
Special Considerations
- Patients with advanced liver disease, alcoholism, malnutrition, or prior encephalopathy require more cautious correction (4-6 mmol/L per day) due to higher risk of osmotic demyelination syndrome 1
- In patients with a short prognosis, strict fluid restriction may not be appropriate if not aligned with goals of care 7
- For neurosurgical patients, distinguishing between SIADH and cerebral salt wasting is critical as treatment approaches differ significantly 1
Common Pitfalls to Avoid
- Overly rapid correction of chronic hyponatremia leading to osmotic demyelination syndrome 1
- Inadequate monitoring during active correction 1
- Using fluid restriction in cerebral salt wasting (which requires volume replacement) 1, 6
- Failing to recognize and treat the underlying cause 1
- Using normal saline (0.9%) in SIADH patients, as it may worsen hyponatremia 4
- Failing to discontinue medications that may be causing SIADH 7
By following these diagnostic criteria and treatment guidelines, clinicians can effectively diagnose and manage patients with SIADH while minimizing the risk of complications.