From the Guidelines
The workup for Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) in the inpatient setting should begin with confirming the diagnosis through laboratory testing, including serum sodium, serum osmolality, urine osmolality, and urine sodium, as outlined in the most recent guidelines 1. The diagnosis of SIADH is characterized by hyponatremia (serum sodium <134 mEq/L), decreased serum osmolality (<275 mosm/kg), inappropriately concentrated urine (urine osmolality >500 mosm/kg), and elevated urine sodium (>20 mEq/L) in the setting of euvolemia, as defined by the American College of Chest Physicians evidence-based clinical practice guidelines 1.
Key Laboratory Tests
- Serum sodium
- Serum osmolality
- Urine osmolality
- Urine sodium Additional tests should include:
- Thyroid function tests to rule out hypothyroidism
- Morning cortisol to rule out adrenal insufficiency
- Kidney function tests to assess renal function Once SIADH is confirmed, investigate the underlying cause through:
- Chest imaging to identify lung malignancies or infections
- Brain imaging for CNS disorders
- Medication review, particularly for SSRIs, carbamazepine, and certain chemotherapeutics
- Appropriate testing for suspected infections
Treatment Approach
Treatment depends on symptom severity and sodium levels.
- For asymptomatic or mildly symptomatic patients with sodium >120 mEq/L, fluid restriction to <1 L/d is the first-line approach 1.
- For moderate to severe symptoms or sodium <120 mEq/L, consider hypertonic (3%) saline at 1-2 mL/kg/hr with careful monitoring to avoid rapid correction, as recommended by the National Comprehensive Cancer Network 1. Sodium correction should not exceed 8 mEq/L in 24 hours to prevent osmotic demyelination syndrome. In refractory cases, vasopressin receptor antagonists like tolvaptan (starting at 15 mg daily) can be considered, though they require specialist consultation due to the risk of rapid sodium correction 1.
From the Research
Diagnosis of SIADH
- The diagnosis of SIADH is confirmed by demonstration of a high urine osmolality with a low plasma osmolality, in the absence of diuretic use 2.
- Appropriate laboratory tests to diagnose SIADH include measurement of urine and plasma osmolality, and serum sodium concentration 3, 4.
- The five cardinal criteria for diagnosis of SIADH are:
- Hypotonic hyponatraemia
- Natriuresis
- Urine osmolality in excess of plasma osmolality
- Absence of oedema and volume depletion
- Normal renal and adrenal function 4
Treatment of SIADH
- Immediate treatment of the symptomatic patient with SIADH includes intravenous furosemide and 3% sodium chloride injection to produce a negative free-water balance 2.
- For chronic SIADH, the treatment of choice is fluid restriction 2, 4.
- If fluid restriction is not tolerated by the patient, demeclocycline can be used to induce a negative free-water balance 2.
- Other treatment options include:
- Hypertonic saline and tolvaptan have been shown to produce the greatest mean rate of serum sodium change 3.
Management Algorithms
- A multidisciplinary Spanish algorithm has been developed for the treatment of hyponatremia due to SIADH, which includes two algorithms:
- Algorithm 1: addresses acute correction of hyponatremia posing as a medical emergency
- Algorithm 2: directed to the therapy of SIADH-induced mild or moderate, non-acute hyponatremia 5