From the Guidelines
Diagnosing SIADH requires laboratory findings of hyponatremia with inappropriately concentrated urine in a euvolemic patient, and treatment depends on symptom severity and sodium levels, with vasopressin receptor antagonists like tolvaptan being effective for chronic SIADH.
Diagnostic Approach
The diagnostic approach for SIADH involves laboratory findings of:
- Serum sodium below 135 mEq/L
- Serum osmolality below 275 mOsm/kg
- Urine osmolality above 100 mOsm/kg
- Urine sodium above 30 mEq/L
- Normal renal, adrenal, and thyroid function
- Absence of diuretic use or volume depletion 1
Treatment Approach
The treatment approach for SIADH depends on symptom severity and sodium levels, with the following options:
- For mild, asymptomatic hyponatremia (sodium >125 mEq/L), fluid restriction to 800-1000 mL/day is the first-line approach
- For moderate cases, salt tablets (1-2 g three times daily) or urea (15-60 g/day) may be added
- For severe or symptomatic hyponatremia, hypertonic saline (3% NaCl) is administered at 1-2 mL/kg/hour, aiming to increase sodium by 4-6 mEq/L in the first 24 hours to prevent osmotic demyelination syndrome
- Vasopressin receptor antagonists like tolvaptan (starting at 15 mg daily, maximum 60 mg daily) are effective for chronic SIADH, but require careful monitoring 1
Key Considerations
Key considerations in the treatment of SIADH include:
- Addressing the underlying cause, such as treating malignancy, discontinuing offending medications, or managing CNS disorders
- Regular monitoring of serum sodium, fluid status, and neurological symptoms is essential during treatment
- The use of vaptans, such as tolvaptan, requires careful monitoring to avoid complications like hypernatremia, dehydration, and renal impairment 1
From the Research
Diagnostic Approaches for 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.
- The five cardinal criteria for diagnosing SIADH include hypotonic hyponatraemia, natriuresis, urine osmolality in excess of plasma osmolality, absence of oedema and volume depletion, and normal renal and adrenal function 3.
- Diagnosis is made on the basis of clinical euvolaemic state with low serum sodium and osmolality, raised urine sodium and osmolality, and exclusion of pseudohyponatraemia and diuretic use 4.
- It is essential to ascertain the euvolemic state of extracellular fluid volume, both clinically and by laboratory measurements, in the diagnosis of SIADH 5.
Treatment Approaches for 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.
- Fluid restriction of 800-1200 mL/24 hours is the mainstay of treatment for SIADH 4.
- For chronic SIADH, the treatment of choice is fluid restriction, and if this is not tolerated by the patient, demeclocycline can be used to induce a negative free-water balance 2.
- Vasopressin receptor antagonists, called vaptans, have been introduced as specific and direct therapy of SIADH, offering a new therapeutic approach 3, 5.
- Hypertonic saline should be used only in severely symptomatic patients, and the increase in serum sodium should not exceed 10 mmol/L in 24 hours or 18 mmol/L in 48 hours to prevent osmotic demyelination 4, 5.
- In any therapy of chronic SIADH, it is crucial to limit the daily increase of serum sodium to less than 8-10 mmol/liter 5.