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What is SIADH? Diagnosis and treatment of SIADH (4+6 marks)

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SIADH AND hyponatremia

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SIADH diagnosis treatment hyponatremia guideline fluid restriction hypertonic saline 2024

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https://academic.oup.com/edrv/article/44/5/819/7090475

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SIADH

Syndrome of inappropriate antidiuretic hormone secretion (SIADH) is a disorder in which ADH (vasopressin) is secreted or acts inappropriately despite low plasma osmolality. This causes excessive renal water reabsorption, leading to dilutional hyponatremia, low serum osmolality, concentrated urine, and usually clinical euvolemia. - Costanzo Physiology, 7th ed., p. 311
Common causes include small-cell carcinoma of lung, pulmonary disease such as pneumonia or tuberculosis, CNS disorders, pain/postoperative state, and drugs such as SSRIs, carbamazepine, cyclophosphamide, and chlorpropamide.

Diagnosis of SIADH (4 marks)

SIADH is a diagnosis of exclusion. Diagnostic features are:
  1. Hypotonic hyponatremia
    • Serum sodium low, usually <135 mmol/L.
    • Serum osmolality low, generally <275 mOsm/kg.
  2. Inappropriately concentrated urine
    • Urine osmolality >100 mOsm/kg, despite hypo-osmolality.
  3. Euvolemic state
    • No signs of dehydration, edema, ascites, or heart failure.
    • Patient is clinically euvolemic.
  4. Urinary sodium is inappropriately high
    • Urine Na+ usually >30-40 mmol/L, provided salt intake and renal function are normal.
  5. Exclude other causes
    • Normal renal function.
    • Exclude hypothyroidism by TSH.
    • Exclude glucocorticoid deficiency by morning cortisol, with ACTH stimulation test if needed.
    • Exclude recent diuretic use, especially thiazides.
    • Low serum urea and uric acid support SIADH.
The standard diagnostic criteria include serum hypo-osmolality, euvolemia, urine osmolality >100 mOsm/kg, increased urinary sodium, and exclusion of adrenal, thyroid, renal disease, and diuretic use. - Miller’s Anesthesia, 10th ed., p. 6371
Low BUN and uric acid are characteristic supportive findings. - Goldman-Cecil Medicine, p. 1201

Treatment of SIADH (6 marks)

1. Treat the underlying cause

  • Stop offending drugs.
  • Treat pneumonia, CNS disease, pain, nausea, etc.
  • Investigate and treat malignancy, especially small-cell carcinoma lung.

2. Fluid restriction

  • First-line treatment for chronic or mild/moderate SIADH.
  • Restrict total fluids to about 800-1000 mL/day.
  • Monitor daily weight, fluid balance, serum sodium, and urine output.

3. Increase solute intake or promote free-water excretion

If fluid restriction fails or is impractical:
  • Oral urea may be used.
  • Salt tablets plus loop diuretic such as furosemide can increase solute and water excretion.
  • Demeclocycline may reduce renal response to ADH but is now used infrequently because of nephrotoxicity and photosensitivity.
  • Vasopressin V2-receptor antagonists, such as tolvaptan, may be considered in selected resistant cases under close monitoring. They can cause overly rapid sodium correction.

4. Severe symptomatic hyponatremia: emergency treatment

For seizures, coma, marked confusion, or severe acute hyponatremia:
  • Give 3% hypertonic saline in a monitored setting.
  • Aim for an initial rise in serum sodium of about 4-6 mmol/L to relieve severe neurologic symptoms.
  • Check serum sodium frequently.

5. Avoid rapid correction

  • Overrapid correction can cause osmotic demyelination syndrome, including central pontine myelinolysis.
  • In most patients, do not increase sodium by more than about 8 mmol/L in 24 hours, with even stricter limits in patients at high risk such as alcoholism, malnutrition, severe hypokalemia, or liver disease.

6. Avoid normal saline in established SIADH

  • Isotonic saline may fail to correct, or can worsen hyponatremia, because sodium is excreted in concentrated urine while water is retained.
Fluid restriction is guideline-endorsed first-line therapy, while hypertonic saline is reserved for severe symptomatic hyponatremia. A recent SIAD review discusses these approaches. Recent systematic reviews also support urea as a treatment option for SIADH-associated hyponatremia, including this 2025 meta-analysis.
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