Syndrome of Inappropriate ADH (SIADH) secretion
Related Subjects:
| Sodium Physiology
| Hyponatraemia
| TURP Hyponatraemia syndrome
| Hypernatraemia
| Diabetes Insipidus
| SIADH
📖 About
- SIADH (Syndrome of Inappropriate Antidiuretic Hormone) is a not uncommon cause of dilutional hyponatraemia 💧.
- Severe hyponatraemia is a medical emergency 🚨 with risk of seizures and coma.
- Correction must be slow and controlled ⚖️ to avoid osmotic demyelination syndrome (central pontine myelinolysis).
🧬 Aetiology & Pathophysiology
- ADH (arginine vasopressin) is made in the hypothalamus and released from the posterior pituitary.
- It normally conserves water in the kidney collecting ducts in response to hyperosmolality or hypovolaemia.
- In SIADH, ADH is released inappropriately ➝ water retention ➝ dilutional hyponatraemia with euvolaemia clinically.
- Diagnosis requires normal adrenal and thyroid function (to exclude mimics).
🛑 Causes (often idiopathic)
- 🧠 CNS: meningitis, encephalitis, Guillain-Barré, subarachnoid haemorrhage, stroke, pituitary surgery.
- 💊 Drugs: SSRIs, TCAs, chlorpropamide, carbamazepine, barbiturates, opioids, vincristine, cisplatin, diuretics.
- 🫁 Respiratory: pneumonia, TB, positive pressure ventilation, pleural effusion.
- 🎗️ Malignancy: especially small-cell lung cancer (ectopic ADH secretion), brain tumours, haematological cancers.
- 🧪 Metabolic: acute intermittent porphyria.
🩺 Clinical
- Patients appear euvolaemic (no oedema or dehydration).
- Symptoms relate to severity & speed of hyponatraemia:
- Mild: headache, lethargy, nausea 🤕
- Moderate: confusion, unsteady gait, cramps 😵💫
- Severe: seizures, coma, respiratory arrest ⚡
🔍 Differential Diagnosis
- Cerebral salt wasting (especially post-SAH).
- Renal sodium losses (diuretics).
- Free water overload ➝ TURP syndrome, excessive IV dextrose.
- Endocrine: hypothyroidism, hypoadrenalism (do TFTs + short Synacthen test).
- Organ failure: heart, liver, renal failure (pseudo-SIADH picture).
🧪 Investigations
- Plasma Na: ⬇️ <125 mmol/L.
- Plasma osmolality: ⬇️ <260 mmol/kg.
- Urine osmolality: ⬆️ >100 mmol/kg despite hypotonic plasma.
- Urine sodium: ⬆️ >30 mmol/L (inappropriately high).
- Exclude adrenal, thyroid, renal, hepatic and cardiac causes.
💊 Management
- 🔄 Address underlying cause (e.g. stop offending drug, treat pneumonia, manage malignancy).
- ⚠️ Sodium correction limit: Do NOT increase serum [Na⁺] by >8–12 mmol/L in 24 hrs ➝ risk of central pontine myelinolysis.
- 💧 First-line: fluid restriction <1000 mL/day.
- 🧂 Consider oral salt tablets ± loop diuretics if more severe.
- 💊 Demeclocycline: induces nephrogenic DI (use in chronic/refractory cases).
- 🧪 Hypertonic saline (3%): in seizures or acute severe hyponatraemia, give small boluses (e.g. 200 mL) with close monitoring.
- 🧬 Vaptans (ADH receptor antagonists e.g. Tolvaptan): useful short-term in resistant SIADH.
📚 References
