Syndrome Of Inappropriate Antidiuretic Hormone Secretion (SIADH)

Syndrome of inappropriate antidiuretic hormone secretion (SIADH) is characterized by the excessive secretion of ADH hormone despite normal or increased plasma volume leading to water retention subsequently results in hyponatraemia.



Criteria To Consider SIADH

  • Normal renal, adrenal and thyroid functions
  • Euvolemic hyponatraemia (Sodium level less than 135mEq/L)
  • Hypotonic plasma osmolality (Less than 270 mOsm/L)
  • Inappropriately increased urine osmolality (More than 500 mOsm/L)
  • Urine sodium more than 30 mEq/L (Inappropriate natriuresis) 
  • Elevated ADH 
 
 
Pathophysiology Of SIADH

Excess secretion of ADH

               ↓
Acts on aquaporine receptors
               ↓
Reabsorption of water from distal convulated tubules and collecting ducts
               ↓
Euvolemic hyponatraemia
 
 
Causes Of SIADH
  • Malignancy 
    • Small cell lung cancer
  • Respiratory 
    • Lung abscess
    • Empyema
    • Pneumonia 
    • Pulmonary TB
  • Neurological  
    • Stroke 
    • Cerebral abscess
    • Meningitis / Encephalitis
    • Intracranial haemorrhage (Subdural haemorrhage, subarachnoid haemorrhage)
    • Intracranial tumors
  • Drugs
    • Sulfonylurea
    • SSRI 
    • Tricyclic antidepressants
    • Carbamazepine 
 
 
Clinical Presentation Of SIADH
 
It is same as hyponatraemia
  • Anorexia, nausea, vomiting
  • Lethargy
  • Headache
  • Personality changes
  • Muscle cramps and weakness
  • Confusion, disorientation, drowsiness
  • Seizures
  • Fluid status should suggest hypervolaemia 
    • Elevated BP and JVP
    • Peripheral oedema
    • Tachycardia
    • Gallop rhythm  
 
 
Investigations For SIADH
  • RFTs – Serum creatinine and blood urea 
  • Serum electrolytes – Sodium, Potassium, bicarbonate
  • Urine osmolality
  • Plasma osmolality
  • TFTs
  • Serum cortisol 
* Before diagnosing SIADH have to exclude cushing’s syndrome and hypothyroidism
 
 
Treatment Of SIADH
  • Fluid restriction to less than 1000 ml/day
  • Democlocycline 
    • Used in the treatment of SIADH secondary to malignancy where fluid restriction is ineffective
  • Vascopressin receptor antagonists (Tolvaptan)
    • May lead to rapid correction 
  • If neurological manifestations present 
    • Slow infusion of 3% saline with simultaneous monitoring of sodium levels  
    • Rapid correction should be avoid as it can lead to central pontine myelinolysis
  • Treat the underlying cause 
 
 
Complications Of SIADH
  • Cerebral oedema leading to herniation of brain 
  • Central pontine myelinolysis can occur following rapid correction 
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