SIADH (= Syndrome of Inappropriate secretion of AntiDiuretic Hormone)

SIADH (= Syndrome of Inappropriate secretion of AntiDiuretic Hormone) is a condition which results in hyponatraemia.

The Bartter and Schwartz criteria for SIADH is:
  • Decreased plasma osmolality (<275 mosm/kg)
  • Inappropriately concentrated urine (>100 mosm/kg) 
  • Euvolaemic 
  • Elevated urine Na (>20 mEq/L) 
  • Normal thyroid and adrenal function 
  • No diuretic use 

ADH itself is made in the supraoptic and paracentricular nuclei of the hypothalamus. It is then stored in the posterior pituitary.

ADH is usually released in response to:
  • raised plasma osmolality (detected by osmoreceptors in the hypothalamus)
  • decreased plasma volume (detected by baroreceptors in the carotids, aorta and left atrium) 
  • limbic system activation – pain, fear, nausea 

It increases synthesis and insertion of aquaporin-2 water channels in the luminal membrane of the collecting ducts, increasing water reabsorption in the kidney

As SIADH causes hyponatraemia, clinically you would expect:
  • Sodium <125mEq/l: weakness, headaches, nausea and vomiting 
  • Sodium <115mEq/l: altered consciousness, seizures, coma 

Causes of SIADH include
  • Drugs
    • Anticonvulsants - Carbamazepine, Sodium valproate
    • Antidepressants - SSRIs, MAOIs, TCAs
    • Haloperidol
    • Amiodarone
    • Ciprofloxacin
    • Chemo
    • Opiates
    • MDMA
  • Malignancy
    • Lung cancer, especially small cell carcinoma
    • Pancreatic cancer
    • lymphoma
    • Head and neck cancers
  • Pulmonary disease
  • CNS
    • Stroke
    • Infection - encephalitis, meningitis, abscess
    • Haemorrhage
    • trauma
    • MS
    • GBS
  • Porphyria

Treatment
  • Treat cause
  • Correct sodium
    • Fluid restriction
    • ? demeclocycline
    • ? hypertonic saline
    • ? furosemide
    • ? vasopressin receptor antagonists (vaptans)
    • ? urea

If acute (48 hrs or less) correction of sodium can be fast
If chronic, correction of sodium should be cautious (around 8meq/day) to avoid osmotic demyelination syndrome.


References:
Esposito, P. et al. The syndrome of inappropriate antidiuresis: pathophysiology, clinical management and new therapeutic options. Nephron Clin Pract. 2011; 119(1):c62-c73. 

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