Classification of SIADH

SIADH can be divided according to the pattern of arginine vasopressin (AVP) secretion across a range of plasma osmolalities.

• type A
  • commonest form of SIADH, is thought to be responsible for 60-70% of cases
  • characteristically seen in lung cancer and in nasopharyngeal tumours
  • patients are prone to develop severe hyponatraemia since plasma AVP concentrations are not suppressed physiologically by drinking
  • there is also a lower osmotic threshold for thirst appreciation
    
    
• type B
  • common, seen in 20-40%
  • secretion of AVP occurs at lower plasma osmolalities than normal
  • since secretion of AVP is supressed at a lower osmotic threshold for AVP release, patients are protected against the progression to severe hyponatraemia
    
    
• type C
  • rare
  • there is failure to suppress AVP at plasma osmolalities below the osmotic threshold, hence inappropriately high plasma AVP levels are seen even at low plasma osmolalities
    
    
• type D
  • rare
  • low or undetectable AVP levels and no detectable abnormality in circulating AVP response o is thought to be due to nephrogenic SIADH (1)

Reference:

1. Hannon MJ, Thompson CJ. The syndrome of inappropriate antidiuretic hormone: prevalence, causes and consequences. Eur J Endocrinol. 2010;162 Suppl 1:S5-12.