• Usually indicative of hypo-osmolality of body fluid due to excess of water relative to solute
  • In dilutional hyponatraemia kidney responds by retaining salt and water as if the individual were intravascular volume depleted
  • Urinary Na+ generally very low (less than 10 meq per litre)
  • Urine osmolality elevated in the absence of diuretics



  • Always present when osmolality less than 240 meq
  • Rate of fall important
  • Increased severity in children and women of child bearing age
  • > 125 mmol/l usually asymptomatic
  • 125 to 115 mmol/l subtle changes in mental status, e.g. confusion
  • < 115 mmol/l stupor ,neuromuscular hyper-excitability (but hyporeflexia, convulsions, coma
  • Initial improvement may be followed by delayed neurological symptoms
  • Central pontine myelinolysis is uncommon


  • Factitious (hyper-proteinaemia / lipidaemia / glycaemia and mannitol)
  • Water retention (renal, hepatic and cardiac failure, hypothyroidism)
  • Polydipsia
  • Salt loss (adrenal cortical insufficiency, diuretics, fluid therapy, post-trauma and stress)
  • Cerebral salt wasting syndrome



  • hyponatraemia
  • hypotonicity
  • urinary sodium > 20 mmol/l
  • urine osmolality > plasma osmolality
  • normovolaemia
  • elevated serum ADH
  • correction with water restriction
  • failure to drop urinary osmolality with fluid challenge
  • absence of renal, hepatic, cardiac, thyroid disease
  • absence of drugs that effect renal water handling

Causes of SIADH

  • tumour
  • neurological
  • pulmonary
  • drugs (tolbutamide, carbamazepine, fluphenazine, anti-depressants, barbiturates)

Treatment of SIADH

  • treat underlying cause
  • fluid restrict to 500 - 1000 mls/day
  • IV normal saline
  • 5% NaCl (1.8ml/kg) if rapid correction required (e.g. seizing)
  • ledermycin (tetracycline with ADH antagonist properties)

Management hyponatraemia

Acute hyponatraemia treated with IV normal saline (particularly in volume depleted)

Chronic hyponatraemia should be corrected slowly

  • Raise sodium to 125 mmol/l and then slowly thereafter
  • Amount of sodium necessary to raise serum sodium to 125 mmol/l can be approximated by sodium (mmol/l = 125 mmol/l - serum sodium (mmol/l) x TBW)

Na+ deficit in hyponatraemia

Na+ requirement (mmol) = total body water x (target Na+ - serum Na+ ).

Rate of infusion (ml/hr) = (Na+ requirement (mmol) x 1000) / (infused Na+ (mmol/L) x time (hours)).

Calculate fluids

0.9% NaCl (154mmol/L) rate: ml/hr for hours.

The above formula does not include insensible water losses.

If SIADH suspected then

  • Treat underlying cause
  • Fluid restrict to 500 - 1000 mls/day
  • IV normal saline
  • 5% NaCl (1.8ml/Kg) if rapid correction required (e.g. seizing)
  • Ledermycin (tetracycline with ADH antagonist properties)