- 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, convulsions, coma, and 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).
- Salt loss (adrenal cortical insufficiency, diuretics, fluid therapy, post-trauma and stress).
- Cerebral salt wasting syndrome.
- urinary sodium > 20 mmol/l
- urine osmolality > plasma osmolality
- 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
- 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)
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)).
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).