Hyponatraemia
Background
- Usually indicative of hypo-osmolality of body fluid due to excess of water relative to solute
- In dilutional hyponatraemia kidney responds by returning 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 diurectics.
Clinical
Symptoms:
- Always present when osmolality less than 240 meq
- Rate of fall important
- Increased severity in children and women of child bearing age
- > 125 mmols/l usually asymptomatic
- 125 to 115 mmols/l subtle changes in mental status, eg confusion
- < 115 mmols/l stupor ,neuromuscular hyper-excitability, convulsions, coma, and initial improvement may be followed by delayed neurological symptoms
- Central pontime myelinolysis is uncommon
Causes:
- Factitious (hyper-proteinaemia / lipidaemia / glycaemia and mannitol
- Water retention (renal, hepatic and cardiac failure, hypothyroidism)
- SIADH
- Polydipsia
- Salt loss (adrenal cortical insufficiency, diurectics, fluid therapy, post-trauma and stress)
- Cerebral salt wasting syndrome
SIADH
Criteria
- 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
- 3% NaCl through central line if rapid correction required
- 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 mmols/l and then slowly thereafter
- Amount of sodium necessary to raise serum sodium to 125 mmols/l can be approximated by sodium (mmols/l = 125 mmols/l - serum sodium (mmols/l) x TBW)
If SIADH suspected then
- treat underlying cause
- fluid restrict to 500 - 1000 mls/day
- IV normal saline
- 3% NaCl through central line if rapid correction required
- ledermycin (tetracycline with ADH antagonist properties)
