EMed

Hyponatraemia

Background

  • 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

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 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

Causes:

  • Factitious (hyper-proteinaemia / lipidaemia / glycaemia and mannitol)
  • Water retention (renal, hepatic and cardiac failure, hypothyroidism)
  • SIADH
  • Polydipsia
  • Salt loss (adrenal cortical insufficiency, diuretics, 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
  • dDrugs (tolbutamide, carbamazepine, fluphenazine, anti-depressants, barbiturates)

Treatment of SIADH

  • Treat underlying cause
  • Fluid restrict to 500 - 1000 mls/day
  • 5% NaCl (1.8ml/kg) only 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 (days)

  • 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)).

If SIADH suspected then

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