Mx of hyponatraemia

see also:

• aetiology of hyponatraemia
• sodium physiology
• dehydration
• for children see: RCH guideline - hyponatraemia in children

• check the other electrolyte levels - if all low, repeat U&E as it may be dilutional from iv fluid
• exclude DDx of hyperglycaemia and apply correction factors (and perhaps consider severe hyperlipidaemia or hyperproteinaemia as a cause of pseudohyponatraemia)
  • for glucose levels up to 22mmol/L, need to apply a correction factor of 1.6mEq/L per 5 mmol/L elevation of glucose
  • for glucose levels above 22mmol/L, need to apply a higher rate of correction of 4mEq/L per 5 mmol/L elevation of glucose above 22mmol/L
  • these calculations are generally used to predict how much serum sodium will rise once hyperglycaemia is corrected

• requires urgent correction to prevent further cerebral injury from cerebral oedema
• correction rate should be no more than 4-6 mEq/L rise 1st 6hrs and then no further correction may be warranted for the remainder of 1st 24hrs, and total correction for the 24 hours MUST be < 9mEq/L.
• 100ml iv 3% saline in adults over 10-60 minutes which should quickly raise serum levels by 2-3 mEq/L, this can be repeated once if needed (and maybe a third time at most)
  • equivalent dose alternatives include: 14mls 20% saline or 50mls 8.4% sodium bicarbonate
• see also seizures

• if it is real and significant, check serum osmolality and send urine sample for urine sodium and urine osmolality to help determine the cause - see hyponatraemia
• marginally low sodium levels may be physiologic (eg. pregnancy) or chronic, asymptomatic and can be managed as an outpatient with follow up
  • patients with chronic levels < 130 mEq/L due to CCF or cirrhosis generally do not warrant Rx as this is usually due to end stage disease (unless definitive Rx such as transplant is being considered)
  • asymptomatic patients with chronic levels < 130 mEq/L due to syndrome of inappropriate ADH secretion (SIADH) may benefit from measures to increase sodium levels to improve cognition and prevent falls
• most patients with acute hyponatraemia (<24hrs onset) with levels below 130mEq/L should be considered for investigation and Mx
• patients who are symptomatic, have rapidly falling levels or levels below 125mEq/L should probably be admitted to hospital for Mx

• fluid restriction to 800ml/day
  • this is likely to be adequate if urine (Na+K) : serum (Na+K) ratio < 0.5 and unlikely to be helpful if this ratio is > 1 and these patients may be helped by Rx with frusemide / furosemide / Lasix
• Rx underlying cause of syndrome of inappropriate ADH secretion (SIADH) if possible
• avoid iv 0.9% saline or 5% dextrose:
  • in the euvolaemic patient with SIADH, administering 0.9% saline is likely to make the hypontraemia WORSE as nearly all the sodium will be excreted due to intact aldosterone and ANP processes, while the high ADH will prevent water excretion
• if sodium must be given due to mod-severe symptomatic hyponatraemia in SIADH, it must be given carefully in a concentration greater than the sodium concentration in the patient's urine to increase the serum level - ie. hypertonic saline
  • usual dose over 1st few hours 1mL/kg lean body weight 3% saline per hour, measuring serum levels every 2-3hrs and ensuring maximum correction rate of 0.5-1mEq/L per hour in the 1st 4hrs is not exceeded
  • maximum correction rate of 4-6 mEq/L over each 24hr period
• consider using oral vasopressin receptor antagonists if patient does NOT have liver disease and is NOT hypovolaemic

• treat the cause if possible without causing too rapid a correction of hyponatraemia:
  • cease thiazides
  • slowly correct hypovolaemia
    • in the hypovolaemic patient, theoretically, each 1L of 0.9% saline iv in an adult will raise serum sodium by 1mEq/L, however, once hypovolaemia is corrected the reduced secretion of ADH may cause an unwanted rapid rise in serum sodium!
  • Rx nausea and pain as these increase ADH secretion
  • patients with adrenal insufficiency (may also have hyperkalaemia):
    • Rx with iv corticosteroids with care
  • Rx hypothyroidism
• restrict water intake to below level of urine output if CCF, cirrhosis, SIADH or advanced renal failure
  • may need to reduce to 60% of daily requirements, but general restriction is to < 800mL/day
• further saline Rx:
  • this can be very difficult to get right and requires great care to avoid rapid changes in serum sodium levels
  • requires individual patient assessment - only those with severe symptoms should need hypertonic saline
• correction rate should be to raise the serum sodium by 4 to 6 meq/L and by less than 9 meq/L over any 24 hour period.