Elderly male from nursing home presents critically unwell with severe hypernatraemia following a seizure.
Issues:
1. Seizure and altered mental state – secondary to high sodium, intracranial haemorrhage, other.
2. Tachycardia and low blood pressure for age – shock, large fluid deficit
3. Life threatening hypernatraemia
4. Possible end of life situation
Severe hypernatraemia and hyperchloridaemia
aim to correct sodium at rate not exceeding 0.5-1 mmol/hr
HCO3 normal
anion gap = 21 – elevated, causes in this patient include :
a. Lactate – shock, sepsis, other
b. Toxins – metformin, paracetamol, iron, salicylates
c. Unlikely here – DKA (glucose 6.9 and no ketones given), renal failure ( normal creatinine)
Urea 23.5 with normal Creatinine – severe volume depletion and pre renal failure
Fluid deficit = assume average weight of 70 kg = 70X0.5X(183/140-1) = 10.75 litres
Total fluid replacement – add ongoing insensible losses and replace over 24 hours
Initial fluid bolus to correct shock ( can use normal saline )
Mildly elevated glucose – stress response, unlikely to cause osmotic diuresis
osmolality = 396 – severe hyperosmolar state, with large fluid deficit (as above)
Interpretation – Elderly male with severe hypernatraemia and large fluid deficit presents following a seizure, in shock. He requires airway support, fluid bolus to correct shock, slow sodium correction, assessment of underlying cause and discussion of possible end of life issues ( severe hypernatraemia associated with more than 50% mortality )
HYPERNATRAEMIA
In general
1. Deranged thirst response or altered behavioural response, usually elderly and institutionalised.
2. Diabetes insipidus – nephrogenic, neurogenic
3. Free water loss
Assessment – aims
1. Fluid/ volume status
2. Acute/chronic
3. Thorough neuro examination
Mechanism – cellular dehydration causes cell shrinkage. The response is altered resting potentials of electrically active membranes. After 1 hour of hypernatraemia there is generation of intracellular organic solutes to restore volume and avoid structural damage. ( too rapid fluid rehydration can cause cerebral oedema). Cell dehydration in the CNS can cause altered mental state, seizures or intracranial haemorrhage ( rupture of bridging veins).
CAUSES
1. Hypovolaemia – renal (osmotic dieresis, post obstructive, kidney disease), extra renal (diarrhoea, vomiting, fistula, burns)
2. Hypervolaemia – sodium gain ( hypertonic saline, NaHCO3, accidental ie. formula problems in infants)
3. Euvolaemia – renal (diabetes insipidus- nephrogenic, neurogenic), extra renal (insensible losses)
DDX – HHS
Additional investigations – urine sodium and osmolality, BSL, CT head
Treatment aims –
1. Restore normal tonicity
2. Investigate and treat underlying cause
FLUID DEFICIT CALCULATION
weight (kg) X %TBW X ( Na/140-1) in litres
TBW
young men 0.6
young women and elderly men 0.5
elderly women 0.4