Hyponatraemia

Question 1

Pathophysiology of hyponatraemia

Answer 1

Acute hyponatraemia - occurs < 24 hours
- Brain rapid adaptation to water gain

Chronic hyponataraemia - occurs > 48 hours
- Brain slow adaptation
Improper therapy -> osmotic demyelination

Question 2

Severe hyponatraemia with CNS compromise

Answer 2

Hypertonic saline infusion - 3% sodium chloride (900mOsm/L)
- Empirical: 150mL over 20 mins
or
- 2mL/kg body weight over 20 minutes
(1mL/kg NaCl 3% increases sNa by 1mmol/L)

Target: 4-5mmol/L - enough to reduce ICP by 50%
Urine output may slowly increase/double

Question 3

Approach to hyponatraemia

Answer 3

1. Is it an emergency? Does the patient require hypertonic saline?
2. Is this hypotonic hyponatraemia? what is the serum osmolality?
3. What is the cause?

Question 4

Determine volume over osmolality in hypotensive patient

Answer 4

Hypo-osmolar serum
- ADH mediated
- High urine Osm > 100

Low urine Na < 30
- Aldosterone maximally re-absorbing Na in DCT

Question 5

In picture of ADH excess and uNa > 40

Answer 5

1. 8am cortisol
2. TFT
3. Check thiazide diuretic use

Question 6

Is it acceptable to do SIADH workup when patient has ongoing IV drip?

Answer 6

Yes, just avoid drip arm

IV drip has negligible effect on urine sodium and urine osmolality
(However diuretics will greatly affect - and not suitable)

Question 7

Pseudohyponatraemia

Answer 7

Hypertonic hyponatraemia due to impermeable solutes other than sodium (glucose, mannitol, glycine, lipids)

Corrected sodium determines Na/osmolality rendered in euglycaemic state
cNa = (serum glucose - 5.5) x 0.4 + current Na

Question 8

When to investigate hyponatraemia

Answer 8

1. Na < 125 regardless well or unwell
2. Symptomatic hyponatraemia

Question 9

Saline paradox

Answer 9

Serum sodium worse with isotonic saline

Assume uOsm = 600 (600mOsm of solute is excreted in 1L water)
Every 1L of normal saline = 308mOsm/kg

Urine volume produced to clear 308mOsm/kg = 308/600 = 500mL

Every 1L given, 500mL out, net balance +500mL
Gain of excess free water increase hypotonicity
Hyponatraemia worsens

Question 10

Osmotic demyelination syndrome

Answer 10

Rare - 0.35% of patients with severe hyponatraemia < 120
Among them 80% had history of alcohol use
Only 40% of them had sodium correction > 8mmol/day

Rondon-Berrios and Sterns, 2024
- Mixed results, inconclusive on slow vs fast correction in mortality (may reduce or increase risk)
- Careful in malnourished or alcoholic patients

Bottom line:
- Stick with correction of < 8-10mmol/day and < 18mmol over 2 days