Hyponatraemia — SAQ

Stem

A 28-year-old woman seizes 36 hours after gynaecological surgery. Maintenance fluid has been 5% dextrose. GCS is 8 after the seizure. Serum Na is 118 mmol/L. There is no oedema and no clinical dehydration. Paired samples: serum osmolality 248 mOsm/kg, urine osmolality 520 mOsm/kg, urine Na 68 mmol/L. Glucose is 5.2 mmol/L. Cortisol and TSH are pending. ^[1]

Questions

a) Define hyponatraemia and give the European 2014 biochemical bands and the separate symptom-severity axis. Why does that split change management? (3 marks)

• Definition: serum sodium under 135 mmol/L.
• Biochemical bands (ion-specific electrode): mild 130 to 135, moderate 125 to 129, profound under 125 mmol/L.
• Onset: acute if documented under 48 h; if onset is unknown, treat as chronic.
• Symptom axis (Table 5), not the same as the biochemical band:
  • moderately severe: nausea without vomiting, confusion, headache
  • severe: vomiting, cardiorespiratory distress, deep somnolence, seizures, coma (GCS 8 or under)
• Why it changes management: severe symptoms are a first-hour 3% saline emergency (cerebral oedema). Profound chronic Na without severe symptoms is an over-correction / osmotic-demyelination problem, not a bolus race. ^[1]

b) Interpret the paired osmolality and volume findings. What syndrome do they support, and which two endocrine exclusions are still required? (3 marks)

• True hypotonic hyponatraemia: Posm 248 (under 275).
• ADH is acting: Uosm 520 (over 100) at a low effective osmolality.
• Euvolaemia plus UNa 68 (over 30) with normal intake fits the essential SIAD criteria.
• Still required before the label sticks: normal adrenal function (cortisol) and normal thyroid, plus no recent diuretics and no renal/cardiac/hepatic failure.
• Supplemental (not required): uric acid under 0.24 mmol/L, urea under 3.6 mmol/L, fractional uric acid excretion over 12 percent, failure of 0.9% saline, response to fluid restriction.
• Mechanism chain: post-operative non-osmotic ADH (pain, nausea, opiates) plus hypotonic maintenance water → the kidney cannot excrete free water → falling Posm → cerebral oedema → seizure. ^[1]

c) Immediate management of her severe symptoms. Give the European 2014 bolus, the first-hour target, and what you do after she improves. (3 marks)

• Airway, oxygen, stop the dextrose, treat the seizure.
• European 2014 severe symptoms: 150 mL of 3% saline over 20 minutes, check Na, repeat 150 mL over the next 20 minutes, and again if needed, until a 5 mmol/L rise in the first hour.
• After improvement: stop 3%, keep a 0.9% saline trickle, then cap further rise at 10 mmol/L in the first 24 h and 8 mmol/L in every 24 h thereafter until 130 mmol/L.
• Do not aim to normalise and do not use the 100 mL finish-line EAH bolus as the hospital European dose.
• Ayus 2019 (US expert practice, not the European recommendation) uses 100 to 150 mL boluses aiming 5 to 6 mEq/L in 1 to 2 hours and notes that European guidance is the 150 mL bolus. ^[1]

d) Give the European 2014 over-correction rule, the monitoring intervals, and the expert-discussed relowering manoeuvre. (3 marks)

• Relower if Na rises over 10 mmol/L in the first 24 h or over 8 mmol/L in any 24 h thereafter.
• Check Na after 20 minutes during 3% boluses; after improvement at 6 and 12 hours; every 4 hours if a 3% infusion continues.
• Stop active treatment and discuss with an expert 10 mL/kg electrolyte-free water over 1 h and i.v. desmopressin 2 micrograms (µg), not more often than every 8 h.
• High-ODS-risk observational series (Bastos 2024): 4 to 6 mmol/L/day, not exceeding 8; 10.4 percent of published ODS cases still demyelinated when reported correction was under 10 mEq/L/day. Do not quote an unsourced 20 to 40 percent ODS mortality. ^[1]

e) Two related settings you must not mix up with this hospital protocol. (3 marks)

• Exercise-associated hyponatraemia (finish-line): severe EAH is 100 mL of 3% NaCl IV, repeated twice if no clinical improvement (EAH Consensus 2015). Oral 100 mL 3% is an option in mild EAH (Owen 2014). Prevent by drinking to thirst, not to a schedule.
• Haemodynamically stable hypovolaemia: European 2014 0.5 to 1.0 mL/kg/h of 0.9% saline or a balanced crystalloid (shock overrides the rate). A urine-output jump over 100 mL/h is the over-correction warning.
• SIAD without severe symptoms: fluid restriction first; second-line urea 0.25 to 0.50 g/kg/day or a loop diuretic plus oral NaCl. European 2014 recommends against vaptans in profound SIAD, does not recommend them in moderate SIAD, and recommends against lithium or demeclocycline. SALT-1/2 showed oral tolvaptan 15 mg (titrate 30 then 60 mg) raises Na at day 4 and day 30 — biochemical efficacy, not a European first-line recommendation. ^[1]

Marking tips

Full marks require the 150 mL / 20-minute / 5 mmol first-hour European severe-symptom sequence, the 10 then 8 daily caps, SIAD uric acid under 0.24 mmol/L and FEurate over 12 percent, and not importing the 100 mL EAH bolus into hospital SIAD care. ^[1]

References1ShowHide

1. [1]Spasovski G, Vanholder R, Allolio B, et al. Clinical practice guideline on diagnosis and treatment of hyponatraemia Intensive Care Med, 2014.PMID 24562549