Hyponatraemia is a serum sodium below 135 mmol/L, and the decision to treat it urgently rests on symptoms rather than the number. According to the Society for Endocrinology's 2022 emergency guidance, severe or moderately severe symptoms are treated with 150 ml of 3% hypertonic saline over 20 minutes, repeated up to twice to achieve a 5 mmol/L rise in the first hour, with the total rise limited to 10 mmol/L in the first 24 hours and 8 mmol/L per 24 hours after that. At the CT1 anaesthetics interview the scenario is a confused post-operative patient with a sodium of 118.
Key takeaways
- Biochemical grades: mild 130 to 135, moderate 125 to 129, profound below 125 mmol/L (Society for Endocrinology 2022)
- Severe symptoms are persistent vomiting, cardiorespiratory arrest, seizures and reduced consciousness with a GCS of 8 or less; moderately severe are nausea without vomiting, confusion and headache
- Management decisions are based on symptoms and signs, not on the degree of hyponatraemia
- Treatment is 150 ml of 3% saline over 20 minutes, sodium checked, repeated twice or until a 5 mmol/L rise
- The rise is limited to 10 mmol/L in the first 24 hours and 8 mmol/L per 24 hours thereafter until sodium reaches 130
- Boluses, not a continuous infusion; no vaptans; no Adrogue-Madias formula; a senior clinician decides and supervises
How is hyponatraemia graded?
Hyponatraemia is graded biochemically by the Society for Endocrinology into mild at 130 to 135 mmol/L, moderate at 125 to 129, and profound below 125. The guidance then says something the station is built on: the clinical picture may not match the number.
A profound sodium can be symptom-free if it developed slowly, while a moderate one can cause seizures if it fell fast. The rate of fall, the brain's ability to adapt, and comorbidity all shape the presentation.
The guidance also notes that severe symptoms are unlikely with a sodium above 130, so a fitting patient with a sodium of 132 needs another explanation.
Which symptoms make it an emergency?
Hyponatraemia becomes an emergency when the symptoms are severe or moderately severe, and the Society for Endocrinology defines both lists. Severe means persistent vomiting, cardiorespiratory arrest, seizures, or reduced consciousness with a Glasgow Coma Scale of 8 or less.
Moderately severe means nausea without vomiting, confusion, or headache. Both groups get hypertonic saline immediately, whatever the cause of the hyponatraemia, according to the guidance.
Mild or absent symptoms are managed by cause rather than by bolus. The first recommendation in the document is that management decisions follow symptoms and signs rather than the biochemical grade.
What do you do first?
The first actions in symptomatic hyponatraemia are ABCDE with the airway protected if the GCS is 8 or less, a repeat sodium on the same analyser, and a senior clinician called to authorise hypertonic saline. The Society for Endocrinology places the decision to treat with a senior clinician with appropriate training.
- Airway and seizure control: a fitting or obtunded patient is the anaesthetist's problem before it is the endocrinologist's
- Confirm the sodium, and keep using the same measurement platform throughout, as the guidance recommends
- Move to an environment that can deliver, monitor and escalate treatment safely
- Take the diagnostic bloods and urine before the bolus if it does not delay it
- Call the senior who will supervise the hypertonic saline
How is hypertonic saline given?
Hypertonic saline is given as 150 ml of 3% sodium chloride, or an equivalent, intravenously over 20 minutes in a closely monitored environment, according to the Society for Endocrinology. The sodium is checked, and a second 150 ml bolus over 20 minutes runs while the result is awaited.
The bolus is repeated twice, or until the sodium has risen by 5 mmol/L, which is the target for the first hour. The guidance recommends boluses rather than a continuous 24-hour infusion because they raise the sodium and improve symptoms faster with less over-correction.
After the 5 mmol/L rise, the hypertonic saline stops, a line is kept open with minimal 0.9% saline, and cause-specific treatment starts. Sodium is checked at 6 hours, 12 hours and then daily until stable.
What is the correction limit?
The correction limit is a rise of no more than 10 mmol/L in the first 24 hours after presentation and no more than 8 mmol/L in each 24 hours after that, until the sodium reaches 130 mmol/L. The Society for Endocrinology sets those limits to avoid osmotic demyelination.
The draft answers candidates carry often quote 8 mmol/L for the first day. That is not the UK figure; the first-day ceiling is 10, with 8 applying thereafter.
Monitoring is increased over the first 24 hours in patients whose sodium rose by more than 5 mmol/L after the first or second bolus, and in anyone who needed 3 or more boluses.
What if the patient does not improve after a 5 mmol/L rise?
If the clinical state has not improved after a 5 mmol/L rise in the first hour, the Society for Endocrinology recommends a further 150 ml of 3% saline over 20 minutes aiming for an additional 1 mmol/L rise. The infusion stops when symptoms improve, when the total rise exceeds 10 mmol/L, or when the sodium reaches 130, whichever comes first.
At that point the guidance says to explore other causes of the symptoms. A patient who is still obtunded with a sodium that has risen 10 mmol/L has something else going on.
How is over-correction managed?
Over-correction is a rise above 10 mmol/L in the first 24 hours or above 8 mmol/L per 24 hours thereafter, and the Society for Endocrinology says it needs active management, not observation. Hypertonic fluid stops, and a clinician experienced in managing over-correction is consulted.
Urine output is monitored throughout, because a rising urine output can be the first sign of aquaresis and a sodium about to climb. Over-correction can occur without hypertonic saline at all, for example after adrenal crisis is treated or when a hypotonic load is excreted.
The guidance recommends against vaptans in symptomatic patients and against the Adrogue-Madias formula, because both are associated with over-correction.
Which causes need thinking about?
The causes of hyponatraemia that an anaesthetic CT1 meets are post-operative hypotonic fluids and pain-driven antidiuretic hormone release, diuretics, adrenal insufficiency, and the syndrome of inappropriate antidiuresis. The Society for Endocrinology's emergency pathway treats the symptoms first and starts diagnosis-specific treatment after the 5 mmol/L rise.
Volume status, paired serum and urine osmolality, urine sodium, thyroid function and a cortisol are the standard work-up. However, none of them should delay the bolus in a seizing patient.
Which traps catch candidates?
The hyponatraemia traps at interview are treating the number, quoting the wrong ceiling, and running hypertonic saline as an infusion. Each contradicts the 2022 guidance.
- Giving hypertonic saline to an asymptomatic patient because the sodium is 118
- Quoting 8 mmol/L as the first-day limit: it is 10, then 8
- Fluid restricting a seizing patient instead of bolusing
- Switching between a blood gas analyser and the laboratory mid-treatment
- Forgetting that the decision belongs to a senior clinician
What is the interviewer listening for?
In a hyponatraemia scenario the assessors are listening for the symptom-led decision, the exact bolus, the 5 mmol/L first-hour target, the 10 and 8 limits, and a monitoring plan with times.
- Symptoms decide treatment, not the sodium
- 150 ml of 3% saline over 20 minutes, repeated to a 5 mmol/L rise
- 10 mmol/L in the first 24 hours, 8 per day thereafter, to 130
- Sodium at 6 hours, 12 hours and daily
- Airway and seizure control, and a senior named
How this comes up at the CT1 anaesthetics interview
Hyponatraemia comes up in the clinical judgement station as the elderly patient on day two after a hemiarthroplasty who is confused, then has a seizure on the ward, with a sodium of 116. The station is one of two 15-minute stations, each marked out of 50, and 60 out of 100 is appointable.
The follow-up asks what you do when the repeat sodium has jumped 9 mmol/L in four hours, which tests whether you know over-correction is actively managed. anaestheticinterview's 340-question bank includes a hyponatraemia scenario with AI-marked spoken practice, where the 150, 5, 10 and 8 can be rehearsed until they are reflex.