PassFinals
Renal & Urology

Hyponatraemia

Hyponatraemia is a low serum sodium, usually reflecting excess water relative to sodium; assess symptoms and osmolality urgently, treat severe neurological symptoms immediately, then classify the cause and control the rate of correction.

In a nutshell

Hyponatraemia is usually relative water excess. Assess symptoms first, then serum osmolality, paired urine studies and volume status. Treat severe neurological symptoms immediately with the correct adult or paediatric hypertonic-saline protocol; thereafter treat the cause and prevent overcorrection.

Classic presentation

An older patient taking a thiazide has nausea, falls and confusion with a sodium of 118 mmol/L; confirm the result, assess symptoms and osmolality urgently, stop the likely cause and escalate for monitored correction.

Key points

  • The emergency decision is symptom-led: severe neurological symptoms need hypertonic saline before a complete cause work-up.
  • Serum osmolality separates true hypo-osmolar hyponatraemia from hyperosmolar and pseudohyponatraemic results.
  • Send paired urine osmolality and urine sodium, but interpret urine sodium cautiously with diuretics or renal impairment.
  • SIADH is a diagnosis of exclusion: check volume status, renal function, thyroid, adrenal function and medications.
  • Rapid correction of chronic hyponatraemia can cause osmotic demyelination; write a correction ceiling and monitor against it.
  • Adult and paediatric hypertonic-saline protocols differ; do not transpose one into the other.

First-line investigation

Repeat sodium with glucose and serum osmolality, then paired urine osmolality and urine sodium plus renal, thyroid, adrenal and medication assessment.

Management

Recognise the neurological emergency

  • Seizures, persistent vomiting, reduced consciousness or coma require immediate senior-led hypertonic-saline treatment in a monitored setting; do not wait for a complete diagnostic classification.1,4

Confirm osmolality and mechanism

  • Check glucose and serum osmolality, then send paired urine osmolality and sodium with renal, thyroid, adrenal and medication assessment; do not diagnose SIADH by urine sodium alone.2,3,6

Correct symptoms without overshooting

  • Use the relevant adult or paediatric hypertonic-saline bolus pathway, then stop the emergency fluid when the symptom-directed rise is achieved and enforce the appropriate correction ceiling with serial sodium monitoring.1,4,5

Treat the cause and prevent recurrence

  • Use cautious isotonic fluid for genuine hypovolaemia, monitored fluid restriction and cause treatment for appropriate SIADH, and disease-specific management for oedematous states; involve renal/endocrine teams for persistent or complex disease.2,8,3,7

Exam traps

  • Treat severe neurological symptoms before waiting for volume status or a full SIADH work-up.
  • Hyperglycaemic and pseudohyponatraemic results are not managed as simple hypo-osmolar water excess.
  • SIADH requires exclusion of adrenal, thyroid, renal and drug causes.
  • A brisk aquaresis after volume restoration can overcorrect sodium unexpectedly.
  • Do not use adult 3% saline boluses and paediatric 2.7% saline boluses interchangeably.
  • Fluid restriction alone is not treatment for acute hyponatraemic encephalopathy.

Key sources

  1. Society for Endocrinology, Emergency management of severe and moderately severely symptomatic hyponatraemia in adult patients (Current UK adult emergency guidance: symptom-led treatment, intermittent hypertonic-saline boluses, correction ceilings, monitoring and overcorrection management; revised 2022)Published 1 Jan 2022
  2. Royal United Hospitals Bath, Hyponatraemia in primary care (UK NHS laboratory guidance on pseudo- and hyperosmolar hyponatraemia, paired serum/urine testing, causes, SIADH exclusion and referral; version approved 20 October 2023)Published 20 Oct 2023
  3. NICE CG174, Intravenous fluid therapy in adults in hospital (Current adult IV-fluid assessment, monitoring and electrolyte-imbalance principles, including the limits of urinary sodium interpretation; published 10 December 2013)Published 10 Dec 2013
  4. NICE NG29, Intravenous fluid therapy in children and young people in hospital (Current paediatric IV-fluid guidance: acute symptomatic hyponatraemia, 2.7% saline boluses, expert advice, hourly sodium checks and the post-symptom correction ceiling; published 9 December 2015)Published 9 Dec 2015
  5. NHS England, Osmotic demyelination syndrome from rapid correction of severe hypo/hypernatraemia (Current NHS patient-safety information identifying rapid sodium correction and osmotic demyelination as a national safety risk)
  6. NICE CKS, Hyponatraemia (NICE CKS topic anchor for adult diagnostic and management pathways; access may require an NHS or institutional login, so specific emergency regimens are taken from the directly accessible Society for Endocrinology and NICE sources)
  7. BNF, fluids, electrolytes, sodium chloride and hyponatraemia-related prescribing (Current BNF prescribing and safety monographs for sodium chloride, fluid restriction, diuretics, vaptans and endocrine treatment; direct monograph access may require NHS login)
  8. NICE NG106, Chronic heart failure in adults: diagnosis and management (Current NICE advice on fluid restriction for dilutional hyponatraemia in heart failure and avoiding routine restriction without clinical need)Published 12 Sept 2018

This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.