PassFinals
Renal & Urology

Hypernatraemia

Hypernatraemia is usually a deficit of water relative to sodium; assess volume status and urine concentration, restore circulation first if shocked, then replace water in a controlled specialist-led plan.

In a nutshell

Hypernatraemia is usually a water deficit. Assess neurological state, volume status and urine concentration; restore circulation first if shocked, then replace water under a monitored plan. In children with hypernatraemic dehydration, obtain urgent expert advice and correct gradually. In known/possible cranial diabetes insipidus, involve endocrinology urgently and do not omit desmopressin. Monitor sodium frequently and avoid over-rapid correction.

Classic presentation

A frail dependent adult with poor intake, dehydration and confusion has a high sodium; alternatively, a patient with cranial diabetes insipidus develops polyuria, rising sodium and reduced access to water during acute illness.

Key points

  • Hypernatraemia is usually too little water, not too much sodium.
  • Restore circulating volume first if the patient is shocked; then replace the free-water deficit.
  • Dilute urine despite high plasma osmolality suggests diabetes insipidus and needs endocrine assessment.
  • Use a senior-led correction target; avoid a rapid sodium fall and monitor closely for cerebral oedema.
  • Children with hypernatraemic dehydration need urgent expert fluid advice and gradual correction.
  • Do not omit desmopressin in a patient with known cranial diabetes insipidus without urgent specialist input.

First-line investigation

Repeat serum sodium with osmolality, glucose, renal function, volume assessment and paired urine osmolality/volume.

Management

Assess shock and neurological risk

  • Seizures, reduced consciousness, shock, severe dehydration or large dilute-urine losses require urgent senior, renal/endocrine and paediatric input as appropriate.4,3,2

Restore perfusion and define the cause

  • Use isotonic fluid for shock, then assess volume status, glucose, renal function, osmolality, urine output and urine concentration before choosing the water-replacement fluid.2,4,3

Replace water under specialist control

  • Use oral/enteral water where safe or an appropriate IV fluid; children and patients with cranial diabetes insipidus need their guideline-specific correction ceiling and frequent sodium checks.5,4,3

Prevent recurrence

  • Treat the underlying loss, maintain access to water, continue a documented desmopressin plan where indicated and monitor sodium, fluid balance, urine output and neurological state.3,5,7

Exam traps

  • In shock, restore circulation with isotonic fluid before free-water replacement.
  • Do not correct chronic or unknown-duration hypernatraemia rapidly.
  • Polyuria with dilute urine and high plasma osmolality is a diabetes-insipidus clue, not simply dehydration.
  • Do not water-deprivation test an acutely dehydrated or unstable patient.
  • Do not omit prescribed desmopressin in known cranial diabetes insipidus without urgent endocrine input.
  • The correction ceiling differs by age and context; use the current NICE or specialist protocol rather than a universal number.

Key sources

  1. NHS, Intravenous fluid therapy and hypernatraemia information (NHS fluid and electrolyte principles supporting assessment of dehydration and IV-fluid complications)
  2. NICE CG174, Intravenous fluid therapy in adults in hospital (Current adult IV-fluid assessment and monitoring principles for resuscitation, replacement and electrolyte abnormalities)Published 10 Dec 2013
  3. Society for Endocrinology, Inpatient management of cranial diabetes insipidus (UK endocrine guidance on hypernatraemia, fluid replacement, desmopressin and controlled correction; published 2018)Published 8 May 2018
  4. NICE CG84, Diarrhoea and vomiting caused by gastroenteritis in under 5s (Current paediatric hypernatraemic-dehydration recommendations: urgent expert fluid advice, isotonic initial replacement, deficit typically over 48 hours and sodium fall below 0.5 mmol/L/hour)Published 22 Apr 2009
  5. NICE NG29, Intravenous fluid therapy in children and young people in hospital (Current paediatric IV-fluid recommendations: gradual hypernatraemia correction, a maximum fall of 12 mmol/L per 24 hours and electrolyte checks every 4–6 hours initially; published 9 December 2015)Published 9 Dec 2015
  6. Society for Endocrinology, Clinical guidance index: cranial diabetes insipidus (Current Society for Endocrinology page linking inpatient cranial diabetes-insipidus guidance and endocrine emergency resources)
  7. BNF, current fluids, electrolytes and desmopressin prescribing information (Use current BNF/BNFC monographs and local fluid protocols for fluid composition, renal adjustment, desmopressin and monitoring; no fixed adult regimen is asserted here)

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.