Synopsis
Restore circulation and free water safely, identify impaired access or renal water loss, and prevent dangerous over-rapid tonicity change.
- Hypernatraemia usually means water deficit relative to body sodium, caused by inadequate access or thirst, extrarenal water loss, osmotic diuresis or arginine vasopressin deficiency or resistance.
- Older, cognitively impaired, intubated, postoperative and care-dependent adults are vulnerable because they cannot obtain water even when renal physiology and thirst would otherwise protect them.
- Assess circulation before calculating free water: shock requires isotonic resuscitation first. In decompensated AVP deficiency with persistent excessive dilute urine, Society guidance advises desmopressin 1–2 micrograms IV or IM and serum sodium every 4 hours during resuscitation.
- Prefer oral or nasogastric water as soon as safe; select intravenous water-equivalent fluid and rate from sodium trajectory, glucose, losses, volume status and local specialist guidance.
- Measure urine volume and osmolality: in hypernatraemia, a kidney that cannot concentrate urine suggests renal water loss from AVP deficiency, AVP resistance or osmotic diuresis.
- Desmopressin is life-sustaining in central AVP deficiency, and omission or delay has caused severe dehydration and death according to the NHS England safety alert.
- Once desmopressin controls a high urine output, ongoing free-water prescription can lower sodium unexpectedly quickly; coordinate the medicine and fluid rather than managing them independently.
- Correction rate depends on duration and symptoms; presumed chronic or unknown-duration disease requires a conservative specialist-agreed trajectory with frequent adjustment, not a one-off deficit calculation.
Key red flags
Lethargy, irritability, confusion, hyperreflexia, weakness, seizure or coma suggests clinically important hypertonicity and requires urgent neurological and biochemical assessment.
Tachycardia, hypotension, poor peripheral perfusion, dry mucosa and oliguria indicate priority restoration of intravascular volume before free-water calculation dominates care.
Large volumes of pale urine, intense thirst, pituitary disease, neurosurgery or missed desmopressin raises central AVP deficiency and must trigger immediate endocrine review.
Hypertonic saline, sodium bicarbonate, concentrated enteral feed without water or dialysis error can raise sodium rapidly and needs immediate source control and specialist correction.
Investigation priorities
Confirm hypertonicity and quantify the actual response to fluid and desmopressin treatment.
Management branches
The hypernatraemic adult has shock, hypotension or clinically important hypovolaemia.
- Use ABCDE, obtain urgent serum and urine studies, secure accurate input and output measurement and identify ongoing gastrointestinal, renal, skin or iatrogenic losses.
- Give isotonic crystalloid resuscitation according to NICE CG174 and repeated haemodynamic assessment despite the raised sodium, because organ perfusion takes immediate priority.
- Once circulation is restored, calculate an initial water plan and move to oral, nasogastric or appropriate intravenous free-water replacement with a documented sodium trajectory.
Haemodynamics are stable but hypertonicity and a water deficit remain.
- Estimate duration, deficit and ongoing losses, then choose the safest enteral or intravenous route considering swallow, glucose, renal function, heart failure and access.
- Set an explicit conservative target for chronic or uncertain-duration hypernatraemia with renal or endocrine advice, prescribing only the next review interval rather than the entire estimated deficit blindly.
- Remeasure sodium and fluid balance frequently, increasing, reducing or pausing water according to the observed slope and neurological and respiratory examination.