Doctor’s Passport

Find your next topic

Explore the current textbook

CardiologyRespiratoryNeurology
Available drafts · Clinical review pending
Membership
Educational draft · awaiting clinical reviewThe full textbook explains uncertainty but does not replace live national or local guidance, specialist advice, or current prescribing information.
Textbook/Neurosurgery/Hyperosmolar therapy principles
Full textbook

Hyperosmolar therapy principles

Choose and monitor hypertonic sodium or mannitol as a time-limited bridge for acute cerebral oedema or intracranial hypertension while preserving perfusion and treating the cause.

24 min full readUK scopeSources checked 13 Sept 2026Clinical review pending
Saved on this device
!
Acute intracranial pressure crisis

Falling consciousness, new pupillary asymmetry, posturing or a monitored pressure crisis suggests threatened herniation and can progress before a complete diagnostic picture is available.

Action: Call critical care and neurosurgery immediately, secure airway and circulation, elevate and align the head, obtain urgent imaging when safe, and give protocol-led hyperosmolar rescue without delaying evacuation, drainage or other definitive treatment.

Open the sections you need. The overview is shown first.
01Purpose and principlesWhat the treatment does and how it fits into care.

The rigid skull contains brain, blood and cerebrospinal fluid. Compensation initially displaces venous blood and CSF, but once compliance is exhausted, small volume increments cause steep pressure rises. Hypertonic sodium raises extracellular tonicity and expands circulating volume; mannitol creates an osmotic gradient and produces diuresis. Both can lower ICP transiently when the blood–brain barrier and gradient permit water movement.

The evidence is disease-specific. NCS 2020 evaluated adults and excluded paediatric studies. It conditionally favours hypertonic sodium over mannitol in TBI and ICH, accepts either in ischaemic stroke and hepatic encephalopathy, and favours symptom-based boluses over sodium-target dosing in SAH. These low-quality recommendations support individualised selection, not a universal hierarchy.

Clinical improvement in pupils or consciousness and reduction in monitored ICP are short-term responses. Neither agent has established that this physiological effect itself improves long-term neurological outcome. Simultaneously correct hypoxia, hypotension, fever, seizure and venous obstruction; seek the lesion that needs evacuation, CSF diversion, decompression or disease-specific treatment.

Key points

  • Hyperosmolar therapy buys time by shifting water from brain tissue into the intravascular space; it does not remove a haematoma, drain obstructive hydrocephalus or reverse the underlying disease.
  • Choose hypertonic sodium or mannitol from the aetiology, serum sodium, volume status, blood pressure, renal function, access and prior response; no single agent or sodium target is universal across causes and ages.
  • Treat the patient and trajectory, not an isolated number: reassess pupils, GCS or sedation-adjusted examination, haemodynamics and measured ICP soon after every rescue dose.
  • In adult TBI, NCS conditionally suggests hypertonic sodium over mannitol for initial ICP or oedema control, but neither has proved improved neurological outcomes; mannitol is an alternative when sodium loading is unsuitable.
  • Paediatric severe-TBI bolus and infusion regimens are weight-based and use a different ICP target; they must not be converted into adult or non-traumatic prescriptions.
  • Monitor sodium, chloride, renal function, acid–base state, fluid balance and line integrity with hypertonic sodium; monitor renal function, volume status and an osmolarity measure, preferably osmolar gap, with mannitol.
  • Avoid prophylactic scheduled osmotic therapy without a defined indication; symptom-based rescue and cause-specific plans reduce unnecessary exposure and false reassurance.
02Indications, selection and cautionsWho may benefit, who needs urgent treatment and important alternatives.
Clinical pressure crisisRed flag

Detect a falling conscious level, pupillary change, new focal weakness, posturing, repeated vomiting or respiratory irregularity. Cushing hypertension and bradycardia are often late and must not be awaited.

Measured ICP riseRed flag

Confirm duration, waveform quality, transducer reference and provoking events before treating a monitor value, unless simultaneous clinical herniation demands immediate rescue. Trend and pressure burden matter more than one artifact-prone sample.

Physiology selecting saline

Hypertonic sodium may support intravascular volume and cerebral perfusion when hypotension or diuresis makes mannitol unattractive, provided baseline sodium, chloride, acid–base state and venous access permit safe delivery.

Physiology selecting mannitol

Mannitol may be useful when further sodium loading is undesirable, but its diuresis can aggravate hypovolaemia and hypotension. Pre-existing renal dysfunction, heart failure and accumulating osmoles increase toxicity concern.

Failure of the bridge

Persistent deterioration or recurrent crises despite appropriate rescue mean the cause remains active. Re-image when safe and escalate for haematoma evacuation, EVD, decompression or other aetiology-specific intervention.

Red flags requiring action

  • A new dilated or poorly reactive pupil with reduced consciousness or hemiparesis is an emergency even before Cushing physiology appears.
  • Hypotension, hypoxaemia, severe hyponatraemia, hyperthermia, seizure or obstructed venous drainage can worsen secondary injury and must be corrected in parallel.
  • Repeated osmotic doses without neurological or pressure response suggest treatment failure, an untreated structural cause or loss of an effective osmotic gradient.
  • Oliguria, rising creatinine, worsening hypovolaemia or a rapidly widening osmolar gap during mannitol therapy signals increased renal and perfusion risk.
  • Severe hypernatraemia, hyperchloraemia, metabolic acidosis or extravasation during hypertonic-sodium therapy requires immediate reassessment of concentration, access and further dosing.
03Assessment before treatmentTests and checks that guide safe selection.
Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

  1. 01
    Immediate neurological examinationFirst step
    Why
    Establish the clinical trajectory and a response baseline before treatment whenever this does not delay rescue.
    Interpretation and limitations
    Improving pupils, motor response or consciousness supports physiological response; no improvement does not prove futility but requires rapid verification of diagnosis, dose delivery and definitive options.
  2. 02
    Non-contrast CT head
    Why
    Identify haemorrhage, mass, hydrocephalus, infarction or diffuse swelling requiring cause-specific treatment.
    Interpretation and limitations
    Midline shift, cisternal effacement or obstructive hydrocephalus changes the definitive pathway. A previously normal scan cannot exclude a later evolving pressure crisis.
  3. 03
    Serum sodium and chloride
    Why
    Quantify baseline sodium-loading capacity and detect hypernatraemia or hyperchloraemia during hypertonic treatment.
    Interpretation and limitations
    Interpret trends with renal function, acid–base status and urgency. NCS gives only a very-low-quality safety range, so a single value is not a universal therapeutic target.
  4. 04
    Renal function and fluid balance
    Why
    Detect AKI, osmotic diuresis and inadequate circulating volume before repeated hyperosmolar exposure.
    Interpretation and limitations
    Oliguria, rising creatinine or negative balance increases risk and prompts dose reassessment, perfusion correction and consideration of an alternative strategy.
  5. 05
    Measured osmolality and osmolar gap
    Why
    Estimate accumulating mannitol and support renal-toxicity surveillance during repeated dosing.
    Interpretation and limitations
    NCS suggests osmolar gap over serum-osmolality thresholds but identifies no validated universal cutoff; interpret the trend with renal function, dose history and response.
  6. 06
    Arterial blood gas
    Why
    Assess ventilation, oxygenation and acid–base disturbance during an acute pressure crisis.
    Interpretation and limitations
    Correct hypoxaemia and major acidosis. Brief controlled hyperventilation is a rescue bridge for herniation, while prolonged hypocapnia risks cerebral ischaemia.
04Treatment approachPreparation, options, escalation and aftercare.
01Emergency bridgeSuspected herniation or acute ICP crisisFirst stepClinical herniation signs or a sustained monitored pressure rise accompanied by concerning examination or imaging.
  1. 1Call neurosurgery and critical care, secure oxygenation and circulation, elevate the head above 30 degrees with neutral neck alignment, control fever and seizure, and stop avoidable noxious stimulation.
  2. 2DefinitiveGive the institution's protocol-led hypertonic sodium or mannitol rescue selected from sodium, volume, blood pressure, renal function and vascular access while definitive imaging and treatment are arranged.
  3. 3Reassess pupils, motor response, consciousness, MAP and ICP within minutes; verify dose delivery and line integrity, and do not repeat automatically when there is no physiological response.
  4. 4Treat the cause through evacuation, CSF drainage, decompression or disease-specific therapy; use brief controlled hyperventilation only as a bridge when active herniation persists during preparation.
02Mannitol pathwayMannitol selected by physiologyFurther sodium loading is unsuitable and blood pressure, circulating volume and renal function can tolerate osmotic diuresis.
  1. 1Confirm an acute treatment indication, obtain baseline renal function, fluid balance and osmolality data, and correct hypotension or hypovolaemia rather than allowing diuresis to reduce CPP.
  2. 2Use a weight-based bolus under local neurocritical-care protocol; the BTF historical adult severe-TBI range is 0.25–1 g/kg, while ENLS uses 0.5–1 g/kg, so the treating protocol defines the regimen.
  3. 3Document clinical and ICP response, urine output, haemodynamics, creatinine, measured osmolality and osmolar gap before deciding on another dose.
03Hypertonic pathwayHypertonic sodium selected by physiologyAcute oedema or intracranial hypertension requires rescue and sodium, chloride, renal function and access allow sodium loading.
  1. 1Choose concentration, bolus volume and route from a neurocritical-care protocol; concentrated preparations require appropriate venous access and monitoring rather than ad hoc peripheral administration.
  2. 2Check sodium, chloride, renal function, acid–base status, fluid balance and the infusion site at a frequency matched to concentration, repeated exposure and rate of change.
  3. 3DefinitiveReassess the pressure and examination response, avoid treating a serum-sodium number as the outcome, and move to definitive or next-tier therapy when crises recur.
05Regimens, contraindications and interactionsTreatment details and the circumstances that modify them.
Creates an osmotic gradient and diuresis that can lower ICP transiently while definitive diagnosis and treatment proceed. In adult TBI, mannitol is an alternative when severe hypernatraemia or volume overload makes further sodium loading unsuitable, with suitability assessed from blood pressure, circulating volume and renal function.

Mannitol intravenous solution

For an adult acute ICP crisis, use the weight-based bolus specified by the neurocritical-care protocol; historical BTF severe-TBI guidance gives 0.25–1 g/kg and ENLS v5 gives 0.5–1 g/kg over 5–15 minutes.

Avoid uncorrected hypotension or hypovolaemia; monitor urine output, circulating volume, creatinine, measured osmolality and osmolar gap. NCS does not support a single universal osmolar-gap or 320 mOsm/kg stopping threshold.

Raises serum tonicity, expands intravascular volume and can reduce cerebral water and ICP during an acute crisis.

Hypertonic sodium chloride

Concentration and bolus volume vary by cause, urgency and access; use the local neurocritical-care emergency regimen with pharmacy-ready preparation and route controls rather than extrapolating paediatric weight-based doses.

Check baseline and serial sodium, chloride, creatinine, acid–base state and fluid balance; prevent extravasation, choose access appropriate to concentration, and avoid severe hypernatraemia or hyperchloraemia.

06Complications, monitoring and follow-upAdverse effects, response and longer-term review.
  • Record the pre-dose and post-dose neurological examination, pupils, MAP and measured ICP where present, including timing, duration and waveform quality of any pressure crisis.
  • For hypertonic sodium, monitor sodium, chloride, creatinine, acid–base state, fluid balance and the vascular-access site; increase frequency when values or interventions change rapidly.
  • For mannitol, monitor urine output, intravascular volume, blood pressure, creatinine, measured osmolality and osmolar gap; rising creatinine or oliguria prompts immediate reassessment. NCS suggests osmolar gap over fixed serum-osmolality thresholds for AKI surveillance; no validated universal osmolar-gap cutoff exists, so interpret trends with renal function, intravascular volume, cumulative dosing and response.
  • Review cumulative doses and response before repeating treatment, because a transient fall in ICP does not show that the structural or disease-specific cause has been controlled.
  • Track hypotension, hypoxaemia, fever, seizure and ventilation in parallel because correcting these secondary insults can be as consequential as the osmotic choice.
  • After stabilisation, agree how osmotic therapy will be reduced and how rebound deterioration will be detected; abrupt unplanned changes can destabilise osmolality and ICP.
07Special situationsVariants, exceptions and circumstances that change the usual approach.

Pressure response is not outcome

Both agents may lower ICP without proven improvement in long-term neurological outcome. Use response to guide the bridge, while definitive treatment and prognosis depend on the underlying injury.

Disease determines the hierarchy

NCS recommendations differ across TBI, ICH, ischaemic stroke, SAH and hepatic encephalopathy. A preferred agent in one condition should not silently become a universal rule.

Children require their own protocol

BTF paediatric severe-TBI guidance supports 3% hypertonic saline 2–5 mL/kg over 10–20 minutes and an ICP target below 20 mmHg; these values do not define adult practice.

Osmolar gap evidence is limited

NCS considers the osmolar gap physiologically preferable to a fixed osmolality threshold for mannitol safety, but found insufficient evidence for a single cutoff value.

Steroids are cause specific

Corticosteroids are contraindicated for severe TBI outcome/ICP treatment and harmful in primary ICH, yet may be indicated for tumour-related vasogenic oedema or particular meningitis contexts.

08Common pitfallsFrequent interpretation and management errors.
  1. 01

    Using hyperosmolar therapy as a substitute for urgent evacuation of a mass, CSF diversion or decompression.

  2. 02

    Importing the adult severe-TBI ICP threshold, CPP target or mannitol range into children or non-traumatic disease without a matched source.

  3. 03

    Repeating an osmotic dose by schedule without documenting a pressure or clinical indication and the response to the previous dose.

  4. 04

    Allowing mannitol-associated diuresis and hypotension to lower cerebral perfusion pressure while focusing only on the ICP value.

  5. 05

    Pursuing a serum sodium, chloride, osmolality or osmolar-gap number as a therapeutic goal without balancing renal, acid–base and circulatory harm.

  6. 06

    Using prolonged aggressive hyperventilation for routine pressure control, risking cerebral vasoconstriction and ischaemia.

Practice

Two practice questions

Question 1 of 20 correct
NeurosurgeryOriginal SBA

Choosing an osmotic agent

An adult with severe traumatic brain injury has an acute intracranial pressure rise, serum sodium 160 mmol/L, clinical volume overload, stable blood pressure and acceptable renal function. Which interpretation should guide the immediate osmotic-treatment discussion?

Sources and review status4 sources · checked 13 Sept 2026 · clinical review pending
Sources

Sources and review status

National guidance is shown before implementation-dependent detail. Typical adult dose examples remain subject to patient factors, contraindications and the live BNF or specialist protocol. Source check completed 13 Sept 2026; clinical approval remains outstanding.

  • Neurocritical Care Society cerebral oedema guidelineCook et al., Neurocritical Care 2020, body read 13 September 2026: methodology and population limits; TBI recommendations; disease-specific SAH, ischaemic stroke, ICH and hepatic encephalopathy sections; hyperosmolar safety and infusion considerations. Adult studies were used and paediatric studies were excluded. Recommendations are mostly conditional with low or very-low-quality evidence. Chapter-specific use: hyperosmolar therapy principles.
  • Emergency Neurological Life Support Intracranial Hypertension and Herniation ProtocolNeurocritical Care Society ENLS version 6.0, last updated September 2024; communication checklist, diagnosis, Tier Zero, Tier One, Tier Two and Tier Three scope read 13 September 2026: crisis recognition, head elevation, SpO2, hyperosmolar treatment, short rescue hyperventilation, CSF drainage and definitive cause control. This multi-aetiology professional consensus protocol requires adaptation to age, aetiology, access and local critical-care protocol and does not make adult severe-TBI thresholds universal. Chapter-specific use: hyperosmolar therapy principles.
  • Brain Trauma Foundation severe TBI guideline, fourth editionCurrent BTF severe TBI recommendation index, fourth edition 2016 with 2020 decompressive-craniectomy update, read 13 September 2026: hyperosmolar therapy, ICP and CPP monitoring, thresholds, ventilation and CSF drainage. Applies to severe traumatic brain injury; older third-edition mannitol statements are explicitly identified by BTF as not supported by evidence meeting fourth-edition standards. Chapter-specific use: hyperosmolar therapy principles.
  • Brain Trauma Foundation paediatric severe TBI guideline, third editionCurrent BTF paediatric severe TBI recommendation index, third edition 2019, read 13 September 2026: ICP monitoring; neuroimaging limitations; paediatric ICP and CPP thresholds; hypertonic-saline regimens and safety; CSF drainage and ventilation. Applies only to children with severe traumatic brain injury and contains mainly weak recommendations. Chapter-specific use: hyperosmolar therapy principles.
Authoring stateComplete draftClinical stateAwaiting reviewJurisdictionUnited Kingdom