01Purpose and principlesWhat the assessment is for and the core concepts behind it.
Invasive ICP monitoring turns pressure into a continuous time series. It is useful when the examination is limited or when severe brain injury carries a high risk of intracranial hypertension. The output must answer a defined management question: whether an intervention is needed, whether it worked, and whether pressure burden is recurring. A catheter never supplies the anatomical diagnosis or a prognosis by itself.
An external ventricular drain is commonly referenced to an anatomical level and can both measure ICP and remove CSF. Its value depends on whether the drain is open or clamped, its prescribed height, zeroing and patency. An intraparenchymal microtransducer provides a local pressure measurement and avoids cannulating a small or displaced ventricle, but it cannot be recalibrated after insertion and does not treat hydrocephalus.
The best-known thresholds come from severe TBI. BTF fourth-edition adult guidance recommends management informed by ICP monitoring, treatment above 22 mmHg and CPP 60–70 mmHg, with clinical and CT integration. BTF paediatric guidance suggests monitoring, an ICP target below 20 mmHg and CPP at least 40 mmHg, generally 40–50 according to age. These weak or setting-specific recommendations are boundaries, not universal normal values.
Key points
- ICP monitoring quantifies pressure and burden; it does not identify the cause, replace serial neurological examination or remove the need for repeat imaging when the patient deteriorates.
- An EVD measures ventricular pressure and can drain CSF but carries infection, haemorrhage, obstruction and over-drainage risks; an intraparenchymal probe is easier to place in small ventricles but cannot drain and may drift.
- Interpret each value with duration, waveform, reference level, head position, stimulation, sedation, pupils, motor response, MAP and imaging; treat the patient and verified trend.
- BTF adult severe-TBI guidance recommends treating ICP above 22 mmHg and targets CPP 60–70 mmHg, while paediatric severe-TBI guidance suggests ICP below 20 mmHg and CPP at least 40 mmHg with age adjustment.
- Those adult and paediatric TBI targets must not be applied automatically to meningitis, tumour, stroke, hydrocephalus or postoperative patients; NICE NG240 advises against routine invasive ICP monitoring in bacterial meningitis.
- Set the EVD reference, drainage height and clamp status exactly to the authorised unit prescription; unplanned repositioning or drainage can produce false readings, over-drainage or herniation risk.
- Monitor insertion-site infection, tract haemorrhage, ventricular collapse, CSF appearance, drainage amount and device patency while minimising unnecessary manipulation and sampling.
02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.
Consider monitoring when a salvageable patient has severe TBI with abnormal CT, high-risk severe TBI despite normal CT, hydrocephalus requiring drainage or another specialist-defined need for continuous pressure data.
If pupils, consciousness or motor response worsen while ICP appears normal, presume the patient may be deteriorating. Check laterality, probe position and waveform while arranging imaging and treatment.
Repeated or sustained elevations with a plausible waveform, reduced compliance pattern, falling CPP or corroborating examination and CT changes are more convincing than single transient peaks.
Recognise new deficit, seizure, insertion-site bleeding, ventriculitis features, CSF leak, over-drainage headache or sudden cessation of drainage as reasons for immediate specialist review.
Coughing, suction, pain, agitation, fever, seizures, hypoxaemia, hypercapnia, hypotension, neck rotation and ventilator dyssynchrony can change ICP or CPP and should be identified before indiscriminate escalation.
03Method and interpretationA systematic approach to the test and its findings.
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.
- 01
ICP value and time burdenFirst step - Why
- Quantify magnitude, duration and recurrence of intracranial hypertension and response to intervention.
- Interpretation and limitations
- In adult severe TBI, a sustained value above 22 mmHg supports treatment, but decisions combine the value with clinical and CT findings; other populations need their own target.
- 02
ICP waveform - Why
- Confirm pulsatile signal and detect changes in intracranial compliance or damping.
- Interpretation and limitations
- A physiological waveform supports signal validity. P2 rising relative to P1 may suggest reduced compliance, while a flat or implausible trace prompts connection, reference, obstruction and device checks.
- 03
Cerebral perfusion pressure - Why
- Relate mean arterial pressure to intracranial pressure and identify threatened perfusion.
- Interpretation and limitations
- CPP is usually MAP minus ICP at a consistent reference. Adult severe-TBI guidance targets 60–70 mmHg; children and non-traumatic conditions require different age- and cause-specific decisions.
- 04
Non-contrast CT head - Why
- Locate haemorrhage, oedema, hydrocephalus, cisternal effacement, infarction or device complication behind a pressure change.
- Interpretation and limitations
- Imaging explains anatomy and directs evacuation or drainage. A normal early CT cannot exclude later intracranial hypertension, particularly in a comatose child with severe TBI.
- 05
EVD drainage record - Why
- Correlate pressure with prescribed drain height, clamp status, patency and CSF output.
- Interpretation and limitations
- No drainage may reflect low pressure, obstruction or malposition; excess output may reflect a low reference or siphoning. Only authorised staff adjust or sample the system.
- 06
Advanced neuromonitoring - Why
- Add brain oxygenation, perfusion or metabolic context when specialist teams face refractory or discordant physiology.
- Interpretation and limitations
- These modalities can identify hypoxia or autoregulatory failure but have device-specific limitations and do not replace ICP, MAP, examination and imaging.
04Clinical next stepsHow the result changes management or prompts escalation.
01Interpretation pathwayA new ICP elevationFirst stepThe monitor shows a new pressure rise or increasing pressure burden, with or without clinical change.+
- 1Look at the patient first: repeat pupils and motor response, review airway, oxygenation, ventilation, MAP, temperature, seizure activity, pain, agitation and recent stimulation.
- 2Validate the signal: confirm the device type, waveform, zero and anatomical reference, head position, EVD clamp and height, connections, patency and whether the displayed value is transient or sustained.
- 3If the rise is verified, correct reversible physiological triggers and apply the aetiology-specific ICP plan while urgent CT and neurosurgical decisions proceed when deterioration or a new structural cause is possible.
- 4EscalationDocument the intervention and time-linked ICP, CPP and examination response; escalate refractory pressure or discordance rather than repeating treatments without evidence.
02Selection pathwayChoose EVD or parenchymal probeA specialist team has decided that continuous invasive pressure information will change management.+
- 1Define whether CSF diversion is required, review ventricular size and distortion, haemostasis, infection risk and the need for transport or repeated imaging.
- 2Choose EVD when ventricular measurement and drainage are both valuable; choose a parenchymal device when drainage is unnecessary or ventricular access is difficult.
- 3Record the insertion plan, reference convention, treatment targets, complication surveillance and which staff may alter, clamp, drain or sample the system.
03Deterioration pathwayClinical decline with a normal readingPupils, consciousness, motor response or respiration worsen despite a normal or unchanged displayed ICP.+
- 1Treat the clinical syndrome as real, activate critical-care and neurosurgical response, and begin immediate measures to prevent secondary injury.
- 2Check for probe displacement, regional pressure gradients, damped EVD waveform, open-drain measurement artifact and extracranial mimics without delaying CT.
- 3Use imaging and the complete physiological picture to diagnose the cause; revise monitoring or treatment if the device is unreliable or sampling only one compartment.
05Risks, monitoring and follow-upComplications, safety checks and further assessment.
- Chart ICP and CPP as trends with episode duration, waveform quality, patient position, stimulation and interventions rather than transcribing isolated hourly numbers.
- Pair every pressure review with pupils, GCS components where assessable, limb movement, respiration, oxygen saturation, blood pressure and temperature; sedation limitations must be recorded.
- For an EVD, record prescribed reference level, drainage height, open/clamped status, hourly output, CSF appearance and patency; only authorised staff manipulate the system.
- Inspect the insertion site and observe for fever, new meningism, unexplained neurological decline, CSF leak or purulence while avoiding routine unnecessary sampling.
- After any intervention, document the time-linked change in ICP, CPP and examination. Failure, recurrence or escalating treatment burden prompts imaging and senior review.
- Audit cumulative complications including tract haemorrhage, infection, over-drainage, obstruction, malposition and monitor drift, and remove the device when its information no longer changes care.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
Local pressure may mislead
An intraparenchymal probe samples one region. Compartmental gradients or device malposition can produce clinical-monitor discordance, so worsening pupils or consciousness still demand action.
Open EVD changes measurement
An open drain relieves and alters ventricular pressure; accurate measurement may require prescribed clamping. Drain state and reference height must accompany every number.
CPP inherits two errors
Because CPP is calculated from MAP and ICP, inconsistent anatomical reference or a faulty signal in either measurement can create a plausible but wrong result.
Normal CT is time limited
BTF paediatric guidance specifically warns that an initially normal CT does not exclude elevated ICP in a comatose child after severe TBI.
Monitoring must change care
Insertion risk is justified only when continuous data or CSF drainage is expected to alter treatment. Routine invasive monitoring is not recommended in bacterial meningitis.
07Common pitfallsFrequent interpretation and management errors.
- 01
Applying the adult severe-TBI 22 mmHg treatment threshold as a universal definition for every age, lesion and neurological disease.
- 02
Treating an isolated cough or suction spike without checking duration, waveform, reference, position and the patient's examination.
- 03
Trusting a normal displayed value over new anisocoria, falling consciousness, posturing or other herniation signs.
- 04
Reporting CPP without stating how MAP and ICP were referenced or correcting hypotension that threatens perfusion.
- 05
Manipulating, lowering, clamping, flushing or sampling an EVD outside the authorised neurosurgical and nursing protocol.
- 06
Leaving a monitor in place after it no longer informs care, thereby prolonging infection and haemorrhage exposure.