Suspected meningitis or encephalitis

ACR AC Headache; NEJM 2001 CT-before-LP criteria

Two questions arrive in one request. Contrast-enhanced MRI answers the diagnostic one; a pre-lumbar-puncture CT answers a procedural one in a defined minority — and neither may be allowed to delay antimicrobial and antiviral treatment.

Fever, headache, confusion, seizures or a reduced conscious level, with or without meningism, where central nervous system infection is being treated empirically while the diagnosis is established.

Referenced decision support — confirm against your local protocol.

Decision support, not a directive. Protocols and timings shown are typical published ones — your local protocol takes precedence, and the vetting radiologist decides.

The request in front of you

Everything is optional. Leave a field alone and the answer assumes nothing — the verdict updates as you go.

Study requested

What the referrer actually asked for. It is evidence of intent, not a constraint on the right answer.

Contrast as written

What the request form says, not what it should say.

Age

Pick a band, or type an exact age if it matters.

years

Pregnancy status

GCS

Focal neurological deficit

Immunosuppressed

Changes the differential rather than the modality.

Renal risk factors

The question a vetter can actually answer from the request. An explicit “none known” is a real answer, and it removes checks rather than deferring them.

Previous contrast reaction

The pathway — tap anything already done

Marking a study complete moves the answer on. A patient arrives partway through a pathway far more often than at the start of one.

Accept as requested

MRI Brain — with gadolinium

MRI Brain

What we'd amend, and why

The findings that make this diagnosis are invisible on CT. Herpes simplex encephalitis appears as medial temporal and insular signal change with restricted diffusion, autoimmune encephalitis as limbic signal abnormality, and meningeal disease as thin leptomeningeal enhancement — all of which require gadolinium, FLAIR and diffusion together. The same study also identifies the complications that change management within hours: empyema, cerebritis, abscess, venous thrombosis and early hydrocephalus.

Nothing needs resolving before this goes ahead. Routine checks below are owned downstream.

How was this decided?

pathwayadult — Adults
rulerule-mr-device-screening — MR safety screening for implants and foreign bodies; checked by Radiographer at the scanner
rulerule-contrast-reaction-premed — Prior contrast reaction and elective premedication; checked by Nurse before the scan
rulerule-gadolinium-renal — Kidney function and gadolinium-based contrast; checked by Radiographer at the scanner
rulerule-paeds-sedation — Sedation or anaesthesia for a child; checked by Nurse before the scan
rulerule-pregnancy-gadolinium — Gadolinium in known or possible pregnancy; checked by Radiographer at the scanner
rulerule-iv-access — Intravenous access adequate for the planned injection; checked by Radiographer at the scanner

Decision support only. Local protocol takes precedence.

Handled at the scanner(2)nothing for you to do

Settled and owned downstream. Each returns to a radiologist only on the stated trigger.

MR safety screening for implants and foreign bodies
Complete the MR safety questionnaire, verify implant labelling and its stated conditions of use against this scanner and this protocol, and ensure no ferromagnetic object enters Zone IV.
Radiographer at the scannerBefore the scan
Flags back if: An implant or retained foreign body that is MR Unsafe, unlabelled, or cannot be identified; or an MR Conditional device whose stated conditions this scanner or the requested protocol cannot satisfy; or a credible unexcluded intraocular metallic foreign body history.
Time-critical: safety screening runs alongside preparation, never as a queue in front of it. If the MR study cannot be delivered within a few hours, proceed with contrast-enhanced CT while the device is identified, and treat empirically throughout.
Intravenous access adequate for the planned injection
Site and test a cannula that supports the protocol flow rate, preferring an antecubital or large forearm vein, and observe the injection for extravasation. A 20-gauge or larger cannula is preferred for flow rates of 3 mL/s or more.
Radiographer at the scannerAt the scanner
Flags back if: No cannula can be sited that supports the protocol flow rate — for example only a 22-gauge hand or foot cannula for a CT angiogram needing 4–5 mL/s; or the only available access is a central line or port that is not labelled power-injectable; or an extravasation occurs.

Worth asking the referrer (1)

None of these hold the request up. They sharpen the protocol or the plan that follows.

Is the scan being requested to permit a lumbar puncture, or to make the diagnosis?
They are different studies. The procedural question is answered by an immediate unenhanced CT; the diagnostic question needs contrast-enhanced MRI with diffusion.

Pathways

Big forks are separate pathways; the first whose conditions match is the one used.

Focal deficit, immunocompromise, a depressed conscious level or age 60 and over — CT before the tap

Role	Study & protocol	Why this answers the question
First line	CT Head CT Head — Unenhanced usually appropriate	This is the group the pre-lumbar-puncture study was defined for, and the order inverts for them alone: a new focal deficit, immunocompromise, a reduced conscious level and age 60 or over are four of the six features that predict an abnormal CT, and the puncture waits for it. The other two — a seizure within the preceding week and papilloedema, alongside known central nervous system disease — cannot be read from a request and must be asked, because their presence puts a patient on this arm who would otherwise be tapped without imaging. The study is procedural rather than diagnostic — it looks for a mass lesion with shift, it cannot diagnose meningitis or encephalitis, and a normal result licenses the tap rather than guaranteeing its safety, because it does not exclude raised intracranial pressure. Antibiotics and aciclovir are given before the patient reaches the scanner; if the imaging is what is delaying them, the pathway has already failed.
Second line	MRI Brain MRI Brain — with gadolinium	The diagnostic study still, and in the immunosuppressed it carries more weight rather than less: the differential widens to toxoplasmosis, cryptococcal disease, progressive multifocal leukoencephalopathy, tuberculosis and lymphoma, and those are separated by enhancement pattern and diffusion, not by CT. Gadolinium, FLAIR and diffusion together also find the complications that change management the same day — empyema, cerebritis, abscess, venous thrombosis and early hydrocephalus. Where the tap is not being done at all, this is the study that should not be deferred behind the CT.

Nothing on this arm is a reason to hold empirical treatment. The CT buys the safety of the puncture, not the safety of waiting.

Adults

Matches your inputsDefault

Role	Study & protocol	Why this answers the question
First line	MRI Brain MRI Brain — with gadolinium usually appropriate	The findings that make this diagnosis are invisible on CT. Herpes simplex encephalitis appears as medial temporal and insular signal change with restricted diffusion, autoimmune encephalitis as limbic signal abnormality, and meningeal disease as thin leptomeningeal enhancement — all of which require gadolinium, FLAIR and diffusion together. The same study also identifies the complications that change management within hours: empyema, cerebritis, abscess, venous thrombosis and early hydrocephalus.
Second line	CT Head CT Head — Unenhanced	A procedural safety study, not a diagnostic one. Its job is to exclude a mass lesion with shift before a lumbar puncture in the defined higher-risk group, and it is fast enough to do that without materially delaying treatment. It cannot diagnose encephalitis or early meningitis, and a normal result must never be reported as excluding either.

Antibiotics and aciclovir are started on clinical suspicion. No part of this imaging pathway is a reason to withhold them.
This arm is the DIAGNOSTIC pathway, not the procedural one. In an alert patient with none of the pre-puncture risk features the answer to "does this patient need a scan before the tap" is no — the lumbar puncture is done and the antibiotics given, and the MRI here answers the encephalitis, empyema and complication question alongside that rather than in front of it. Holding the tap for an MRI slot is the same failure as holding it for a CT, and it costs the same hours.

Pitfalls

Allowing the scan to delay antibiotics or aciclovir. Time to treatment is the outcome that matters, and imaging is the commonest reason it slips.
Reporting a normal CT as excluding meningitis or encephalitis. It excludes neither, and it does not exclude raised intracranial pressure either.
Omitting diffusion, which distinguishes empyema and abscess from sterile effusion and is the most specific early sign in herpes encephalitis.
Missing early hydrocephalus and venous sinus thrombosis, both of which complicate bacterial meningitis and both of which change management the same day.
Requesting CT before lumbar puncture in a patient with none of the recognised risk features, which delays treatment for no diagnostic gain.
Omitting age from the pre-puncture risk assessment. Age 60 and over is one of the six predictors of an abnormal CT alongside a focal deficit, immunocompromise, a depressed conscious level, a seizure within the preceding week and known central nervous system disease — and an older patient whose meningism is actually a mass lesion is the specific harm the rule exists to prevent.
Waiting for an MRI slot before the lumbar puncture. The MRI answers the diagnostic question, not the procedural one; it does not license the tap and it must never postpone it or the antimicrobials.

Priors — what to pull first

Prior imaging is decisive for interpreting temporal lobe signal change, which is otherwise easily confused with a post-ictal state or an old injury.
In the immunosuppressed, compare with any earlier study before attributing new lesions to infection rather than to progression of the underlying disease.

What makes a good request

Published evidence supports imaging before lumbar puncture in a defined group — new focal neurological deficit, immunocompromise, recent seizure, reduced conscious level, papilloedema, older age or known central nervous system disease — rather than in everyone. Absence of those features has a high negative predictive value for an abnormal CT.
A normal CT does not exclude raised intracranial pressure, so it licenses the puncture rather than guaranteeing its safety.
Imaging before lumbar puncture measurably lengthens the time to antibiotics, which is the harm this pathway has to design around: treat first, image second.

How these studies are acquired

Contrast, phases and timing for every study on the pathways above.

CT Head — Unenhanced MRI Brain — with gadolinium

Confirm locally

MRI Brain — with gadolinium: timings are typical — confirm against local protocol.
Timings, contrast volumes and rates above are typical published values. Your department's protocol, scanner and patient population decide the actual numbers.
Safety thresholds and premedication policy follow local policy where it differs from the cited guidance.

References

Implemented from the cited published sources. Educational and workflow support only; confirm against current guidelines and local policy before clinical use.