01Purpose and principlesWhat the assessment is for and the core concepts behind it.
The decision to image begins with phenotype: onset speed, first or worst status, evolution, associated neurology, systemic illness, age, pregnancy, trauma, cancer and immune state. A positive diagnosis of migraine or tension-type headache is safer than saying “nothing serious” because the diagnosis also defines which future change should trigger reassessment.
CT is fast and sensitive for acute blood but uses ionising radiation and has limited posterior-fossa and soft-tissue sensitivity. MRI better shows tumour, inflammation, venous complications and posterior-fossa disease but takes longer and may require device, motion or monitoring planning. CTA and venography answer vascular questions and should not be added indiscriminately.
Imaging can harm through incidental findings, anxiety, contrast exposure and cascades, so reassurance alone is not a sufficient indication. Conversely, avoiding low-value imaging must never become a reason to ignore red flags. The useful report answers the suspected secondary cause and states limitations and the next test when concern remains.
Key points
- Do not image a diagnosed stable tension-type headache, migraine, cluster headache or medication-overuse headache solely for reassurance when no secondary warning feature is present.
- Image to answer a defined dangerous differential: non-contrast CT for acute haemorrhage, CTA for aneurysm or arterial disease, venography for venous thrombosis and MRI for mass, inflammation or pressure disorders.
- A normal scan is modality- and timing-specific: early ischaemia, later-presenting SAH, cerebral venous thrombosis, meningitis and small posterior-fossa lesions can survive an inappropriate negative test.
- Typical migraine aura is fully reversible, develops gradually over at least five minutes and lasts 5–60 minutes; motor weakness, diplopia, monocular symptoms, poor balance or reduced consciousness warrants further investigation or referral.
- For suspected SAH, urgent non-contrast CT comes first; the need for LP after a negative scan depends on whether imaging was within or beyond six hours and whether the scan was adequate and radiologist-reported.
- Use age and population boundaries: adult and over-12 primary-headache guidance does not replace the under-16 red-flag pathway, pregnancy assessment or acute infection care.
02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.
Establish time to peak precisely; maximal intensity within minutes activates the acute SAH and vascular differential even if pain later improves.
Progressive morning headache, vomiting, transient visual obscurations, pulsatile tinnitus, diplopia or papilloedema requires imaging for mass and venous disease before selected LP pathways.
Fever, neck stiffness, rash, immunocompromise or altered consciousness raises infection and inflammatory causes; antibiotics and stabilisation must not be delayed for routine imaging.
Weakness, aphasia, field loss, ataxia, cranial-nerve deficit, cognitive change or seizure requires urgent cause-specific imaging rather than classification as primary headache.
Reversible visual, sensory or speech symptoms that spread gradually over at least five minutes and last 5–60 minutes support aura when the overall pattern is stable.
Motor weakness, diplopia, monocular visual symptoms, poor balance or decreased consciousness prompts further investigation or referral even when migraine is possible.
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
Urgent non-contrast CT headFirst step - Why
- Detect acute subarachnoid or other intracranial haemorrhage, hydrocephalus and major mass effect in a red-flag presentation.
- Interpretation and limitations
- Sensitivity depends on timing, quality and reader expertise. A negative later scan does not close suspected SAH and should follow the NG228 LP branch when appropriate.
- 02
MRI brain with tailored sequences - Why
- Assess progressive headache, focal signs, posterior-fossa disease, tumour, inflammation or another structural cause unresolved by CT.
- Interpretation and limitations
- Choose diffusion, susceptibility, contrast and sellar or posterior-fossa sequences from the suspected diagnosis; a generic protocol may miss the target.
- 03
CT or MR venography - Why
- Investigate cerebral venous thrombosis in pregnancy or postpartum, hypercoagulability, raised-pressure syndrome or atypical haemorrhage.
- Interpretation and limitations
- Review source images for true filling defects and congenital asymmetry. A routine non-contrast CT does not exclude venous thrombosis.
- 04
CT angiography - Why
- Identify aneurysm or arterial disease after haemorrhage or when the vascular presentation remains compelling.
- Interpretation and limitations
- CTA follows the clinical and non-contrast CT pattern. A technically limited or negative study may require neurovascular review and selective DSA.
- 05
Formal fundoscopy and visual assessment - Why
- Confirm papilloedema and measure acuity and fields in a suspected raised-pressure disorder.
- Interpretation and limitations
- Absence of obvious papilloedema does not exclude every pressure emergency; atypical discs need ophthalmic confirmation and imaging precedes LP when mass or venous disease is possible.
- 06
Lumbar puncture - Why
- Test selected suspected SAH or infection after the correct imaging and safety sequence.
- Interpretation and limitations
- For SAH after negative CT beyond six hours, wait at least 12 hours from onset for bilirubin spectrophotometry. Do not perform through mass effect, obstructive hydrocephalus or instability.
04Clinical next stepsHow the result changes management or prompts escalation.
01Worked emergency pathwayThunderclap headache with normal examinationFirst stepAn adult has a sudden maximal-at-onset headache with vomiting and meningism but no focal deficit.+
- 1Treat the onset pattern as possible SAH and obtain urgent non-contrast CT; a normal examination does not reduce this to routine migraine care.
- 2If CT is negative, check exact onset-to-scan interval, image quality and radiologist reporting; within six hours routine LP may be avoided, while beyond six hours consider LP when safe and at least 12 hours after onset.
- 3EscalationEscalate confirmed blood or persistent aneurysmal suspicion through the specialist neurovascular pathway.
02Stable primary pathwayTypical recurrent migraine without changeA person over 12 has a stable primary-headache pattern, typical aura if present and normal examination.+
- 1Make and explain a positive diagnosis, review medication overuse and provide clear change-based safety netting.
- 2Do not request neuroimaging solely for reassurance; reconsider if a new feature from the secondary-headache list develops.
- 3Use a headache diary when it will refine frequency, triggers or treatment response rather than as a substitute for urgent assessment.
03Pressure pathwayPapilloedema or progressive morning headacheHeadache is accompanied by papilloedema, visual obscurations, diplopia or persistent unexplained vomiting.+
- 1Arrange urgent MRI or CT for a mass and add venography for venous thrombosis according to stability and availability.
- 2Obtain formal visual assessment and involve neurology or ophthalmology; treat any acute visual decline urgently.
- 3Proceed to pressure measurement only after imaging excludes a dangerous pressure gradient and other procedural contraindications are addressed.
05Risks, monitoring and follow-upComplications, safety checks and further assessment.
- For a non-imaged primary headache, document the positive diagnosis and explicit triggers for urgent return: onset change, persistent vomiting, focal signs, altered consciousness or seizure.
- After negative emergency imaging, retain ownership of the residual differential and complete the specified SAH, venous or infection pathway rather than discharging on the word normal.
- Use a headache diary over at least eight weeks when helpful to record frequency, duration, severity, associated symptoms, medicines, precipitants and menstrual relationship.
- Re-examine patients whose symptoms evolve, especially visual fields, pupils, gait, coordination, cognition and fundoscopy where competence and equipment allow.
- Track incidental findings to a named clinician and avoid repeating imaging unless the lesion-specific recommendation or a clinical change creates a new question.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
Reassurance needs an explanation
A positive primary-headache diagnosis and change-based safety net are more useful than an incidental-finding-prone scan requested without a clinical question.
Onset timing changes meaning
The same negative CT carries different residual SAH probability at two hours and ten hours, so exact chronology belongs in the request.
Aura evolves gradually
Typical aura spreads over minutes and resolves within an hour; sudden fixed negative symptoms should remain in the vascular differential.
Venous disease needs venography
Routine brain CT or MRI can be normal or nonspecific in cerebral venous thrombosis unless the venous circulation is deliberately assessed.
Children have distinct red flags
Age under four and the under-12 red-flag set trigger neurological assessment and should not be diluted by adult reassurance rules.
07Common pitfallsFrequent interpretation and management errors.
- 01
Ordering MRI solely because a person with stable diagnosed migraine requests reassurance, without explaining the primary-headache diagnosis.
- 02
Using a normal neurological examination to dismiss a thunderclap headache or suspected subarachnoid haemorrhage.
- 03
Calling a later negative CT definitive for SAH without accounting for onset time, quality and the lumbar-puncture branch.
- 04
Applying CG150 over-12 advice to younger children without checking the separate NG127 age-specific red flags.
- 05
Requesting generic MRI when venography, dedicated sellar imaging or another cause-specific protocol is needed.