Synopsis
Decide when headache requires emergency CT, urgent or planned MRI, vascular imaging or no imaging, using age-specific red flags and the limitations of each negative test.
- 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.
Key red flags
Headache reaching maximum intensity within minutes, especially with exertion, collapse, vomiting or neck stiffness, requires urgent assessment for subarachnoid haemorrhage.
New focal deficit, altered consciousness, cognitive change, personality change or seizure prompts urgent investigation for vascular, inflammatory or mass lesions.
Papilloedema, new persistent vomiting, immunocompromise or cancer history requires cause-directed imaging and referral rather than routine primary-headache management.
New headache during pregnancy or postpartum, with fever, or after head trauma requires the matching obstetric, infection or trauma pathway and tailored vascular imaging.
Children under 12 with headache plus morning waking, progressive symptoms, vomiting, ataxia, altered consciousness or squint need same-day neurological assessment; all children under four with headache need urgent neurological assessment.
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.
Investigation priorities
Detect acute subarachnoid or other intracranial haemorrhage, hydrocephalus and major mass effect in a red-flag presentation.
Management branches
An adult has a sudden maximal-at-onset headache with vomiting and meningism but no focal deficit.
- Treat the onset pattern as possible SAH and obtain urgent non-contrast CT; a normal examination does not reduce this to routine migraine care.
- If 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.
A person over 12 has a stable primary-headache pattern, typical aura if present and normal examination.