When to Order a Non-Contrast Head CT for New-Onset Headache
Order a non-contrast head CT immediately for any patient with thunderclap headache (severe headache reaching maximum intensity within 1 minute), any abnormal neurological finding, or any red flag feature—these presentations carry significant risk of life-threatening intracranial pathology including subarachnoid hemorrhage, which has a 30% 90-day mortality rate. 1, 2
Immediate CT Indications (High-Risk Features)
Thunderclap Headache Characteristics
- Severe headache reaching maximum intensity within 1 minute warrants immediate non-contrast CT, as subarachnoid hemorrhage (SAH) accounts for 10-25% of thunderclap presentations 2, 3
- Headache reaching at least 7/10 intensity within 1 minute requires urgent evaluation 4
- CT performed within 6 hours of symptom onset has 98.7% sensitivity for SAH, missing fewer than 1.5 in 1000 cases 1, 5
- CT sensitivity drops considerably beyond 6 hours (≤90%), making timing critical 1, 5
Ottawa SAH Rule Red Flags
Apply the Ottawa SAH Rule for alert patients >15 years with new severe non-traumatic headache reaching maximum intensity within 1 hour. Order CT if ANY of the following are present 1:
- Age ≥40 years
- Neck pain or stiffness
- Witnessed loss of consciousness
- Onset during exertion
- Thunderclap headache (instantly peaking pain)
- Limited neck flexion on examination
The Ottawa SAH Rule has 100% sensitivity but only 15.3% specificity, meaning it identifies all SAH cases but results in many negative CTs 1
Additional Red Flag Features Requiring CT
- New-onset headache in patients ≥50 years old 6
- Progressively worsening headache over days to weeks 6
- Any abnormal neurological examination finding (focal deficits, altered mental status, papilledema, abnormal cranial nerves, gait disturbance) 1, 6
- Headache awakening patient from sleep 6
- Headache worsened by Valsalva maneuver 6
- New focal neurological deficit (fixed or worsening) 1
- Altered mental status, confusion, or decreased level of consciousness 1
- Seizure activity 1
- Head trauma 1
Migraine-Specific Considerations
When CT Is NOT Indicated for Migraine
- Typical migraine presentation with completely normal neurological examination has only 0.2% prevalence of significant intracranial abnormality 6
- Avoid reflexive imaging in patients meeting migraine criteria without red flags, as testing should not be ordered if it won't change management 6
When CT IS Indicated for Migraine
- New-onset migraine or worsening migraine pattern with any atypical features 6
- Abnormal neurological examination increases likelihood of intracranial pathology including tumor, arteriovenous malformation, or hydrocephalus 6
- Atypical features or headaches not fulfilling migraine criteria warrant lower threshold for imaging 6
Critical Time-Dependent Considerations
The 6-Hour Window
- Within 6 hours of onset: Non-contrast CT alone may be sufficient to exclude SAH when interpreted by fellowship-trained, board-certified neuroradiologists using high-quality CT scanners 1
- Beyond 6 hours of onset: If CT is negative but clinical suspicion remains high, lumbar puncture for xanthochromia evaluation is mandatory 1, 2
Follow-Up After Negative CT
- If CT is negative within 6 hours and clinical suspicion is low, SAH is effectively ruled out 1
- If CT is negative but performed >6 hours after onset, lumbar puncture with spectrophotometric analysis for xanthochromia has 100% sensitivity and 95.2% specificity 1, 5
- If both CT and LP are negative, consider CT angiography (CTA) or MR angiography to evaluate for reversible cerebral vasoconstriction syndrome, cervical artery dissection, cerebral venous thrombosis, or other vascular pathology 1, 4, 7
Special Populations
Pediatric Patients
- CT is NOT routinely indicated for pediatric headache with normal neurological examination, as diagnostic yield is <1% 8
- MRI without contrast is preferred over CT for non-emergent evaluation in children due to superior sensitivity for tumors and no radiation exposure 8
- CT is appropriate for pediatric thunderclap headache or suspected acute hemorrhage requiring immediate assessment 8
- Red flags in children include occipital location (rare in children), progressive symptoms, altered mental status, papilledema, or abnormal neurological findings 8
Patients with Known Intracranial Pathology
- Non-contrast CT is first-line for acute or worsening mental status changes in patients with known mass, recent hemorrhage, recent infarct, or CNS infection to assess for complications including progressive mass effect, increasing edema, hydrocephalus, or new hemorrhage 1
Common Pitfalls to Avoid
- Do not rely solely on CT timing beyond 6 hours—sensitivity declines significantly and LP becomes mandatory if clinical suspicion persists 1, 5
- Do not skip lumbar puncture after negative CT if presentation was >6 hours from onset and SAH suspicion is high 1
- Do not assume all post-SAH headaches are benign—rebleeding rates are 7-26% in the first 2-8 weeks before aneurysm repair 9
- Do not order routine CT for typical migraine with normal examination—yield is only 0.2% and exposes patients to unnecessary radiation and cost 6
- Do not delay CT if MRI is unavailable when acute stroke or hemorrhage is suspected 1
- Do not dismiss atypical presentations such as primary neck pain, syncope, or seizure—these still warrant appropriate imaging 1
- Do not use contrast-enhanced CT as first-line—non-contrast CT is superior for detecting acute hemorrhage 6
Algorithm Summary
- Thunderclap headache OR any red flag feature → Immediate non-contrast head CT
- CT within 6 hours + negative + low clinical suspicion → SAH ruled out
- CT >6 hours OR negative CT with high suspicion → Lumbar puncture for xanthochromia
- Negative CT and LP → Consider CTA/MRA for vascular causes
- Typical migraine + normal exam + no red flags → No imaging indicated
- Pediatric patient with normal exam → No CT; consider MRI only if red flags present