What are the indications for a head computed tomography (CT) scan in patients presenting with headache?

Indications for Head CT in Patients with Headache

Head CT is indicated when specific "red flag" features are present, particularly in emergency settings where rapid exclusion of life-threatening pathology is essential, while routine imaging is not warranted for typical primary headaches with normal neurological examination.

Emergency/Acute Indications for Head CT

Suspected Subarachnoid Hemorrhage (SAH)

• Non-contrast head CT is the imaging modality of choice for thunderclap headache or suspected SAH due to superior sensitivity for detecting acute blood products 1
• CT performed within 6 hours of symptom onset has 98.7% sensitivity for SAH, missing fewer than 1.5 in 1000 cases 1
• Modern CT imaging is sufficient to exclude 97.5% of SAH in patients with "worst headache" presentation 2
• If CT is negative but clinical suspicion remains high (especially >6 hours from onset), lumbar puncture for xanthochromia evaluation should follow 1

Ottawa SAH Rule Application

CT is indicated if any of the following criteria are met in alert patients >15 years with new severe headache reaching maximum intensity within 1 hour 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

Traumatic Brain Injury

Head CT is indicated in minor head injury patients with any of these high-risk factors 1:

• Failure to reach Glasgow Coma Scale score of 15 within 2 hours
• Suspected open skull fracture
• Sign of basal skull fracture
• Vomiting more than once
• Age >64 years

Red Flag Features Warranting CT Imaging

Clinical History Red Flags

• New-onset headache in patients ≥50 years old 1, 3, 4, 5
• Progressively worsening headache over days to weeks 1, 3, 4
• Headache awakening patient from sleep 1, 3, 4
• Headache worsened by Valsalva maneuver 1, 3, 4
• Rapid increase in headache frequency 1, 3
• Presentation within 1 hour of headache onset 5
• History of aphasia 5

Examination Findings

• Any unexplained abnormal finding on neurological examination is an indication for neuroimaging 1, 3, 6
• Focal neurological deficit at examination 5
• Abnormal neurologic examination increases likelihood of intracranial pathology including brain tumor, arteriovenous malformation, and hydrocephalus 1, 3

Additional High-Risk Features

• Immunocompromised state 7
• Active cancer 7
• Pregnancy 7
• Headache related to specific activity or position 7

When CT is NOT Indicated

Typical Migraine with Normal Examination

• Neuroimaging is not usually warranted in patients with migraine and normal neurological examination 1, 3
• The prevalence of significant intracranial abnormality is only approximately 0.2% in this population 1, 3, 4
• This yield is equivalent to the general asymptomatic population 3

Tension-Type Headache

• Insufficient evidence exists for routine imaging in tension-type headaches with normal neurological examination 1
• Studies show no lesions found in patients with tension-type headaches and normal neurological examination 1

CT vs. MRI Considerations

When CT is Preferred

• Emergency settings requiring rapid diagnosis 4
• Suspected acute hemorrhage (CT superior to MRI for acute SAH) 1, 3
• Faster acquisition time with decreased safety screening requirements 4
• Excellent for detecting hemorrhage, mass effect, and hydrocephalus 4

When MRI is Preferred Over CT

• Elective/outpatient evaluation of persistent headache 3
• Suspected brain tumor or structural lesions 1, 3
• Pediatric patients (to avoid radiation exposure) 1
• When focal neurological findings are present (MRI with contrast provides superior soft tissue resolution) 6

Diagnostic Yield in Emergency Settings

Overall Yield

• In patients with acute severe headache, normal neurological examination, and normal non-contrast CT, cranial imaging leads to diagnosis of underlying pathology in approximately 1 in 7.6 patients (13%) 5
• CTA after normal CT and examination has 7.4% yield for vascular abnormalities, but only 1.6% for abnormalities clearly related to headache 8

Specific Pathology Detection

• Unruptured intracranial aneurysms found in 5.4% of patients undergoing CTA after normal CT 8
• Most aneurysms detected are likely incidental findings 8
• Number needed to scan to find any abnormality is 14; for abnormality other than aneurysm is 61 8

Key Principles for Imaging Decisions

Three Consensus-Based Principles 1, 3:

1. Avoid testing if it will not lead to a change in management
2. Testing is not recommended if the patient is not significantly more likely than the general population to have a significant abnormality
3. Testing may be appropriate in individual cases where excessive worry about serious pathology exists

Common Pitfalls to Avoid

• Do not reflexively order imaging on every migraine patient - typical migraine with normal examination has extremely low yield (0.2%) 4
• Do not use MRI when CT would be more appropriate for detecting acute hemorrhage 3
• Do not delay imaging when red flags are present - absence of red flags is less reliable than their presence 1
• Do not misdiagnose migraine with cranial autonomic symptoms as "sinus headache" 3
• Do not assume migraine can cause any neurological symptom - focal deficits warrant investigation 6
• Do not omit contrast when focal neurological findings are present on MRI - may miss enhancing lesions 6