When is a cranial CT (Computed Tomography) scan recommended for a patient with a headache?

When to Perform Cranial CT in Patients with Headache

Cranial CT scanning is indicated for headache patients when specific "red flag" clinical features are present that suggest life-threatening secondary causes, particularly to detect subarachnoid hemorrhage, intracranial bleeding, mass lesions, or other acute neurosurgical emergencies. 1

Primary Indications for Cranial CT

Acute Emergency Situations

Noncontrast CT is the cornerstone imaging modality for thunderclap headache (sudden severe headache) due to its 98% sensitivity and 99% specificity for detecting acute subarachnoid hemorrhage (SAH). 2 CT is preferred over MRI in this acute setting because of its superior ability to detect acute blood products, rapid availability, and lack of need for sedation. 2

Red Flag Clinical Features Warranting CT

CT scanning should be performed when any of the following high-risk features are present:

• Age ≥60 years - This demographic factor alone increases risk of intracranial injury and warrants imaging 3
• Thunderclap headache - Sudden onset of severe headache reaching maximum intensity within seconds to minutes 2
• Headache awakening patient from sleep - Nocturnal headaches suggest increased intracranial pressure 1
• Progressive worsening over 3 weeks - This pattern justifies imaging even with normal neurological examination 1
• Headache worsened by Valsalva maneuver - Suggests increased intracranial pressure 1
• Focal neurological deficits - Any abnormal neurological examination finding mandates imaging 1, 4
• Altered mental status or confusion - Including "foggy sensation" or difficulty concentrating 3
• Signs of increased intracranial pressure - Such as papilledema 1
• History of head trauma - Particularly in elderly or anticoagulated patients 3

Clinical Presentation Patterns

In patients presenting to the emergency department with headache and clinical warning criteria, CT detects intracranial pathology in approximately 13-19% of cases. 5, 6 A multivariate analysis identified four factors significantly associated with abnormal CT results: age ≥50 years, presentation within 1 hour of headache onset, clinical history of aphasia, and focal neurological deficit. 5

When CT is NOT Indicated

Migraine with Normal Neurological Examination

Neuroimaging (either MRI or CT) is not usually warranted in patients with migraine who have normal findings on neurological examination, as the prevalence of significant intracranial abnormalities is only approximately 0.2%. 1 This represents the same risk as the general asymptomatic population. 1

Primary Headache Disorders

In patients with typical features of primary headache disorders (migraine, tension-type headache) and completely normal neurological examinations, routine CT scanning has extremely low yield. 1 Studies show that less than 1% of such patients have relevant findings to explain their headaches. 1

CT vs MRI Decision-Making

When CT is Preferred

• Acute hemorrhage detection - CT has 98% sensitivity for acute SAH, superior to MRI 2
• Emergency settings - Faster acquisition, more readily available, no sedation required 2
• Suspected acute intracranial bleeding - CT without contrast is the gold standard 2
• Rapid triage needed - When immediate neurosurgical intervention may be required 2

When MRI is Superior

MRI is more sensitive than CT for detecting subtle findings adjacent to calvarium or skull base, small white matter lesions, and chronic sequelae of previous injuries, but may be no more sensitive than CT for clinically significant acute pathology. 2, 1 However, for acute SAH detection specifically, CT remains superior. 2

Special Populations Requiring Lower Threshold for CT

• Anticoagulated patients - Risk of intracranial hemorrhage increases from 1.5% to 3.9% 3
• Patients on antiplatelet agents (beyond aspirin) - Higher bleeding risk 3
• History of previous aneurysm or cerebral venous thrombosis - Warrants imaging despite normal initial evaluation 7
• Pediatric patients with sickle cell disease - Higher risk of acute CNS events 1

What CT Evaluates For

When CT is performed for headache, it specifically investigates for:

• Subarachnoid or intraparenchymal hemorrhage 1
• Brain tumors or metastases 1
• Hydrocephalus 1
• Cerebral venous thrombosis 1
• Mass lesions requiring urgent intervention 1
• Signs of increased intracranial pressure 1

Common Pitfalls to Avoid

• Do not image patients with typical migraine and normal neurological examination - This leads to unnecessary radiation exposure and detection of clinically insignificant incidental findings 1, 8
• Do not misdiagnose "sinus headache" - This is commonly a misdiagnosis among migraine sufferers, and neuroimaging is not indicated 1
• Do not delay CT when red flags are present - Age ≥60 years or severe headache alone warrant imaging 3
• Do not use MRI when acute hemorrhage is suspected - CT is superior for detecting acute blood products 2

Evidence-Based Principles

The U.S. Headache Consortium advocates two key principles:

• Testing should be avoided if it will not lead to a change in management 1
• Testing is not recommended if the patient is not significantly more likely than the general population to have a significant abnormality 1

In clinical practice, approximately 61% of ED headache patients undergo cranial CT, which detects underlying pathology in 1 in 7.6 patients (13%). 5 This relatively high imaging rate reflects the difficulty in differentiating benign from dangerous headaches based on clinical features alone, but adherence to evidence-based red flag criteria can optimize appropriate utilization. 5, 6