Head trauma: who needs a CT (Canadian CT head rule)
- Choosing a study
Most bumps to the head do not need a scan. Simple rules find the patients who do, and spare the rest radiation and waiting.
What it is
Non-contrast head CT is the study for acute head injury: it shows bleeding, fractures, swelling and herniation quickly. Patients with a moderate or severe injury (Glasgow 12 or less) or a neurological deficit get CT at once, together with imaging of the cervical spine. For minor head injury, clinical decision rules avoid scans that will be normal. The Canadian CT Head Rule applies to adults with Glasgow 13 to 15 who had loss of consciousness, amnesia or disorientation; it lists high-risk findings (need for neurosurgery) and medium-risk findings (brain injury on CT). PECARN does the same for children, with separate rules for those under and over two years, and offers observation as an option for intermediate risk. Rules do not apply to patients on anticoagulants, with seizures after the injury or a suspected abusive injury. MRI is a second step for symptoms that CT does not explain, such as diffuse axonal injury, and in the workup of suspected child abuse.
How to decide
- Canadian rule, high risk: Glasgow under 15 two hours after injury, suspected open or depressed skull fracture, any sign of basal skull fracture, two or more episodes of vomiting, age 65 or older.
- Canadian rule, medium risk: amnesia for more than 30 minutes before the impact, or a dangerous mechanism (pedestrian struck by a vehicle, ejection from a vehicle, fall from about one metre or five stairs).
- Signs of basal skull fracture: blood behind the eardrum, «raccoon eyes», bruising behind the ear (Battle sign), CSF leaking from nose or ear.
- PECARN under two years: altered mental status or a palpable skull fracture means CT; a non-frontal scalp haematoma, loss of consciousness of five seconds or more, a severe mechanism or a parent saying the child is not acting normally means observation or CT.
- PECARN two years and older: altered mental status or signs of basal skull fracture means CT; any loss of consciousness, vomiting, severe headache or a severe mechanism means observation or CT.
- Diffuse axonal injury: CT is often normal or shows small bleeds at the grey-white junction, corpus callosum or brainstem; MRI shows it better.
| Patient | Image now | Notes |
|---|---|---|
| Glasgow 12 or less, focal deficit, or deteriorating | CT head now, with cervical spine imaging | Call neurosurgery; no rule needed |
| Adult, minor head injury, any high-risk finding | CT head | Canadian rule: Glasgow under 15 at two hours, skull fracture signs, repeated vomiting, age 65 or older |
| Adult, minor head injury, medium-risk finding only | CT head | Long pre-impact amnesia or dangerous mechanism |
| Adult, minor head injury, rule negative | No CT | Written head-injury advice and a responsible adult at home |
| Adult on anticoagulants or with a bleeding disorder | CT head, low threshold | Rules exclude these patients; delayed bleeding can occur |
| Child, PECARN high risk | CT head | Altered mental status, palpable or basal skull fracture |
| Child, PECARN intermediate risk | Observe or CT | Decide with parents; worsening or several findings favour CT |
| Infant with unexplained injury or unclear history | CT head, then MRI brain and spine | Plus skeletal survey and eye exam for retinal haemorrhages |
| Persistent symptoms with normal CT | MRI brain | Diffuse axonal injury, small contusions, posterior fossa |
Common mistakes
- Applying the Canadian rule to patients it excludes: children, anticoagulated patients, seizures after injury, no clear history.
- Scanning every child with a minor bump: CT radiation matters more in children, and PECARN allows observation at intermediate risk.
- Forgetting the cervical spine in a patient who needs a head CT after significant trauma.
- Blaming alcohol for a low Glasgow score without scanning.
- Dating subdural bleeds in a possibly abused infant by CT density alone: ages overlap and need expert reading and MRI.
- Discharging after a normal CT without return advice: delayed bleeding and worsening can still occur.
Next step
- Rule positive: order a non-contrast head CT and assess the cervical spine with its own rule.
- Any bleed, fracture or swelling on CT: call neurosurgery and repeat the neuro exam often.
- Rule negative or normal CT: discharge with written warning signs (worsening headache, vomiting, drowsiness, weakness, seizure) and an adult to watch.
- Child at intermediate risk: observe for several hours; scan if symptoms persist or worsen.
- Suspected abusive head trauma: admit for safety, order skeletal survey and eye exam, MRI brain and spine, and notify child protection as local law requires.
- Normal CT with persisting deficit or coma out of proportion: ask for MRI and discuss with the radiologist.
Test yourself
Think of the answer, then tap the question to see it.
To which patients does the Canadian CT Head Rule apply?
Adults with minor head injury (Glasgow 13 to 15) who had loss of consciousness, amnesia or disorientation, without anticoagulation, seizure or obvious open fracture.
What are three high-risk findings of the Canadian CT Head Rule?
Any three of: Glasgow under 15 at two hours, suspected open or depressed skull fracture, signs of basal skull fracture, two or more vomits, age 65 or older.
In PECARN, what can be done for a child at intermediate risk instead of an immediate CT?
A period of observation, with CT if symptoms persist or worsen, decided together with the parents.
Which study shows diffuse axonal injury better than CT?
MRI of the brain, especially sequences sensitive to small bleeds, which show lesions at the grey-white junction, corpus callosum and brainstem.
Elsewhere in DurgiPedia
- Level of consciousness and the Glasgow Coma Scale
- Pupils: light reflex and abnormal pupils (Marcus Gunn, Horner, Argyll Robertson)
Connections
Educational content; it does not replace a radiologist's report and is not a clinical instruction. Each study is ordered on the patient's assessment and your institution's protocols.
Sources: Traumatic Brain Injury (StatPearls (NCBI Bookshelf), 2025); Pediatric Head Trauma (StatPearls (NCBI Bookshelf), 2024); Pediatric Abusive Head Trauma (StatPearls (NCBI Bookshelf), 2025).
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Representative model, for educational purposes only.