What is the purpose of the Canadian Head Computed Tomography (CT) scan?

The Canadian CT Head Rule: A Decision Tool for Identifying Patients Who Need Head CT Scans

The Canadian CT Head Rule is a highly sensitive clinical decision rule used to identify patients with minor head injury who require computed tomography (CT) scanning, with the goal of reducing unnecessary imaging while ensuring detection of clinically important brain injuries that may require neurosurgical intervention. 1

High-Risk Criteria (100% Sensitive for Neurosurgical Intervention)

The Canadian CT Head Rule consists of five high-risk factors that indicate when CT scanning is necessary:

1. Failure to reach GCS score of 15 within 2 hours of injury
2. Suspected open skull fracture
3. Any sign of basal skull fracture
4. Vomiting more than twice
5. Age greater than 65 years

These high-risk criteria are 100% sensitive for predicting the need for neurosurgical intervention while requiring only about 32% of patients to undergo CT scanning. 2

Medium-Risk Criteria (For Clinically Important Brain Injury)

The rule also includes two medium-risk factors:

1. Amnesia before impact >30 minutes
2. Dangerous mechanism of injury (e.g., pedestrian struck by vehicle, ejection from motor vehicle, fall from height >3 feet or 5 stairs)

When combined with the high-risk factors, these medium-risk criteria are 98.4% sensitive for predicting clinically important brain injury while requiring only about 54% of patients to undergo CT scanning. 2

Clinical Application

The Canadian CT Head Rule applies to patients with:

• Minor head injury (Glasgow Coma Scale 13-15)
• Loss of consciousness, amnesia, or witnessed disorientation
• Injury within the past 24 hours

Comparison with New Orleans Criteria

Both the Canadian CT Head Rule and the New Orleans Criteria have been validated as 100% sensitive for detecting injuries requiring neurosurgical intervention. However, they differ in specificity and potential CT reduction:

• New Orleans Criteria: Higher sensitivity for any traumatic finding (97.7%-99.4%) but very low specificity (3.0%-12.7%), reducing CT scans by only about 3% 1, 3, 4
• Canadian CT Head Rule: Slightly lower sensitivity for non-surgical traumatic findings (83.4%-87.2%) but much higher specificity (37.2%-50.6%), potentially reducing CT scans by about 37% 1, 3, 4

Application in Minimal Head Injury

Research has shown that the Canadian CT Head Rule can also be applied to patients with minimal head injury (no loss of consciousness or disorientation):

• In a study of 240 patients with minimal head injury, the rule was 100% sensitive and 29% specific for detecting intracranial hemorrhage 5
• All cases of intracranial hemorrhage occurred in patients who met either high-risk (age) or medium-risk (mechanism) criteria 5

Clinical Pitfalls and Limitations

1. Neither rule applies to patients on anticoagulants - these patients typically require CT scanning regardless of other criteria 1

2. Both rules require careful interpretation - studies show physician misinterpretation rates of 4.0% for the Canadian CT Head Rule and 5.5% for the New Orleans Criteria 4

3. Non-clinical factors often influence CT decisions - including physician reassurance (24.6%), patient reassurance (24.2%), patient expectation (14.6%), and legal concerns (11.7%) 5

4. The rules do not apply to all head trauma patients - specifically excluding those with unstable vital signs, obvious skull fracture, seizure, coagulopathy, or focal neurologic deficits 1

The Canadian CT Head Rule offers a standardized, evidence-based approach to head CT decision-making that balances the need to identify clinically significant injuries while reducing unnecessary radiation exposure and healthcare costs.