Two recent studies have refuted a commonly held belief that loss of consciousness or posttraumatic amnesia is a prerequisite for significant intracranial injury.

In a Class II study, Ibanez et al. prospectively studied 1,101 mild TBI patients older than 14 years who had a GCS score of 14 or 15.⁷ A comprehensive clinical variable data collection sheet was used, and all patients had a head CT regardless of loss of consciousness or amnesia. Of the 491 patients in this study who did not have loss of consciousness, 1.8% had an intracranial injury and 0.6% required neurosurgery.

In a Class II study, Smits et al. analyzed a prospectively collected database of mild TBI patients older than 15 years with a GCS score of 15.⁸ Of 2,462 patients, 754 had neither loss of consciousness nor posttraumatic amnesia. There was an 8.7% occurrence of an intracranial injury in those patients with loss of consciousness or posttraumatic amnesia, versus 4.9% in those without; the need for neurosurgical intervention was 0.4%, versus 0.5% in patients with no loss of consciousness or posttraumatic amnesia.

These studies have thus challenged the previously held tenet that loss of consciousness can reliably be used to decide which mild TBI patients require neuroimaging.

These studies serve to reinforce the instruction that although both the New Orleans Criteria and the Canadian CT Head Rule have been validated, they must be applied within the limits of their inclusion criteria, and the clinician should understand their sensitivity and specificity both for neurosurgical lesions and for intracranial injury. Specifically, these rules are valid when applied to patients who have had loss of consciousness or amnesia and who are not on anticoagulants.

2. Is there a role for head MRI over noncontrast CT in the ED evaluation of a patient with acute mild TBI?

• Level A recommendation. None specified.
• Level B recommendation. None specified.
• Level C recommendation. None specified.

There are no well-designed studies that specifically examine the use of MRI within 24 hours of injury in mild TBI patients. Therefore, at this time no evidence-based recommendations can be made regarding the use of MRI compared with CT in the ED setting. Most studies comparing CT results with MRI results in patients with TBI do not distinguish between mild TBI and more severe TBI.

Four studies were identified in which concussion patients with mild TBI can be isolated; the prevalence of abnormal MRI in these four studies ranged from 10% to 57%.^14-17 None of these studies demonstrated clear clinical relevance of these abnormal MRI scan results in patients with mild TBI, and none was conducted within a time frame relevant for ED disposition of patients.