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The Mayo Brain Injury Classification system: Is this tool fit for purpose?

9 July 2024   |   Gyrus Group

Identifying the severity and long-term impact of a traumatic brain injury is critical for legal proceedings.

Dr Doug McCorry and Dr Faith Borgan, in the first of an occasional series of posts, consider brain injury classification. They review how the Mayo Classification system was developed, and its limitations in determining the severity of traumatic brain injuries and predicting outcomes.

Why is this important for medicolegal practice?

Determining the severity and long-term impact of a brain injury is critical for medicolegal practice. The Mayo Classification System for Traumatic Brain injury (Malec et al., 2007) is commonly used by medical experts can help establish a brain injury. However, the descriptive terminology used in the system is not of predictive help to the court in understanding the extent of brain damage and the likely outcome.

How are traumatic brain injuries typically assessed?

Individuals who have suffered traumatic brain injuries will have documented features within their records, such as loss of consciousness, confusion, a Glasgow Coma Score, and brain imaging that may show the effects of the trauma. There are numerous available classification systems available that help combine these features. However, there is no uniformly accepted system in medical practice that is used by experts for legal reporting.

What is the Mayo classification system?

The Mayo classification system is a measure of traumatic brain injury that combines subjective and objective assessments, outlining three categories for injury classification, including Symptomatic (Possible), Mild (Probable), and Moderate-Severe (Definite) (Malec et al., 2007).

How was the Mayo classification system developed?

The Mayo classification system was developed in 2007 using medical health records from 1,501 patients, who had documented evidence of traumatic events. The tool was developed with the view of classifying injury severity from medical records alone. This tool was not developed to comprehensively assess injury severity, or the likely long-term functional impact of an injury.

When the tool was developed, it excluded patients experiencing symptoms indicating mild traumatic brain injuries, such as focal neurological symptoms, temporary confusion, blurred vision, and nausea, even if these symptoms occurred in the context of traumatic brain injuries. Authors did not include these symptoms when they developed the model because they are not only seen in the context of traumatic brain injuries. In addition, specific types of events that did not improve the predictions of the tool where excluded even if they provided clear evidence of traumatic brain injury, including seizures related to traumatic brain injuries, hydrocephaly, leakage of CSF, brain surgery, and retrograde amnesia.

As a result, the tool may not be fit for purpose for classifying specific types of injuries or mild brain injuries. In line with this, previous literature has shown that patients diagnosed with mild traumatic brain injury, as assessed using the Mayo classification system, exhibit functional outcomes of independence which are more consistent with a Moderate-Severe injuries (Garlander et al., 2018).

Another limitation of the tool is that combines the incidence of death and the presence of a 30-minute loss of consciousness into a single category called “Moderate-Severe” (Malec et al., 2007). As a result, the tool lacks sensitivity to be able to distinguish mild to severe head injuries, which is particularly important for medicolegal practice. Therefore, the e; the descriptive terms does not relate to expected recovery.

Another limitation of the tool is that it was developed based only on the presence of positive evidence included in medical records. This means that if medical records used to develop the tool found no evidence of a brain injury on a brain imaging scan, this information was not used to improve the predictions of injury severity when the tool was developed. This limitation also impacts the clinical use of the measure. If for example, a patient did not show evidence of a head injury on a brain imaging scan, a head injury might not have been considered. As a result, the other evidence to support the diagnosis of a head injury may not have been acquired or documented. Since the absence of positive events is arguably as important as the presence of positive events for determining injury severity, it’s likely that the failure to include all clinical data is likely to impact the accuracy, reliability and sensitivity of the tool.

What are the considerations for medicolegal practice?

Since the Mayo Classification system may be unable to determine the severity or long-term functional impact of a traumatic brain injury, injury severity classifications derived from this tool should be interpreted with caution. We will discuss other systems based upon the presence and extent of post-traumatic amnesia in this series.


Garlanger, K.L., Beck, L.A., Cheville, A.L.. (2018). Functional outcomes in patients with co-occurring traumatic brain injury and spinal cord injury from an inpatient rehabilitation facility’s perspective. J Spinal Cord Med, 41(6):718-730. doi: 10.1080/10790268.2018.1465744. Epub 2018 May 1. PMID: 29714644; PMCID: PMC6217473.

Malec, J.F., Brown, A.W., Leibson, C.L., Flaada, J.T., Mandrekar, J.N., Diehl, N.N., Perkins, P.K. (2007). The mayo classification system for traumatic brain injury severity. J Neurotrauma, 24(9):1417-24. doi: 10.1089/neu.2006.0245. PMID: 17892404.