Discussion
The GCS score has been adapted into categories of severity and are ubiquitous in the trauma setting and trauma treatment guidelines.5 17–19 In the standard GCS categorization, all patients are classified as a severe TBI based on GCS 3–8, moderate based on GCS 9–12, and mild based on GCS 13–15. The usage of GCS scores of 3 to 8 to denote severe TBI is simple to recall, but this categorization scheme assumes that all patients with GCS 3–8 fall into the same TBI severity regardless of age; this convention appears to be inappropriate. In this study of over half a million patients with TBI, we identified a significant interaction between GCS score and age for mortality following TBI and created revised GCS categories that account for the variability in prognosis by age. We propose that these revised categories will improve severity assessment and early prognosis of TBI based on the initial GCS score.
Our study objective was to create a straightforward revision of the GCS to take into account an age interaction. While these findings may be intuitive, the GCS has not previously been objectively recalibrated for this reason. The predictive performance of the revised GCS categories compared with the standard GCS categories, as measured by the AUROC, was statistically significantly improved. This study did not address whether this effect size is clinically meaningful; we suspect there exists a threshold to which the predictive performance of the GCS can increase because it is an imperfect predictor of mortality. Other studies have recognized the limitations of the GCS score,20–22 especially for risk prognostication.23–26
The majority of the TBI population presented to the ED with GCS scores at the extreme end of the range (GCS 3, GCS 15) where revision of the GCS categories was not affected. The revised GCS had the greatest impact on patients in their 70s and 80s, leading to recategorization of 25% of the population. One likely reason for this finding is that any deviation from a normal (GCS 15) at the initial assessment may be clinically meaningful in an older patient because they are more likely to have cerebral atrophy and occult findings due to blunted or delayed response to injury, or polypharmacy that masks the injury response.27 28 Alternatively, it might suggest their risk of death is not attributable to the TBI, but rather to their pre-injury health and frailty status and lack of compensatory mechanisms.29 30 Regardless of the reason, these insights might allow for more accurate evaluation of an elderly patient’s clinical status, potential outcome, and precise discussion and prognostication with patient’s family.
The revised GCS risk categories have implications for TBI research as part of study/trial selection criteria, as well as adjustment for baseline severity. For research studies or clinical trials of patients with severe TBI (GCS 3–8), younger patients will be disproportionately represented, and older patients will be underrepresented because their GCS scores are typically less severe. The GCS is commonly included in outcome assessment models as a means for adjusting the population based on head injury severity in order to make comparisons between groups of patients, but this adjustment may not lead to a meaningful correction among older patients. Finally, the GCS is used in risk prediction models, such as the revised trauma scale, the trauma injury severity score, and the APACHE II risk scoring systems. We suspect that current risk prediction models might be systematically misclassifying older patients due to reduced validity of the original GCS score over time.
The GCS categories are also used to guide clinical practice. For instance, the Brain Trauma Foundation (BTF) publishes guidelines for the management of severe TBI by synthesizing published studies of patients with GCS 3–8 and then translating the findings into recommendations.31 There already exists age disparities in following BTF guidelines.32 Our findings suggest this age disparity, or systematic bias, might be even more profound because the guidelines are based on a definition of severe TBI using GCS 3–8. Older adults with a true severe TBI may be underrepresented because their presenting ED GCS score may not reflect the severity of their injuries; conversely, younger adults may be overrepresented in severe TBI management guidelines, in that their TBI does not result in neurosurgical intervention or poor morbidity or mortality.
In selecting our study population, we excluded patients who were sedated, intubated, or chemically paralyzed at the time of their ED GCS assessment to improve accuracy, but this resulted in exclusion of a quarter (24.6%) of the TBI population. We performed a sensitivity analysis to test whether the results held true when this population was included in the development of the recalibrated GCS and examined in the validation dataset (online supplemental table 1). The majority of patients who were sedated, intubated, or chemically paralyzed presented to the ED with a GCS 3 but also had elevated mortality and were therefore properly categorized in the high-risk category, resulting in minimal change to the overall revised GCS categorization. The sensitivity analysis demonstrated that the revised GCS categories still provided significantly better discrimination for mortality than the standard GCS categories (c-statistic: 0.845 vs 0.822, p<0.001). Including these patients did not affect the results; still, we excluded them from the final analysis population for accuracy.
This study has limitations. First, the GCS has undergone minimal change since its creation in 1974. However, the GCS40 is an updated version of the GCS that incorporates a pupillary reactivity score that is subtracted (−2 when both pupils are unreactive), resulting in GCS40 scores ranging from 1 to 15. The GCS40 has been incorporated into the NTDB since 2019; additional validation (or recalibration) of the GCS categories will be required. Second, our findings are not meant to recommend changes to hospital protocols or algorithms, but to provide an effective way to convey mortality risk from TBI based on two major prognostic indicators: age and GCS score. We hypothesize that incorporation of the revised GCS categories into hospital guidelines may have an effect on outcomes, but this research needs to be conducted. Third, we used older NTDB data (admission year 2010–2015) for the analysis for consistency; the NTDB is currently updated through 2017, but the international classification of diseases changed from version ICD-9 to ICD-10 at the end of 2015. Fourth, the AIS is not a mandatory variable in the NTDB and was only documented for 17% of the population, which limited our ability to examine patients with isolated TBI and, conversely, patients with extracranial injuries. Fifth, we do not know the cause of death and if it was TBI related, nor do we know whether an unfavorable discharge destination was due to the TBI or an extracranial injury such as an orthopedic injury. Finally, the NTDB is a convenience sample of patients from over 400 trauma centers and is not nationally representative, as trauma patients treated at non-trauma community hospitals will be underrepresented and patients treated at children’s hospitals may be overrepresented.33