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The Glasgow Coma Scale (GCS) is used frequently to define the extent of neurologic injury in patients with a traumatic brain injury (TBI). Whether age affects the predictive ability of the GCS for severity of TBI (determined by the Abbreviated Injury Scale [AIS] score) remains unknown.To investigate the effect of age on the association between the GCS and anatomic TBI severity.We examined all patients with a TBI, defined by diagnostic codes 850 to 854 from the International Classification of Diseases, Ninth Revision, Clinical Modification, who were admitted to 2 level I trauma centers from January 1, 2008, through December 31, 2012.We compared elderly (≥65 years) and younger (18-64 years) adults with TBI.We examined differences by age in GCS category (defined by emergency department GCS as severe [3-8], moderate [9-12], or mild [13-15]) at each level of TBI severity (head AIS score, 1 [minor] to 5 [critical]). Cochran-Armitage χ2 trend tests and stepwise multivariate linear and logistic regression models were used.During the study period, 6710 patients had a TBI (aged <65 years, 73.17%). Significant differences in GCS category by age occurred at each AIS score (P ≤ .01 for all). In particular, among patients with an AIS score of 5, most of the elderly patients (56.33%) had a mild neurologic deficit (GCS score, 13-15), whereas most of the younger patients (63.28%) had a severe neurologic deficit (GCS score, 3-8). After adjustment, the younger adults had increased odds of presenting with a severe neurologic deficit (GCS score, 3-8) at each of the following AIS scores: 1, 4.2 (95% CI, 1.0-17.6; P = .05); 2, 2.0 (1.0-3.7; P = .04); 3, 2.0 (1.2-3.5; P = .01); 4, 4.6 (2.8-7.5; P < .001); and 5, 3.1 (2.1-4.6; P < .001). The interaction between age and GCS for anatomic TBI severity remained significant after adjustment (estimate, −0.11; P = .005).Age affects the relationship between the GCS score and anatomic TBI severity. Elderly TBI patients have better GCS scores than younger TBI patients with similar TBI severity. These findings have implications for TBI outcomes research and for protocols and research selection criteria that use the GCS.