The Estimated Verbal GCS Subscore in Intubated Traumatic Brain Injury Patients: Is it Really Better?

Abstract

The Glasgow Coma Scale (GCS) has limited utility in intubated patients due to the inability to assign verbal subscores. The verbal subscore can be derived from the eye and motor subscores using a mathematical model, but the advantage of this method and its use in outcome prognostication in traumatic brain injury (TBI) patients remains unknown. We compared the validated "Core+CT"-IMPACT-model performance in 251 intubated TBI patients prospectively enrolled in the longitudinal OPTIMISM study between November 2009 and May 2015 when substituting the original motor GCS (mGCS) with the total estimated GCS (teGCS; with estimated verbal subscore). We hypothesized that model performance would improve with teGCS. Glasgow Outcome Scale (GOS) scores were assessed at 3 and 12 months by trained interviewers. In the complete case analysis, there was no statistically or clinically significant difference in the discrimination (C-statistic) at either time-point using the mGCS versus the teGCS (3 months: 0.893 vs. 0.871;12 months: 0.926 vs. 0.92). At 3 months, IMPACT-model calibration was excellent with mGCS and teGCS (Hosmer-Lemeshow "goodness-of-fit" chi square p value 0.9293 and 0.9934, respectively); it was adequate at 12 months with teGCS (0.5893) but low with mGCS (0.0158), possibly related to diminished power at 12 months. At both time-points, motor GCS contributed more to the variability of outcome (Nagelkerke ΔR²) than teGCS (3 months: 5.8% vs. 0.4%; 12 months: 5% vs. 2.6%). The sensitivity analysis with imputed missing outcomes yielded similar results, with improved calibration for both GCS variants. In our cohort of intubated TBI patients, there was no statistically or clinically meaningful improvement in the IMPACT-model performance by substituting the original mGCS with teGCS.

Keywords: Glasgow Coma Scale; estimated verbal GCS; motor GCS; outcome prognostication; traumatic brain injury.

FIG. 1.

Flow diagram of the final study cohort. We show the flow diagram of the final cohort of 251 intubated TBI patients from the OPTIMISM study. Of 393 consecutive OPTIMISM patients, 5 were excluded due to “hanging” as the trauma mechanism, 128 were excluded as they lacked intubation, and 9 were excluded as they were enrolled in the study early on before we had IRB approval to perform long-term follow-up. We had 199 patients with complete 3-month, and 185 with complete 12-month follow-up GOS available for the primary analysis. Multiple imputation methods were employed to impute the missing GOS for the sensitivity analysis. GOS, Glasgow Outcome Scale; IRB, Institutional Review Board; TBI, traumatic brain injury.

FIG. 2.

Receiver operating characteristic (ROC) curves. Shown are the ROC curves for the complete case analysis for the 3-month (top row, A–C) and 12-month outcomes (bottom row, D–F), and for the three different GCS variants (from left to right: mGCS, GCS_total, and teGCS). The corresponding C-statistic (area under the ROC curve) is shown under each graph. Notably, there is minimal or no difference in the ROC curves and the C-statistic between the GCS variants at each time-point. GCS, Glasgow Coma Scale; GCS_total, total GCS with the verbal subscore set as “1”; mGCS, motor GCS; teGCS, total estimated GCS.