Glasgow: The Power Is in the Motor

Abstract & Commentary

Source: Healey C, et al. Improving the Glasgow Coma Scale score: Motor score alone is a better predictor. J Trauma 2003;54:671-680.

The Glasgow Coma Scale (GCS) has served as an assessment tool in head trauma, as a measure of physiologic derangement in outcome models, and often is used to rapidly assess neurologic status. Its value as a predictor of survival never has been prospectively validated. The authors used a large trauma data set (National Trauma Data Bank, N = 204,181), and compared the predictive power and calibration of the GCS to its component scores (motor, eye, verbal). The authors discovered that different combinations summing to a single GCS score often have very different mortalities. For example, the GCS score of 4 can represent any of three motor/verbal/eye combinations: 2/1/1 (survival 0.52), 1/2/1 (survival = 0.73), or 1/1/2 (survival = 0.81). In addition, the relationship between GCS score and survival is not linear, but decreases linearly from a GCS of 15 to 11, remains unchanged to a score of 7, and then decreases linearly again to a score of 3. The motor component of the GCS, by contrast, not only is related linearly to survival, but also preserves almost all the predictive power of the GCS. The authors conclude that the motor component of the GCS contains virtually all the information of the GCS itself, offers advantages over the other components (e.g., can be measured in intubated patients), and is much better behaved statistically than the GCS. They further state that the motor component of the GCS should replace the GCS in outcome prediction models.

Commentary by Richard J. Hamilton, MD, FAAEM, ABMT

The GCS is a score from 3 to 15, right? Wrong! It’s actually a collection of 120 different combinations of neurological abnormalities. A GCS of 13 could be someone who withdraws to pain, or speaks in incomprehensible words, or opens his or her eyes to painful stimuli, or is confused and only opens his or her eyes to speech. However, it’s hard to imagine that every single one of those patients has the same survival rate or severity of outcome. In fact, the 120 combinations end up being represented by a few scores with greater frequency than the others because of the specific pattern with which trauma patients deteriorate as measured by the scale. In this database of 204,181 patients, 80% of patients had a GCS of 15, 6% had a GCS of 14, 6% had a GCS of 3, and the rest of the scores were represented with a frequency of 1% or less.

Thus, most patients exhibit a score of 15, 14, or 3. Furthermore, this study demonstrates that survival is the same for GCS scores in the range of 7 to 11, although it does decrease linearly (as expected) for all other scores. Thus, it appears that GCS is a good tool only for predicting outcome when the score lies between 11 and 15 or between 3 and 7. The differences in the middle scores are meaningless.

Why is this? It turns out the strength of the correlation with survival is exclusively in the motor score. As the motor component goes from 6 to 1, survival decreases linearly. As the eye and verbal scores decrease, survival remains the same until the lowest possible score. The original authors of the GCS first intended it to be a three-score system, but later modified it to be a complete additive score. They never had a large number of patients to prospectively validate their findings. For example, when the score goes below 8 in a head trauma patient, we historically have taken that as a predictor of a bad outcome and instituted airway intervention. This probably makes little sense, because according to this large analysis, a score of 8 is no worse than a score of 7, 9, 10, or 11. One clearly can see the confounding issues (intoxication or behavioral abnormalities) in the verbal and eye components and why the motor scale makes clinical sense as well as statistical sense as a useful tool alone.

In conclusion, if you’re using the GCS, you had better pay more attention to the motor scale, or use the motor scale exclusively. Every time the patient drops a point on that scale, it means something important.

Dr. Hamilton, Associate Professor of Emergency Medicine, Program Director, Emergency Medicine, MCP Hahnemann University, Philadelphia, PA, is on the Editorial Board of Emergency Medicine Alert.