The modern version of the Glasgow Coma Scale, often referred to as GCS-P (Glasgow Coma Scale with Pupil reactivity), is a crucial prognostic update to the standard neurological scale used to objectively assess consciousness level, especially after traumatic brain injury. The original GCS score assesses three components: Eye Opening (E, scored 1-4), Verbal Response (V, scored 1-5), and Motor Response (M, scored 1-6), resulting in a total score from 3 (indicating deep coma) to 15 (fully conscious). The innovation of the GCS-P is the direct integration of pupillary reactivity to light, a vital indicator of brainstem function. After determining the GCS score (sum of E+V+M), a 'Pupil Score' (P) is calculated by subtracting the number of non-reactive pupils from the GCS score. Alternatively, and more commonly for prognosis, a Pupil Reactivity Score (PRS) is determined (0 = both pupils reactive; 1 = one pupil non-reactive; 2 = both pupils non-reactive). The final GCS-P score is then calculated as GCS - PRS. For example, a patient with a GCS of 5 and one non-reactive pupil (PRS=1) would have a GCS-P of 4. A patient with a GCS of 3 and both pupils non-reactive (PRS=2) would have a GCS-P of 1, the worst possible score. This combined approach has been shown to be a significantly more accurate predictor of mortality and unfavorable neurological outcome than the GCS score alone, as it quantifies the added impact of brainstem injury.