In this study, the relationship between GCS score during the chronic stage of TBI and the DTT results for the ARAS was investigated in patients who showed impaired consciousness at TBI onset. We observed the following: first, the FA values of the lower dorsal ARAS and the upper ARAS in patient subgroup A were lower than those of patient subgroup B and the control group; second, the FA value of the lower dorsal ARAS and the TV of the upper ARAS had moderate positive correlations with the GCS score obtained during the chronic stage of TBI in the patient group. Furthermore, the FA value of the upper ARAS had a strong positive correlation with the GCS score during the chronic stage in the patient group.
Among the DTT parameters, FA and TV are most commonly used in evaluating the status of neural tracts in patients with brain injury [19, 20]. The FA value represents the state of white matter organization by indicating the degree of directionality and integrity of white matter microstructures such as axons, myelin, and microtubules, with a low FA value suggesting a loss of white matter integrity [19]. The TV value is determined by the number of voxels included in a neural tract, thereby suggesting the total number of fibers within the tract [20]. Therefore, low FA and/or TV values for a neural tract indicate an injury of that neural tract [19,20]. As a result, in this study, the low FA values of the lower dorsal ARAS and upper ARAS in patient subgroup A (patients exhibited impaired consciousness during the chronic stage of TBI) indicate injuries of these neural tracts.
Regarding the correlation between GCS score and DTT parameters at the chronic stage in the patient group, we observed that the FA value of the lower dorsal ARAS and the TV of the upper ARAS had moderate positive correlations with GCS score, and the FA value of the upper ARAS had a strong positive correlation with the GCS score. These results suggest that the injury severities of the lower dorsal and upper ARAS were related to the level of consciousness in the chronic stage of TBI. In particular, compared to the lower dorsal ARAS, the upper ARAS was more closely associated with the level of consciousness [18].
Since the introduction of DTI, a few studies have demonstrated that the injuries of several neural structures such as the corpus callosum, internal capsule, thalamus, and brainstem were related to consciousness in patients with TBI [3-5]. A few case reports have demonstrated ARAS injuries in patients with impaired consciousness following TBI [12, 15, 16]. In 2013, Edlow et al. reported on a patient with coma following a severe TBI who showed complete disruption of white matter pathways connecting the brainstem arousal nuclei to the basal forebrain and thalamic nuclei, as well as partial disruption of the pathways connecting the thalamus and basal forebrain to the cerebral cortex [12]. Subsequently, Jang et al. [2015] demonstrated recovery of an injured lower dorsal ARAS concurrent with impaired consciousness in a patient with severe TBI [15]. In 2016, Jang et al. demonstrated the recovery of an injured upper ARAS with concomitant recovery of impaired consciousness in a patient with TBI and hypoxic-ischemic brain injury [16]. As a result, to the best of our knowledge, the present DTT-based study is the first to demonstrate a relationship between consciousness and the ARAS in a large number of patients with TBI. However, some limitations of this study need to be considered. First, brain regions with fiber complexity and crossing can prevent the full DTI visualization of the underlying fiber architecture; therefore, DTI may underestimate or overestimate the state of some fiber tracts [21]. Second, this retrospective study included a relatively small number of subjects. Third, because this study was performed retrospectively, we were not able to obtain the other neuropsychological data, except for GCS. Thus, prospective studies that include a larger number of subjects should be encouraged.