Injury of the Ascending Reticular Activating System in Patients With Fatigue and Hypersomnia Following Mild Traumatic Brain Injury: Two Case Reports

We report on patients with post-traumatic fatigue and hypersomnia who showed injury of the lower portion of the ascending reticular activating system (ARAS) between the pontine reticular formation (RF) and the intralaminar thalamic nucleus (ILN) following mild traumatic brain injury (TBI), using diffusion tensor tractography (DTT). Two patients with mild TBI resulting from a car accident were enrolled in this study. Patient 1 was a 51-year-old woman showed abnormalities as 6.9 (cut off: 3.7 points) and 18 (cut off: 10) on the Fatigue Severity Scale and the Epworth Sleepiness Scale at 11 months after onset. Patient 2 was a 64-year-old woman who revealed abnormalities on the Fatigue Severity Scale and the Epworth Sleepiness Scale with 6.8 and 19 at 3 months after onset. In both patients, the upper ARAS in which the neural connectivity of the ILN to the cerebral cortex did not show significant abnormalities. However, we observed the narrowing of the left dorsal lower ARAS between the pontine RF and the ILN in both patients and the tearing (patient 1) and narrowing (patient 2) of the left ventral lower ARAS between the pontine RF and the hypothalamus. Injuries of the dorsal and ventral lower ARAS were demonstrated in patients with fatigue and hypersomnia following mild TBI. We believe that these injuries of the ARAS might be a pathogenetic mechanism of fatigue and hypersomnia in patients with TBI.