Diffuse Axonal and Vascular Pathology in the Gyrencephalic Brain after High-Energy Blunt Injury: Clinicopathological Correlations Involving the Brainstem

Abstract

Traumatic brain injury (TBI) after high-energy, behind helmet blunt trauma (BHBT) is an important but poorly understood clinical entity often associated with apnea and death in humans. In this study, we use a swine model of high-energy BHBT to characterize key neuropathologies and their association with acute respiratory decompensation. Animals with either stable or critical vital signs were euthanized within 4 h after injury for neuropathological assessment, with emphasis on axonal and vascular pathologies in the brainstem. The majority of cases were characterized by fractures of the cranium at or about the impact site, extensive subarachnoid hemorrhages, coup and contrecoup contusions, and primarily diffuse axonal and vascular lesions throughout the cerebrum, particularly in the brainstem. Absence of spontaneous respiration that was encountered frequently was associated with both severity of impact and the severity of brainstem axonal and vascular lesions. A focused regional examination of brainstem pathology indicated a link between adverse outcomes and diffuse axonal lesions within the medial medulla or vascular lesions within the anteroventral brainstem, a pattern suggesting that injury to brainstem respiratory centers may play a role in apnea following BHBT. In addition, while the overall burden of diffuse axonal and vascular pathologies correlated with each other, we found minimal overlap in their regional distribution. Our findings indicate that high-energy, blunt-force impact TBI causes diffuse lesions in axons and blood vessels associated with poor outcomes. They also suggest that axons and vessels may have distinct responses to tissue deformation and that commonly used markers of vascular pathology, for example, in diagnostic radiology, cannot be used as direct surrogates of diffuse axonal injury. In concert, our study underscores the role of regional axonal and vascular injuries in the brainstem in acute respiratory decompensation after high-rate blunt TBI, even in the presence of head protection; it also emphasizes the importance of detailed clinicopathological work in complex brains in the field of TBI.

Keywords: head injury; impact brain apnea; respiratory group; traumatic axonal injury.