Intensity- and Interval-Specific Repetitive Traumatic Brain Injury Can Evoke Both Axonal and Microvascular Damage

In the experimental setting several investigators have recently reported exacerbations of the burden of axonal damage and other neuropathological changes following repetitive traumatic brain injuries (TBI) that were sustained at intervals from hours to days following the initial insult. These same studies also revealed that prolonging the interval between the first and second insult led to a reduction in the burden of neuropathological changes and/or their complete elimination. Although demonstrating the capability of repetitive TBI to evoke increased axonal and other neuropathological changes, these studies did not address the potential for concomitant microvascular dysfunction or damage, although vascular dysfunction has been implicated in the second-impact syndrome. In this study we revisit the issue of repetitive injury in a well-controlled animal model in which the TBI intensity was bracketed from subthreshold to threshold insults, while the duration of the intervals between the injuries varied. Employing cranial windows to assess vascular reactivity and post-mortem amyloid precursor protein (APP) analysis to determine the burden of axonal change, we recognized that subthreshold injuries, even when administered in repeated fashion over a short time frame, evoked neither axonal nor vascular change. However, with an elevation of insult intensity, repetitive injuries administered within 3-h time frames caused dramatic axonal damage and significant vascular dysfunction bordering on a complete loss of vasoreactivity. If, however, the interval between the repetitive injury was extended to 5 h, the burden of axonal change was reduced, as was the overall magnitude of the ensuing vascular dysfunction. With the extension of the interval between injuries to 10 h, neither axonal nor vascular changes were found. Collectively, these studies reaffirm the existence of significant axonal damage following repetitive TBI administered within a relatively short time frame. Additionally, they also demonstrate that these axonal changes parallel changes in the cerebral microcirculation, which also may have adverse consequences for the injured brain.