Abstract
Mild traumatic brain injury and repetitive head impacts alter brain network functional connectivity. This review aimed to systematically analyze literature surrounding alterations in connectivity in functional networks after mild traumatic brain injury and repetitive head impacts, demonstrated with fMRI. A systematic review, adhering to PRISMA-guidelines was performed. Databases included PsycINFO, PubMed, SportDISCUS, Web of Science. 3,784 articles we returned from the databases. After inclusion screening, data were extracted from 98 articles, and analyses focused on within network connectivity. In the acute phase of injury (0-3 days), more studies presented within-network hyperconnectivity, compared to hypoconnectivity (12 hyper, 10 hypo). However, this ratio is inverted, with more studies presenting hypoconnectivity in both the subacute (14 hyper, 23 hypo) and chronic (18 hyper, 31 hypo) phases of mild traumatic brain injury. Additionally, there was hyperconnectivity observed in studies of athletes after a season of repetitive head impacts (9 hyper, 2 hypo). This finding was driven primarily by hyperconnectivity of the DMN (7 hyper, 2 hypo). Also, there was more evidence to support hyper-, in comparison to hypoconnectivity, between networks, at all phases of injury (11 hyper, 5 hypo). Finally, there were three studies that all report within-network and whole brain region hyperconnectivity in the frontal lobe simultaneous with hypoconnectivity in the parietal lobe and “posterior regions”. These findings indicate that increased or decreased connectivity in the whole brain and individual networks after mild traumatic brain injury or repetitive head impacts may provide an important biomarker for injury recovery and long-term functional deficits, thus aiding in the clinical diagnostic relevance of fMRI after mild traumatic brain injury and repetitive head impacts.