Repeated Head Impacts May Compromise White Matter Integrity and Bimanual Coordination in American Football Players

Background: Repeated head impacts (RHI) in American Football may contribute to subtle white matter (WM) abnormalities and functional deficits even in the absence of clinically diagnosed concussion. Hypothesis: Contact athletes would demonstrate tract-specific differences in fractional anisotropy (FA) and mean diffusivity (MD) compared with noncontact controls, and high-exposure (HE) athletes would show greater within-season diffusion change, worse bimanual coordination, and greater symptom burden than low-exposure (LE) athletes. Study Design: Prospective cohort study. Level of Evidence: Level 3. Methods: Twenty-two male varsity American Football players and 13 male noncontact controls underwent preseason diffusion tensor imaging. Football athletes were monitored over a single season using helmet-mounted accelerometers and stratified into HE and LE groups. Tract-specific FA and MD were derived using probabilistic tractography. Pre-post SCAT3 symptom and severity checklist data were collected from all football athletes, and pre-post Kinarm Ball-on-Bar (BOB) bimanual coordination data were available for 12 athletes. Results: At preseason, contact athletes demonstrated higher FA in 15 of 16 tracts compared with noncontact controls, with lower FA in the forceps minor. MD was lower in 10 of 16 tracts, higher in the bilateral inferior longitudinal fasciculus, and similar in the corticospinal tract and superior thalamic radiation. Over the season, HE athletes sustained more impacts ( P < 0.001) and greater cumulative g -force ( P = 0.001) than LE athletes. On the BOB task, LE athletes showed modest improvement, whereas HE athletes worsened, yielding a between-group difference in change of 0.59 (95% CI, 0.22-0.96; P = 0.005). Generalized estimating equation models accounting for within-athlete tract correlations did not detect HE-versus-LE differences in FA change (Wald χ²(1) = 0.14; P = 0.71) or MD change (Wald χ²(1) < 0.01; P > 0.99), and SCAT3 symptom and severity scores remained low with no meaningful between-group differences. Conclusion: In 22 contact athletes, repeated head-impact exposure was associated with preseason WM differences and worsening bimanual coordination across a single season, while longitudinal tract-level DTI and symptom-based clinical measures showed limited between-group change. Clinical Relevance: Objective motor assessment and advanced neuroimaging may help characterize subtle consequences of RHIs that are missed by routine symptom-based tools.