Brain MRI findings associated with soccer heading: a systematic review and meta-analysis

Repetitive head impacts from soccer heading may cause subclinical brain alterations, but the magnitude and consistency of neuroimaging abnormalities remain uncertain.

A systematic review and meta-analysis of brain MRI studies in soccer players was performed. Diffusion MRI was the primary outcome; magnetic resonance spectroscopy (MRS) and structural MRI morphometry were secondary outcomes.

Thirteen independent studies were included. Five diffusion MRI studies (n≈300 players) were synthesised using Fisher's z. Soccer heading exposure was associated with significant white-matter microstructural alteration (pooled Fisher's z - 0.73, 95% CI - 1.02 to - 0.44), corresponding to an equivalent correlation of r≈ - 0.62 and a standard mean difference of approximately - 1.5. Between-study heterogeneity was moderate (I²≈68%). Individual studies reported increased radial or axial diffusivity (Hedges g up to + 3.00) or reduced fractional anisotropy (z range - 0.27 to - 1.40). Four morphometry studies showed small, non-significant differences in cortical thickness or volume (pooled Hedges g - 0.10, 95% CI - 0.22 to 0.10). Four MRS studies demonstrated predominantly increased choline- and myo-inositol-related metabolites (Hedges g ≈0·3-0·8), with minimal or inconsistent N-acetylaspartate change.

Soccer heading is associated with moderate-to-large effect sizes in diffusion MRI metrics of white-matter microstructure, whereas metabolic and structural MRI findings are smaller, less consistent, and of uncertain clinical significance.

This meta-analysis provides the first quantitative evidence that soccer heading is associated with a robust, moderate-to-large diffusion MRI signal of white-matter microstructural alteration. It further shows that diffusion MRI is more sensitive than spectroscopy or structural morphometry for detecting subclinical brain effects of repetitive head impacts.