Évaluation multimodale de l’impact de coups répétés à la tête chez des athlètes sans historique de commotion cérébrale

Now that the deleterious effects of sport-related concussions on brain integrity are better understood, the question arises as to whether the practice of a contact sport, through repeated exposure to head blows, could be associated with similar effects even in the absence of a concussion. With neuropsychological assessment and magnetic resonance imaging methods, the main objective of this thesis was to assess the cognitive, metabolic and microstructural effects of repetitive head impact exposure in university-level soccer and rugby players without a history of concussion. A second objective was to determine whether the response to so-called subconcussive impacts could be modulated by the effects of physical fitness on the brain. To do so, three groups were compared: athletes practicing a contact sport (soccer and rugby, A-CS), athletes practicing a noncontact sport (swimming, A-NCS), and non-athletes (NA).  Three papers are presented in this thesis. The first one is a systematic review of the literature reporting the effects of concussions on the corticospinal and intracortical excitability of the primary motor cortex (M1), measured with transcranial magnetic stimulation. The description of previous studies highlighted the existence of specific neuronal mechanisms at the neurochemical level, and helped guide hypotheses for study 2.  The second paper compares the three groups of participants on neurometabolic (magnetic resonance spectroscopy; MRS), neurological (susceptibility-weighted imaging; SWI), and cognitive measures.  MRS revealed significantly elevated levels of myo-inositol in the M1 of the A-CS group compared to A-NCS and NA. Furthermore, in the A-CS group, decreased levels of prefrontal glutamate+glutamine (Glx) and GABA were observed compared to A-NSC only. No significant differences were found between groups for microhemorrhages and for attentional, executive and memory function. However, a positive correlation was observed between Glx concentration and performance at a memory task in A-CS.  The third paper focuses on white matter microstructure as measured by diffusion weighted imaging (DWI). Overall, the A-NCS group exhibited significantly higher fractional anisotropy (FA) values than the A-CS group in the corpus callosum and the corticospinal tract, and in the anterior section of the corpus callosum compared to the other two groups.  No significant correlation was found between FA values and cognitive tasks.  In chapter 5, additional data that was not included in the experimental papers are presented. First, cortical thickness data showed no significant differences between the three groups. Second, exploratory intersex analyses were performed. MRS data showed lower NAA concentrations in female A-CS compared to female A-NCS.  DWI data showed significant intergroup differences in men only for various segments of the corpus callosum.  In sum, exposure to repetitive subconcussive hits to the head in university-level contact sports is associated with metabolic and white matter microstructural alterations. However, the overall lack of metabolic and structural difference between the A-CS and NA groups, as well as the lack of differences in the cognitive profile of the three groups suggest limited short term clinical significance of repeated exposure to head blows in the absence of a history of concussion.

Keyphrases: Cognition, Subconcussive hits to the head, Sport-related concussion, Physical fitness, University-level athletes, Magnetic resonance imaging, Magnetic resonance spectroscopy, Diffusion weighted imaging