The effect of repetitive head impacts in sensory reweighting and human balance

Abstract

Repetitive subconcussive head impacts are common in contact sports such as football, ice hockey, and soccer. Subconcussive head impacts are mild head impacts that do not result in acute clinical signs and symptoms of a concussion but the exposure to repetitive head impacts (RHI) is suggested to cause significant current and future detrimental neurological effects. These RHI may be associated with short-term and long-term white matter microstructural changes and impaired cognitive performance, as well as later-life behavioral and mood changes. However, contemporary studies do not explore potential deficits in balance and sensory reweighting. Sensory reweighting is the process through which the central nervous system adapts the processing of a particular sensory input due to neurological injury or when environmental conditions change. For example, when visual cues are diminished after entering a dark room, the nervous system must increase its emphasis on somatosensory and vestibular information to maintain upright balance. The fusion of visual, proprioceptive and vestibular inputs (i.e., multisensory fusion) has been shown to play a key role in quiet standing balance in humans, and the lack of sensory reweighting is related to a central processing impairment. Following mild traumatic brain injury (mTBI), or concussion, there are deficits in sensorimotor function. In addition, previous research has suggested that even repetitive subconcussive head impacts may lead to subtle balance disturbances during standing. Specifically, our research group demonstrated vestibular dysfunction following subconcussive impact as evidenced by the diminished response to galvanic vestibular stimulation (GVS) while standing with eyes closed on foam and increased medial-lateral trunk displacement and velocity during treadmill walking after mild head impact. This disruption in vestibular processing could be an underlying mechanism of balance problems after head impact. This project investigated the effect of repetitive head impacts on sensory reweighting in college athletes that participate in contact sports. Understanding changes in sensory reweighting in this population may help in early brain damage detection and prevent future injuries through the development of better training and rehabilitation for those with such deficits. ☐ We studied sensory reweighting through a series of three experiments. The first and second experiments investigated the effect of soccer headings on balance control during walking (first experiment) and upright quiet stance (second experiment). For both experiments, the participants were randomly assigned in two groups: a soccer heading group and a control group. They were tested in three sessions: a baseline (Pre), a post soccer heading (post 0h) and 24h (post 24h) post soccer heading session. To assess whether vestibular processing was affected during walking following soccer headings, we attempted to isolate this system by having the subjects walk on a foam surface, blindfolded while perturbing the vestibular system with galvanic vestibular stimulation (GVS). We then calculated the response to GVS on balance mechanisms. No differences in GVS response on balance mechanisms post soccer headings were found. Our results suggest that an acute bout of soccer headings does not result in a balance deficit during walking. To study the effect of soccer headings in sensory reweighting in upright stance we implemented a multisensory fusion paradigm, consisting of simultaneous visual, somatosensory and vestibular perturbations. The response of the trunk/leg segment movement was calculated relative to each modality (i.e., gain), pre and post soccer headings. This experiment showed no alterations in sensory reweighting pre and post soccer heading. To understand the effect of the continuous practice of collision sports and the effect of repetitive subconcussive head impacts in collegiate athletes, our third experiment used the same multisensory paradigm. We compared trunk/leg gains relative to each sensory modality between collision sports players and no contact players. No differences were found in sensory reweighting between collision and non-contact sports athletes. Our result suggests that RHI are not sufficient to change sensory reweighting and balance in collegiate athletes. We believe that head impact tolerance might play a role in our results and more studies should be conducted to understand the effect of repetitive head impact force and frequency on human balance.