Long-term Effects of Concussions on Cerebral Autoregulatory Mechanisms in Retired Contact Sport Athletes

Abstract

Increase in public awareness of the immediate effects of concussions has fueled researchers to better understand the injury from both a clinical and physiological standpoint in hopes to improve preventative and rehabilitation practices. However, there is very little information regarding the long-term change in the physiological systems in individuals with a history of previous concussions. Therefore, the purpose of this thesis was to explore how multiple concussions effected cerebral haemodynamics in the three major cerebral autoregulatory mechanisms in retired contact sport athletes. Specifically, three studies were conducted to focus on the different cerebral autoregulatory control mechanism. Study one (Chapter 3) focused on the neurovascular coupling mechanism (NVC), study two (Chapter 4) focused on cerebrovascular reactivity (CVR), and study three (Chapter 5) focused on dynamic cerebral autoregulation (dCA). 55 retired contact sport athletes, between the ages of 40-80 years of age (mean=59±8 yr), with a history of previous concussions were recruited with 29 non-contact athletes (mean =64±8 yr), with no history of concussion recruited as a control group. All participants underwent physiological testing using non-invasive near-infrared spectroscopy (NIRS). Each of the three protocols began with 5-minute of seated rest to determine physiological baselines. NVC was assessed using a 5-minute (20-second eyes closed:40-second eyes open x 5 repeats) object identification protocol (“Where’s Waldo”), CVR was assessed with a 5-minute hypercapnic challenge (20-second breath-hold:40-second normal breathing x 5 repeats), and dCA was assessed with a 5-minute squat-stand baroreflex maneuver (10-second squat:10-second stand; 0.05 Hz frequency). Results showed significant differences in all three mechanisms, primarily in the left prefrontal cortex between the previously concussed group (mTBI) and the healthy controls (CTRL). Similar increases were observed in HbDiff ΔMAX and O2Hb ΔMAX in the assessment of CVR and dCA but not seen in NVC. These findings provide evidence that there are autonomic nervous system dysregulation occurring in individuals with a history of multiple concussions due to evidence that cerebral autoregulation is impaired in all three autoregulatory mechanisms.