Disclosure: This is about a video series on The Athletic. However, I believe it is unlocked to non-subscribers.
I just watched this video series (Outside the Lines-Style) about the concussion research at UNC. As someone in a research field adjacent to this, I know Kevin Guskiewicz's name - he is at the top of the field. Basically, UNC had a high number of football players diagnosed with ADHD. It is suspected that this was done in order to get athletes to stay eligible by allowing them access to learning aids or ADHD medication to help them keep grades up.
Subsequently, UNC football players were the subjects of many studies done on concussions over a number of years by the UNC brain research center. The investigation looked over all of these papers and there is no mention of excluding players with ADHD diagnosis. In one specific masters thesis, they mentioned that there were players with ADHD in the study and that it is a limitation off the study. When the study was published in a journal (which is where people would actually read it), it was not mentioned.
If this is true, this is a big deal. This research is used by the NFL, NHL, NCAA, and every other concussion researcher in the US. (The reason it's a big deal is because people with ADHD perform worse in concussion tests as shown by previous research by other groups). I saw a couple papers in the video with some researchers at VTs helmet lab listed in the author block. If the results of these studies aren't accurate, there could be a whole lot more that we don't know about concussions than we even realize. I highly recommend watching the videos (I believe there's also an unlocked article) if you have some time.