https://en.m.wikipedia.org/wiki/Cerebrospinal_fluid
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I hope your friend is recovering well, and I wish them the best of luck in recovery. Brain injuries are frightening and frustrating, but the brain is amazing in its plasticity, too.
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[edit] for some reason, I can't reply to the comment below, so I'm posting my answer here, so you don't think I am ignoring your point. Personality change after TBIs is not a good thing. For a representative example: https://www.google.com/amp/s/www.psychologytoday.com/blog/pr...
Reminds me of a documentary I saw recently about snowboarder Kevin Pearce called The Crash Reel (2013) [1]. Kevin Pearce was changed by his TBI also but the one I came to think of when you said this was another person that was affected even worse from a TBI they got from a snowboarding accident.