I had a mTBI a couple of years' ago, and still have some kind of post-amnesia type symptoms now. I also have aphantasia as a result - an inability to see anything in my mind's eye - which is pretty devastating given that I had a brilliant imagination before. 
I always worry about the risk of dementia later on as I really value my intelligence and mind - without it, I would feel very hopeless and depressed.
I hope to God that if or when the time comes, medical science will have found some kind of effective treatment. If they haven't, I'm signing up to Dignitas asap.