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Interesting research, and it's true that the pre-injury state tends to be overlooked, yet it is as important as what happens post-injury.
However I would like to draw attention to another cause of cognitive defects after concussion. This is post-traumatic hypopituitarism (PTHP). I am puzzled at the lack of attention it receives, because unlike chronic traumatic encephalopathy it is treatable, and growth hormone replacement in particular has been shown to improve cognition substantially.
 High WM et al, Effect of Growth Hormone Replacement Therapy on Cognition after Traumatic Brain Injury, Journal of Neurotrauma 2010, 27:1565-1575
 Reimunde P et al, Effects of growth hormone (GH) replacement and cognitive rehabilitation in patients with cognitive disorders after traumatic brain injury, Brain Injury 2011;25(1):65-73