Chronic traumatic encephalopathy. After repeated concussions and hits to the head, the brain can change but this disorder is only possible to diagnose during an autopsy. It can cause major changes to personality, increased aggressiveness, impulsive behavior, poor judgment, suicidal ideation, so forth, plus major neurological defects as it progresses like dementia. Football players, hockey players and wrestlers are usually the most at risk groups since they take major blows to the head. You may remember Chris Benoit, the wrestler, who murdered his family and then committed suicide. He was found to have CTE and it has been floated as a possible cause for his behavior.
There are so many symptoms that are usually attributed to other causes like psychiatric disorders until it is found on an autopsy, but you have to know to look for it on that autopsy. Some athletes have tried to sue their former professional organizations claiming they likely have CTE, but they can't prove it. It also significantly decreases your lifespan in severe cases.