"We think this is how chronic traumatic encephalopathy starts," said McKee. "This is speculation, but I think we can assume that this would have continued to expand."
CTE has thus far been found in the brains of six out of six former NFL players.
"What's been surprising is that it's so extensive," said McKee. "It's throughout the brain, not just on the superficial aspects of the brain, but it's deep inside."
CSTE studies reveal brown tangles flecked throughout the brain tissue of former NFL players who died young -- some as early as their 30s or 40s.
McKee, who also studies Alzheimer's disease, says the tangles closely resemble what might be found in the brain of an 80-year-old with dementia.
"I knew what traumatic brain disease looked like in the very end stages, in the most severe cases," said McKee. "To see the kind of changes we're seeing in 45-year-olds is basically unheard of."
The damage affects the parts of the brain that control emotion, rage, hypersexuality, even breathing, and recent studies find that CTE is a progressive disease that eventually kills brain cells.