Boston University researchers revealed Thursday that former New England Patriots tight end Aaron Hernandez had one of the most severe cases of chronic traumatic encephalopathy ever discovered in a person his age when he hanged himself in a Massachusetts prison in April. He was just 27 years old yet showed signs of Stage III CTE, a neurological disorder that afflicts people who have suffered numerous head injuries.

As described in a 2012 study of 85 brains by the Boston University neurology journal Brain, there are four stages of CTE. Hernandez’s case is so striking because his case was so advanced for someone his age.

Stage I

Symptoms: Mostly headaches and loss of attention and concentration.

Age of symptom onset in 2012 study: CTE can only be evaluated after a person has died, so the 2012 Boston University study had to rely upon medical histories and anecdotal evidence from the people who knew the deceased to determine when they first started showing symptoms of the disease. For Stage I CTE, first-symptom ages ranged from 17 to 30.

Stage II

Symptoms: Many of the Stage I symptoms are present here, only more pronounced. Plus, the subjects started having trouble managing their emotions.

Age of symptom onset in 2012 study: 26 to 47.

Stage III

Symptoms: Memory loss, executive dysfunction, explosivity and difficulty with attention and concentration.

Age of symptom onset in 2012 study: 27 to 63.

“In this age group, he’s clearly at the severe end of the spectrum,” Ann McKee, the head of BU’s CTE Center, said Thursday of Hernandez, who was in prison for the murder of Odin Lloyd in 2013. “There is a concern that we’re seeing accelerated disease in young athletes. Whether or not that’s because they’re playing more aggressively or if they’re starting at younger ages, we don’t know. But we are seeing ravages of this disease, in this specific example, of a young person.”

Stage IV

Symptoms: All of the above plus dementia and severe memory loss, language difficulties, aggressive tendencies, paranoia and gait and visuospatial difficulties.

Age of symptom onset in 2012 study: 35 to 83.

Hernandez was only 23 years old when he murdered Odin Lloyd, the crime for which he was serving time in prison, and he died at 27. People at that age usually are only showing the first signs of CTE, if they have it. In Hernandez’s case, the disease was much more advanced.

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