WASHINGTON -- A new study of brains donated after death details the degenerative brain disease that afflicted 68 of 85 subjects who suffered multiple concussions during stints in the military or in organized sports. Among the deceased athletes whose brains were examined for the study were NFL Hall of Famers John Mackey, a tight end, and running back Ollie Matson, both of whom died in 2011 of dementia complications.

Among those diagnosed post-mortem as suffering from chronic traumatic encephalopathy, 26 percent were considered suicidal at some point in their lives, and at least seven ultimately took their own lives, the study found. In addition to difficulties with attention, memory and judgment, most of the affected subjects whose brains were examined by experts at Boston University also suffered from explosivity, aggressive tendencies, paranoia and depression.

More than one in three of the people whose brains were examined in the current study also had a diagnosis of another degenerative disease of the brain, including Parkinson's Disease, Lewy body dementia, motor neuron disease, Alzheimer's disease or frontotemporal lobar degeneration.

The study came just days after the football world was rocked by the suicide of Kansas City Chiefs linebacker Jovan Belcher, after the player allegedly fatally shot his girlfriend, Kasandra Perkins. Although Belcher was reported not to have suffered concussions during his play with the Chiefs, he was briefly listed as injured with a head injury in November 2009.

Among the brains studied in the latest research, 35 came from men who had played football at the high-school-and-above level. Of those, all but one showed signs of the brain atrophy, protein tangles and cognitive difficulties that have become hallmarks of chronic traumatic encephalopathy, and researchers characterized 16 of those cases as being "stage 3" or "stage 4."

At that level of disease progression, the affected patient's brain would be shrunken and riven with tangles and inflammation, its ventricles would be swollen, and there would be gaps in the layers of fatty white tissue that conduct electrical signals from cell to cell and region to region. Of the 34 former professional football players whose brains were examined, 94 percent were reported to have suffered memory loss, problems with attention and concentration, and difficulty with organization and reasoning.

The latest research, which also found chronic traumatic encephalopathy in the brains of veterans of military service, is the first to detail four stages of the disease, and to distinguish its effect on the brain's structure from that of other diseases with similar symptoms. It was published Monday in the December issue of the journal Brain.

But the authors, who have pioneered work on brain trauma and degeneration in athletes, cautioned that they are far from fully characterizing the disorder: They still want to know who is most vulnerable to developing chronic traumatic encephalopathy, what overlap it may have with other brain diseases, and how common it is among a general population. The ability to diagnose the disorder in patients who are still alive is "an important next step," said co-author Robert Stern, a professor of neurosurgery at Boston University.

Emerging details of chronic traumatic encephalopathy have underscored that the cumulative effect of concussions -- even mild ones -- can be devastating over time. But co-author Robert Cantu of Boston University's Center for the Study of Traumatic Encephalopathy cautioned that it still may be a rare disease.

"While it remains unknown what level of exposure to brain trauma is required to trigger CTE, there is no available evidence that occasional, isolated or well-managed concussions give rise to CTE," Dr. Cantu said.

First Published: December 9, 2012, 5:00 a.m.