During the mid-1990s, he began to show emotional instability and developed insomnia that afflicted him until his death, the suit said. He awoke at 1 a.m. or 2 a.m. and couldn't fall back asleep.

He became forgetful about discussions and appointments, and he was unable to concentrate or focus, the suit said.

He exhibited "self-destructive, aggressive and violent behavior" and suffered extreme depression, withdrawing from his family and children, court documents said.

His children "would look into his eyes and not recognize the person with whom they were now dealing," the suit said.

Off the field, his one-time ability to show good business sense dissipated, and he made "impulsive, ill-advised decisions," the documents said.

He drank to cope, "entered a devastating cycle of depression and alcohol abuse," and became a manic, compulsive gambler in which he lost a "significant amount of money in an attempt to make back business losses," the lawsuit said.

Seau's family donated his brain to the NIH for research, and this month it released a statement saying "abnormalities were found that are consistent with a form of (CTE)."

According to the NIH's pathology report, five researchers -- two NIH neuropathologists and three independent experts -- examined slides of Seau's brain, and all confirmed that there were signs consistent with CTE. None of the researchers was aware that the brain they were examining was Seau's.

In a recent study, researchers found CTE in 34 of 35 deceased NFL players whose brains were donated by family members.

A brain with CTE is riddled with dense clumps of a protein called tau. Under a microscope, tau appears as brown tangles similar to dementia. However, the Boston study showed this progressive, tau protein array in football players much too young for a dementia diagnosis, which typically occurs in people in their 70s or 80s.