Washington, Jul 26 (IANS): A progressive, degenerative brain disease known as chronic traumatic encephalopathy (CTE) was found in 99 per cent of former National Football League (NFL) players whose brains were donated for research after their deaths, a study said.

The study, published in the Journal of the American Medical Association (JAMA), on Tuesday said CTE may be more common among football players than previously thought, Xinhua news agency reported.

"The data suggest that there is very likely a relationship between exposure to football and risk of developing the disease," Jesse Mez, a Boston University School of Medicine (MED) assistant professor of neurology and lead author of the study, said in a statement.

Mez and colleagues examined the donated brains of 202 former football players for evidence of CTE, which has been linked to repeated blows to the head.

Researchers believe CTE is caused by the buildup of an abnormal form of a protein called tau in the brain that can lead to brain cell deaths and a variety of clinical symptoms, including memory loss, confusion, impaired judgment, depression, suicidality and eventually progressive dementia.

Of the 202 brains studied, the group diagnosed 177 with CTE.

The study has several limitations including that it is a skewed sample based on a brain donation programme because public awareness of a possible link between repetitive head trauma and CTE may have motivated players and their families with symptoms and signs of brain injury to participate in this research.

As a result, the researchers urged caution in interpreting the high frequency of CTE in this study, stressing that estimates of how prevalent CTE may be cannot be concluded or implied.

They noted that the study -- the largest and most methodologically rigorous CTE case series ever published -- offers important information and direction for further research.

"I think the data are very surprising," said corresponding author Ann McKee, a MED professor of neurology and pathology and director of BU's CTE Centre. "We've sort of become accustomed to it, but it is very shocking."

The NFL said it contributed $100 million to neuroscience research last year in addition to the $100 million that the league and its partners are already spending on this area.