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Football players appear to have an increased risk of dying from neurodegenerative causes, although whether that's a result of repetitive brain trauma is uncertain, a retrospective study suggested.

Among a cohort of National Football League members who played between 1959 and 1988, the standardized mortality ratio for amyotrophic lateral sclerosis (ALS) was 4.31 (95% CI 1.73 to 8.87), according to Everett J. Lehman, MS, and colleagues from the National Institute for Occupational Safety and Health (NIOSH) in Cincinnati.

In addition, the mortality ratio for Alzheimer's disease was 3.86 (95% CI 1.55 to 7.95), the researchers reported online in Neurology.

Almost 2 decades ago, NIOSH examined the causes of death among players in the NFL, and noted that there had been four cases of ALS, but little further research was done until recent years, when a series of players were recognized as having severe brain dysfunction and atrophy following multiple concussions.

Research into these highly publicized cases of chronic traumatic encephalopathy (CTE) among athletes suggests that trauma may have had a role in the neurodegeneration experienced by the earlier NFL players, and that some deaths may have been misclassified because of the similarity of symptoms.

Although not included in the International Classification of Diseases as a cause of death, CTE is now considered a distinct condition in which neural function deteriorates, usually many years after repetitive concussions sustained during contact sports.

Affected individuals experience a loss of mental function, depression, and physical difficulties such as loss of balance. In the most disastrous cases, suicides have occurred.

To explore the overall mortality associated with neurodegenerative causes among these athletes, Lehman and colleagues identified 3,439 former players who had played for at least 5 years.

They noted that certain players are at particularly high risk for concussions "because of the high acceleration, rotational acceleration, and multiple impacts they experience during games."

The researchers classified these as "speed" players, such as quarterbacks, running backs, halfbacks, and linebackers, who "are those who are able to build up considerable momentum before the point of being tackled or tackling another player."

Among the overall cohort, 60% were white, median age was 57, and 62% were speed players.

A total of 10% of the cohort died. Compared with the overall U.S. male population, mortality was lower, with a standardized mortality ratio of 0.53 (95% CI 0.48 to 0.59), according to the researchers.

Neurodegenerative disease was classified as an underlying cause of death for 10 players -- two from Alzheimer's disease, 6 from ALS, and 2 from Parkinson's disease -- and as a contributing cause in 17 player deaths.

For all causes of neurodegenerative mortality combined, mortality was increased (SMR 3.26, 95% CI 1.90 to 5.22).

Further analysis determined that speed players had greater mortality than nonspeed players from all neurodegenerative causes, with a standardized rate ratio of 3.29 (95% CI 0.92 to 11.7).

Speed players also had higher rates of Alzheimer's disease and ALS, although these were not statistically significant.

"Although the results of our study do not establish a cause-effect relationship between football-related concussion and death from neurodegenerative disorders, they do provide additional support for the finding that professional football players are at an increased risk of death from neurodegenerative causes," the researchers stated.

Limitations of the study included the small number of deaths and the study population of exclusively professional players.

The researchers also had no specific information on concussions or other potential contributors.

In particular, genetics should not be overlooked, say other experts.

"Other risk factors for neurodegenerative disease weren't looked at in the study," said Murtaza Akhter, MD, of Emory University in Atlanta, who was not involved in the study but has an interest in traumatic brain injury.

"This could bring up multiple confounders, including the genetic one, which we know plays a role in neurodegeneration," Akhter wrote in an e-mail to 番茄社区app and ABC News.

"Additional studies to quantify the cumulative effects of brain injuries, in particular the relative effects of concussive-level injuries, will be of particular importance in understanding the underlying disease mechanisms," the researchers concluded.

Further research also should expand surveillance of current players, according to Louis Cornacchia, MD, of Brookdale University Hospital in Brooklyn, N.Y., who also was not involved in the research.

"There should be a multiple head trauma databank established to collect neurologic data at least biannually for all NFL players during and after their careers, with neurocognitive testing each year," Cornacchia said in an e-mail to 番茄社区app and ABC News.

This article was developed in collaboration with ABC News.

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