Athletes who lose consciousness after concussions may be at greater risk for memory loss later in life, a small study of retired National Football League (NFL) players suggests.

Researchers compared memory tests and brain scans for former NFL players and a control group of people who didn't play college or pro football. After concussions that resulted in lost consciousness, the football players were more likely to have mild cognitive impairment and brain atrophy years later.

"Our results do suggest that players with a history of concussion with a loss of consciousness may be at greater risk for cognitive problems later in life," senior study author Munro Cullum, chief of neuropsychology at the University of Texas Southwestern Medical Center in Dallas, said by email. "We are at the early stages of understanding who is actually at risk at the individual level."

Cullum and colleagues recruited 28 retired NFL players living in Texas: eight who were diagnosed with mild cognitive impairment and 20 who didn't appear to have any memory problems. They ranged in age from 36 to 79, and were an average of about 58 years old.

All but three former athletes experienced at least one concussion, and they typically had more than three.

Researchers compared these men to 27 people who didn't play football but were similar in age, education, and mental capacity to the retired athletes, including six with cognitive impairment. These men were 41 to 77 years old, and about 59 on average.

To assess memory and cognitive function, researchers gave participants a common verbal test measuring how well they can recall lists of words and understand how the words are related. The tester might list fruits on a grocery list, for example, and then see if the participant could repeat all of the items and also understand that they are part of the same food group.

Athletes who had a concussion history as well as mild cognitive impairment got the lowest scores on this memory test.

In addition, brain scans in the study participants with mild cognitive impairment showed that the retired athletes in this subgroup had significantly smaller volume in the left hippocampus, a region of the brain involved in memory.

One drawback of the study is its reliance on players to report their own concussion history, the researchers acknowledge in JAMA Neurology.

"Retrospective recall of concussions by athletes is notoriously inaccurate," Dr. John Leddy, medical director of the University at Buffalo Concussion Management Clinic, said by email.

Another shortcoming is the study's use of an outdated system for rating concussions based on whether athletes lost consciousness, a common practice before the late 1990s that has since given way to a focus on the duration and severity of post-concussion symptoms, said Leddy, who wasn't involved in the study.

"Nobody finds it useful to grade concussions any longer because the initial treatment is the same for all concussions: remove immediately from at-risk activity," Leddy said.