Concussions and Permanent Cognitive Harm — A Dose-Response Relationship

A large, long-term study shows a dose-response relationship with concussions — one is bad, three or more result in "significant long-term cognitive deficits"

What’s the Claim?

A long-term cohort study of more than 15,000 patients found that individuals who’d experienced at least one moderate to severe traumatic brain injury (TBI, defined as an injury causing loss of consciousness [LOC] of 30 minutes or longer) had poorer scores for attention, executive function, and cognitive processing speed than individuals who reported no prior head injury. Those who experienced one mild TBI (mTBI, defined as a head injury with LOC of <30 minutes or a dazed or confused episode) reported somewhat lower scores for attention, but no other detectable deficits. What’s most interesting about this study is the dose-response curve — with each mTBI episode, things looked a little worse; for example, those who reported three mTBIs had not just poorer attention scores (more so than the one mTBI group) but also poorer scores for executive function, and those with four mTBIs had even worse scores for attention and poorer scores for processing speed and working memory. Finally, these changes seem fixed — once they happen, they don’t seem to improve or worsen.

How’s It Stack Up?

This extends on what was known of the idea that once cognitive changes appear, they generally stay stable over time. The follow-up span in this study and its size make its findings more helpful and convincing than prior efforts on the same topic. It also appears to differ a bit from a smaller study that had a shorter follow-up period in terms of the cognitive domains most affected — that smaller study found memory loss to be one of the more pronounced findings after TBI, while memory loss was not so prominent here. This likely was an artifact of how memory was tested. In the earlier study, it was verbal memory, whereas here verbal memory was assessed under “executive function” and what this study called “memory” was more a kind of visual memory. This was a somewhat subtle distinction relative to the main, and most-interesting, finding — there is a large and convincing dose-response relationship associated with several smaller hits to the head. We note, though, that even the mild TBIs here were not what sometimes are referred to elsewhere as "subconcussive blows" of the kind that occur in collision sports. The “mild” TBIs in this study certainly were concussions by any definition.

What’s Our Take?

Team docs, this one’s for you. This robust and methodologically intense study, which most team physicians probably did not see because of where it was published, absolutely should influence how those team physicians counsel their athletes. Even one “real concussion” takes its toll, and each successive ringing of the bell after that makes things worse. Last word goes to the study’s authors, who noted that “each additional mTBI increases risk of substantial cognitive decline,” and that three or more resulted in “significant long-term cognitive deficits.”


Lennon MJ, Brooker H, Creese B, et al. Lifetime Traumatic Brain Injury and Cognitive Domain Deficits in Late Life: The PROTECT-TBI Cohort Study. J Neurotrauma. Published online January 27, 2023. DOI: 10.1089/neu.2022.0360.