Subconcussive hits
Recent biomechanical studies suggest that athletes in collision sports such as football may experience over one thousand hits during the course of a single season,[26, citing studies] precisely the kind of "repeated sublethal brain trauma" a recent study suggests may ultimately lead in an unknown percentage of cases to CTE even if none, individually, leads to a formal diagnosis of concussion. [Indeed, the 2009 Randolph study [30] hypothesized that concussion per se was unlikely to be as signficant a factor in the development of age-related neurodegenerative diseases like MCI, AD and PD as such routine repetitive trauma.]
In a groundbreaking 2010 study [5, 46] by researchers at Purdue University, athletes repeatedly subjected to such so-called "sub-concussive hits" (particularly offensive and defensive linemen) were shown to have measurable short-term impairment of neurocognitive function (primarily visual working memory) on neurocognitive tests, as well as altered activation in neurophysiologic function on sophisticated brain imaging tests (fMRI), even though they displayed no clinically-observable signs of concussion,
Moreover, even though the players in the Purdue study who suffered short-term cognitive impairment from repeated sub-concussive blows appeared to fully recover cognitive function before the next season, exhibiting results on fMRI and neurocognitive tests comparable to their previous baseline scores, the Purdue researchers cautioned that return to baseline did not necessarily mean that there was 100% recovery.
The findings led Randall Benson, a neurologist at Wayne State University in Detroit, to speculate in Sports Illustrated [31] that the Purdue researchers may have taken what amounted to a "real-time snapshot" of the early stages of CTE, and that it was possible that the damage would only be known over the long term, years later.
Since publication of the first Purdue study, similar findings about the effects of RHI, both in the short- and medium-term, have been reported by many other researchers [45, 47-62], with several of the studies finding changes which persisted for weeks and even months after a football season ended.
Summarizing the state of the research in 2015, a study by researchers at the University of Virginia[[53] found that, in the short term, RHI has been "linked to increased susceptibility to concussion, decreased cogntitive function, altered gray matter functional connectivity, and changes in white matter microstructure," and that, in the long term, "retired football players who have sustained high levels of subconcussive impact over their careers have been hypothesized to have an increased risk of developing neurodegenerative disorders, like amytrophic lateral sclerosis [e.g. Lou Gehrig's disease], Alzheimer's disease, Parkinson's disease, and chronic traumatic encephalopathy (CTE)."
While the severity of CTE seems to correlate with the length of time engaged in the sport and the number of traumatic injuries, whether a single TBI can trigger the onset of CTE, and the number of concussions and/or amount of cumulative subconcussive trauma necessary to produce such pathology, remains unknown, leading some experts to conclude that, as of yet, "no reasonable basis exists to predict which athletes might be at risk other than perhaps to identify very broadly those involved in sports with exposure to repeated high-impact forces to the head (e.g boxing, American football)."[13]
It is important to note, however, that, while few studies have evaluated repeated concussive head trauma as a risk factor for later neurodegenerative disease, two studies that did failed to identify an increased risk of neurodegeneration among participants in contact/collision sports.[33,38]
A 2012 Mayo Clinic study of high school students who played football from 1946 to 1956 [33] found that, 50 years later, the did not appear to be at increased risk of later developing dementia, PD or ALS compared with their non-football playing high school male classmates.
The authors, however, were careful to note the many differences between today's high school football players and those of the distant past: while today's players have better equipment, trainers and physicians who are more knowledgeable about concussions, play under rules which prohibit - at least in theory - leading with the head when blocking and tackling, and may be marginally more likely to report concussive symptoms than players from a prior era, they also tend to be larger and quicker, increasing the force of impact, and wear helmets which, while "dramatically different from the marginally protective headgear of the earlier era, do not eliminate concussions and may give them a false sense of protection."
As a consequence of these differences, they cautioned that the results of their study, while they could be viewed as "somewhat reassuring to high school players from 50 years ago, they should give no reassurance to today's players."
A 1990 study [38] of individuals with Alzheimer's disease also found no association between risk of disease and participation in contact sports, although, as Jordan noted in his 2013 paper,[20] the robustness of the study's finding was limited by its small sample size.
