Recent research indicates that head impacts occur quite commonly during contact sports (most commonly in American Football), in which visible signs/symptoms of neurological dysfunction may not develop despite those impacts having the potential for neurological injury. The associated cumulative impact burdens over the course of a career are equally if not even more important. Clinical studies have also identified athletes with no readily observable symptoms but who exhibit functional impairment as measured by neuropsychological testing and functional MRI (fMRI) which are further corroborated by diffusion tensor imaging (DTI) and most excitingly via blood biomarker studies, demonstrating axonal injury in asymptomatic athletes at the end of a season.  This emerging biomarker data have demonstrated significant axonal injury, blood-brain barrier permeability, and evidence of neuroinflammation, all in the absence of behavioural changes.

Such data suggest that subconcussive level impacts can lead to significant neurological alterations, especially if the blows are repetitive. 1.


Nuts and Bolts

Brain trauma among football players of all codes (and athletes in other sports such as lacrosse and ice hockey), may be less the result of violent collisions that cause concussions than as the cumulative effect of repetitive head impacts (RHI).  The discovery has lead to increased calls by a number of experts to take steps at all levels of sports, from professional down to the youth level, to limit exposure to such repetitive trauma. A shrinking number (including the majority of US professional and college codes), continue to urge a more cautious approach until more is known or at least definitively researched.

Although scientists have long suspected that RHI caused brain damage, especially in boxers, a 2010 study of high school football players by researchers at Purdue University was the first to identify a completely unexpected and previously unknown category of players who, though they displayed no clinically-observable signs of concussion, were found to have measurable impairment of neurocognitive function (primarily visual working memory) on computerized neurocognitive tests, as well as altered activation in neurophysiologic function on sophisticated brain imaging tests (fMRI).1.

Indeed, researchers found, the players with the most impaired visual memory skills were not those in who had been diagnosed with concussions but were in the group which, in the preceding week, had experienced a large number of RHI (around 150 hits), mostly in the 40 to 80 g range of linear acceleration. (It is thought that rotational force is the BIGGER 'culprit').

​Number of hits and force levels aside, the take home message is that 'undiagnosed' and 'untreated' concussion may lead to serious problems if correct 'Return to Play' (RTP) decisions are not made.

In Summary

1. Escaping detection.  Because they have not suffered damage to areas of the brain associated with language and auditory processing, they are unlikely to exhibit clinical signs of head injury (such as headache or dizziness), or show impairment on sideline assessment for concussion, all of which test for verbal, not visual memory.

2. Didn't know they were injured.  If working memory deficits are sufficiently small, a player may not be aware of the additional effort required to complete everyday tasks, and therefore not think to bring the problem to anyone's attention. 

3. Facing an uncertain future.  Even though the players in the original Purdue study who suffered short-term cognitive impairment from repeated sub-concussive blows exhibited results on fMRI and ImPACT tests administered before season #2 comparable to the baseline results before season #1, their return to baseline did not necessarily mean that there was 100% recovery, as several of the subsequent Purdue studies demonstrated. It is possible that the damage will only be known over the long term; years later.

Side note

Patients with moderate and severe brain injuries often have focal deficits and occasionally profound brain damage. However, it should be noted that the severity of the initial injury does not correlate in a linear fashion with the severity of the brain damage. They may need ongoing cognitive and vocational rehabilitation, case management, and pharmacological intervention to return to their highest level of function.

Similarly, with 'mild' cases where level of blow does not correlate with level of injury.



1. Julian E. Bailes, et al. Role of subconcussion in repetitive mild traumatic brain injury. J Neurosurg. 119:1235–1245, 2013.

​2. Talavage T, Nauman E, Breedlove E, et. al. Functionally-Detected Cognitive Impairment in High School Football Players Without Clinically-Diagnosed Concussion. J Neurotrauma. 2010; DOI: 10.1089/neu.2010.1512.
3. Talavage TM, Nauman EA, Breedlove EL, et al. Functionally-detected cognitive impairment in high school football players without clinically diagnosed concussion. J Neurotrauma. 2014;31(4):327-338. doi:10.1089/neu.2010.512.