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Several sports are vulnerable to the impacts of concussions and a growing body of research is documenting its long-term effects. Governing bodies of American and European football (soccer), boxing, and mixed martial arts have implemented concussion protocols to deal with injuries to the head.

However, subconcussive injuries that do not result in loss of consciousness or other typical concussion symptoms often fly under the radar in team sports. The violent blows to the head of Jamie Vardy of Leicester City two weeks ago and Loris Karius, formerly of Liverpool, during last year’s Champions League final are cases in point.

Keisuke Kawata studied the effects of subconcussive blows to the heads of high school football players over the course of a season. He discussed his research with SCINQ.

SCIENTIFIC INQUIRER: What prompted you to do this study? What is the significance of subconcussive head impacts?

KEISUKE KAWATA: As awareness of chronic traumatic encephalopathy (CTE: a neurodegenerative condition unique to athletes and veterans) grew, researchers slowly began shifting their scope from concussive hits (that induce symptoms like headache, disorientation) to the importance of studying repetitive subconcussive head impacts happening in a various contact sports setting.

The problem of these head impacts is that athletes are asymptomatic, meaning they feel fine after head hits, but in microscale cellular and molecular standpoint, there are subtle but detectable levels of brain damage occurring from subconcussive head impacts. Because it is subclinical (they don’t have symptoms), they keep on playing and incur more head impacts. The cumulative subconcussive head impact effects, that we think, may be more harmful than a single concussion since, after a concussion, patients are obligated to go through step-wise recovery protocol to ensure they are fully recovered before returning to the full sports activity.

Two decades of research consistently underpin that the oculomotor system is sensitive to concussive trauma. And we were one of the very original groups to investigate the subconcussive effects on eye movement functions. The idea for the current study derived from our previous sequential publications (1,2): we introduced these studies in the introduction of the paper. These 2 studies were conducted in college-age athletes, so we needed to know whether, and to what extent, subconcussive head impacts influence adolescents eye movement function in a longitudinal setting (one season).

SI: Can you first discuss what near point of convergence is and its basis in the brain? How is it measured?

KK: The oculomotor system orchestrates accommodation and vergence, and their concomitant adjustments enable individuals to visualize an object at various distances and directions. Accommodation changes the shape of the lens by the contraction of the ciliary body, which enables accurate focus on an object, whereas vergence refers to the complex movement of both eyes, which is important for binocular vision. For example, to view an object moving back and forth directly in front of the left eye, only the right eye has to move. More precisely, convergence involves adduction of the eyes by contracting the medial rectus muscles, which are controlled by cranial nerve III (oculomotor nerve).

The near point of convergence (NPC) measures the closest point to which one can maintain convergence while focusing on an object before diplopia occurs. The strength of this test is very quick and simple, thus anyone can use this. That’s why NPC measurement is widely used in clinical settings (sports field, athletic training room, doctor’s office). We use an accommodative ruler and players were instructed to focus on a single letter (14-point font size letter). The tester slides down the target at a rate of 1-2cm/second. The NPC is measured when eye mal-alignment (left and right eye movement don’t synchronize) was observed by the tester or when the player verbally signaled once he experienced double vision or no longer perceived a single target.

Another strength of the NPC is that the test isolates oculomotor nerve function from other nerves that control a different direction of eye movements. The NPC does not involve cognitive function, so the result is purely reflective of the oculomotor nerve integrity.

SS: People are familiar with concussions, particularly with everything that has gone on in the National Football League. How does subconcussive head impacts differ from full concussions? Do they have a cumulative effect that, for lack of a better word, adds up to full on concussions down the road?

KK: The difference between concussion and subconcussion was discussed earlier, but very very simply put, when you hit your head and you have a headache or any kind of symptom, then you are diagnosed with a concussion, whereas if you have no symptoms from the hit, then it is considered as subconcussion or subconcussive head impacts. In my opinion, as subconcussion research grows, we are getting more confused as to what really is a concussion? Because if he/she lies to say “I’m totally fine” in fact they are feeling some symptoms, technically that person won’t be diagnosed with a concussion and continues to play game/practice.

To the second point whether subconcussive head impacts add up to be a concussion, a simple answer is “we don’t know just yet.” Concussion exerts symptoms, while subconcussive head impacts do not cause symptoms, no matter how many hits player sustains. A study (3) by Beckwith et al. indicates that subconcussive head impacts if received repetitively, make he/she susceptible to sustain a concussion. It means that concussion was diagnosed in those who received many subconcussive head impacts, while those who did not receive many subconcussive head impacts are less likely to be diagnosed with a concussion.

I believe subconcussive head impacts will weaken one’s resiliency to subsequent hits, so the more you sustain head hits, the more likely you develop concussive symptoms – then diagnosed with a concussion. I know it’s confusing…

SI: Can you describe how you designed your study?

KK: One of the absolute novelties of this study was the study design. We conducted the study in a span of single high school football season (Aug 8th to Nov 10th to be exact). For the entire season, players wore sensor-installed mouthguards (called Vector mouthguard) to detect frequency and magnitude of every hit from every practice and game.

During each game, we measured players’ heart rate and estimated excess post-exercise oxygen consumption (EPOC), which reflect physical exertion. This EPOC did not have any interaction with NPC change, by the way, so it’s a minor element for this paper. We collected NPC data from 14 different time points as listed in the right figure.

SI: What were your findings?

KK: The most astonishing finding of the study was that oculomotor nerve function tested by NPC showed a bell curve-like change during the season. NPC began worsening from the summer camp (August) toward mid-season (early October), which was correlated with the number and magnitude of head impacts players sustained up to the mid-season.

Surprisingly, from mid-season, NPC started to normalize to the baseline level and was almost completely recovered by the beginning of November when players were incurring the most frequent hits and greatest magnitude while trying to make it to the playoff stage. I found this oculomotor adaptational property to subconcussive head impact very fascinating.

SI: Can you speculate why your test subjects’ NPC begin to return to baseline after peaking somewhere around the midpoint of the season? Neuroplasticity?

KK: This is a difficult question to answer, given that we could not find any literature indicating any hints for this unique oculomotor response. When you think about other organs/systems (i.e., skeletal muscle, bone, immune system, cardiac muscle), they thrive when these organs are subjected to just enough stress that is above their natural state.

When you lift/workout, skeletal muscles experience soreness, followed by hypertrophy. Cardiac muscle increases its capacity when subjected endurance type of stimulus. Bones as well, it’s good to apply a stimulus to build strong and resilient bones, especially after bone fracture. The immune system, as you can imagine, fights with a new pathogen (cause fever, aches, etc), but after a while, it learns how to fight back some strain of pathogen because it builds resiliency from the first encounter.

Neuroplasticity is more so to form new neuronal connections to adapt to a new environment, information, situations and to compensate damaged/diseased neurons. One season of football subconcussive head impacts DO cause damage to neuronal axons, by increasing diffusion of molecules in and out of axons more freely than usual. Axonal damage is heightened in post-season compared to pre-season baseline and mid-season, showing a very different pattern from NPC. There are many gaps in knowledge, but if this is part of neuroplasticity against traumatic stress which is specific to oculomotor function, then our results are an absolutely novel initial discovery of such adaptation.

SI: Your study focused on a group of high school football players, teenagers essentially. Would you expect the normalization of NPC to differ, perhaps disappear, in professional football players who have had nearly two decades worth of subconcussive head impacts?

KK: This is another difficult one but very interesting question. No data on pro football players are available, but we did test NPC response in D-1 college football players (1) and found out that NPC recovered in a 2wk post-season follow up. I think early career (young) NFL players and D-1 college football players do not differ substantially, but long-career NFL players may respond differently from younger counterparts.

They continue to endure high frequency of subconcussive head impacts as they march on their professional career, while age effects begin to emerge. I would expect them to have a slower recovery of NPC than high school and college football players, or worse, they may continue to elevate NPC even in post-season follow up.

But one thing I have to stress is that based on our sequential findings in high school (4) and college football players, (1) we do not think NPC is a good measure to reflect chronic subconcussive neural damage, because at some point, it naturally comes back in most people, for whatever the reason may be. Instead, this study does call for a caution to clinicians that when they observe an increased NPC, they should not jump to conclusion that they have a concussion. We did confirm that not only concussion, but subconcussive head impacts can also impair NPC especially in the early phase of the contact sports season.

SI: Is NPC sufficiently robust enough to be used as a sole indicator?

KK: Clinicians should not rely on NPC as a sole indicator to diagnose both concussion and subconcussive neural damage. If anything, our study introduced more questions to the research society rather than providing a solution. NPC along with other objective measures can be a useful tool in helping to accurately diagnose brain injury IF they are used as a panel of tools. I encourage clinicians to build their panel or battery of tests for brain injury. Consumption of evidence-based literature can be difficult, but research evidence can give them a reassurance such that your tools/measures have been rigorously tested by researchers.

SI: What is next for you in terms of research?

KK: Next step on our horizon is to conduct a longitudinal study to track adolescent athletes for several seasons while measuring various objective markers including oculomotor testing like NPC and smooth eye pursuit, neuroimaging, and blood biomarkers. We are currently preparing to begin such a study from this summer. As I stated above, an evaluation of subconcussive neural damage requires an array of sensitive measurements. My team’s strength also lays on the use of blood biomarkers and neuroimaging. Collectively, we will be able to truly evaluate the value of NPC in reflecting subconcussive neural damage.


1. Kawata K, Rubin LH, Lee JH, et al. Association of Football Subconcussive Head Impacts With Ocular Near Point of Convergence. JAMA Ophthalmol. 2016;134(7):763-769.
2. Kawata K, Tierney R, Phillips J, Jeka JJ. Effect of Repetitive Sub-concussive Head Impacts on Ocular Near Point of Convergence. International journal of sports medicine. 2016;37(5):405-410.
3. Beckwith JG, Greenwald RM, Chu JJ, et al. Timing of concussion diagnosis is related to head impact exposure prior to injury. Med Sci Sports Exerc. 2013;45(4):747-754.
4. Zonner S, Ejima K, Fulgar CC, et al. Oculomotor response to cumulative subconcussive head impacts in high school football players: a pilot longitudinal study. JAMA Ophthalmol. 2018;In Press.

For more information about Keisuke Kawata and his research, visit his Indiana University page.

WORDS: Marc Landas

IMAGE SOURCE: Creative Commons

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