Subconcussion in traumatic brain injury.

Recent scientific data and news stories in the lay press have challenged our ideas of how blows to the head in contact sports are diagnosed and treated. The classically held belief that concussed players can return to play as soon as they’ve regained their senses has given way to concepts such as “second impact syndrome” and “cognitive exertion.” Indeed, there is mounting evidence that mild brain trauma can result in changes in neural function and vascular reactivity far more extensive and long lasting than previously thought. Players suffering a concussion are now subject to much greater scrutiny by coaches, trainers, and medical staff. They must now undergo extensive neuropsychological test batteries, sideline tests, and detailed medical assessments before being allowed to return to the field of play. This is clearly to the player’s benefit and makes contact sports a safer venture for these athletes. Another issue associated with sports-related concussions is the topic in the following paper by Bailes et al., who begin with an examination of what constitutes a meaningful head injury.1 With current efforts aimed at reducing morbidity following concussion per se, it is theoretically possible that milder forms of head injury could have a deleterious effect on players, unrecognized under current criteria. These so-called subconcussive injuries are a bit tougher to accurately define. Not every incidental blow to the head is pathological. Bailes and colleagues define these injuries as cranial impacts or acceleration/ deceleration injuries that do not result in a clinically diagnosed concussion. They argue that it is the cumulative effect of these smaller blows that can cause neurological dysfunction. They review the pertinent laboratory and clinical evidence, arguing that subclinical impacts to the head should not be ignored in our efforts to make contact sports safer and that the number of such impacts should be tracked and minimized. This, of course, is a tall order. The impacts are extremely difficult to quantify, and not every subconcussive injury has the same significance. Is a soccer “header” of the same import as a nonconcussive blow to the head in a football game? Indeed, is every soccer header to be considered the same, or does a change in angular acceleration of the head make a difference in how significant this is from a subconcussive standpoint? We already face significant challenges in trying to keep track of actual concussive injuries in contact sports; tracking those events resulting in subconcussive forces is likely to be truly formidable. Moreover, the notion that the mere number of these events is the most important determining factor of long-term injury vastly oversimplifies matters; it is likely that there are genetic differences in the players themselves, placing certain individuals at higher risk of long-term neurological dysfunction than others. Still, the concept of repetitive subconcussive brain injury is an important one. Efforts to get a better handle on the number, types, and degrees of severity of these injuries will go a long way to improving our understanding and protecting our young players from long-term harm. Bailes and colleagues are to be commended for focusing our attention and providing the readership with a thoughtful synthesis of current evidence on this problem. (http://thejns.org/doi/abs/10.3171/2013.4.JNS13538)