JAMA Ophthalmology, March 2019
Repetitive subconcussive injury in athletes has become a major public health concern. Although most head injuries appear asymptomatic, they can have serious neurologic effects if sustained continually. The near point of convergence (NPC), denoting the closest point of focus before diplopia occurrence, has been shown to detect subclinical neuronal damage. Yet the longitudinal pattern of NPC changes due to subconcussive injury is unclear. Zonner et al. studied the NPC response to recurring subconcussive impact and found that initial disruption eventually led to adaptation of the oculomotor system to the subclinical brain injury.
The authors’ study included 12 U.S. varsity football players (mean age, 16.4 years) from a single high school, who were followed throughout a season. NPC assessments were made prior to the season, before and after six games, and when the season concluded. An embedded accelerometer mouth guard measured the frequency of impact to the head and the magnitude of impact from practices and games.
During the games, players wore chest-strap heart monitors to record heart rate and to estimate excess postexercise oxygen consumption, accounting for possible physical-exertion effects on NPC values. The players participated in practices and games with no restrictions.
During the football season, there were 8,009 head impacts, 177,907 g of peak linear acceleration, and 16,123,371 radians per second squared (rad/s2) of peak rotational acceleration. NPC rose significantly until midseason (5.25 cm at baseline vs. 6.42 cm before game 3; p = .01), which correlated highly with the frequency and magnitude of impact. However, NPC began normalizing toward baseline after midseason (5.75 cm before game 6; p = .32), despite the continuation of such injuries. A significant quadratic trend also was observed (β = −0.002 cm/d; p = .003).
These results indicate that although NPC can be perturbed for an initial period of repetitive subconcussive trauma, it may normalize over time, even with additional injury.
The authors acknowledged that the mechanism by which this apparent “tolerance” develops is uncertain and warrants exploration. (Also see related commentary by Ann C. McKee, MD, and Michael L. Alosco, PhD, in the same issue.)
The original article can be found here.