Repetitive , low-grade head impacts among football players could impair eyeball, eyelid movement and oculomotor function which detects symptomless brain injury, according to the researchers of Indiana University.
But the function may adapt mid-season, even as athletes continue to incur head impacts.
"Repetitive subconcussive head impacts have quickly become one of the most complex public health issues," said Kei Kawata, assistant professor at the IU School of Public Health-Bloomington, who lead the study published in JAMA Ophthalmology.
Head injuries in sports remain a focus of researchers throughout the world. Much of this work focuses on understanding repetitive subconcussive head impacts -- hits that don't meet the criteria for a concussion diagnosis but still may have long-term effects.
The goal is to identify a "safe" threshold for subconcussive brain injury that does not necessarily trigger immediate symptoms, such as headache, dizziness and disorientation.
The study, a collaboration between Kawata and Steve Zonner, a sports medicine physician in Washington Township Medical Foundation, measured the near point of convergence -- the measurement of the closest point of focus before double vision occurs -- in more than a dozen high school football players over one season.
Near point of convergence was measured 14 times: a baseline measurement; a pre- and post-game measurement during six in-season games; and post-season follow-up. Players experienced more than 8,000 recorded hits over this period, and the researchers found a significant increase (up to 33 percent) in near point of convergence resulting from subconcussive head impact frequency, up to the middle of the football season.
However, near point of convergence began to move back toward the baseline measurement from mid-season on, even though players continued to experience subconcussive head impacts.
"Based on our previous studies, we thought that near point of convergence would be impaired throughout the entire season," Kawata said. "Instead, near point of convergence was normalized to the baseline by the last quarter of the season, when players play their hardest to make it to the playoff stage."
The researchers conclude that, while near point of convergence is one of the most prevalent clinical assessments for concussions, further study is necessary to determine whether it is the best tool in tracking long-term subconcussive brain damage.
Kawata and his partners are planning a larger-scale, longitudinal study to further investigate the use of oculomotor function testing on repetitive subconcussive head impacts.
"Our study further lays the groundwork for understanding the usefulness -- and limitations -- of convergence as a clinical biomarker for understanding acute and chronic subconcussion," Zonner said. "When evaluating the severity of brain damage, it is important to assess not only oculomotor function but also other neurological variables including fluid biomarkers and neuroimaging in the ultimate goal of brain injury prevention."