To examine the effect of subconcussive head impacts on neuro-ophthalmologic function tested by the King-Devick test (KDT)

Subconcussive head impacts can lead to insidious neurological impairment if sustained repetitively. The oculomotor system is sensitive to brain trauma; however, neuro-ophthalmologic response to subconcussive head impacts remain unclear.

Sixty-eight healthy adult soccer players with at least 5 years of soccer heading experience were randomized into either a soccer heading group (n=36) or soccer kicking-control group (n=32). The independent variables were group (heading vs. kicking control) and assessment time points (pre-, 0hr-, 2hr-, and 24hr-post-heading/kicking). At each time point, KDT performance was assessed. Between the pre and 0hr post time points, heading subjects then performed 10 soccer headers, with the ball traveled at 25 mph, over the course of 10 minutes. The control subjects kicked the soccer ball 10 times. 

The heading group experienced a median linear head acceleration of 31.8g per head impact (IQR: 31.1-34.5g) and a median rotational head acceleration of 3.56 krad/sec² per head impact (IQR: 2.93–4.04 krad/sec²). Conversely, soccer kicks did not result in a detectable level of head acceleration. The kicking control group showed a significant improvement in KDT performance over time, indicating the neuro-ophthalmologic adaptability and learning effect (pre, 42.5±5.7sec; 0hr-post, 39.3±5.9sec; 2h-post, 38.5±5.8sec; 24h-post, 37.3±6.1sec). However, such improvement was blunted by repetitive soccer headings in the heading group (pre, 43.6±7.8sec; 0hr-post, 42.7±7.9sec; 2h-post, 42.6±7.8sec; 24h-post, 40.7±7.3sec), as illustrated by significant time x group interactions at 0hr (p=0.012) and 2hr (p=0.002) post-heading/kicking time points in the mixed-effect regression model.

Our data suggest that the neural circuit encompassing the brain and oculomotor system has tremendous abilities to adapt and improve its function if administered repeatedly, but subconcussive head impacts induced by soccer heading can transiently impair neuro-ophthalmologic function and its adaptive property.