More than 70% of the brain is involved in vision, so it is no wonder that both mild traumatic brain injuries and traumatic brain injuries will often negatively affect the visual system. This is also true for other systems, such as the vestibular system, which relies on immediate and accurate visual input to function properly. Convergence insufficiency is the most common post-traumatic visual impairment, followed by focusing problems, and then eye movements, photosensitivity, vision motion sensitivity and visual processing speed. It is the effects on eye movement and tracking skills that we will concentrate on with this article.
Eye-tracking movements are also often referred to as smooth pursuits, which is a much more descriptive and accurate term. It takes incredible precision to keep the fovea of one eye fixated on a moving target, and yet under normal circumstances we are able to do that while keeping the second eye aligned within a degree or two of the first eye to prevent double vision. We can often track or follow targets of relatively small size up to 20 miles per hour before having to switch to jump or saccadic eye movements. The neuro pathways that control this process are quite complex and incorporate almost all the geographic areas of the brain. This results in an opportunity for a blow or injury to almost any portion of the head to negatively affect the process.
While the majority of discussion on eye movements rightly concentrates on the visual pathways and control of the six extra-ocular muscles, there is also a much bigger picture to consider. The extra-ocular muscles of the eye have the highest ratio of nerve fibers to muscle fibers in the entire body, which helps yield the required precision, but those nerve fibers also come from areas outside of the traditional visual pathways. We cannot forget the contribution of the vestibular system in the control of these eye muscles and the possible involvement of the neck and even torso muscles in keeping the eyes on target. On average, the eyes move approximately 17 degrees before the head, and possibly the other large muscle groups of the torso and legs turn to assist the eye muscles to keep the fovea on task. With today’s advancements in technology, we can accurately measure not only the speed, precision, and latencies of the eye movements, but also monitor the head movement, position, and stability using IMU’s and other electro-diagnostic devices.
Although the use of eye tracking has been around for quite some time, personally having done thousands of measurements on patients with reading difficulties and other developmental issues, it lacked the sampling speed and accuracy to analyze the subtle changes we desired. Today we have devices that sample at over 500 hertz instead of the original 30 hertz, with highly amplified and filtered signals that give us incredible detail. With these advances, it is no wonder that eye tracking is fast becoming a significant metric in the diagnosis and treatment of concussion and traumatic brain injury. Numerous studies have shown high sensitivity and specificity rates, including a 75.3% accuracy in determining concussions in the pediatric population. (1) It is not only a very sensitive and objective biomarker for diagnostic purposes, but also shows great promise in helping to determine when it is safe for a patient to return to play or be safely released from therapy.
About the Author:
Charlie Shearer, OD has been a nationally recognized leader in the areas of vision therapy, visually related learning problems and sports vision enhancement. He has worked with US Olympic athletes and teams, professional and amateur athletes, and coordinated national screening of both Special Olympics and Junior Olympics. Charlie is a leader in the field of instrumentation development as related to his role of Vision Science Consultant for Bertec Corporation, as well as his work with other companies. Charlie was born in Mishawaka, Indiana. He received his undergraduate and optometry degrees from Indiana University, upon graduation he returned to Mishawaka, Indiana, where he is in private practice. He has been a leader and innovator in of areas of pediatric vision, neuro-optometry, vision training and visual perceptual problems. He also has an interest in sports vision and continues to consult with the Colorado Rockies Major League Baseball team.
(1) Oculomotor Behavior as a Biomarker for Differentiating Pediatric Patients with Mild Traumatic Brain Injury and Age Matched Controls Melissa Hunfalvay1, Nicholas P. Murray2 *, Claire-Marie Roberts3, Ankur Tyagi1, Kyle William Barclay4 and Frederick Robert Carrick5,6,7
