Concussion is a mild form of traumatic brain injury (mTBI). It is often insidious due to the lack of physiological finding on MRI, CT, or other imaging. Concussion is defined as an immediate acceleration and deceleration or stopping event, resulting in temporary or permanent damage to the “structures” of the head and can produce extensive diffuse axonal injury (DAI). This injury is likely associated with low levels of axonal stretch resulting in temporary changes in neurophysiology[i]. It is well established in the literature that mTBI can produce reduced activities of daily living (ADL), poor reading (eye movement), cognitive, balance, and other difficulties[ii].
All eye movements are cortical responses that are integrated onto subcortical pathways enabling a person to voluntarily and involuntarily put both eyes where they want to in a binocular and efficient manner. These systems are well integrated in the athlete population. The pathways themselves are not disturbed in concussion, but the functional outcomes are decreased due to the interactions with the other sensory systems that build our visual, auditory, vestibular, proprioceptive, and tactile spatial map. The subcortical pathways that have cortical integration are able to produce useful eye movements due to the vestibulo-ocular reflex (VOR) and optokinetic nystagmus system (OKN) which consists of smooth pursuit and saccades. The development of these systems starts with subcortical pathways that are reflexive at or near birth and develop into voluntary cortically controlled movements up through our teens and later with normal development.
VOR – The first and fastest ocular response is the VOR. It has a latency of about 10-16ms. The job of this reflex is to keep the eyes pointing in a position for short periods of time with head movement. The gain of the VOR is the amount of perceived motion to actual motion and it decreases as cortical control develops. The gain drops from 1.0 at or near birth to 0.6-0.8 in adult like responses. What this means is that the subcortical process gives us a 1:1 motion to reaction ratio at birth and 1:0-6-0.8 motion reaction in adult like cortical control. This reaction is subcortically driven by the vestibular system early on in development and is cortically overridden or integrated by the visual system when adult like responses occur.
OKN – The OKN (140ms latency) responses are for more sustain movement or image movement on the retina. It consists of 2 types of movements pursuits (latency of 90-150ms), and saccades (latency of 150-200ms). Pursuits are smooth tracking of an object following retinal slip. Saccades are large and fast catch-up or return movements of the eyes where the vestibular system shuts of retinal slip (eliminating blur) while the eye moves to the position of interest (voluntary or involuntary). This process implies a visual spatial map of the visual system. These are large visual-motor movements planned and implemented without peripheral feedback during movement which… implies peripheral “map” with schema holding muscle tension, velocity, size information to get from point A to point B[iii].
It has been shown that eye movements are affected with concussion and can be rehabilitated[iv]. This is due to the integration of brain processing of the other sensory systems that affect eye movement. So what does this have to do with vision? There are companies that are coming out with eye tracking abilities to diagnose concussion from the very small movement and acceleration/deceleration speeds and there are other companies that are coming out with eye tracking/treatment devices that will be able to treat eye movement dysfunction. With the knowledge of how eye movements happen, it is truly amazing to see what will be happening in the near future.
Jason Clopton, OD, FCOVD
Center of Vision Development
1080 Neal Street, suite 300
Cookeville, TN 38501