The Effects of Concussion on Gait and Posture in Individuals With Peripheral Vision Loss

Traumatic brain injury (TBI) results when an external force injures the brain and is a major cause of death and disability. In 2013 alone, an estimated 2.8 million people in the United States sustained a traumatic brain injury, most commonly from falls, being struck by or against an object, and motor vehicle accidents. The majority of traumatic brain injuries occur in patients aged 15-64 years during participation in sports. A frequent result of TBI is concussion caused by the rapid acceleration of the brain so that it impacts the inner walls of the skull which can cause both focal lesions such as cerebral laceration and hemorrhage and more diffuse damage resulting in edema and axonal injury. Retrospective studies show that 65 to 79% of patients with mild traumatic brain injury (mTBI) - a term often used interchangeably with concussion - report subjective visual complaints. The most common vision problems related to concussion include photophobia, visual processing problems, blurred vision, double vision and loss of vision (including decreased peripheral vision). For example, in 100 subjects 11-17 years of age with concussion, 69 percent had at least one visual problem, most commonly affecting accommodation, convergence, or saccadic and smooth pursuit movements. Also, in pediatric patients with concussion, 13 percent of 275 patients had persistent near point convergence abnormalities which were referred for vision therapy. The high frequency of visual involvement in concussion is not surprising, since more than half of the brain's pathways are dedicated to vision and eye movement control. The cognitive control of vision, in particular of eye movements, needs coordination of reflexive and voluntary activity, including frontoparietal circuits and subcortical nuclei; these pathways are vulnerable to injury in mTBI. While the topic of sports concussion has received a lot of media attention as a result of increased recognition of potential long-term deleterious effects of repeated concussive events, such as depression, altered cognition, and neurodegenerative diseases (e.g. chronic traumatic encephalopathy and Alzheimer's disease), the vision issues described above can also affect posture and balance. Postural control is essential to lead an independent life, perform activities of daily living, and ensure a correct gait.

Postural stability is maintained through the sensory integration of visual, vestibular, and proprioceptive inputs. The visual field is considered the most relevant visual function in falls. In humans, the visual field is divided into central vision (CV) and peripheral vision (PV). CV detects objects in the gaze direction, and PV detects the existence of surrounding objects, although imperfectly. There is some dispute in the literature as to the roles that central and peripheral visions play in maintaining balance. Three general hypotheses of visual field influences on postural control have been put forth in the literature: (1) the peripheral dominance theory is that peripheral vision is more important than central vision in postural control; (2) the retinal invariance hypothesis suggests that central vision is just as important as peripheral vision in the control of posture; and (3) the functional sensitivity hypothesis suggests that the periphery of the retina is most susceptible to lamellar optic flow and the central part is most sensitive to radial optic flow, and thus, both central and peripheral have important, but functionally different, roles in maintaining posture. Despite these three competing hypotheses of visual field influences on postural control, it is agreed upon that PV is closely related to postural control and sway and that those who have problems with PV have problems with postural control. Due to alterations in vestibular-visual feedback and sensorimotor control, individuals who have experienced a concussion frequently demonstrate gait abnormalities due to altered static and dynamic postural control. In addition to increased anterior-posterior (forward-backward) and medial-lateral (side-to-side) postural sway, concussed individuals have demonstrated decreases in gait velocity and step/stride length, wider step width, reduced cadence, and increased stance and double leg support time compared to normal, uninjured subjects. While the influence of PV loss on gait and posture has been studied in normal, healthy individuals, gait abnormalities in concussed individuals who have experienced PV loss has not been examined. Gait analysis has the benefit of detecting changes from a concussion that will be present after other criteria have been used to return individuals to participation. More specifically, there is preliminary evidence that gait deficits may exist beyond the typical 10-day recovery period and return to activity. Consensus about the most important gait parameters for concussion diagnosis and clinical management are lacking. Therefore, it is important to develop a better qualitative and quantitative understanding of changes in gait and posture in concussed individuals with PV loss. Analysis of gait may allow for the treatment of concussion patients to improve overall balance, movement efficiency, and overall quality of life.