Vision and vestibular function are closely linked, particularly as they relate to gait and postural stability. Visual acuity is independent of vestibular function only when the head is perfectly still. With any movement of the head, we rely on the vestibular ocular reflex (VOR) to provide visual stability. Any significant deficiency of the VOR can result in visual blurring (oscillopsia) with head movement, even in a person who has normal visual acuity when his or her head is stationary. Independent of VOR function, there are many visual changes that inevitably occur with aging, and these can increase the risk of falling.
There is a gradual reduction in eye pupil size with aging, resulting in reduction in light at the retina. For example, in the light a 20-year-old receives six times more light than an 80-year-old. In the dark, the difference is more extreme with the 20-year-old receiving 16 times more light. This has been described as the equivalent of a younger person wearing sunglasses in the dark.
Darkness adaptation (the time it takes to adjust to lighting changes) is also much slower in the elderly population. This is a result of slower pupillary reaction to changes in light. This can result in insufficient light when entering a darkened room, or being temporarily blinded (dazzled) when going outdoors or into a more brightly lit environment. Fotiou et al (2007) studied the specifics of pupillary reaction in the elderly, and reported that latency of response was unchanged with age. However, baseline size of the pupil was significantly smaller and constriction velocity was significantly slower in the aged population. In other words, the eye starts adjusting to lighting changes quickly, but the elderly eye takes a lot longer to complete the adjustment.
Contrast sensitivity has been defined as “a person’s ability to detect edges under low contrast conditions.” Diminished contrast sensitivity may decrease the ability to detect ground level hazards and increase the chances of older people tripping over obstacles. As the crystalline lens becomes less clear and more yellow, there is a diminished ability to discriminate colors. When two similar colors are juxtaposed (e.g., black and blue, or red and pink) the older person has difficulty distinguishing where one color (or object) begins, and the other ends.
Binocular vision is critical for efficient depth perception. With the loss of vision in one eye, depth perception is reduced. There is a direct and significant correlation between reduced depth perception and increased risk of falling.