Vision and vertigo

Some people may experience vertigo as a result of being overstimulated by complex visual environments where there is a lot of movement. This is known as visual vertigo.

Visual vertigo was first defined by Bronstein (1995) who wrote an article listing the situations which cause problems: passing trains; passing pedestrians; flickering light; intolerance of scrolling on visual display units (VDUs) – some people, for example, can’t use their computers for more than 10 minutes without feeling sick. This is really quite disabling and needs a fair amount of intervention to reduce this sensitivity.

One of the functions of the vestibular system is to control eye positions so that when your head moves, your eyes can automatically stay fixed on something you want to look at. This control is called the vestibular ocular reflex (VOR) and is easily demonstrated during walking. When you walk, your head moves up and down, but your visual world stays stable. A ratio exists between the amount of head movement required for stability of gaze (what you are looking at). If you turn your head so many degrees, your eyes have to turn just the right amount to keep your gaze steady. Your brain calculates this ratio and remembers it.

Wearing glasses

When you put on glasses, the size of your visual world changes (getting bigger or smaller depending on whether you are far or near sighted), and your brain has to recalculate the ratio between the amount of head movement and the amount of eye movement. This is why sometimes you might have trouble adjusting to new glasses.

The problem for the brain is fairly simple if the lenses have only one optical power. If you wear bifocals, the brain has to use two ratios, one for the top of the glasses and another for the bottom because of the different portions of the lenses. It also has to switch ratios back and forth rapidly as you look through the tops or bottoms. This problem is more complex with trifocals and progressive addition lenses. With progressives, gradual change in optical magnification occurs as you look from the middle of the lens to the reading area. This means that your brain must change the VOR ratio constantly as you look up and down through the lens. This extra work is not fun for an already overloaded brain. In addition, the progressives have zones of distortion in the lower corner of the lenses; the distortions make the brain work even harder.

Many people also have significant troubles with bifocal contact lenses because these lenses project multiple images (different images for different distances) on the retina of the eye, and the brain must sort out which image to use. This takes practice and a fair amount of brain power, which is difficult for many with vestibular disorders.

Clear vision is dependent upon a normal functioning vestibular system. Because vision is linked to the vestibular system, people with vestibular disorders have special needs when selecting glasses. Discussion of these needs with your optician may help you avoid problems when you use glasses.

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