Vision Library

What Are Prisms For?

Have you ever looked over the edge of a tall building or side of a steep mountain and felt a sudden change in your stability and posture? It can be a very exciting feeling or terrifying, depending on your nature. That sensation of change in posture and stability is similar to what many vision therapy patients might feel the first time they wear prisms designed for therapy. The typical comment is “Whoa, everything is curved.” This type of comment denotes good visual awareness. If they describe feeling a postural change like “It feels like I’m walking up a hill,” we like to say they have good postural awareness. So, beyond the “gee-wiz!” factor, why are prisms used in optometry and vision therapy?

Prisms have been used by developmental optometrists to influence a wide variety of visual functions. Prisms are an optical lens that refracts light. The amount of refraction depends on the angle of the prism. The prism angles for those used for in-office vision therapy range from 5, 10, and 20 degrees. The prisms bend light coming into the eyes, therefore changing how the brain interprets the image. In eyeglass prescriptions, prisms are used primarily to resolve double vision. Typically the angles are very low but can go higher if warranted by the patient’s diagnosis.

Double vision (diplopia) occurs when someone sees two individual images due to their eyes not being properly aligned or they are unable to converge or diverge effectively. Typically our brain fuses the separate images from each eye and we perceive only one image. Diplopia can be very distressing. Children who suffer from double vision often do not say anything because they lack the context in which to describe it or they say some words or letters seem to float or move. They can exhibit behaviors that can be misunderstood as poor behavior when it comes to school work, like avoidance or frustration (homework wars). Prescription glasses that contain prism lenses help align the eyes so that the brain can fuse the images more easily. Not unlike the “Ahhh” when you take the first drink of a cold glass of water on a hot summer day, prisms will provide relief that the child or adult suffering from diplopia will describe almost instantly.

In order to provide astronauts practical experience in zero gravity orientation, they are put through the rigors of zero gravity parabolic flights. These brief experiences in antigravity stimulate the astronaut’s vision and vestibular system, improving their orientation in space. In vision therapy prisms are used to challenge a patient’s vision and vestibular systems in a similar way. While wearing therapeutic prisms the patient must make visual and postural changes in order to adapt to the orientation of the prism. For instance, a prism that has its base (thicker part of the lens) in the up position will view flat objects, like the floor, as curved so that the middle seems higher than the sides. When the patient walks forward, it can give the sensation or feeling of walking down a slope. This challenge to the visual system will increase the patient’s general visual and postural awareness, which enhances their orientation when they are not wearing the prism.

The use of prisms in vision therapy can have a dramatic effect on children who are toe walkers or have other orientation difficulties related to vision. Prisms are also very useful for the treatment of vision problems related to head injuries or some chronic diseases (post trauma vision syndrome). If you have concerns related to your or a loved one’s vision, contact us. DrsTod Davis and Amy Carlyle at Virginia Vision Therapy Center and their staff is ready to receive you. We provide the very best vision assessment and vision therapy that is backed up by over thirty years of experience. Our mission is your best vision.


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