Playing Tricks on the Eyes

Many patients with low vision make appointments with us hoping we’ll be able to improve their vision. Although we are rarely able to change their vision, we can alter their environment and give the illusion of improved vision. Magnifiers can enlarge close-up text, high plus lenses can allow a closer working distance by using relative distance magnification, and tinted lenses can increase contrast, but sometimes even our most successful low vision tips and tricks can leave a patient struggling. However, thinking outside the box and taking the time to listen to your patients can allow a sort of magic to happen.

OUTSIDE THE BOX

Prism is typically used to alleviate double vision, but less commonly, it can also ease nystagmus, improve behavior, and reduce the effect of hemianopsia.^1,2 In other cases, just as a magician turns an audience’s focus away from the real action, yoked prism can be used to redirect a patient’s visual world so they miss the illusion right in front of their eyes.

A CASE EXAMPLE

A 52-year-old woman presented to the low vision clinic after recent vision loss in her better-seeing right eye. Despite her longstanding struggles with pathologic myopia in both eyes and a choroidal neovascular membrane that left her with BCVA of 20/400 OS, she had been able to continue to work as a nurse for years.

In the fall of 2021, however, the patient experienced a nonarteritic ischemic optic neuropathy in her right eye, which resulted in further deterioration of her central vision and a dense inferior altitudinal scotoma. To help this patient continue working, we first trialed computer-only lenses, clip-on magnifiers, and computer programs, such as ZoomText, that enable detailed magnification. Even with these recommended visual aids, however, she struggled to see well enough to perform her daily tasks. Although her central blur was bothersome, it became apparent that the inferior scotoma was causing the biggest problem. Not only was she unable to track words as her eyes moved lower down on the computer screen, but she also found herself manipulating her neck to see around the scotoma. We decided to try a prism to redirect her visual world.

PRISM REFRESHER

Prism is used to redirect an image’s apparent location in space, where the image shifts in the direction of the apex of the lens. For example, with base out prism, the base of the lens is aligned with the temporal aspect of the eye, and the perceived image is shifted toward the patient’s nose. This setup can be used to correct esotropia or diplopia stemming from an inward turning eye. Prism used in this manner can be placed over the involved eye, or more commonly, divided between the two eyes. The opposite is true for treatment of exotropia, which requires base in prism over each eye.

Vertical prism to alleviate diplopia has the base situated opposite each other (that is, base up is used over one eye and base down over the fellow eye). If you consider double vision that is situated one above the other, base up of equal amounts over each eye would not alter the position of the images relative to each other as desired. Instead, it would cause the image for each eye to shift downward by an equal amount but remain an equal distance apart. For this reason, if you want to align the image that is separated vertically between the two eyes, you need base up in one eye and base down in the fellow eye, with the base down situated over the hyper eye. (For example, if the right eye is sitting higher, or hyper, compared with the left eye, then the image of the right eye is likely lower than the image in the left eye. To move the right image upward to align with that of the left eye, you would need base down prism over the right eye.)

Conversely, instead of moving the image of the right eye upward, you could move the image of the left eye downward with base up prism. Or, more conventionally, you could divide the amount and separate it over each eye. Although this may be a review for most ODs, it is important to understand the basics before delving into yoked prisms.

Because prism is typically used when the posture of the eyes is misaligned, it is easy to think that the prism realigns the eyes. This is not the case and is why an eye turn may cosmetically appear worse when wearing prism. Rather than thinking of prism as a way to realign the eyes, it is more accurate to imagine the eyes sitting in their natural posture while the prism works to move the visual world to meet each eye, rather than the reverse.

Consider the vertical double vision discussed above. If you placed base up lenses over each eye, what would you expect? This is the concept of yoked prism and, as explained above, this type of prism would simply shift both images down, rather than closer together. Although individuals with double vision do not typically benefit from yoked prism, the idea of two separate images shifting in unison allows easier understanding of the concept. For those with a hemianopsia or scotoma, the visual shift of a single image, rather than the syncing of a double image, is generally the goal.

REVEALING THE SECRET

Like any good magic trick, the illusion, rather than the reality, is key. Such is the case with yoked prism. Although it is impossible to change the location of a nonmoving object in space, it is possible to change the perception of its location. It is understood that prism shifts the visual world 1 cm for each 1 prism diopter when an object is located 1 m away.³ As noted above, vertical prism of equal amounts oriented in the same direction (ie, either both base up or both base down) creates a movement of an object’s apparent position in space. Therefore, for our patient with an inferior scotoma, base down prisms over each eye shifted her visual world upward (Figure), creating the illusion that the scotoma had shifted downward, out of her line of sight.

For prescribing, the amount of prism necessary is directly proportional to the degree of shift you desire and can be easily trialed with the patient in-office. For the patient in our case, a shift of 10 prism diopters (~ 10 cm) moved her visual world up far enough to allow her to continue to work comfortably.

You may think the eyes would simply follow the image shift, and therefore, the scotoma would move, too; however, this is not the case. When prism is oriented in a symmetrical vertical direction with equal power, the eyes themselves do not move. Instead, they remain fixated forward while the image moves.

PICK YOUR PRISM WISELY

Although there is some debate regarding the phenomenon of yoked prism adaptation and foveal refixation in full-time wear, such problems are unlikely to interfere when used only in certain situations (ie, when the eye is undergoing numerous refixations, as it is while reading or using the computer).^3,4

In addition to the possibility of adaptation, yoked prism should not be used full time because doing so can make it difficult to properly navigate spaces. If your patient is already worried about climbing the stairs with their progressive lenses, for example, imagine their difficulty wearing glasses that shift their visual world 10 cm higher than reality.

Although our case example demonstrates yoked prism in a vertical direction, horizontal prism can be used in this manner as well, such as for a patient with a left hemianopsia. Base left prism, or a prism situated with base out over the left eye and base in over the right eye, would shift the missing left visual world to the right, into the patient’s line of sight.

THE POWER TO SHAPE REALITY

Using a yoked prism may require some time on the patient’s part to adjust, but with the right motivation, you can use the power of redirection to help your patient experience an almost magical visual improvement.