The nearpoint demands our patients encounter in their daily lives are increasingly challenging their visual systems, resulting in exponential cases of asthenopia, which can lead to diplopia. (See “Signs of binocular system stress,” below.)

For example, in a 2022 study published by the National Institutes of Health, 75% of 784 participants older than age 18 had asthenopia symptoms after using digital devices at the end of the day.¹ While this is alarming, digital device use is here to stay. Aside from the use of digital devices, other nearpoint activities that involve prolonged use of the eyes, such as reading, painting, and jewelry making, can also challenge the visual system.² 

Thus, as clinicians, it’s imperative we discern when and how to prescribe prism to these patients to provide them with optimal and comfortable vision. This article answers those questions. 

When the use of prism should be considered
Among those who may benefit from prism are usually symptomatic (e.g., headaches, etc.) non-strabismic patients with probable binocular vision disorders, strabismic patients, patients dissatisfied with their current prescription after seeing multiple eye doctors, patients experiencing deteriorating vision/increasing myopia, and those who have concussion/traumatic brain injury.

The simplest way of determining when a patient could benefit from prism is to observe them. Observation includes, but is not limited to, noting patient posture while they walk to the exam room and while seated in your exam chair, acquiring a thorough case history about your observation, and performing a comprehensive eye exam to reinforce what you suspect.

• Noting patient posture. Often, patients in need of prism will exhibit a noticeable head tilt and, sometimes, a head turn. This is because these actions help compensate for the conflict between both eyes to work together and are often indicators of a possible binocular vision disorder. When head posture is skewed, the rest of the body will also be skewed due to compensating for the head tilt or turn. Think of the head as one triangle, the shoulders and torso the second triangle, and the hips and legs as the third triangle. When one of those triangles are misaligned, the rest of the body has to change to maintain some type of balance or equilibrium.

• Acquiring a thorough case history. While looking at the patient, ask questions about their head tilt or turn, without making them self-conscious. This dialogue may start out with “I notice you tilt your head to the side a little; do you ever see double?” If they respond “yes,” ask whether it occurs in one or both eyes, whether the images are separated up and down, side by side, or a combination, how often they notice this, when they first noticed it, and during what activity. (The latter three questions enable you to ascertain the seriousness of their double vision and when it is most prevalent.) Also, ask the patient whether they cover or, perhaps, close an eye to make their double vision go away, whether they experience headaches, fatigue when reading or while on the computer, blurred vision while reading or on the computer, difficulty switching focus from near to far, or neck and back aches. “Yes” answers to these questions indicate they may benefit from the application of prism in their glasses.

• Performing a comprehensive eye exam. While every clinician’s definition of a “comprehensive” exam may differ, the ultimate goal is to take care of your patient. Listen to the reason for the patient’s visit to lead the direction of your exam. Also, be aware of the effort it takes for them to go through your exam. Barring learning challenges, if a patient laborious-ly reads the Snellen chart, grimaces, sees better monocularly than binocularly, opens their eyes wide, rubs their eyes, or turns their head to see better, they are showing signs of an inefficient binocular vision system.

Cover and pursuit testing are essential to a “comprehensive exam” and will confirm your suspicions of a binocular vision disorder. Specifically, look carefully for subtle vertical phorias and extremes of horizontal exophorias or esophorias at both distance and near, and measure them with your prism bar in room lighting that is bright enough to allow you to see these subtle eye movements. The red glass test is another method to use to quickly determine the presence and type of  diplopia the patient is experiencing. As the patient holds the red lens over an eye and looks at a white fixation light across the exam room, they report where the red light is in relation to the white fixation light.

Of note: If you suspect a patient’s diplopia is organic in nature, meaning the patient reports an acute onset, perform red cap desaturation testing or color vision testing, automated perimetry or a MRI before proceeding with the recommendation of prism. This is because the diplopia may be associated with a life-threatening systemic disease, such as uncontrolled diabetes and/or hypertension, optic neuritis, uncontrolled thyroid issues, or tumors.

Also of note: Patients who present with a convergence insufficiency may not benefit from prism.^3,4 This seems especially true in children.⁵ However, some eye care providers have found that adult patients can benefit from base-in (BI) prism.⁶ Patients with eccentric fixation and anomalous correspondence with long-standing deviations require a more cautious approach due to the fact they can prism adapt and possibly fluctuate from their original fixation spot to the newly established fixation spot.

In addition to the cover test, the Maddox Rod test with fixation disparity testing (i.e., the Thorington Test Card or the Wesson Fixation Disparity Card) can help determine the patient’s horizontal and vertical phoric posture. A caveat: If the patient holds the occluder, their ability to report which letter or number the perceived red Maddox line goes through on the card may not be reproducible. However, if the clinician holds the Maddox Rod and the fixation disparity card with good stability, this test allows for a great starting point in determining how much prism to prescribe. These methods are also quicker than the Von Graefe testing mentioned below and occur in free space as opposed to being behind the phoropter.

Lastly, the Von Graefe test with the phoropter is commonly used in many practices that specialize in evaluating binocular vision disorders, as it aids in determining fusional vergence. A prism bar in free space can also be employed to determine vergences. Both tests are more reproducible than the Maddox Rod test, though require more of a time commitment initially. However, with practice, either test becomes quick and easy to utilize.

How to determine how much
After obtaining phorias and fusional vergences using the Von Graefe test, the following formula from Sheard’s Criterion indicates how much prism the patient requires:

Prism needed = 2/3 (phoria) - 1/3 (compensating fusional vergence) to help the clinician determine how much prism the patient can tolerate and utilize comfortably. So, if a patient has 6∆ exophoria and base-out (BO) to blur is 6∆, the prism needed would be 2/3(6) - 1/3(6), or 4 - 2. You would then prescribe 2∆ base-in (BI), since deviation is exophoria. This formula is useful in checking your tentative prescription of prism, but it is still highly recommended that you recheck yourself out of the phoropter in free space. This is because doing so allows for viewing the patient’s response under more “normal” conditions.

As a review, exophoric patients need BI prism, esophoric patients need BO prism, and patients who have vertical misalignments generally benefit from base-down (BD) prism on the hyper eye. It is aesthetically better to prescribe equal quantities with the same base direction in the patient’s glasses. If the magnitude of prism is greater than 4 D to -6 D of prism, the product may result in heavier and more noticeable edges in the patient’s glasses. Avoiding this depends on the alignment of the frame on the patient’s face, the centration of their eyes in the frame, and the frame size chosen. For larger magnitudes of prism (greater than six prism diopters), Fresnel prism can be applied. A caveat: The thickness and change in weight of the lenses in the patient’s frame should be discussed with the patient before ordering their glasses. Show the patient an example of the edge of prisms, utilizing one from your trial lenses sets or loose prisms, and get their approval before processing the order.

If the prism contains a large vertical disparity, it is better to divide the amount of prism in half and apply BD to the hyperphoric eye and base-up (BU) on the hypophoric eye.

To be precise and more confident in your final determination of prism, I recommend simulating the prescription in a trial frame in free space and presenting varying magnitudes of loose prisms that bracket your tentative prescription. These action steps significantly cut down on the time needed to determine “how much prism,” and also show patients what they’ll see and how the prism will make them feel.

If you’re prescribing a small amount of prism (2 pd or less), I also suggest you try the whole amount on one eye and compare it to evenly splitting the prism between the two eyes. The reason: If the patient tends to have both horizontal and vertical components and the vertical component is consistently present in their strabismic or non-strabismic alignment, often correcting for the horizontal component remedies the vertical component, negating the need for correcting both.⁷

When the patient finds no relief, adapted too quickly, is still symptomatic after the application of prism, or an appropriate amount of prism could not be determined, vision therapy (VT) is a viable option. A caveat: Patients with vertical deviations alone are more challenging because training them to fuse vertically is challenging. Those with horizontal and vertical deviations often benefit from VT because addressing the horizontal component often takes care of the vertical component.

Enhancing quality of life
Prism for symptomatic patients solves a problem they may have thought they were stuck with. In many ways, you may have improved their quality of life, saved their job, or prevented them from failing a class. This is how powerful detecting the need for and prescribing prism can be. OM

References

1. Abuallut I, Qumayi EA, Mohana AJ, et al. Prevalence of Asthenopia and Its Relationship with Electronic Screen Usage During COVID-19 Pandemic in Jazan, Saudi Arabia: A Cross-Sectional Study. Clin Ophthalmol. 2022:16:3165-3174. doi: 10.2147/OPTH.S377541.

2. Chu GCH, Chan LYL, Do C, et al. Association between time spent on smartphones and digital eyestrain: A 1-year prospective observational study among Hong Kong Children and Adolescents. Environ Sci Pollut Res30, 58428–58435 (2023). https://doi.org/10.1007/s11356-023-26258-0.

3. Carter, DB. Fixation Disparity and Heterophoria Following Prolonged Wearing of Prism. Am J Optom Arch Am Acad Optom. 1965:42:141-52. doi: 10.1097/00006324-196503000-00001.

4. Scheiman, M, Wick, B. Clinical Management Binocular Vision; Pennsylvania, Lippincott Williams and Wilkins; 2002; 105-109.

5. Scheiman, M, Cotter S, Rouse M, et al. Randomized clinical trial of the effectiveness of base-in prism reading glasses versus placebo reading glasses for symptomatic convergence insufficiency in children. Br Ophthalmol. 2005;89(10):1318-23. doi: 10.1136/bjo.2005.068197.

6. Abdi S, Kangari H, Rahmani S, Baghban AA, Rad ZK, Home vision therapy and prism prescription in presbyopic persons with convergence insufficiency: study protocol for a randomized controlled trial. BMC Ophthalmol. 2024;24(1):169. doi: 10.1186/s12886-024-03411-y.

7. Caloroso, EE, Rose MW. Clinical Management of Strabismus, Massachusetts; Butterworth-Heinman, 1993:279, 284-85

Debra A. Shim, OD practices at VisualEyes of the Palm Beaches, in Jupiter, Fla. She is a Fellow of College of Optometrists in Vision Development, and member of the Florida State and local Optometric Association.