How to Prescribe Prism with Confidence

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Four words in a patient’s chart can strike doubt in the mind of even the most accomplished optometrist: “Complains of double vision.”

What caused the diplopia? Is it sudden-onset or longstanding? Have they ever had prism in their glasses before? What about prior strabismus surgery? Could it just be undercorrected astigmatism or a corneal condition?

A comprehensive description of the diagnosis and management of all medical and ocular conditions that cause diplopia could fill several textbooks. This article will narrow that exhaustive list down to non-urgent, binocular vision causes of diplopia that are best managed with prism.

Prescribing prism is more of an art than a science, and can be very subjective. That subjectivity is part of the reason so many optometrists hesitate to tackle it.

Discover the patient’s main concern.

Constant, comitant diplopia due to strabismus or muscle paresis is the simplest case to manage.

The prism amount will most likely be stable, with no adaptation, and immediate patient relief. However, many binocular vision cases present with vague symptoms.

A young lawyer may only notice diplopia after several hours of intense computer work. An elderly patient may have had a small amount of occasional diplopia for several years, but now comments that it is happening more often and to a greater degree. An elementary school student may not complain at all, but consistently closes one eye to read comfortably.  All of these patients may benefit from a prism prescription, and they may have gone years without proper binocular evaluation.

Binocular vision testing should be performed during all comprehensive eye exams.

This does not have to be a long process; a quick NRA/PRA after refraction can reveal an accommodative issue that may be subtly compromising binocular function. Technicians can be trained to perform cover test as part of the pre-testing process, and any questionable findings can be re-checked by the optometrist in the exam room.

Obtain objective and subjective measurements.

There are several articles that describe methods to measure and apply prism.

Ultimately, the choice of technique is up to each individual clinician, and several measurements may be prudent to determine the “best” value (1).

For example, a patient with a small vertical phoria may appear orthophoric to the clinician during a cover test, but when prompted, may note the target appearing to move up and down as each eye is uncovered.

Loose prisms may be used to measure this subjective motion, and the result compared to a Von Graefe phoria.

In the case of a vertical heterophoria, even a prescription with 0.5 prism diopter base up or down may be clinically significant (2).

The important factor is not which tests are performed, nor how many tests are performed, but rather, the consistency and validity of the results.

If four tests are performed, and four wildly different values are obtained, the patient will likely not benefit from a prism prescription. However, if the results of two tests make sense clinically when compared to patient complaints and exam findings, they may be enough to diagnose and manage a binocular condition.

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About Janna Mixan

Janna Mixan
Janna earned an engineering degree from the University of Iowa before attending Southern College of Optometry. She and her husband both graduated from SCO in 2015. After graduation, she worked as an associate for a private practice in Iowa before moving to Omaha, Nebraska. She currently works as an independent optometrist, and focuses in pediatric and binocular vision. Outside of optometry, her interests include biking, baking, and playing board games.

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