What are Prism glasses?
You may have heard or seen some people talk about prism glasses. This isn't as common as what people traditionally know in glasses such as astigmatism, farsightedness or nearsightedness. We'll explain what prism is and if it's something you should think about.
What is prism?
You may be familiar with a prism breaking up white light to its individual colors. In optometry we use a different prism, where all colors of light are bent into a new direction. In essence, the image simply shifts to a different direction.
Prism is used for patients that need help aligning light into their eyes. Some are sudden like post stroke patients while others over time an eye alignment gets worse. That is where we use prism glasses.
Eye Alignment
Our eyes work together to create a single and clear image. It takes coordination from the brain, moving the extracoular muscles to align images so the brain can fuse and process each separate image the eye sees.
Most people’s eyes appear to look perfectly straight and parallel. In reality, many people’s eyes sit slightly inward (esophoria) or outward (exophoria.) The brain subconsciously moves the eye muscles from this misalignment to appear straight.
For example, in stroke, a part of the brain responsible for a muscle may be impaired and now the eyes will move unevenly. This will cause double vision. It’s also why if you notice double vision, you could be having an impending stroke!
Types of eye misalignment:
Transient (phorias)
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Exophoria (outwards)
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Esophoria (inwards)
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Hyperphoria (one up)
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Hypophoria (one down)
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Persistent (strabismus)
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Exotropia (one or both constant outwards)
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Esotropia (one or both constant inwards)
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Hypertropia (one up constant)
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Hypotropia (one down constant)
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What's interesting is this can alternate between eyes or be one or both eyes.
Prism
Prism is used to help shift an image so that the eye does not have to fully move to “lock” on to the object it is focusing on. This provides relief since prism is really shifting an image towards its natural resting position. So the eye and its muscles will work less to fuse and focus on an image. If an eye is looking inwards, then we use a prism to shift the image inwards so the eye doesn't have to go fully inwards.
Some patients can benefit from prism.
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if a patient is experiencing eye strain,
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double vision or diplopia
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fatigue,
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blurry vision,
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nausea,
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motion sickness
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headaches
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reading difficulty
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depth perception
Some formal diagnosis of people who may need prism
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strabismus
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convergence insufficiency
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cranial nerve palsies
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post concussion or neural trauma
Should I get evaluated for prism?
The first question is to ask yourself if you notice any of those symptoms above. Secondly, are you are using a current prescription? Lastly that you aren’t exhibiting signs of an impending stroke or TIA—that needs to be addressed first.
To determine if prism would be applicable to you and help, the next step is to find an optometrist who understands prism and binocular vision. Dr. Matthew Pham in Katy is, having been a teacher aid for Dr. Wensveen’s binocular vision lab at the University of Houston college of optometry. He has prescribed prism for many patients including a retired optometrist colleague!
Here’s a case study of a recent prism prescription Dr. Pham did:
Patient with 3BO and 1 BD prism. seen 4 docs with varying shifts in prescription.
What to expect during a prism evaluation exam
It’s really just a typical eye exam with some added testing. Some additional testing includes a cover test, von graefe phorias, maddox rods, parks 3 step, prism ranges and a binocular refraction. The doctor will synthesize all these info and will have you test it in office. Either with a loose prism or with a stick on prism called a fresnel prism.
Treatment
After confirmation that the prism provides relief, a careful fitting of the glasses and measurements are done. If a glasses does not sit properly it will induce unwanted prism, from there the lens needs to go to the lab.
Sometimes prism isn’t needed and lifestyle adjustments or vision therapy could help.
After the glasses are made further real life testing is done and tweaks are done. Checking yearly to see if different prism amounts are needed.
Using prism glasses
In terms of how to use prism glasses, it only involves you just wearing glasses like normal! Your eyes should adapt over a couple weeks. To help with adapting, relax your eyes! We find patients do well with some prism even if the amount isn’t a lot.
FAQ's
What is prism?
Prism is used to bend light towards a certain description.
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How is prism used by the eyes?
By bending light into an area where the eye is able to navigate towards more easily, it helps reduce the effort the eye would do
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How does one know if prism could work for them?
An eye exam is the best way to determine eye alignment, prescription, and strain. In office testing before getting the glasses made is helpful to pinpoint the amount of prism to use.
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Is prism a prescription?
Yes but not in the traditional sense of a typical glasses prescription. The measurement and units are different.
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Can I get prism in contacts?
It's not feasible to do so
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What are the directions of prism that can be used?
We have in, out, up and down; and of course any combination of those!
Conclusion
If you’ve tried multiple prescriptions and still do not have vision where you want it, it’s a good idea to go to an optometrist who can further evaluate your binocular vision and hence decide if prism is needed. If you are in the houston area, make the trip to Katy and visit us at Eye Eye Doc! We have optometrists that are not intimidated by prism and binocular vision. These are often considered outside a typical scope of a general optometrist. Not only are our doctors willing to try prism for our patients, we often find a solution that gives patients comfort.
Service Hours
Contact Information
Monday
9:30 AM - 5:00 PM
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​Tuesday
9:30 AM - 5:00 PM
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Wednesday
9:30 AM - 5:00 PM
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Thursday​
9:30 AM - 5:00 PM
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Friday
9:30 AM - 3:00 PM
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Saturday​
8:00 AM - 12:00 PM
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​Sunday
Closed
24826 Morton Ranch Rd.
Ste 300
Katy, TX 77493
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About the Author
Dr. Matthew Pham has practiced optometry since 2017. Before opening his own office he had worked for two other offices in Houston, TX.