What is "prism"?
  • Prism is a type of lens that is shaped in a way to bend the light that is coming into the eye.  This means that we can "move" images seen by the eyes/brain to a place that is more comfortable for the eyes to see - allowing for single, clear, and comfortable vision.

What are the different types of prisms and what are they used for?

  • Most often prism is used to help with eye alignment/double vision.  When the two eyes are not pointed in the same direction then the brain receives two different images and interprets the world as being seen in double.  By using a type of prism called compensating prism, we can bend the light traveling into each eye in a way that moves the images closer to each other to align what each eye is seeing (even though they are pointed in different directions) so that the brain perceives only one image instead of two.

  • A different type of prism we can prescribe is called yoked prism.  In yoked prism we move the images of both eyes in the same direction.  Yoked prism is helpful for patients who have gaze palsies (both eyes can't look up or down - often due to conditions like brainstem stroke), restrictive strabismus (e.g., thyroid eye disease), nystagmus, abnormal egocentric localization, as well as cervical disorders of the neck that may cause patients to be unable to look in a certain gaze due to physical limitations of the neck's range of motion.  

  • A third class of prism we can utilize is called visual field expansion prism.  There are many types of "field expansion" prisms including Peli Prisms, Gottlieb prisms, and yoked prism, which can be used for patients who have neurologic visual field loss like homonymous hemianopsia.  While this prism does not give patients their full peripheral visual field back, this prism can expand the field by 3-25 degrees which can help patients with things like reading, walking/ambulating in public.

What if I tried prism before and it didn't "work" for me?

  • Prescribing prism is an art.

  • The eyes are dynamic and can fluctuate throughout the day, so sometimes it can take a few attempts to get the prism correct.  If Dr. Theis is concerned that your double vision fluctuates or the prism prescription may need a trial period, she will prescribe a temporary prism called a "Fresnel" prism, which is a sticker lens that can be placed on your current glasses to trial the prism and adjust as needed until the correct prism combination is found and works for you throughout the day.  

  • Once the correct prism combination is found and stable, she can prescribe a more permanent ground in prism.

 

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