Prism Correction and Double Vision

  • BY Zenni Optical

In order to see well, it’s important for your eyes to have good binocular vision, or the ability to work together as a team. When your eyes aren’t working in tandem you experience eye muscle imbalance, which can result in focus issues and double vision.

What Causes Double Vision

Also known as diplopia, double vision is when you see two separate images or overlapping images. A common cause of double vision is strabismus, a condition where one or more of the six muscles that control your eye movements do not coordinate correctly with the other muscles. When this happens you may experience trouble with focus, eye fatigue, limited depth perception, and double vision. The American Association for Pediatric Ophthalmology and Strabismus estimates that 4% of the American population has some form of strabismus.

Strabismus hampers the ability of the brain to fuse the two images that your eyes see into one coherent 3D image, causing one of your eyes to look at a different image than the other. The terms that describe the directions in which your eyes point relative to center gaze are “tropia” and “phoria.”

Below are common tropias and phorias, and what they mean for one or both eyes:

Exotropia/Exophoria: eye turns outward
Esotropia/Esophoria: eye turns inward
Hypertropia/Hyperphoria: eye turns upward
Hypotropia/Hypophoria: eye turns downward

How To Correct Double Vision

The most commonly prescribed way of alleviating eye muscle imbalance that leads to double vision is prism correction. When adding corrective prisms to your lenses they are incorporated into the lens material. While you may not see a physical difference in the lens, your brain will process the image shift the prism creates and fuse the images from your eyes together into one cohesive whole.

Vision therapy is another treatment to help resolve binocular vision problems, where the patient goes through vision exercises to improve abilities such as eye teaming and focusing, often in conjunction with using eyeglasses containing prism. Eye surgery may be needed if prism and/or vision therapy are not able to sufficiently resolve the binocular vision issues.

When to Talk to Your Eye Doctor About Prism Correction

If you wear prescription glasses and notice you are still experiencing double vision and an inability to focus, these symptoms may be linked to a phoria muscle imbalance causing you eyestrain. Staring at a computer screen all day for work or focusing on small text can aggravate these symptoms even more. Talk to your eye doctor about prism lenses and protect your eyesight from HEV blue light by trying blue blocker lenses.

If your eye doctor determines that you need an eyeglasses prism correction you may see a triangle on your vision prescription (the prism “base”), followed by a direction indicator and a numerical measurement (showing which direction the base is pointing – up, down, in, or out – and how strong the prism is). Prism is indicated on an eyeglass prescription with a base direction and a power. Prism base is abbreviated for up, down, in, or out, while prism power is measured in Prism Diopters (PD; not to be confused with Pupillary Distance) on a scale of every half centimeter, such as 1.0 or 3.5. Prism may be prescribed in just one eye or both eyes.

Below are some additional common abbreviations that you may see on your prescription:

BI = Base In
BO = Base Out
BU = Base Up
BD (or BDn) = Base Down

How to Order Your Prism Lenses from Zenni

When ordering any pair of frames from Zenni Optical, you want to be sure to enter your prism values.
1. On the Order Page, first choose your type of prescription.
2. Enter your Sphere (SPH), Cylinder (CYL), and Axis numbers for each eye and then click “Yes” under Prism Values.
3. Select your Prism Horizontal (for Base In/Base Out) and Prism Vertical (for Base Up/Base Down) values in diopters and indicate the direction for each value on your prescription for each eye. (To learn more about the Cylinder, Sphere, and Axis values on your prescription read our breakdown of astigmatism correction.)
4. Complete your order and wait for your new glasses to arrive in the mail!

Start shopping today for your new prism prescription glasses!