Home SEE WELL What are Prism Correction Glasses?

What are Prism Correction Glasses?

by Michael Healy

Eyeglasses are typically prescribed for two main reasons:

  • astigmatism
  • near- or farsightedness

Another common reason is to correct double vision, or diplopia. Read on to understand the warning signs, diagnostics, and whether or not you might be in need of prism correction glasses.

What causes the need for prism correction glasses?

If your eyes are working in accordance with each other, you have what is known as binocular vision. When light passes through the cornea, the dome-shaped tissue lining the outside of the eye helps to focus the light before it reaches the retina, the light-sensitive tissue lining the back surface of the eye.

Light that’s been converted by the cornea is sent to the brain as electrical impulses along the optic nerve to be translated into images. Typically, the eyes work together to see one, single image because light falls on the same spot in each retina.

However, in someone with diplopia, light falls on different parts of the retina, causing two separate images to be seen.

Diplopia is not the only reason for potentially needing prism correction glasses. Other causes include:


Strabismus is described as a turning of the eye and can occur in one of four ways:

  • Esotropia – inward turn
  • Exotropia – outward turn
  • Hypertropia – upward turn
  • Hypotropia – downward turn

The most common types of strabismus are eso- and exotropia, although, a person can experience any combination of the four.

When a child is born with strabismus, the brain may completely cut-off vision from the affected eye so that one image is seen out of the unaffected eye. The result is a lazy eye that never has an opportunity to strengthen and develop properly.

When strabismus occurs in adulthood, symptoms could include migraines and nausea, causing an inability to perform daily tasks.


Heterophoria is a condition where the eyes point in different directions while at rest. It can occur as a result of eye strain from seeing double, forcing the muscles that facilitate binocular vision to overwork. To compensate, the eye will continuously move up and or down in order to bring an image into singular focus. Eventually, the eye muscles weaken and can give out altogether, resulting in one or more of the following:

  • dizziness
  • migraines
  • anxiety while driving or in locations with a lot of visual stimulation
  • poor balance
  • neck pain
  • nausea

Hemianopia or quadrantanopia

These two types of double vision involve blindness in half or a quarter of the eye. The eyes could be affected the same way or differently, and hemianopia, especially, usually occurs to the right or left of a person’s field of vision. Rarely does it occur above or below regular field of vision. Quadrantanopia, however, can affect the upper, lower, right, or left field of vision.

Brain trauma

Double vision could be the result of a neurological problem. Traumatic brain injury, brain tumors, chronic migraines, and even stroke can lead to double vision. People who have had a stroke or a traumatic brain injury are commonly afflicted with hemianopia.

Nerve-related issues

Double vision could also result from a nervous disease such as multiple sclerosis (MS) when inflammation or permanent damage happens to the nerves of the eye that control movement.

Double vision can also be the result of diabetes, as spikes in blood sugar can affect vision. When diplopia results in response to a blood sugar spike, conversely, vision can return to normal once blood sugar levels are under control.

Diagnosing for diplopia

One or more of the following tests may be necessary to diagnosis diplopia and a need for prism correction glasses:

Cover tests

Cover tests are just as the name implies—they all involve covering one eye in some way to either diagnose diplopia or to measure the severity of it.

There are three potential cover tests:

  • Cover-uncover test: one eye is covered, and your doctor looks for movement in the uncovered eye
  • Simultaneous prism and cover test: one eye is covered and a prism is placed in front of the uncovered eye to discern the prism strength needed to bring vision into alignment
  • Alternate cover test: a prism is placed in front of the eye as the cover is alternated in order to measure the difference between the two eyes, and thus, decide the prism strength required to repair double vision

Hirschberg test

The Hirschberg test uses a pen light directed at the eyes. Typically, the light reflecting off the cornea will appear in the center of the pupil of each eye. When the reflection is off-center, strabismus may be the diagnosis.

Krimsky test

Used to measure the degree of misalignment within the eye, the Krimsky test is similar to the Hirschberg test, only it uses a prism in addition to a pen light. While looking at a pen light, various prisms are placed in front of the eye. These prisms are all of different strengths, and when the reflection of the light is brought to the center, the necessary prism prescription is discovered.

Maddox rod test

This test is used to ascertain the type of double vision. Through one eye, a person will look at a horizontal or vertical line. Through the other eye, that person will see a white light. In binocular vision, the light will superimpose the line. When looking at a vertical line, if the light deviates to one side of the line, or above or below the line when looking at one that is horizontal, a doctor is able to measure the prism strength required to correct the person’s vision.

Treatment for double vision

Once double vision is diagnosed and measured, your optometrist may prescribe vision therapy, a series of exercises to strengthen the muscles of the eye in order to help them focus as one.

More than likely, however, your doctor will prescribe prism correction lenses. A prism is then ground into the eyeglasses’ lens, and while the physical lens might not look that different, your brain will begin to process images as one instead of two.

Double vision, as already indicated, could be a sign of something more serious going on. It’s important to schedule an appointment with your eye doctor to have your eyes checked right away.

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