What Are Prism Lenses?

Reviewed by: HU Medical Review Board | Last reviewed: September 2023

If you have thyroid eye disease (TED), the swelling and inflammation from the condition can lead to problems with your vision. One problem that can occur is seeing double. This is known as diplopia (dip-low-pee-uh).1

While seeing double can be temporary, it could be a sign of something more serious. It is crucial to seek immediate care if you have changes to your vision, even if you think it relates to TED. Only a trained professional can determine the cause of your vision changes.

For those who see double, prism glasses and lenses might be an option to help.2

What are prism lenses?

Prism lenses are special lenses placed in eyeglasses that bend light before it reaches the eye. When the light is redirected, it reaches the retina in the right spot to allow the brain to make a single, clear image.2

Why are they used in TED?

The retina is a layer of tissue that lines the back of the eye. The retina senses light and sends a signal to the brain so an image can be seen. We normally see a single image through 2 eyes because incoming light is directed to the same spot on the retina in each eye.2,3

Because TED affects the muscles of the eyes, your eye movement can be affected. The result can be seeing 2 of a single object or misaligned eyes. This is known as strabismus (struh-biz-mus). TED can also cause your eyes to dry out, which can lead to temporary double vision.2

Getting prism lenses

Your TED specialist or eye doctor prescribes prism lenses. Once your doctor determines that prism lenses can treat your double vision, you will need a full eye exam and fitting. Your TED specialist may use 1 or more tests during your exam:

Corneal light reflex test (Hirschberg test)

This test is a simple and quick way your doctor can check the alignment of your eyes. Your doctor will shine a small light and ask you to focus on the light. Next, your doctor will check the reflection of the light off the surface of your eye (cornea). In those with normal eye alignment, the light reflection is even in both eyes. For those with misaligned eyes (strabismus), this reflection varies in each eye.2

Cover tests

Cover tests help your TED specialist determine the severity of your eye misalignment. There are different variations to this test. In 1 method, your TED specialist will cover 1 of your eyes and look at the uncovered eye to see if the movement in the uncovered eye is normal. Then your doctor will check your other eye.2,4

Krimsky test

Like the corneal light reflex test, this test involves your TED specialist shining a small light in your eyes and asking you to focus on the light. However, with this test, prisms of different strengths are placed in front of your eye. Your doctor will look for the right prism that aligns the reflection in both your eyes to determine which prism works best.2

What do they look like?

There are different kinds of prism lenses. Sometimes, a temporary vinyl sticker prism (Fresnel prism) may be added to your normal prescription lenses.2

If you need a longer-term solution, a prism can be made directly into the lenses of eyeglasses. This causes the lens to be thicker on 1 side and may be noticeable. If this is something that bothers you, there are eyeglass frames that can help with the cosmetic appearance this may cause.2

Do these eyeglasses cause side effects?

Like other prescription eyeglasses, it may take a couple of days to get used to the vision correction caused by prism lenses. Rarely, these lenses may cause symptoms that are often temporary, including:2

Prism lenses are a powerful tool that can greatly improve the symptoms of double vision. Make sure you are seeing a TED specialist who will be able to determine if this treatment is right for you. They can also make sure you are given the right prescription.

Choosing the right frame and lens combination can ensure the best vision correction and cosmetic outcome.

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