Prism correction is used in eyeglasses for people who have diplopia, or double vision. Diplopia is when a person sees two separate images of one object and can be present in either one or both eyes. In monocular diplopia, the double vision is present in only one eye and will remain even when the other eye is shut. Binocular diplopia, or double vision in both eyes, is caused by a misalignment of the eyes.
People with binocular diplopia are most eligible for prism correction in their eyeglasses, though people with all types of diplopia may be helped with prism lenses. Prism glasses help align the two images caused by diplopia so that only one image is seen.
How Do Prism Glasses Work?
Light enters each eye through the cornea and falls on the retina, the tissue that lines the back of the eye. With normal binocular vision, the eyes will work together to provide a single image to the brain. The eyes are able to do this because light falls onto the same place on the retina in both eyes.
In a person with diplopia, the light will fall on different parts of the retina in each eye, producing two images.
This is where prism glasses come in. Prism lenses will bend the light before it enters the eye, directing it so that it falls correctly on each retina. The brain will then be able to do its job and use feedback from both eyes to create one image rather than two.
Who Might Benefit From Prism Glasses?
Prism lenses can be used for diplopia from eye misalignment caused by:
- Eye muscle problems such as myasthenia gravis, Graves’ disease, or strabismus, also known as crossed eyes
- Neurological issues such as traumatic brain injuries, strokes, migraines, or tumors
- Nerve-related problems like multiple sclerosis or diabetes mellitus
There are many underlying causes of double vision, and prism glasses won’t always help. If you are experiencing double vision suddenly for unknown reasons, it could be a sign of a serious health condition, and you should see a doctor immediately.
How are Prism Glasses Prescribed?
There are multiple tests that can be done to determine if prism lenses are necessary and if they are, the type and strength of prism that will most benefit you.
Here are some of the more common types of tests used to prescribe prisms:
This is a basic test for strabismus, or crossed eyes. The procedure is relatively simple: a penlight will be aimed at the eyes. When the patient attends to the light, it will reflect in the center of each pupil, in a person with normal vision. If the light is off-center in either eye or in both eyes, there may be strabismus. Those who suffer from strabismus are eligible for prism correction eyeglasses.
A Krimsky test helps measure the actual degree of misalignment present, and the strength of prism that will be required to rectify the misalignment. This test is actually performed with prisms but is otherwise similar to the Hirschberg test. While the penlight is being shined into the eyes, prisms of varying strength are placed in front of the eye. When the light is reflected in the center of each eye, the correct prism strength has been selected.
There are three types of cover tests. With the cover/uncover test, one eye is covered and the other is left uncovered. The doctor will look for any movement in the eye that is left uncovered.
For the prism and alternate cover test, the eye that is covered will be alternated, and a prism will be placed in front of the uncovered eye. This will help measure the difference between the two eyes and help determine which prism is right for your eyes.
For the Maddox test, a line, either vertical or horizontal, is placed in front of one eye. A white light is placed in front of the other eye. Whether or not the light superimposes the line helps determine deviation in the eyes. There is a variation of this test where two lines are placed before the eyes and rotated, and the angle of rotation necessary to make the lines parallel can help determine what type of double vision is present. Prisms can also be used to determine the distance between the two images.
Prism Glasses and Prescription Contact Lenses
Prism glasses, like normal eyeglasses, are measured and prescribed with a unit of measurement. For prism lenses, this unit of measurement is called prism diopters. As well as the strength of the prism, the positioning of the prism inside the lens, whether it be horizontal or diagonal, is also determined by the type of double vision you have. Prisms can be placed in one or both lenses.
If you or your doctor are unsure if a prism will work for you, a thin, press-on vinyl sticker called a Fresnei prism can be placed on your glasses. It is slightly visible from the outside and is thus not ideal for the long term, but it allows you to test drive a prism and see how it works for you. If for any reason your prism prescription may not be stable, Fresnei prisms can also be used.
For those with longer-term diplopia, the prism can be ground into the lens of your eyeglasses. In this case, the eyeglasses will, for the most part, look the same as any other glasses on the outside, though one lens may be slightly thicker than the other.
Contact Shreveport Eye Specialists
If you would like more information about prism glasses, please feel free to contact us, (318) 771-7597. Our team is here to answer all questions you have regarding any eye conditions you may be suffering from. We always work diligently to assist our patients efficiently and answer any questions you may have.
To provide adequate one-on-one time with our patients, our office is by appointment only. Please contact our front office staff prior to visiting so we can confirm.