Yes, prism glasses can correct certain types of double vision. Double vision occurs when a person sees two images of a single object, which can be disorienting and disruptive to daily life. Prism glasses are specialized lenses designed to bend light before it enters the eyes, effectively realigning the images so the brain can fuse them into a single, clear picture.
Understanding Double Vision and Its Causes
Double vision is a symptom that signals a breakdown in the visual system, which normally coordinates the input from two eyes into one cohesive image. To determine if prism glasses are an appropriate solution, eye care professionals first categorize the double vision into one of two main types.
The first type is monocular diplopia, where the double image persists even when the person covers one eye. This form of diplopia is usually related to issues within the eye itself, such as cataracts, corneal irregularities, or severe astigmatism. Prism lenses are generally not an effective treatment for monocular diplopia because the problem originates within a single eye’s optical system.
The second, and most common, type is binocular diplopia, where the double vision disappears when either eye is covered. This condition is the result of eye misalignment. Binocular diplopia is the condition that prism glasses are specifically designed to treat.
Binocular diplopia results from eye misalignment caused by various issues. These include problems with the eye muscles, such as strabismus (crossed eyes or wall eyes), or conditions like Graves’ disease or myasthenia gravis. Neurological events, such as a stroke, head injury, or cranial nerve palsies, can also weaken the nerves controlling eye movement. This lack of coordination forces the brain to receive two slightly different images it cannot successfully merge.
How Prism Lenses Realign Vision
Prism lenses function by using a principle called refraction to compensate for the eyes’ physical misalignment. Unlike standard corrective lenses that simply focus light onto the retina, a prism lens shifts the image’s perceived location.
A prism is essentially a wedge-shaped piece of lens material with a thick edge, called the base, and a thin edge, called the apex. Light entering a prism is always bent toward the base, which means the image the person sees is shifted toward the apex. By carefully orienting the base of the prism in the glasses, the light rays entering the misaligned eye are redirected.
The light bending ensures the image from the misaligned eye falls onto the fovea, the area of the retina responsible for sharpest central vision. This optical trick moves the image without forcing the eye muscles to strain for physical alignment. When images from both eyes are directed onto corresponding points on the retinas, the brain successfully fuses the two signals into a single, clear picture.
Prism prescriptions are measured in prism diopters, which indicate the strength of the light-bending capability needed to correct the deviation. The prescription also specifies the base direction—in, out, up, or down—which tells the lab how the thickest part of the prism must be oriented to counteract the specific direction of the eye misalignment. This prism correction is separate from any standard prescription for nearsightedness or farsightedness, though it can be incorporated into the same lens.
The Process of Obtaining and Using Prism Glasses
Obtaining prism glasses begins with a comprehensive eye examination by an eye care professional who specializes in binocular vision. These specialists use specific diagnostic tests, including the cover/uncover test and the Maddox Rod test, to precisely measure the degree of eye deviation. This measurement is crucial for determining the exact strength and orientation of the prism required.
The prism correction can be incorporated into glasses in two main ways. For a permanent solution, the prism is ground directly into the lens itself, often split between both lenses to maintain a more uniform appearance and reduce lens thickness. If a large amount of prism is needed, or if the eye condition is temporary or fluctuating, a Fresnel prism may be used.
A Fresnel prism is a thin, flexible vinyl sheet that is temporarily pressed onto the surface of an existing lens. While they can slightly reduce visual clarity and are more visible than ground-in prisms, they allow for easy adjustments to the prism strength as the patient’s condition changes. They are also often used as a trial to test if prism correction will be effective before a permanent lens is ordered.
Patients should anticipate an initial adaptation period when first using prism glasses. The brain must learn to interpret the slightly altered visual input, and some people may temporarily experience mild side effects like a feeling of imbalance, slight distortion, or headaches. Prism glasses are often a long-term management tool, but they may need periodic adjustments, particularly if the underlying cause of the eye misalignment is progressive.