Diplopia (double vision) is a rare but worrying condition. It happens when images of objects in front of you form in different parts of the retina of your two eyes. Many eye and nervous system disorders cause you to see two of everything. This condition can make your daily activities difficult. Prismatic lenses have the property of changing the direction of light rays. They can be used to ensure that both of your eyes are forming images in the same place. Prismatic lenses are useful both for assessing double vision and for reversing it.
What is double vision?
Diplopia, or double vision, is a condition in which you see two of everything. The two images can be above and below each other, side by side, tilted, or any combination thereof.
The eyeball has six muscles attached to it, which are controlled by three cranial nerves. If one of these muscles isn’t working as it should, your eyes don’t move in pairs and become misaligned. It makes you see double. Diplopia makes judging distances difficult, and reading and daily tasks may become impossible.
Double vision has several causes:
Double vision can be a sign of a serious illness, so you should always see your doctor if it happens. They will try to find the cause and begin treatment. They will also refer you to an ophthalmologist or optometrist for relief of double vision.
Double vision can sometimes be treated with eye exercises to improve weak muscle strength. Your doctor can give you a prism to stick on your glasses temporarily. If you see better with these temporary prisms, the prisms can be added to your prescription glasses. Sometimes your eye doctor may give you a prism to further separate the two images so that one is ignored. If none of these methods work, you can cover one eye at a time to stop seeing double.
What are prismatic lenses?
Prismatic lenses are made of plastic or glass and are wedge or triangle shaped. Light rays change direction as they enter the prism lens and again as they exit. This property is used to ensure that images of objects are formed in the same area of the retina in both eyes.
Prismatic lenses, despite their name, have no focusing power. They do not correct refractive errors such as farsightedness (farsightedness) or nearsightedness (myopia). Their purpose is to change the location of the image in your eye and improve eye alignment.
Temporary prism lenses are vinyl and attach to your regular lenses. These temporary prisms are called Fresnel prisms. They allow you to try out prism lenses before you get permanent prisms. They are also useful if your diplopia is likely to be temporary or changing. The vision through these temporary adhesive prisms is not completely clear.
How do prism glasses work?
Prism glasses work by changing the direction of light rays. Usually your eyes move together in all directions. Their axes are aligned and the images of both eyes are formed on the same parts of their retinas.
When your eyes are misaligned, they don’t move properly together. Images form on different parts of your retinas and you see double. Prism glasses compensate for eye misalignment by redirecting light rays to align the two images.
Prismatic lenses are measured and prescribed in prismatic diopters. The axis must also be specified — horizontal, vertical, or oblique. If the power required is high, it can be split in two because the alignment is a function of the two eyes. Your eye doctor will have you try on prism glasses by attaching temporary prisms to your existing glasses. Once you find them comfortable, they can be ground into new glasses.
What is the purpose of prism glasses?
Prism glasses have two uses when dealing with diplopia. Your doctor will measure the misalignment of your eyes by placing prism glasses of different strengths in front of your eyes until your double vision disappears.
The Krimsky test uses prisms to measure the degree of eye misalignment. Your eye doctor will ask you to look at a penlight placed in front of you. They will put prisms of different strengths in front of your eye until the reflection in both eyes is in the center. This tells your doctor the required prism power.
The other use, of course, is to correct your double vision. Your doctor will prescribe the strength and alignment of prism glasses you need. You can have these grinded in your prescription glasses.
What do prism glasses look like?
When using a prism for the first time, your doctor will give you temporary prisms (Fresnel prisms) to stick on your glasses. These are slightly visible on your glasses.
Once it is clear that you need prisms and the prisms correct your double vision, your eye doctor will prescribe prismatic lenses for long-term use. These prisms will be ground into your usual glasses. Your glasses will look exactly the same as before, but one of the lenses might be thicker.
What are the side effects of wearing prism glasses?
Any new pair of glasses can cause eye strain for the first few days. Other symptoms are quite rare but can include:
- eye pain
- Double vision
Prism glasses are more likely to cause such problems when the prescription is incorrect for your eyes. This may happen because your misalignment changed after your eyes were tested. You should see your eye doctor again. They can test your eyes again and prepare a new prescription. You should never wear prism glasses unless your eye doctor has prescribed them for you.
Double vision can be disabling. Driving is prohibited in some countries if you have it. Treatment with prisms can restore your vision and improve your quality of life. Prism glasses are successful in 68-88% of people with diplopia and are a valuable non-surgical treatment option. Strabismus surgery is an option if prism glasses don’t work.