You may have heard the term ‘prism glasses’ when searching
for a new pair of specs, or have been prescribed them yourself. If you still
need a hand getting to grips with the term or are unsure if you need them, read
on to find out what prism glasses are and how they help correct your vision.
What are prism glasses?
Prism glasses are prescribed to those who have diplopia,
also known as double vision. They have a slimmer apex and thinner base than
regular glasses, allowing light to bend differently to sharpen your focus. This
aligns the two images caused by double vision and instead trains the eye to see
a single object as seen by people without this issue.
How do prism glasses work?
To understand how prism correction works, you must first
understand how we see things through our eyes. Light enters each eye which then
passes through the cornea and onto the retina. With regular vision, the eyes
work simultaneously to ensure the brain sees a single image when viewing an
object as the light falls on the same part of the retina in each eye.
When you have double vision, two images are seen instead of
one. This occurs because the light falls in different places on each retina,
resulting in two images of one object to be seen.
Prism glasses work by bending the light before it hits your cornea and retina, ensuring it falls in the right place. This makes the brain work hard to combine the two images, fusing them together to result in a single image.
Who should have prism correction?
Prism correction is for people with double vision. This
issue can be caused by a manner of things, including:
- Neurological problems, such as a stroke, head injuries
or migraines - Issues with the eye muscle, including Graves’ disease
or strabismus. - Nerve issues, including multiple sclerosis
You must see your doctor if your double vision occurs
suddenly or there is an unknown reason for it. They may recommend a different
form of treatment based on your personal circumstances and issues surrounding
your vision.
How do I get prism glasses?
If you think you need prism correction, you’ll undergo one
or more of the following tests. This largely depends on the severity of your
condition, as well as the potential reasons behind it:
- Hirschberg test: A penlight is shone into the eyes and
when the patient looks at it the corneal reflection should show up in the
centre of each pupil. If the reflection is off-centre in either eye, prism
glasses may be needed and prescribed.
- Krimsky test: This is used to measure how severe the
misalignment of the eyes is. This also uses a penlight, but this time, prisms
of various strengths are held in front of the eye. In this case, you have found
the correct prism measurement when it brings the reflection of the eye to the
centre.
- Cover tests: There are a few different types of cover
tests; the type used depends on the potential problems at-hand. The most common
of all is the cover/uncover test, which is where a doctor covers one eye and
leaves the other uncovered.
As mentioned, the type of test used depends on the issues
and your prior health history. If you’re prescribed prism glasses, the prism
itself will be placed vertically, horizontally or diagonally. This depends on
your specific issue and may be placed in one or both of your lenses.