What are they and how do they work?
Our eye movements are controlled by twelve different muscles, and if only one of these
muscles is slightly weak, you can experience eye strain, headaches or even double
vision. If your optometrist spots an eye muscle weakness, they can incorporate a prism
into your normal lenses to make your vision more comfortable.
Treatments of double vision
In severe cases, eye muscle weaknesses can result in double vision. This is when you see two images of a single object at the same time. The images can be completely separate or overlapping, they can also be side by side or above and below each other.
Depending on the underlying cause, people can experience double vision constantly, or sometimes it’s only an occasional problem.
Double vision can sometimes be a sign of something more serious, so if you also experience eye or head pain with your double vision, call 111 or go to your nearest A&E as soon as you can.
How do prism correction lenses work?
In order for us to see a clear image, light coming into the eye must focus on an area at the back of the eye, called the retina. This is where the light messages are translated and sent on to the brain to produce the images we see.
For people who are long- or short-sighted, the light either focuses just in front of the retina, or behind it, which makes the images we see blurry. Glasses for these eye conditions are designed to bend the light coming into the eye so it focuses on the retina, producing a clear image.
If you have an eye muscle weakness, the remaining muscles work harder to prevent you seeing double vision, which can result in eye strain and headaches. In severe cases it can cause double vision.
Prism lenses work by bending the light in such a way that it compensates for an eye muscle weakness, aligning your eyes and helping to maintain comfortable vision, as well as preventing double vision.
Small amounts of prisms are usually incorporated straight into your lenses. If larger amounts of prisms are needed, then they can be trialled using a stick-on lens before adding into the lenses.
Your prism prescription and glasses
Your glasses prescription is an instruction for the lab to use when making up your glasses. It’ll show
the type of vision you have, with numbers to indicate how strong your lenses need to be and where
they should be positioned within your frame.
If there is a number in the ‘PRISM’ section of your prescription, this shows that you have an eye
muscle weakness which needs correcting and how strong that correction needs to be. The ‘BASE’
section tells the lab how the prism should be oriented. A prism can be prescribed in one or both
lenses, but is often split between the two to improve the appearance of the lenses.