Loose prisms help to train relative vergences which involve changing the alignment of our eyes while keeping the object we are looking at clear and single. Prisms are triangular lenses that move the appearance of the world in the direction of the narrow edge when you look through them. When you look with one eye through a prism the eyes either have to converge (move together in the direction of the nose) or diverge (move apart) in order to not get double vision. We make these vergence movements every second of the day to focus on near and far objects.
Most patients have a range of prism strength that they can look through without getting double vision. Many patients with binocular vision problems have a reduced vergence range which may lead to double vision, blur, fatigue and headache. For example, someone with reduced convergence may get double vision when looking through a 10ʌ prism but a normal patient may be able to still see single with a stronger 25ʌ prism. This can be improved with loose prism training. Having a normal vergence range will help the visual system change and maintain its focus across a range of distances.
Do 3 of these 1 minute cycles, three times per day.
If you are doing well at your follow-up appointments, your optometrist may swap your prism for a higher power to make the exercises more difficult. Doing these prism exercises on a far away object will be harder than near. If the pen is easy to cycle through at 40cm, move to a clear object such as a door knob at 1 metre. Your optometrist may recommend that you watch TV whilst dropping your prism in and out every 3 seconds!
Remember: For these exercises to be effective you need to notice that the pen goes double first, then becomes single. If the pen never becomes double and someone watching your eyes whilst you do the exercises cannot see your eye behind the prism moving left and right, you may be suppressing one eye - discuss this with your optometrist. Please ask us if you have any other questions about the exercises.