Vision Training With Loose Prisms

What Is This Exercise For?

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.

What will you need?

  • One loose prism (your optometrist will prescribe a certain power for you) from Innovative Eye Care
  • A pen/pencil

Instructions:

  1. First identify which side of the prism has the thick side - this is called the ‘base’. Imagine an Egyptian pyramid - the thickest side closest to the ground is called the ‘base’ of the pyramid! Your optometrist will have told you whether the base of the prism is to be on the outside (further from the nose) or the inside (closer to the nose).
  2. Hold your pen at 40cm from your nose in front of you. Focus on the pen.
  3. Move the prism with the correct base direction in front of one eye ‒ see image (it does not matter which eye). You will see the pen go double.
  4. Voluntarily bring the two images together to make the pen appear single and clear. Hold this for 2 seconds.
  5. Remove the prism. Once again the pen may go double quickly before your eyes- make the two images single again. Continue these cycles for 1 minute (20 cycles) then relax for 30 seconds.

Do 3 of these 1 minute cycles, three times per day.

CAPTION: Holding pen out 40cm and prism in front of eye

  • 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.