It is important to measure monocular PD because it can lead to a diagnosis of amblyopia. There are many ways you can measure Monokular PD, but the most accurate way is by using cycloplegia, which requires an eye patch and instilling atropine or cyclopentolate drops in one eye for 10-15 minutes. It’s also possible to use vertical prism diopter (VPD) testing and full-field auto refraction with pupil accommodation, but these tests may not be as accurate.
What is monocular PD and why is it important to measure?
Monocular PD is the difference in diopters between the two eyes of a patient. For example, if someone has a prescription of -1.50 (-0.75 in each eye) in one eye and +0.25 (-0.5 in each eye) in the other, then their monocular PD would be 0.25 mm (-0 + 1). Monitoring monocular PD can help with diagnosis of certain eye problems or can be used to determine whether surgery is necessary for the patient’s eyesight to be corrected.
A higher PD value indicates that the patient may need surgery on one or both of their eyes to have better vision. It also indicates that there is an issue with the way their eyes are working together to produce vision. The opposite is true if the PD value is low, since it means that surgery may not be necessary and that there are no issues with their eyes working together. However, a low monocular PD should still be checked regularly since it can change over time, particularly in young children who are still developing.
Monocular PD needs to be measured using a phoropter which contains lenses of different dioptric powers (and therefore magnifications). This equipment enables you to test both eyes at once, and swap between them quickly so you do not have to keep swapping lenses back and forth each time you want to test another eye. Phoropters are available for rent or purchase from most vision care providers, some of which will let you rent or purchase the equipment for a month at a time.
How to measure monocular PD?
Place both lenses in the phoropter so that they are facing the patient (e.g., right lens on one side of phoropter and left lens on other). Have patient look through first lens in phoropter at near target. While patient looks through the first lens, adjust dioptric power until patient has a clear image of the target without straining their eyes or moving their head up or down excessively. Slide second lens into place over first with help from aid and then have a patient look through the second lens at near target. While patient looks through a second lens, adjust dioptric power until patient has clear vision with their eyes parallel and not diverging or converging excessively. Record measurements of each eye.
We hope this blog post has given you some insight into how to measure monocular PD. If you are unsure about any of the steps we have outlined, call us at your convenience and one of our customer service representatives will be happy to help answer questions you may have.