The College of Optometrists

hiya all hope all is well and good

I had a question with regards to taking distance PD's by someone who is monocular. Other canthus to inner canthus is too variable and taking the near PD and adding 3/4mm is incorrect. So how does a monocular individual take distance PD's.

Many thanks

You can use a pupilometer. I know of no other way that is accurate. However, you can get a decent reading by using a light from a torch placed just under your dominant eye to make a reflex on the cornea.

if you line the ruler up on the corneal reflex of the px eye opposite yr dominant eye - butyou twist yr face towards that you - then you twist yr face the other way and read the ruler at the other eyes corneal reflex. You have to twist your face as far as you would for your pd - which is like a little wag which you can judge by practicing in a mirror. hope it helps - palfi

many thanks for your reply, however using a pupilometer is rather cumbersome in a routine. i do not understand the second cornal reflex method you have mentioned, would you be kind enough to elaborate further and what do you mean by twisting, is this a literal twist of the head or is it a lateral movement of the head across to the px's other eye. many thanks

its easy --- you look straight on to yr px and line up the ruler with yr good eye to his eye that is oposite yr good eye. Then you (as my tutor said) put your nose over nipple (by twisting yr face) so your good eye is now opposite the other eye - read ruler. Quite accurate. I will also explain this mether on theoptom.com website for benefit of other pre-reg students. palfio

A common rule-of-thumb for estimating the near PD from the distance PD is to minus 2.5 mm.

You could invert this guide, ie measure near PD, then add 2.5 mm. I accept that it will not be perfect, but is very unlikely to be out by > 1mm. This is adequate for refraction purposes.

If you subsequently dispense, then a pupillometer can be used for greater accuracy.

-which the same as doing the half pd ( you put the '0' of ruler to middle of bridge - and then measure where the corneal reflex comes on the ruler) and doubling = which could possibly be more accurate, I wonder.

problem then if Px has assymetric pds... Use the pupillometer, or get good at your error correction judgement.