The College of Optometrists

hi all

just wondering, how do we work out the amount of prism we would give to a patient if mallet unit results aren't available and we just have near and distance phoria measurments?(i can work out ac/a ratios with n and d phoria results but wouldn't know how to estimate prism.)

thanks

Hi irb

there is a couple of basics before we begin.

First is that with spex - you are limited to about 3 prism dioptre per lens depending on the power of the lens.

Second is that the correcting prism mat even be contrary to any dissociated phoria findings.So prisms based on phorias may be wrong. I tried hard to find a way of calculating a prism from ACA's and phorias -and never found a working and reliable answer!

An objective way is to watch the eyes movements while conducting cover test - the 'correct' prism will give a quick fusional movement when the cover is removed.

A subjective way is to use a prism flipper - which is made from a 2 diopter prism for horizontal, and a half dioptre prism for verticals.

You use this like a crossed cyl - which is better one or two? Listening for the patients responses - you adjust the prism until both flips give the same response.

Hi

thanks for your reply.

was also trying to apply the above to the samlple cdm question on the college website. the patient has a 4 dioptre add and a 2 dioptre xop at distance and 9 dioptre xop at near,clearly some base in prism will help. if we weren't given the near fixation disparity how could we estimate the amount of prism the patient would need based on the n and d phoria measurements.

would it be based on your first point about +/- 3 dioptres per lens/ on ac/a ratio?

thanks

Sorry - I missed your new question. There is no hard and fast rules for this. A few theoretical methods date back to the 1920's and are not very acurate either.

I forget the example given -so only trusting memory. The px is too old to have much accomodation and relies on +4 reading add to help with her reading, which is compromised by early AMD. She has 9d prisms XOP - but 'normal' fusion ability is to absorb only 5 XOP. -So she has to actively converge the extra 4 prisms - and this can be straining -so you give 2 in R and L. This will help with any binocular strain.

You notice that she reads at 25cm - and its this that causes the excess XOP at 'near'. So you can assume she has a slightly reduced ability to converge - which is typical in the elderly.

Quick question re: FR. Does anyone know what the norms are, as I would assume this is important for us to know in order to determine what prism to prescribe based on Mallet Unit findings (which is what we will be given on CDM)