Guidelines for predicting performance with low vision aids
This paper is now available in the low vision feature issue of Optometry and Vision Science here. I am pleased with this paper, as it has potential impact on how low vision clinicians carry out their visual function assessments: I’ll be amending my undergraduate teaching this year to take the findings into account, and would be very interested to hear from anyone who puts the recommendations of the paper into practice in their clinical assessments.
In essence, what we have found is that how well a visually impaired patient will read with a low vision aid / magnifier can be predicted from just a couple of standardised tests of clinical visual function. Firstly, measure acuity on a reading chart at a fixed distance with an appropriate add (we used an MNRead chart at 40cm with a +2.50D, but a Bailey Lovie chart or other log scaled chart, and other distances such as 25cm with +4.00D would be just as good). Those with acuity better than 0.85 logMAR should be able to read print of 1M size (around N8-N10: see the appendix of the paper for a discussion!) and be able to read fluently (>80wpm) with an optical aid of appropriate magnification. For those with acuity worse than this, whether they will be able to read 1M print with a magnifier is then dependent on contrast sensitivity: if CS is better than 1.05logCS (normal or noticeable loss) success is likely, whereas worse CS than this (moderate or severe loss) suggests a patient will be unable to read 1M print, even with a magnifier. Those with acuity worse than 1.00logMAR, or who read slowly with a fixed add, are also less likely to read fluently with a magnifier.
As far as appropriate magnification is concerned, we also looked at how much magnification was needed to raise patients from just being able to see print of a certain size to being able to read it comfortably (at their maximum reading speed). For most people, 2x the magnification to just see print was suitable for reading close to their max reading speed, but a quarter of people required more (sometimes a lot more) than 2x extra magnification to reach maximum reading speed: my clinical interpretation of this would be to always try a bit more magnification than you think is needed for a specific print size to see if reading speed improves.
I hope these guidelines will help clinicians to be able to determine more quickly what the best options are for their patients, working out from just a couple of standard measures who is likely to be able to achieve their reading goals with optical aids, and who might be better served by early referral towards sensory substitution aids.