Welcome to question of the day #230

Welcome to question of the day #230

I’m setting up my own practice. It’s my first and although I’m pretty sure what general equipment I need, I’m really unsure what near visual acuity test I should buy. What should I do?

It is confusing. There is a range of near vision tests. In my practice, I use the Faculty of Ophthalmologists near reading print chart and note the patient’s preferred working distance in the clinical records. However, I think I should invest in an M notation chart. I’ll speak more about this towards the end of this piece.

It’s important to know when doing near visual acuity testing, reduced Snellen visual acuity is only accurate at a fixed testing distance, which is 40 cm for most near visual acuity charts. I prefer to know the habitual working distance of each patient.

Some people use the Jaeger chart and not the near visual acuity in J numbers. However, J numbers have no precise meaning. The lack of external definition of Jaeger numbers makes them extremely variable. One study found that the size of J numbers varied from chart to chart, that is, for example, J4 in one version might be smaller or larger than J4 in another version. It is better not to use Jaeger charts for measuring near visual acuity.

I think the best method for near visual acuity testing uses the M-unit, which is the only letter size unit that is well defined. A 1M letter subtends five minutes of arc at 1M. A 1M sized letter is equivalent in size to newspaper print, 0.5M is equivalent to print half the size of newsprint and 2M is equivalent in size to 18-point large print and 0.5M is equivalent to print half the size of new.

When measuring near visual acuity, the patient should use their reading correction. Ask the patient to hold the card at their normal reading distance. Near visual acuity can be recorded as M-units at the testing distance, for example, 1.25M at 40 cm.