Once it has been concluded that refractive correction alone will not be sufficient to improve vision to the required level, magnification will be needed. This may be combined with other strategies, such as training in eccentric viewing, nonoptical aids or tints, in order to optimise performance. Multiple aids and strategies are likely to be required: it is unlikely that one approach would be appropriate for all tasks.
Each task will require consideration, so it is important to prioritise the most significant concerns at the first assessment. For example, if someone is struggling to stay at work or is unable to manage their medication then this should be addressed at the first visit, with less important tasks left for a future assessment.
For each task, the following routine should be followed:
- 1.
Determine the strategy most likely to help (e.g. an optical low vision aid, electronic magnification, sensory substitution, or another approach).
- 2.
In the case of magnification, predict the likely magnification required.
- 3.
Determine whether binocular or monocular correction would be preferable.
- 4.
Select an appropriate low vision aid, if required.
- 5.
Demonstrate the device on the required task, and modify it if necessary.
- 6.
Determine the required spectacle correction.
- 7.
Provide instruction for the low vision aid, and consider if training is required.
- 8.
Issue the aid for trial in the ‘real world’.
- 9.
Make any referrals needed to other services.
- 10.
Plan a suitable review visit.
It may be that some tasks, such as driving, cannot be safely performed with any aid for legal or practical reasons. This should be sensitively but clearly discussed with the patient. If a task cannot be performed with magnification, then it is far better to explain this in the consultation room rather than to prescribe an aid which is unlikely to help: ‘false hope is a cruel deception’. It can also be helpful to manage patient expectations with an explanation of the nature of magnifiers: for example, that these change the way the task is performed, or make the task slower, and that some practice is needed to get the best performance.
Determine the Strategy Most Likely to Help
Consider the task that the patient would like to perform, their level of vision, and their ability to use different approaches. For example, if someone would like to read printed bank statements, would they find it easiest to use online banking and a screen reader? If an academic is struggling to see slides at scientific conferences, would it be more plausible for her to use a distance telescope or to take and enlarge pictures using her iPad? Would a retired person wanting to read paperback books be more likely to succeed with a text-to-speech system like Orcam, or with a high-powered hand magnifier?
Discussing the possible options with the patient will help identify the best strategies: the approach would be very different for someone who enjoys trying new things on their computer, for someone who falls asleep when listening to audiobooks, and for someone who does not own a mobile phone.
Predict the Magnification Required
Predicting what magnification will be required for a task means that you can more quickly select a magnifier from the many possibilities you have. Showing a magnifier which is too weak and cannot perform the task can cause the patient to become disillusioned; showing a device which is too strong might be difficult for the patient to align and give the false impression that low vision aids are hard to use.
Predicting the likely magnification required also helps to check how the patient responds to magnification. If the acuity achieved with a device is less than expected, it may be that other strategies such as eccentric viewing training are required; if the acuity is better than predicted, the presenting acuity may have been measured incorrectly and it should be assessed again, perhaps with more encouragement. For example, if acuity is much better than predicted using a telescope, it may suggest glare problems which are aided by the restricted aperture; if much worse than expected for reading, it often suggests a central scotoma.
Although predictions are useful to assess the magnification level, they should only be taken as a first approximation and will not determine the success with using a magnifier. Vision is complex and success with a magnifier is multifactorial: contrast sensitivity, scotoma size and position, eye movements, fixation stability, binocular interactions, magnifier handling and cognitive factors will all affect the final performance of everyday tasks with a device.
Distance Magnification
For distance tasks, the Snellen equivalent of the visual acuity (VA) can be used to predict magnification. If someone wants to read television subtitles of size 6/9 and their acuity is 6/18, then they will need 2× magnification, or to halve their viewing distance, to be able to resolve the target. They might be able to see the subtitles with a 10× telescope, but this would have an unnecessarily small field of view and higher aberrations than a 2× or 4× system, which may be perfectly adequate for the task.
The calculation performed is simple:
Magnification required=(required VA)/(present VA)
For example, in Snellen notation to improve from 6/60 to 6/6:
Magnification required=(6×60)/(6×6)=10×
Or to improve from 2/36 to 6/18:
Magnification required=(6×108)/(6×18)=6×
(note that the Snellen value was converted to 6 m, by multiplying 36 by (6/2)).
The same method can also be used to assess the improvement which might be achieved with a particular device:
Magnification used=(achieved VA)/(present VA)
so
Achieved VA=(magnification used)×(present VA)
For example, a patient with VA of 6/36, using a 4× telescope, should achieve VA of:
Achieved VA=4×(6/36)=6/9
Note that performing this calculation on a calculator gives a decimal acuity (in this case 0.67). The lower value of the Snellen fraction can then be found using the sum (6/0.67). Someone with a VA of 6/18, using a 2× telescope to view a letter chart at 3 m, should have an acuity of:
Achieved VA=2×(6/18)=6/9=3/4.5
so the final acuity would be 3/4.5 (or 3/5 as that is the next largest letter size on a standard chart).
These calculations are easy to make when using Snellen notation. When using a logMAR chart, it can be remembered that the angular size of letters halves after three lines: for example, 0.7 logMAR is half the size of 1.0 logMAR, and 0.1 logMAR is half the size of 0.4 logMAR.
Each line on the logMAR chart is 1.25 times smaller than the line above, so an alternative way to calculate this is to use the formula:
Magnification=(1.25)n
where n is the ‘number of steps’.
As noted earlier, the change of three lines in logMAR VA is a magnification = 1.25 3 = 1.95 ≈ 2×
For example, if the current acuity is 0.5 logMAR, and 0.1 logMAR is required, then:
Magnification required=(1.25)4=2.44×
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