Many patients believe one pair of spectacles will do the job for every task, but this may change over time. That’s why practitioners need to prepare for this scenario, including conversations around a second pair, writes NICOLA PEAPER.
The frequent question that lens sales reps and customer service operatives are asked is: “Which of your multifocal lens designs will work best for a patient who wants to drive and use a desktop computer?” The reply is, unless the patient still has a level of accommodation, none. To have an ergonomically correct position for a desktop screen the patient needs to be able to see around 80 cm with their head in a straight-ahead position. This means that the power at pupil height, made up of accommodation and lens power, needs to be +1.25 D. If it is less than that the patient will have to lift their chin, potentially exposing them to computer back/neck syndrome.
Consider a first time presbyope with a +1.25 D addition. They have at least 1.25 D of accommodation left
which is sufficient to see a screen at arm’s length. Looking through a point at the fitting cross, any progressive lens design will work to give them clarity for screen use. As the presbyopia goes up the patient will probably start to lift their chin to access some of the add but, as the add is low the subsequent corridor width is high, they will probably have enough width for a couple of screens.
When the addition goes up to
+2.00 D and there is only +0.50 D of residual accommodation, they will now need at least +0.75 D of power from the corridor of the multifocal. To access this power they will need to lift their chin. If the corridor is 16 mm long and we assume the power comes in at a rate of 0.13D/mm (2.00D/16mm), to achieve at least +0.75 D, the patient must use a point 5.8 mm below the fitting cross. This will probably be in the narrowest part of the corridor.
One regular response is to shorten the corridor and ‘push’ power up the lens. With a 14 mm corridor, the power rate is 0.14D/mm. Now the point at which
0.75 D is achieved is 5.4 mm down. This 0.4 mm gain is not going to be noticed by the patient. However, the subsequent reduction in corridor width and increase in head movement to see the full screen width will be.
The solution is to have an occupational lens for computer use, and the patient needs to be warned that this may happen at every point of their presbyopic journey. Patients can be very tolerant and so to warn that problems may occur and, if they do, there is a solution will set both practitioner and patient up for a second pair conversation.
When considering an occupational lens, choose one that is suitable for the visual task that will deliver the correct power at pupil height.
Digressive lenses were developed for close visual tasks and there are many different ‘PC’ type lenses on the market. These designs tend to have long corridors of 24 mm or more and the pupil sits about a third of the way down
They will always deliver the correct amount of power so that, depending on the level of residual accommodation, the patient will be in a comfortable posture for looking at screens. As the digression carries on above pupil, they also give depth of clear vision beyond the screen. These lenses are ordered with distance script and add as the digression will differ depending on accommodation available.
Because digressives have a corridor, as the addition increases the available width for screen use goes down. By the time the addition is over 2.00 D there probably will not be enough width for multiple screen use. In these cases, one option is to use a single vision lens with a boost of power, so called anti-fatigue lenses. Ordered with an intermediate script for screen use and a boost sufficient for working at 40 cm, this can be a good solution. The drawback is that the patient has less depth of vision and will not be able to see clearly beyond their screen.
The fact remains that many patients think that one pair of spectacles will always suffice for every task, and with low additions this maybe the case. However, good practice dictates that practitioners should warn that this may change and then be familiar with suitable second solutions.
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