Every day, many of us prescribe contact lenses to correct our patient’s vision. Although much attention is paid to assuring the lenses are a good fit and are comfortable for the wearer, optimal correction of the wearer’s ametropia and improvement of their visual performance critically hinges on harmonisation of the lens optics with the wearer’s own optics.
There are a number of optical considerations worth bearing in mind when a patient moves from a spectacle to a contact lens correction, considerations that all too often are ignored. The case scenario for this exercise was chosen to highlight these for discussion.
Unlike the previous exercises, there were far fewer responses submitted for this exercise. Either this is a topic too few feel comfortable discussing, or maybe the thought of discussing optics during the early winter months was less than appealing. Most people were comfortable in discussing a toric option for the right eye, many chose a multifocal option as first choice (including the possibility of a toric multifocal for the right eye), but too few considered the implications of impact upon binocular status and stability. This was perhaps unfair, as it did not feature in the source material, and so is the main focus for this feedback feature here.
Case Scenario
A 48-year-old patient attends your practice and asks about the possibility of contact lenses. They have the following spectacle refraction:
- R: -4.75/-0.75 x 180 (6/5)
- L: -5.00/-0.50 x 180 (6/5)
- Add: +1.00DS (N6 R+L, range 1m to 20cm)
- Cover test:
- Distance: orthophoric with and without Rx
- Near:
- With Rx (at 40cm): orthophoria
- Without Rx (at 20cm): 4Δ XOP, good recovery
They are now struggling to read with their spectacles in place and find the working distance for reading too close when removing their spectacles. They have had multifocal spectacle options explained to them already, but are interested to know if there might be a contact lens alternative as, ideally, they would ‘love not to have to wear specs.’
For discussions, the following questions were posed:
- When a myope moves from spectacles to contact lenses, are there any optical considerations that might affect the binocular status?
- What might be your first choice recommendation for a contact lens correction and why?
- What would be your priorities when first assessing the fit and vision through the lenses?
- What advice might you give to the patient before they first trial your choice to maximise the potential for their adaptation to the correction?
Binocular Considerations
Spectacle lenses have the potential for inducing prism. With a hyperopic correction, where the lens is thicker in the centre, any point on the lens outside the centration point will offer base in prism when viewing temporally, base up inferiorly, base down superiorly and base out nasally. For negatively powered lenses, as in this case, the opposite is the case: base out temporally, base up superiorly, base down inferiorly and base in nasally.
The prescription here is roughly isometropic and so, for versional movements where both eyes coordinate to look along the same direction, the prism presented before each eye should be similar for vertical movement and cancel out for lateral movement. Obviously, in cases of anisometropia, lateral induced prism is usually well absorbed by the horizontal fusional reserves, while even small differences in vertical prism may lead to binocular instability due to the restricted vertical fusional reserves we all have. This is for a discussion elsewhere.
When moving from spectacles to contact lenses, it is for vergence movements, where each eye moves in a different direction as with convergence, that prismatic influence may need consideration. Consider the cover test results above which, at first glance, seem completely normal. With spectacles in place, the patient is orthophoric at distance and near. Without their spectacles, however, there is a small exophoria (albeit a compensated one) detected when a target at 20cm is viewed.
At 20cm, this patient will not require any significant accommodation. However, with their spectacles in place, each eye will have a significant base-in prism to support the convergence. Using everybody’s favourite centimetre-based formula, if a near target requires viewing through each reading lens around 1cm nasally to the centration point, this will offer almost 4Δ base-in prism for each eye. The extra convergence required without the specs, and indeed if viewing through contact lenses, is likely the reason for the manifest phoria.
Early presbyopia somewhat complicates the result, in that accommodation itself is associated with convergence. This is why some younger myopes presenting with near exophorias causing problems when reading without their spectacles are often best managed by simply encouraging them to wear their distance spectacles all the time. The extra accommodation this requires for near work may well overcome any decompensation of a near exophoria.
As a general rule, the higher a myopic correction, the more help with convergence is provided with spectacles and the greater the extra convergence required when moving to contact lenses.
One further point. A discussed in the review of toric contact lenses on page 20 of this issue, toric contact lenses rely on differences in thickness in order to stabilise the lens rotation to correct astigmatism. In this patient, a toric contact lens might be sensible for the right eye as, at 0.75DC, the refractive error just meets the level of cylinder likely to be appreciatively improved with a toric correction. Depending on the design of toric lens chosen, and bearing in mind that the effect will be small in this case, there is the potential to introduce some vertical prismatic difference between the two eyes.
Lens Choice
The prescription was at a level where back vertex distance would require some change to the power of the contact lenses; less minus would be required. This was remembered by most, as was the need to consider a toric lens option for the right eye. Suggesting a multifocal option was not universal. This was considered by some to be potentially problematic as the patient is myopic and therefore might be less likely to adjust to any compromise in their near vision if they had previously been viewing near targets uncorrected. However, with improvements in design of simultaneous vision lenses and the advent of toric multifocals makes this a viable first option; if offered appropriately and positively.
Initial Assessment
Key observations of the initial fit must always include checking for good centration; this is key to any successful multifocal fit. Observation of rotational movement is essential where a toric is introduced.
Adaptation is essential for any contact lens fit. This is especially so when a multifocal simultaneous vision option is prescribed. During the testing up to the point of application of the first lens, eye care professionals should be able to gauge the likely blur sensitivity of a patient (figure 2); in other words, predict the expectations the patient has of the vision with the lenses and be able to communicate effectively to ensure a realistic goal is expected and achieved.
Trial
The key to initial adaptation, especially for a simultaneous vision correction, is to encourage the patient to adapt to their lenses within their own visual world. They should always be encouraged to view, binocularly, real life targets (smartphones, tablets, televisions, cinema screens and so on) while remembering that they are now so doing without any spectacles. As the always insightful Professor Lyndon Jones pointed out, when you first prescribe progressive power spectacle lenses, you do not concentrate on the likely blur and distortion the patient is going to experience. So why is this all too often the case when prescribing multifocal contact lenses?
- Neil Retallic is President of the BCLA
Source Material
- Richdale K, Cox I, Kollbaum P, Bullimore MA, Bakaraju RC, Gifford P, et al. BCLA CLEAR – Contact lens optics. Contact Lens and Anterior Eye, 2021;44(2):220-39. https://doi.org/10.1016/j.clae.2021.02.005