Tips for maximizing comfort, stability for contact lens wearers with astigmatism
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Astigmatic contact lens wear poses two key challenges: comfort and stability of vision.
Comfort, of course, can be a challenge for wearers of any type of contact lens, so understanding the material and mechanical properties of a lens allows us to maximize comfort for our patients.
The edge profile of any soft contact lens has been proven to drive comfort, with a thin infinity edge being the most comfortable. When we consider toric lenses, the prism- and peri-ballasted stabilization methods can significantly increase the edge profile and thickness, resulting in lower lid sensation and decreasing comfort. Choosing a thin edge design that is not weighted, or ballasted, in the lower portion of the lens can reduce this sensation of discomfort; only Acuvue (Johnson & Johnson) toric lenses meet this criteria.
We also want to choose the lens material and replacement schedule carefully. To stay up-to-date on the latest lens technology and study results, I frequently turn to ClinicalTrials.gov, where lens performance and comfort studies conducted by different lens manufacturers are posted. Most of us now recognize that a single-use lens is the most comfortable — and the most compliant — modality for most patients, so a daily disposable lens, rather than 2-week or monthly replacement, is typically my first choice for astigmatic wearers.
I am a strong advocate of wetting agents that are embedded throughout the matrix of the lens, rather than being applied as surface treatments only. For me, the wetting agent of choice that seems to hold up well for all-day comfort is long-chain, high molecular weight polyvinylpyrrolidone, which effectively reduces friction and acts as an excellent lubricant.
Toric stability
If I correct for astigmatism in a given axis and the lens is not rotationally stable on the eye, the patient will obviously note a fluctuation in vision as the lens rotates on and off axis.
Traditionally, toric lens designs have relied on ballasting techniques that weigh the lower portion of the lens to help it stabilize with gravity. In addition to the impact the lower lid-lens interaction has on comfort, I also find that gravity makes stabilization somewhat unpredictable with a patient who has multiple activities throughout the day. If you are playing golf and twisting your head as you swing the golf club, or reclining on a couch or chair to watch television — essentially any activity where the head is not perfectly upright — gravity can rotate the lens incorrectly.
Another problem with prism-ballasted toric lenses is that, by definition, they introduce vertical prism into the optic zone of the lens. That’s not something we would consider acceptable in a spectacle lens, as it violates the ANSI standard for spectacles, so why would we accept that in a contact lens?
If I were to prescribe a prism-ballast lens in both eyes, the prism in these lenses equals out: Light is displaced equally in both eyes and will not be noticed by the patient. Alternatively, if I fit a toric lens in one eye and a spherical lens in the other eye — a common situation for many of our patients — the binocular disparity in vertical image displacement can inhibit fusion and could result in asthenopic complaints.
For both of these reasons, I prefer a non-ballasted, blink-stabilized design that relies on nasal and temporal stabilization zones to stabilize the lens with the blink pressure. This method of stabilization is not dependent on head position, is less disrupted by rapid eye movements and doesn’t introduce vertical prism.
Fitting tips
A careful lens fit is particularly relevant for a toric wearer. A good refraction is always necessary, and it is important to remember to vertex correct for both the sphere and cylinder.
Many times I find the higher minus patient with cylinder correction of –3.00 D and above can be corrected with a readily available lens because of the vertex correction. The lens should be allowed to stabilize for about 5 minutes on the eye before measuring rotation. Checking rotation after dynamic eye movements — looking up and to the right before fixating on a central visual target, for example — is ideal.
With contemporary toric lenses, I find that rotation is minimal. When it does occur, we can compensate for that rotation in the contact lens prescription, with a method such as the left add/right subtract, or LARS, rule.
Astigmatic wearers do face some challenges compared with their spherical counterparts, but it is worth addressing these with the best lens technology available. Some practitioners still fit a spherical lens and over-minus the patient to “mask” their astigmatism for those lower astigmatic patients. I don’t find that acceptable in a spectacle correction, nor in a contact lens. It has been noted that masking as little as –0.75 D of astigmatism with the spherical equivalent could result in slowing of reading speed by 24%, emphasizing why we should always strive to maximize our patients’ vision.
With attention to lens material, edge design and stability method, we can provide astigmats with similarly stable, clear vision that spherical patients enjoy.
References:
- Chamberlain P, et al. Optom Vis Sci. 2011;doi:10.1097/OPX.0b013e31820ea1ea.
- McIlraith R, et al. Cont Lens Anterior Eye. 2010; doi:10.1016/j.clae.2009.08.003.
- Sulley A, et al. Cont Lens Anterior Eye. 2015;doi:10.1016/j.clae.2015.02.006.
For more information:
Robert L. Rosenthal, OD, practices at EYECARE for You in Newtown, Pennsylvania. He can be reached at rosenthalfamilyeyecare@gmail.com.
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