Refractive Error and Corneal Curvature Issues with Silicone Hydrogel Lens Wear

Kathryn Dumbleton - BSc (Hons), Uni of Wales1984 MCOptom 1985, MSc Uni of Waterloo 1988

Senior Researcher
Centre for Contact Lens Research (CCLR)
University of Waterloo, Ontario, Canada

Introduction

Silicone hydrogel (SH) materials have allowed clinicians to fit their patients with hydrogel lenses that supply sufficient oxygen to eliminate hypoxia. Interesting findings are frequently reported in the early phases of experience with new products and the impact and importance of these findings are often not fully understood. Such is the case with apparent changes in refraction and corneal curvature that may be observed in SH lens wearers.

Studies Investigating Changes in Refractive Error Associated with Extended Wear

Chronic corneal anoxia has been blamed for the myopic creep or shift associated with the wear of low oxygen transmissibility (Dk/t) hydrogel lenses1-5. An early clinical trial with silicone hydrogel (SH) materials reported no change in refractive error of eyes wearing high Dk/t SH lenses on an extended wear (EW) basis but a small increase in myopia in subjects contralateral eyes following EW of low Dk/t conventional hydrogel lenses6.

This finding was investigated further as part of a subsequent clinical trial conducted at the Centre for Contact Lens Research (CCLR) in which the overall clinical performance of high and low Dk/t lenses was investigated7. In this study an analysis was conducted to determine if refractive error and keratometry altered over a period of 9 months of 6 night EW with conventional low Dk/t (etafilcon A) lenses and up to 30 night EW with high Dk/t fluorosiloxane hydrogel lenses (lotrafilcon A) in a prospective parallel group study. Refractive error and corneal curvature were measured without contact lenses in place during the baseline, three, six and nine month visits for 62 subjects who completed the trial. Thirty nine participants were randomized to wear the lotrafilcon A lenses (Focus Night & Day™) and the remaining 23 participants wore the etafilcon A (Acuvue™) lenses.

The mean spherical refractive error increased by –0.30 DS ± 0.45DS (p<.0001) in subjects wearing the etafilcon A lenses but did not change in subjects wearing lotrafilcon A lenses (Figure 1).

There was however considerable variation in the degree of refractive error change between subjects. When stratified by baseline degree of myopia into groups with low (up to –3.00D) and moderate myopia (>-3.00D to –6.00 D), spherical refraction in etafilcon A wearers was found to increase to a greater extent in the subjects with low myopia than in the subjects with moderate myopia (p=0.005). Eight percent (8%) of the lotrafilcon A lens wearers and thirty percent (30%) of the etafilcon A wearers experienced an increase in myopia of at least -0.50 D.

Thirteen of the 23 subjects wearing the low Dk/t lenses in the original study were followed in a separate three month study to investigate the effect of refitting them with the high Dk/t lenses. This sub-group had shown an increase in myopia of 0.25D (p=0.004) in the first nine (9) months of low Dk/t lens wear and then became less myopic by 0.37D (p=0.003) after three (3) months of EW of the high Dk/t lenses (Figure 2).

Similar results have also been reported in studies conducted at another centreby Jalbert et al8 at the Cooperative Research Centre for Eye Research and Technology in Sydney and with balafilcon A (PureVision™) lenses Pritchard et al9 at the Centre for Contact Lens Research in Waterloo.

Visit

Figure 1 : Mean change in refractive error over time for High Dk (lotrafilcon A, n = 39) and Low Dk (etafilcon A, n = 23) wearers

Etiology of Hypoxia Related Refractive Error Changes

The designs of the studies conducted to date have not allowed the mechanism behind the change to be fully investigated. The changes were however noted early in the trials rather than later, ruling out a dose dependent response which would continue with sustained EW. The change has been reported with two low Dk/t materials7;9 supporting the theory that the effect appears to be driven by hypoxia rather than lens specific factors such as lens design or modulus of elasticity.

A change in the corneal index of refraction is a factor which could influence the overall refracting power of the system. The index would be altered slightly in conditions of increased hydration from edema associated with low Dk/t EW. In addition, differential swelling response due to varying thickness across a lens could contribute to increased myopia. Further study into the possible mechanisms for the refractive changes is required.

Time (Months)

Figure 2: Change in refractive error over time – Subset of 13 subjects crossed-over from Low Dk (etafilcon A) to High Dk (lotrafilcon A) lenses.

Clinical Significance of Refractive Error Changes

While the mean increase in myopia following EW with low Dk/t lenses is small, the degree of change may be significant for some individuals. In clinical practice when patients are refitted from low Dk/t lenses to hight Dk/t SH lenes, a reversal of the myopic shift may result. For this reason, approximately one month after refitting, all patients should be carefully over-refracted since the patient may then be wearing a lens which is over-minused or under-plussed, which could result in near vision problems, particularly for a patient who is on the verge of presbyopia. While myopic patients may appreciate this reduction in their prescription, hyperopic patients may be less content and require careful counseling about the health benefits associated with their “apparent” increase in prescription.

Other Optical Considerations with Silicone Hydrogel Lens Wear

There have been a number of anecdotal reports of patients requiring higher powers with Focus Night & Day™ SH lenses than conventional lenses. The apparent requirement for additional power is related to the aspheric design of the Focus Night & Day™ lenses and a resultant relative decrease in spherical aberration compared to other spherical lens designs. This is most noticeable in the higher minus or plus designs. For example if a patient is wearing a -9.00 D spherical lens as their current lens, that lens is -9.00 in the centre but will have several dioptres of extra minus spherical aberration across the optical zone. This effectively makes the average power across the optic zone somewhat higher than -9.00 D. When the aspheric design Focus Night & Day™ lens -9.00 D is placed on the eye, there is less spherical aberration and thus the average power is somewhat lower than its spherical counterpoint (but in actual fact closer to labeled power). There have been reports of -0.50 to -1.00 D "extra" power being required in some patients.

Corneal Curvature Changes and Silicone Hydrogel Lens Wear

In several studies, central corneal curvature has been reported to decrease or flatten following EW with SH lenses7-9. The degree of flattening reported is small, ranging from 0.16 D to 0.35 D. In one study however, there was no reported change in central corneal curvature following EW with SH lenses10. EW with low Dk/t lenses resulted in a small degree of corneal steepening in another study8 but no change in central corneal curvature were reported in two studies conducted at the CCLR with two different low Dk/t lens materials7;9.

Conclusions

Small changes in refractive error and corneal curvature may occur in some patients wearing SH lenses. Although these changes appear to be related to the alleviation of chronic hypoxia from previous lens wear, the precise etiology behind these changes is not clear and requires further investigation. Disparities may also occur in the powers required in SH lenses compared to conventional lens designs. These are thought to occur as a result of differences in spherical aberration between lens types, however other factors may also influence these findings.

References

1. Barnett WA, Rengstorff RH. Adaptation to hydrogel contact lenses: variations in myopia and corneal curvature measurements. Journal of the American Optometric Association 1977;48:363-6.
2. Grosvenor T. Changes in corneal curvature and subjective refraction of soft contact lens wearers. American Journal of Optometry and Physiological Optics 1975;52:405-13.
3. Harris MG, Sarver MD, Polse KA. Corneal curvature and refractive error changes associated with wearing hydrogel contact lenses. American Journal of Optometry and Physiological Optics 1975;52:313-9.
4. Hill JF. A comparison of refractive and keratometric changes during adaptation to flexible and non-flexible contact lenses. J Am Optom Assoc 46, 290-294. 2003.
5. Binder PS. Myopic extended wear with the Hydrocurve II soft contact lens. Ophthalmology 1983;90:623-6.
6. Fonn D, MacDonald KE, Richter D, Pritchard N. The ocular response to extended wear of a high Dk silicone hydrogel contact lens. Clinical and Experimental Optometry 2002;85:176-82.
7. Dumbleton KA, Chalmers RL, Richter DB, Fonn D. Changes in myopic refractive error with nine months` extended wear of hydrogel lenses with high and low oxygen permeability. Optometry and Vision Science 1999;76:845-9.
8. Jalbert I, Holden B, Keay L, Sweeney DF. Refractive and corneal power changes associated with overnight lens wear: differences between low Dk/t hydrogel and high Dk/t silicone hydrogel lenses. Optom Vis Sci 76[12s], 234. 2001.
9. Pritchard N, Fonn D. Myopia associated with extended wear of low-oxygen-transmissible hydrogel lenses. Optom Vis Sci 1999;76:169.
10. Omar R, Mutalib HA, Rahim HA, et al. Corneal changes in silicone hydrogel contact lenses wearers: a Malaysian experience. Optometry and Vision Science 79[12s], 259. 2002.