Refractive Error and Corneal Curvature Issueswith 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 tofit their patients with hydrogel lenses that supply sufficientoxygen to eliminate hypoxia. Interesting findings are frequentlyreported in the early phases of experience with new productsand the impact and importance of these findings are often notfully understood. Such is the case with apparent changes in refractionand corneal curvature that may be observed in SH lens wearers.

Studies Investigating Changes in Refractive Error Associatedwith Extended Wear

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

This finding was investigated further as part of a subsequentclinical trial conducted at the Centre for Contact Lens Research(CCLR) in which the overall clinical performance of high andlow Dk/t lenses was investigated7. In this study an analysiswas conducted to determine if refractive error and keratometryaltered over a period of 9 months of 6 night EW with conventionallow Dk/t (etafilcon A) lenses and up to 30 night EW with highDk/t fluorosiloxane hydrogel lenses (lotrafilcon A) in a prospectiveparallel group study. Refractive error and corneal curvaturewere measured without contact lenses in place during the baseline,three, six and nine month visits for 62 subjects who completedthe trial. Thirty nine participants were randomized to wear thelotrafilcon A lenses (Focus Night & Day™) and the remaining23 participants wore the etafilcon A (Acuvue™) lenses.

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

There was however considerable variation in the degree of refractiveerror change between subjects. When stratified by baselinedegree of myopia into groups with low (up to –3.00D)and moderate myopia (>-3.00D to –6.00 D), sphericalrefraction in etafilcon A wearers was found to increase toa greater extent in the subjects with low myopia than in thesubjects 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 myopiaof at least -0.50 D.

Thirteen of the 23 subjects wearing the low Dk/t lenses in theoriginal study were followed in a separate three month studyto investigate the effect of refitting them with the high Dk/tlenses. 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 wearand 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 conductedat another centreby Jalbert et al8 at the Cooperative ResearchCentre for Eye Research and Technology in Sydney and with balafilconA (PureVision™) lenses Pritchard et al9 at the Centre forContact Lens Research in Waterloo.

Visit

Figure 1: Mean change in refractive error over time for High Dk (lotrafilconA, 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 allowedthe mechanism behind the change to be fully investigated. Thechanges were however noted early in the trials rather than later,ruling out a dose dependent response which would continue withsustained EW. The change has been reported with two low Dk/tmaterials7;9 supporting the theory that the effect appears tobe driven by hypoxia rather than lens specific factors such aslens design or modulus of elasticity.

A change in the corneal index of refraction is a factor whichcould influence the overall refracting power of the system. Theindex would be altered slightly in conditions of increased hydrationfrom edema associated with low Dk/t EW. In addition, differentialswelling response due to varying thickness across a lens couldcontribute to increased myopia. Further study into the possiblemechanisms for the refractive changes is required.

Time (Months)

Figure 2: Change in refractiveerror over time – Subset of 13 subjects crossed-overfrom 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/tlenses is small, the degree of change may be significant forsome individuals. In clinical practice when patients are refittedfrom low Dk/t lenses to hight Dk/t SH lenes, a reversal of themyopic shift may result. For this reason, approximately one monthafter refitting, all patients should be carefully over-refractedsince the patient may then be wearing a lens which is over-minusedor 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 theirprescription, hyperopic patients may be less content and requirecareful counseling about the health benefits associated withtheir “apparent” increase in prescription.

Other Optical Considerations with Silicone Hydrogel Lens Wear

There have been a number of anecdotal reports of patients requiringhigher powers with Focus Night & Day™ SH lenses thanconventional lenses. The apparent requirement for additionalpower is related to the aspheric design of the Focus Night & Day™ lensesand a resultant relative decrease in spherical aberration comparedto other spherical lens designs. This is most noticeable in thehigher minus or plus designs. For example if a patient is wearinga -9.00 D spherical lens as their current lens, that lens is-9.00 in the centre but will have several dioptres of extra minusspherical aberration across the optical zone. This effectivelymakes the average power across the optic zone somewhat higherthan -9.00 D. When the aspheric design Focus Night & Day™ lens-9.00 D is placed on the eye, there is less spherical aberrationand thus the average power is somewhat lower than its sphericalcounterpoint (but in actual fact closer to labeled power). Therehave been reports of -0.50 to -1.00 D "extra" powerbeing required in some patients.

Corneal Curvature Changes and Silicone Hydrogel Lens Wear

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

Conclusions

Small changes in refractive error and corneal curvature mayoccur in some patients wearing SH lenses. Although these changesappear to be related to the alleviation of chronic hypoxia fromprevious lens wear, the precise etiology behind these changesis not clear and requires further investigation. Disparitiesmay also occur in the powers required in SH lenses compared toconventional lens designs. These are thought to occur as a resultof differences in spherical aberration between lens types, howeverother factors may also influence these findings.

References

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