Increases in myopia or ‘myopic shifts’ during both daily and extended wear with traditional soft lenses have been reported since the 1970’s (1-3). Some of these myopic shifts are less than -0.50 diopters and are usually considered too small to be of much clinical relevance. However significant increases in myopia up to and over 1 D have also been observed (4, 5). Soon after the release of silicone hydrogel lenses, there were several anecdotal reports of large hyperopic shifts in silicone hydrogel lens wearers. These unexpected results stimulated researchers to characterise the spectrum of changes in refractive error with soft lens wear (Figure 1), and to better understand why such changes occur.
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| Figure 1 |
Figure 1. Change in refractive error in patients wearing low Dk hydrogel lenses for 6-nights with weekly replacement (n = 51) and silicone hydrogel lenses for 30-nights with monthly replacement (n = 41) after six months of lens wear. Data published in Jalbert et al. Changes in myopia with low-Dk hydrogel and high-Dk silicone hydrogel extended wear.
Optom Vis Sci. 2004;81:591-6.
Generally the myopic shift associated with traditional soft lens wear is attributed to lens-induced hypoxia (4, 6, 7) . It is well established that traditional hydrogel lens materials fail to deliver oxygen to the cornea at levels sufficient to avoid hypoxic consequences, particularly during overnight wear (8) . The corneal oedema that results from low Dk soft lens wear is thought to modify corneal shape and induce changes in corneal curvature and refractive index that bring about the increase in total refractive power of the eye.
Silicone hydrogel lenses made from materials of high oxygen permeability (Dk) have practically eliminated the corneal overnight swelling typically induced by lower Dk hydrogel lens materials (9) . Therefore it was anticipated that high Dk silicone hydrogels would have a minimal effect on a wearer’s refractive state. Because most wearers transferred from low to high Dk lens wear, the unexpected changes in refractive error seen with silicone hydrogel extended wear were initially attributed to a simple recovery from patients’ previous wear histories (4, 10) . Anecdotal reports of large increases in correction in the direction of hyperopic changes as well as evidence from more recent clinical trials (11) suggest that there might be more to this story. Indeed, we recently demonstrated that the small decrease in myopic correction observed during soft silicone hydrogel extended wear occurs even in subjects who have not worn lenses previously (11) . An alternative explanation must therefore be proposed in these subjects as the theory of recovery from hypoxia-induced corneal swelling due to previous low Dk lens wear can no longer hold true in such cases.
Aside from the tremendously improved oxygen permeability, silicone hydrogel materials significantly differ from classic hydrogel polymers in their modulus of elasticity as they can be up to four times stiffer than their counterpart (12) . Therefore a possible cause for the changes in refractive error seen with silicone hydrogels may be related to pressure-induced changes in corneal shape. In this hypothesis, the central cornea is flattened by the relatively stiff silicone hydrogel materials leading to an ‘orthokeratology’-like effect. This hypothesis is supported by the fact that central corneal flattening is typically observed during silicone hydrogel wear (4, 11) . It also follows that these effects would be more pronounced in prolate corneas and/or with increasing lens central thickness which occurs with high hyperopic corrections.
Given that the amount of refractive change measured with silicone hydrogels is more than 0.50 diopters in approximately 30% of cases and can be as high as 1.00 diopter in 5% of cases, significant visual complaints may develop if the refractive change is allowed to persist undetected. This is best highlighted by one of John Mountford’s patients whose initial refraction was -11.00D sph and who was prescribed a –10.00D silicone hydrogel lens. After 6 weeks of continuous wear, the patient returned complaining of blurred distance and near vision. Examination revealed the patient’s refraction had changed to –8.75D without lenses, an overall change of +2.00D (see Unintended Orthokeratology effect of Silicone Hydrogels on Hypermetropic patients). Practitioners would therefore be wise to adopt the following practices:
- Full refraction should be undertaken before and regularly reviewed in the initial weeks/months following the initial fitting of soft silicone hydrogel lenses.
- Corneal topography and/or central corneal thickness measurements should be included whenever possible. Keratometry is required as a minimum.
- Early follow-up visits should be scheduled in high risk cases such as high hyperopes.
- Subjects should be advised to return for follow-up should they experience any unusual visual symptoms.
Finally, whilst more studies are required to confirm the ‘soft lens orthokeratology’ theory with silicone hydrogels, it may have interesting implications for contact lens practitioners. The evidence on the effectiveness of rigid contact lenses in slowing myopia progression in children is currently under review but one can speculate on a possible role for soft silicone hydrogel lenses in the control of myopia progression.
References
1. Hill J. A comparison of refractive and keratometric changes during adaptation to flexible and non-flexible contact lenses. J Am Optom Assoc 1975;46:290-4.
2. Barnett W, Rengstorff R. Adaptation to hydrogel contact lenses: variations in myopia and corneal curvature measurements. J Am Optom Assoc 1977;48:363-6.
3. Binder PS. Myopic extended wear with the Hydrocurve II soft contact lens. Ophthalmol 1983;90:623-6.
4. Dumbleton KA, Chalmers RL, Richter DB, Fonn D. Changes in myopic refractive error in nine months' extended wear of hydrogel lenses with high and low oxygen permeability. Optom Vis Sci 1999;76:845-9.
5. Nizam A, Waring Gr, Lynn M, Group PS. Stability of refraction during 11 years in eyes with simple myopia. Invest Ophthalmol & Vis Sci [ARVO Abstract] 1996;37:S1004.
6. Grosvenor T. Changes in corneal curvature and subjective refraction of soft contact lens wearers. Am J Optom & Physiol Optics 1975;52:405-13.
7. Fonn D, MacDonald K, Richter D, Pritchard N. The ocular response to extended wear of high DK silicone hydrogel contact lenses. Clin Exp Optom 2002;85:176-82.
8. Holden BA. The Glenn A. Fry award lecture 1988: The ocular response to contact lens wear. Optom Vis Sci 1989;66:717-33.
9. Fonn D, du Toit R, Simpson T, Vega J, Situ P, Chalmers R. Sympathetic swelling response of the control eye to soft lenses in the other eye. Invest Ophthalmol Vis Sci 1999;40(13):3116-21.
10. McNally J, Chalmers R, McKenney C. Factors associated with change in refractive error during a year of 6 night or 30 night extended wear. [ARVO Abstract]. In: The Association for Research in Vision and Ophthalmology; 2002; Florida, USA: http://www.arvo.org; 2002.
11. Jalbert I, Stretton S, Naduvilath T, Holden B, Keay L, Sweeney D. Changes in myopia with low Dk hydrogel and high Dk silicone hydrogel extended wear. Optom Vis Sci 2004;81:591-6.
12. Tighe B. Silicone hydrogel materials - how do they work? In: Sweeney D, editor. Silicone hydrogels: the rebirth of extended wear contact lenses. Oxford: Butterworth Heinemann; 2000. p. 1-21. |
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