Vision and Silicone Hydrogels

Recently there have been several studies examining refractive changes with silicone hydrogels. ^1-3 These studies clearly show that the refractive response to silicone hydrogel lenses differs when compared to conventional hydrogel materials. Conventional hydrogel contact lenses lead to small increases in myopia whilst changes with silicone hydrogels rarely occur. The effect with conventional hydrogel materials is believed to be a result of hypoxia, and has been shown to reverse when patients are dispensed with lenses with higher oxygen transmissibility. For a review of the general issues surrounding refractive changes and silicone hydrogel lens wear see Kathryn Dumbleton’s excellent editorial Refractive Error and Corneal Curvature Issues with Silicone Hydrogel Lens Wear.

Unintended ‘orthok’?

In addition to the studies described above there have been a few anecdotal reports from practitioners describing a significant increase in hyperopia or reduction in myopia, particularly with patients with relatively high refractive error, whilst wearing their silicone hydrogel lenses.⁴ Very recently at the International Contact Lens Congress Meeting in Australia, Ray Fortescue, a practitioner with more than 20 years private practice in Sydney, reported that approximately 12% of his hypermetropic patients experience up to a +2.00D shift with silicone hydrogel lenses. He associated these effects with corneal flattening and has found that if patients are changed to a daily wear schedule, or are refit with a lower modulus lens material, then the effects can be reduced.

The cause or mechanism behind these changes is still unclear. The refractive changes do not occur immediately or consistently in the same people and in some instances the changes are unilateral. Practitioners agree that the evidence points towards corneal moulding or “ortho k” type effects. Indeed John Mountford has reported topographic changes to the cornea characterised by central corneal flattening and a ring of mid-peripheral corneal steepening, similar to that observed after reverse geometry lens wear in orthokeratology (see Unintended Orthokeratology effect of Silicone Hydrogels on Hypermetropic patients).

One explanation for why these large refractive shifts are occurring is that patients are inadvertently wearing their silicone hydrogel lenses inside-out. This would explain the inconsistencies in the occurrence of the problems. Also, when observing the tear profile beneath an inverted lens and comparing this with a lens correctly inserted, the evidence for this hypothesis is quite startling (Figures 1, 2 and 3). As can be observed in the picture of the reverse geometry lens used for intentional orthok (Figure 1), there is minimal central corneal clearance and relatively high corneal clearance in the midperiphery with the inverted silicone hydrogel lens (Figure 2). The same silicone hydrogel that is inserted the correct way (Figure 3) has a more even distribution of corneal clearance. It is interesting to note that the higher the power of the silicone hydrogel lens (whether it be positive or negatively powered), the more the inverted fluorexon image resembles a reverse geometry orthok lens fluorescein pattern.

The corneal re-shaping effects orthok lenses have on the cornea are believed to be the result of positive and negative forces present in the tears. Therefore, positive pressure would be present beneath the central portion of the lens where there is minimal clearance producing corneal compression or flattening and negative “suction” pressures would be present beneath the reverse curve where the tear profile is thickest producing corneal steepening.⁵ It is thought that these hydraulic forces in the tears, rather than the physical pressure of the lens itself, is responsible for the corneal re-shaping.

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Figure 1: Fluorescien pattern of an R&R Reverse geometry lens from Danker Laboratories (USA)	Figure 2: Fluroexon pattern of an inverted +5.00 inverted Focus Night and Day lens	Figure 3: Fluorexon pattern of a correctly inserted +5.00 Focus Night and Day lens on the same eye as Figure 2

Comfort and Vision with Inverted Silicone Hydrogel Lenses

Lens handling is difficult even for experienced wearers and hyperopes are more at risk of problems with handling due to their reduced visual capabilities at near. Practitioners generally advise their patients that they are likely to be aware that their lenses are inverted as they will not be as comfortable and vision may be affected. Although published work on this is scarce, clinical experience with conventional contact lens materials makes this fairly obvious. However, our experience with silicone hydrogel lenses indicate that patients are less likely to be aware of incorrect lens insertion with silicone hydrogel lenses than with conventional hydrogel lenses if they are to rely on comfort and vision cues.

We observed this after conducting a short pilot study looking at the vision and comfort of inverted lenses compared to lenses worn the correct way. Both comfort and vision were dramatically lower with inverted Acuvue 2 lenses compared to Acuvue 2 inserted the correct way but there were no differences in comfort and vision between the majority of inverted and correctly inserted Focus Night & Day lenses tested ( Figure 4). Interestingly, the performance of some of the inverted Focus Night & Day lenses was actually better compared to when they were inserted the correct way!

Figure 4: Subjective comfort and vision ratings comparing inverted lenses to those inserted the correct way for –3.00 Acuvue 2 and –3.00 Focus Night & Day lenses

Summary

With all the evidence so far, it is clear that patients are able to wear their silicone hydrogel lenses comfortably while they are inverted. It is also clear that inverted silicone hydrogel lenses can produce significant changes in refractive error (i.e an increase in hyperopia or decrease in myopia), particularly higher powered lenses. The question now is, are all the anecdotal reports from practitioners regarding “unintended” orthok solely due to incorrect lens insertion? Some practitioners are adamant that their patients are having the problems despite correct lens insertion, therefore other issues may still need to be considered.

1. 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.
2. McNally J, McKenney C. A clinical look at a silicone hydrogel extended wear lens. Contact Lens Spectrum 2002;January:38-41.
3. 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;Accepted for publication, March 2004.
4. Saks A. Beware extended wear. NZ Optics Magazine 2003;August:14-5.
5. Mountford J, Noack D. BE Optimal Orthkeratology, BE Enterprises QLD, 2001.