Unintended Orthokeratology effect of Silicone Hydrogels on Hypermetropic patients

John Mountford, FAAO, FCLSA, FVCO, Dip. App. Sc

John Mountford graduated from the Queensland University of Technology in 1976. He is a Fellow of the American Academy of Optometry, a Foundation Fellow of the Contact Lens Society of Australia and a Fellow of the Victorian College of Optometry. He is a Visiting Lecturer to the Departments of Optometry at QUT and UNSW. In private practice, he specializes in advanced contact lens applications including keratoconus, Post P-K, Post refractive surgery, pediatric aphakia and anisometropia, orthokeratology and all types of scleral lens fitting.

He has written and co- authored numerous papers on orthokeratology, as well as a chapter on the topic for the 4th Edition of Phillips and Speedwell’s Contact Lenses, Hom’s “Contact lenses” and Bennet and Weismann’s Contact lens text. He has also published papers on Silicone lenses, polarized biomicroscopy and scleral lens fitting, and co-authored other papers on contact lens related topics. He is the co-author of Theory and Practice of Orthokeratology, which is due for release in March 2003.

Over the past 3 years, more than 200 hypermetropic patients have been fitted with both Night and Day and Purevision lenses in my practice, predominantly in monovision mode. In some cases, there has been a relatively rapid increase in plus power, past that normally expected for the age and refractive error of the patient. This “hyperopic creep” has some interesting clinical aspects, and raises some interesting questions as to the effect of the lens material and design on corneal shape.

A review of the patient files shows that an unexpected increase of greater than +0.50D occurs in the first year of lens wear in approximately 12% of hyperopic patients whose previous records show stabilization of refractive error. Greater refractive change (=> 1.00D) associated with induced astigmatism occurs in approximately 3% of patients seen.

Of interest are the following clinical observations:

1. The time of onset appears to be variable, from weeks to months.
2. The eye with the highest plus lens (the reading eye) is the more likely to be affected.
3. The presenting complaint is increased difficulty with close work.
4. Cessation of lens wear results in a return to baseline refraction within 10 days, associated with a return to baseline corneal topography.
5. Changing the base curve of the lens, or changing from Night and Day to Purevision or vice versa has no effect: the increased hypermetropia returns within 1-2 weeks.
6. A change form Extended wear to daily wear has no effect.
7. A return to hydrogel lenses results in a return to baseline refraction and topography.

A typical case is that of JC, a 58 y.o. female, who had successfully worn Acuvue lenses on a daily wear monovision schedule for 5 years, with little or no change to her distance Rx of R&L +1.75D, and a mild increase in the Left near vision lens from +3.75 to +4.00D the year before.
She was fitted with silicone hydrogel lenses to the following Rx:

OD +1.75 (6/5)
OS +4.00 (n.5)

Extended wear was initiated in November 2001, and was uneventful. The patient returned in April 2002 with symptoms of blurred near vision. The distance vision remained unchanged at 6/5 with an over-refraction of plano.
The near vision could not be improved with the addition of low plus lenses, so the lens was removed and refraction performed. The result was:
Rx +4.25/-1.50 x 125 (6/6).

The patient was advised to cease lens wear, and returned 1 week later. The refraction had returned to baseline, with no astigmatic component present.
The patient was returned to lens wear, but this time with a different brand of silicone hydrogel lenses lenses (OD +1.75, OS +4.25) and after 2 weeks, returned with the same problems with near work. Once again, refraction showed an increase in both hypermetropia and astigmatism to +3.75/-1.00 x 120.

Lens wear was again ceased for 2 weeks, and refraction and topography once again returned to baseline. Due to the unwanted changes, the patient returned to daily wear disposable lenses (Acuvue). Review over the following year has not resulted in any refractive change.

Figure 1. The top left plot shows the cornea following lens removal. Note the “red ring” of corneal steepening at the 6.00mm zone. The inferior left map shows the cornea after 1 week of no lens wear. The subtractive map (right) shows that the cornea has steepened by 2.50D since lens wear was ceased. Also note the irregularity of the central pupil zone resulting in the induced astigmatism. The “blue ring” show where the cornea has flattened following cessation of lens wear.

A subtractive tangential power topography map of the cornea immediately on lens removal and following a return to baseline is shown in Figure 1. Note the apparent steepening of the central cornea following cessation of silicone hydrogel wear. The analogy is that lens wear resulted in a marked degree of central corneal flattening. The topography map on the top left hand side bears some remarkable similarities to those seen following orthokeratology lens wear (see Figure 2). The central corneal flattening is associated with a ring of mid-peripheral steepening. The difference map in figure 1 shows this area is having flattened (blue ring) following cessation of lens wear.

Figure 2 . A post- wear topography map of a cornea following orthokeratology. Note the flattening of the central cornea, and the mid-peripheral “red ring” of steepening. A similar effect appears in post-wear hypermetropic silicone hydrogel lens wear.

Reverse geometry lenses produce a change in corneal shape due to the differences in tear layer fluid forces acting under the lens. They do not compress the cornea into a new shape, but rather tend to use negative force or suction in order to “draw” the corneal epithelium into a shape similar to the back surface geometry of the lens.

The higher modulus of silicone hydrogels (especially in the plus lens design) may mean that they do not, like high water content soft lenses. “wrap” onto the cornea and mimic its shape. They may, in fact, have a relatively deep tear layer trapped between the lens and the corneal surface that, due to the aspheric back surface of the lens and relatively flat base curve to cornea fitting relationship is deeper at the edge of the optic zone than in the centre. This is similar to the tear layer shape caused by reverse geometry lenses. It is interesting to postulate that the increased plus seen in some patients may be due to corneal flattening as a result of the same type of squeeze film forces present in reverse geometry lenses, albeit to a much lesser degree.

The recent interest in tear layer thickness research under silicone hydrogels may shed more light on the topic.

In conclusion, an interesting patient presented for review 3 weeks ago. The initial refraction was -11.00D sph. After 6 weeks of continuous wear of a –10.00D silicone hydrogel lens, he returned complaining of blurred distance and near vision. Over-refraction with the lens in place was +2.00D. Refraction without the lens in-situ was -8.75D. The lens was changed to -8.00D, which, after 2 weeks required a change back to – 10.00D. Topography plots have been taken, and it will be interesting to see if the reduction in myopia occurs again.

Silicone hydrogels cause refractive changes in some patients, with the changes appearing to be greater in hypermetropes, and being totally due to some form of corneal molding by the lens. Perhaps the decrease in minus seen in previous soft lens wearers may also be influenced by alterations to corneal shape.

To quote and old saying….”curioser and curioser”.