Unintended Orthokeratology effectof Silicone Hydrogels on Hypermetropic patients

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

John Mountford graduated from the Queensland University of Technologyin 1976. He is a Fellow of the American Academy of Optometry, aFoundation Fellow of the Contact Lens Society of Australia anda Fellow of the Victorian College of Optometry. He is a VisitingLecturer to the Departments of Optometry at QUT and UNSW. In privatepractice, he specializes in advanced contact lens applicationsincluding keratoconus, Post P-K, Post refractive surgery, pediatricaphakia and anisometropia, orthokeratology and all types of sclerallens fitting.

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

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

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

Of interest are the following clinical observations:

1.The time of onset appears to be variable, from weeksto months.
2. The eye with the highest plus lens (the reading eye) isthe more likely to be affected.
3. The presenting complaint is increased difficulty with closework.
4. Cessation of lens wear results in a return to baseline refractionwithin 10 days, associated with a return to baseline cornealtopography.
5. Changing the base curve of the lens, or changing from Nightand Day to Purevision or vice versa has no effect: the increasedhypermetropia 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 baselinerefraction and topography.

A typical case is that of JC, a 58 y.o. female, who had successfullyworn Acuvue lenses on a daily wear monovision schedule for 5years, 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 followingRx:

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 blurrednear vision. The distance vision remained unchanged at 6/5with anover-refraction of plano.
The near vision could not be improved with the addition oflow 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 weeklater. The refraction had returned to baseline, with no astigmaticcomponent present.
The patient was returned to lens wear, but this time with a differentbrand of silicone hydrogel lenses lenses (OD +1.75, OS +4.25)and after 2 weeks, returned with the same problems with nearwork. Once again, refraction showed an increase in both hypermetropiaand astigmatism to +3.75/-1.00 x 120.

Lens wear was again ceased for 2 weeks, and refraction and topographyonce 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 refractivechange.

Figure 1. The top left plot shows the cornea followinglens removal. Note the “red ring” of cornealsteepening at the 6.00mm zone. The inferior left map showsthe cornea after 1 week of no lens wear. The subtractivemap (right) shows that the cornea has steepened by 2.50Dsince lens wear was ceased. Also note the irregularity ofthe central pupil zone resulting in the induced astigmatism.The “blue ring” show where the cornea has flattenedfollowing cessation of lens wear.

A subtractive tangential power topography map of the corneaimmediately on lens removal and following a return to baselineis shown in Figure 1. Note the apparent steepening of the centralcornea following cessation of silicone hydrogel wear. The analogyis that lens wear resulted in a marked degree of central cornealflattening. The topography map on the top left hand side bearssome remarkable similarities to those seen following orthokeratologylens wear (see Figure 2). The central corneal flattening is associatedwith a ring of mid-peripheral steepening. The difference mapin figure 1 shows this area is having flattened (blue ring) followingcessation of lens wear.

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

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

The higher modulus of silicone hydrogels (especially in the pluslens design) may mean that they do not, like high water contentsoft lenses. “wrap” onto the cornea and mimic itsshape. They may, in fact, have a relatively deep tear layer trappedbetween the lens and the corneal surface that, due to the asphericback surface of the lens and relatively flat base curve to corneafitting relationship is deeper at the edge of the optic zonethan in the centre. This is similar to the tear layer shape causedby reverse geometry lenses. It is interesting to postulate thatthe increased plus seen in some patients may be due to cornealflattening as a result of the same type of squeeze film forcespresent in reverse geometry lenses, albeit to a much lesser degree.

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

In conclusion, an interesting patient presented for review 3weeks ago. The initial refraction was -11.00D sph. After 6 weeksof 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. Refractionwithout the lensin-situwas -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 interestingto see if thereduction in myopia occurs again.

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

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