Modification of an Ortho-K lens to prevent central corneal staining

This case report describes how to modify lens parameters to reduce the risk of developing central corneal staining following Orthokeratology lens wear. The use of Ortho-K in monovision correction for a presbyopic myope is also discussed.

Case Report

In 2010, a 49-year-old Caucasian female attended an examination. She had been using spectacles with monovision correction. The right lens was powered for the distance and the left for reading. She wanted to explore the possibility of being spectacle free with Orthokeratology.

Her subjective refraction was

R: -3.25/-1.00x80 (6/6)

L: -2.75/-0.25x70 (6/6)

Ocular health examination showed no contraindication to Ortho-K lens wear (Table 1).

Table 1. Subjective refraction, ocular health and corneal topography results in 2010

After discussing the pros and cons of Orthokeratology, the patient decided to proceed with Ortho-K for her right eye for distance and the left eye was to be left unaided for near. A Forge Ortho-K lens was ordered empirically (Table 2).

Table 2. Parameters of the lens prescribed 0n 14-Aug-2015

The lenses fitted well and achieved an unaided visual acuity of 6/6 (RE) following two weeks of overnight lens wear. In the subsequent regular reviews, no specific issues with lens wear were noted. The patient had been using a non-preservative lubricant before lens wear and before lens removal as advised.

Follow up exam 06-Feb-2018

The patient was reviewed regularly, and at an aftercare visit on 06-Feb-2018, the optometrist noted Grade 1+ superficial central corneal staining (Figure 1) with a corresponding distortion of the mire map (Figure 2).

Figure 1. Gr 1+ superficial central corneal staining on the right eye on 06-Feb-2018

Figure 2. The mire of the right central cornea eye was distorted on 06-Feb-2018

A new lens with increased central tear film thickness (cTFT) profile was then ordered on 12-Mar-18, with the cTFT increased from 2.9um to 10.2um (Figure 3 and Figure 4).

Figure 3. The simulated fluorescein pattern with the old lens on the right eye on 14-Feb-2018

Figure 4. The simulated fluorescein pattern with the new lens on the right eye on 17-Apr-2018

The patient was asked to wear the new lens. With this modification, no central corneal staining was observed in the following visits including the most recent one on 17-Apr-2018 (Table 4 and Figure 5).

Table 4. Subjective refraction, ocular health and corneal topography results on 17-Apr-2018

Figure 5. Tangential difference map for the right eye on 17-Apr-2018

The central corneal mires were intact without distortion in the corneal topography map (Figure 6).

Figure 6. The mire of the Right central cornea eye showed no distortion on 17-Apr-2018

The unaided distance vision of the right eye was 6/6, with near VA of N20 while the unaided distance vision of the left eye was 6/60 with near VA of N3. The patient was satisfied with distance and near vision under binocular viewing conditions.

Discussion

Central cornea staining and lens binding

In Ortho-K lens wear, lens binding has been reported as the most frequent troubleshooting problem. Around 40% of these patients showed various grades of corneal staining in the first-day post-lens wear with the majority demonstrating central corneal staining. Central corneal staining is believed to relate to poor lens fitting which may cause lens binding on the cornea. Chan et al. reported the incidence of central corneal staining at day one following overnight lens wear was 22% and at day 7 was 17% among 51 pediatric wearers.

In contrast to day wear RGP, the Ortho-K lens is worn overnight in a closed eye environment and lens movement is almost absent. A closed eye exerts additional pressure on the cornea through the lens. Swarbrick and Holden suggested that large flat fitting lenses, poor axial edge lift, the change of the post-lens tear thickness along with tear viscosity, and eyelid pressure could contribute to lens binding. Chui and Cho commented that “settling down” of an Ortho-K lens can reduce the sag height of a lens by up to 20 microns, and this will depress the lens closer to the corneal surface. A cornea that responds relatively fast in curvature change with low rigidity was proposed to associate with lens binding after the first overnight of lens wear.

The patient in our case had Grade 1+ central corneal staining and associated lens binding following overnight lens wear. Simulated fluorescein fitting showed that the central tear film thickness (cTFT) was 2.9 microns. The thin post-lens tear layer suggests that the lens surface might directly touch the corneal epithelium during sleep due to the mechanical pressure from the eyelid or the rate of lens depression.

Lens parameters were modified to increase the cTFT to 10.2 microns. Based on the tangential difference map on 17-Apr-2018 (Figure 5), the centre of the treatment zone was almost at the pupil centre. The prediction of cTFT for the right eye was likely to match well with the real lens fit in a closed eye scenario. As fluorescein is not visible when its thickness is below 15 microns, the increase of cTFT from 2.9 microns to 10.2 microns was not noticeable under slit-lamp examination. In this case, the simulated fluorescein pattern provided more valuable information in refining the lens fit. The patient had been using a non-preserved lubricant before lens insertion and before lens removal to reduce the chance of lens binding. Use of lubricant before and after lens insertion as well as before lens removal has also been proposed by practitioners to alleviate the possibility of lens binding and central corneal staining. Minimising corneal staining also lowers the chance of a corneal infection due to a compromised corneal epithelium.

Use of Orthokeratology in myopic presbyopic adults to achieve monovision correction

Although Orthokeratology serves as an effective intervention to slow the progression of myopia in children its use is not limited to the pediatric population. Myers et al. fitted a young athlete with Ortho-K lenses who achieved monocular VA of 20/20 in both eyes after three weeks of lens wear. Gifford and Swarbrick fitted a group of 16 emmetropic presbyopes with Ortho-K lenses empirically. Only the non-dominant eye of each patient was fit with an Ortho-K lens targeted for a hyperopic correction of +2.00D for near while the other eye was unaltered to maintain distance vision without Ortho-K treatment. While myopic Ortho-K correction aims to flatten the central cornea, hyperopic Ortho-K was designed to induce central corneal steepening. The hyperopic lens caused -1.00D of refractive shift after the first night of lens wear and a similar correction was maintained following a week of overnight lens wear. When utilising monovision, the binocular distance VA of the participants remain unchanged with improved binocular near VA.

The patient in our case was myopic in both eyes (R: -3.25/-1.00x80 L: -2.75/-0.25x70). As she had been wearing monovision spectacle correction, she was a suitable candidate to maintain monovision with Ortho-K lens wear in the right eye to correct for distance vision. She well adapted from her spectacle wear to use her near addition in the left eye.

Conclusion

In this case report, lens parameters were modified to increase the central tear film thickness between the lens and corneal surface and therefore preventing central corneal staining due to lens binding. Fitting a presbyopic myope with an Ortho-K lens in one eye has helped the patient achieve successful monovision without the day time visual aids.

References

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