The Use of SiH Lenses in the Management of High Refractive Error & Anisometropia

Susan Kovacich, OD, FAAO

Susan Kovacich received both her undergraduate and O.D. degrees from Indiana University. After graduation, she completed a hospital-based optometric residency at the John Cochran Veterans Administration Medical Center in St. Louis, Missouri. Following her residency, she practiced in St. Louis at a multidisciplinary clinic, then with a corneal specialist. In l998, Dr. Kovacich returned as faculty to the IU School of Optometry, where she is currently an Associate Clinical Professor. After initially being assigned to the Primary Care Clinic, she is now Co-Director of the Cornea and Contact Lens Clinic. Dr. Kovacich is a Fellow in the American Academy of Optometry and has been active in the American Optometric Association, Indiana Optometric Association, National Board of Examiners in Optometry, and Association of Optometric Contact Lens Educators. She has published in Optometry, Contact Lens Spectrum and Optometric Management.

A 48 YO Female wanted to be fit with contact lenses, specifically a contact lens that would help reading with her left eye. Her ocular history included high myopia with reduced vision OD secondary to posterior staphyloma. The patient also wanted to be able to best use the available vision in her right eye. She was wearing a soft contact lens with unknown parameters in the left eye for distance only. Distance entering acuities were:

CF OD uncorrected, 20/25+ OS corrected with SCL. Refraction: -27.00D OD (20/100-), -1.75 -0.50 X 150 with a +1.50 ADD OS (20/20 distance and near).

The patient had an intermittent RXT which was more apparent when she grew tired at the end of the day. The external exam was within normal limits and the internal exam was remarkable for the posterior staphyloma OD.

After considering the options, including GP lenses, the decision was made to fit the patient with an O2Optix Custom lens in the right eye. Vertexing -27.00D resulted in a
-20.00D contact lens power. The vertexed power and Ks were given to the consultant at CIBAVision, who calculated the base curve and diameter.  Following lens insertion, the lens moved and centered well.  After dispensing the -20.00D/ 7.7/ 13.2 O2Optix Custom right lens, a +2.50 OR was found. The final lens ordered was a -17.50D lens, which resulted in 20/80- vision in the right eye.

The patient was fit with a -2.00 D Low Add/ 8.6/14.0 Purevision Multifocal lens OS, which resulted in 20/20 vision for distance and near. The patient was very happy with the end result and the ability to better use her central and peripheral vision with the right eye.

Discussion
Posterior staphyloma tends to be a unilateral condition characterized by elongation of the globe in the posterior pole secondary to a presumed disruption in scleral development. The affected retina around the optic nerve is stretched, thinned and de-pigmented. The elongation of the eye results in a very high myopic refractive error. There is a variable reduction in vision dependent on the degree of involvement of the macula.

Several challenges are presented in this case.

Challenge # 1 The most obvious challenge in this case is the patient’s high refractive error OD due to the increased axial length. Contact lenses are the refractive system of choice because less anisokonia results from a contact lens compared to a spectacle lens, due to the reduction in image size. Furthermore, high power soft contact lenses are thicker than low power lenses, which further limit the amount of oxygen transmitted through the lens.  Thick, low-Dk lenses can cause hypoxic conditions and complications.  Fortunately, silicone hydrogel lenses are available in this patient’s power. It is also difficult to accurately calculate the vertex power with a very high refractive error. The calculated vertex power was -20.00D, which was +2.50D too strong. After our predicted outcome proved to be incorrect, my colleague, Neil Pence, OD, suggested piggybacking two -10.00D lenses and overrefracting that system. The OR was +2.50, accurately predicting the -17.50D power of the final soft contact lens order.

Challenge # 2 The second challenge is the reduced vision in the right eye. The refractive endpoint is difficult to assess, as there is some variability in the patient response to the visual acuity chart. One way to more accurately predict the final contact lens prescription would be to put a lens as close to the predicted power as possible. Again, overrefracting the contact lens(es) on the eye could help better predict the final refractive outcome.

Challenge # 3 The last challenge is to educate this monocular patient about protecting the good eye. The highest Dk lenses available should be offered to the patient. This patient should only wear the lenses on a daily wear basis, and should be informed about the risks that comes with wearing contact lenses. The patient should discontinue contact lenses wear and return to the clinic if the eye doesn’t “see good, feel good or look good” and the importance of contact lens compliance should be emphasized at each visit.

Conclusion
This patient was a challenge because of her high refractive error and reduced vision in one eye and her desire to wear contact lenses in both eyes. Fortunately, today there are silicone hydrogel lenses available for a patient with a high refractive error or anisometropia as reported here.

1. Alexander, L. Primary Care of the Posterior Segment. 2nd ed. East Norwalk: Appleton Lange; 1994:106-7