contact lens case reports
Kids and Orthokeratology
Corneal reshaping (orthokeratology) has proven successful among adolescents as a visual aid in school, for sports and recreation and, perhaps its most exciting prospect, for myopia control.
Research from the University of Houston College of Optometry and the Institute for Eye Research in Sydney, Australia, indicates that foveal imagery doesn't necessarily play the dominant role in axial development. It may be the images that focus on the peripheral retina that control the ultimate growth of the eye. This work suggests that we may control myopia in young children by creating optical aberrations that manipulate the peripheral optics of the eye.
Myopia Control in Action
Three years ago patient CF, a -3.25D 9-year-old with progressive myopia since age 6, was placed into CRT (Paragon Vision Sciences) lenses for overnight ortho-k. At his last examination, his VA without correction was 20/20 OU. Of greatest interest was that he has required no change in the base curve radius of his lenses, implying that his axial length hasn't increased.
While this is a single case report, recent controlled studies by Cho and Walline have demonstrated a trend toward myopia control in some children wearing ortho-k lenses.
How it Works
The mechanism for this effect appears related to the post-ortho-k corneal shape. Figure 1 shows CF's right eye with a prefitting corneal shape factor of +0.41 across the center 6mm of the cornea. Post-ortho-k the cornea assumes a shape factor of -0.43, indicating that the cornea is flatter in the center and gradually steepens across the pupil.
It's been well documented that myopic eyes (corrected with either spectacles or contact lenses) exhibit significant hyperopic defocus in the area of the peripheral retina. In this situation the central rays of light come to focus on the fovea while the peripheral rays of light fall posterior to the retina, allowing the eye to grow more myopic (Figure 2). Conversely, the oblate optics of a post-ortho-k cornea generate a negative curvature of field in which the peripheral rays of light are focused anterior to the retina, creating the environment for a myopia control effect (Figure 3).
Figure 2. Myopic eyes corrected with glasses or CL's exhibit a hyperopic defocus and a positive curvature of field.
Patrick Caroline is an associate professor of optometry at Pacific University. He is also a consultant to Paragon Vision Sciences and SynergEyes, Inc. Mark Andre is an associate professor of optometry at Pacific University. He is also a consultant for Alcon Labs, CooperVision and SynergEyes, Inc.
For references, visit www.clspectrum.com/references.asp and click on document #138.