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Researchers confirm safety of overnight orthokeratology

Issue: October 2004

ByBarbara Anan Kogan, OD

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Orthokeratology pioneer Stuart Grant, OD, told Primary Care Optometry News that with a 10-year history, ortho-K has shown “no negative effects from the movement of the corneal epithelial layer.

“Ortho-K patients, unlike conventional contact lens patients, were watched every day and then weekly to monitor corneal health without today’s diagnostic instrumentation,” he said. “And we did not have today’s materials, designs such as reverse geometry nor Food and Drug Administration-approved GP contact lenses.

Corneal stress

Jou-Chun Lin, MD, who practices in Kaohsiung, Taiwan, where 21% of 18-year-olds are myopic, investigated how the morphological changes in the endothelial layer indicate corneal stress. Thirty-six children (13 boys and 23 girls, mean age of 9.9 years) with mean myopia of 3.24 to +1.81 D were evaluated after 6 months of ortho-K (7 to 9 hours a night) with gas-permeable lenses (Dk of 100 [ISO/Fatt] and center thickness of 0.18 to 0.20 mm).

Researchers used specular microscopy to evaluate central and peripheral corneal endothelial cell morphometry at baseline and 3 and 6 months after ortho-K.

“The patient’s age needs to be considered, as parameters of cell density and morphology change at a higher rate in children than adults,” Dr. Lin said.

Dr. Lin found no significant change in corneal endothelial density nor morphology of corneal endothelium over 6 months follow-up. She concluded that high-Dk GP lenses are a safe and effective treatment device.

Dr. Lin added that hypoxia-induced, contact lens-related keratopathies can best be eliminated by prescribing a high-Dk GP lens in children who wear ortho-K lenses for a number of years.

Corneal topography, corneal shape

According to Australia’s Helen A. Swarbrick, OD, PhD, FAAO, “The typical corneal shape is a prolate (flattening) ellipse, which can be quantified clinically by using paracentral keratometry readings; peripheral, axial or tangential radii of curvature; or various corneal shape indices from corneal topographers.” The limitations of corneal shape indices arise from topographer manufacturers’ choice of appropriate shape indices and a lack of transparency in computational algorithms used to describe unusual corneal shapes that are ortho-K induced, she said.

After ortho-K, the center of the cornea may be flatter than the periphery, creating an oblate corneal shape. The ability of a topographer to describe corneal shape depends on the accuracy with which it can measure or calculate corneal elevation. “This,” she said, “is even more critical when considering unusual cornea shapes, such as post ortho-K and post-refractive surgery.

“Because the cornea does not sphericalize after ortho-K,” Dr. Swarbrick continued, “the amount of achievable refractive change may not be limited by the baseline shape.” The continuing safety of overnight ortho-K will require development of international minimal clinical standards, including: appropriate patient selection and lens designs, Dks of >87 to avoid overnight hypoxia, safe and effective lens care, and patient education and compliance, she said.

Central corneal thinning

A number of worldwide studies have shown central corneal thinning with overnight ortho-K, said Sarita Soni, OD, of Indiana University. “Choo et al. have shown compression of epithelial cells, especially basal cells, after 8 hours of lens wear in cats and a loss of cells after 14 days of continuous wear,” she said. (Choo JD, Caroline PJ, Harlin DD, Meyers W. Morphologic changes in cat epithelium following overnight lens wear with the Paragon CRT lens for corneal reshaping. Invest Ophthalmol & Vision Sci. 2004;1552/B363)

She added that humans experience a decrease in cell desquamation rate after 2 weeks of lens wear.

A short-term recovery investigation where an overnight ortho-K lens was worn for 1 month followed by 2 weeks of no lens wear documented significant central epithelial thinning and full recovery. Corneal oxygen consumption and permeability increased slightly with lens wear, returning to baseline following lens removal. “Even though these changes were not statistically significant, in the 10 eyes that we examined, the recovery data is promising,” Dr. Soni said.

Ortho-K vs. extended wear

In a pilot study, principal investigator Eric R. Ritchey, OD, MS, looked at the refractive Quality of Life (QoL) when comparing overnight ortho-K lenses with up to –4.00 D of sphere and –1.25 D of cylinder with 30-day extended-wear silicone hydrogels in successful contact lens patients at least 18 years old.

Eight patients wore Paragon CRT Lenses and 10 patients wore CIBA Night & Day Lenses.

Using the National Eye Institute’s Refractive QoL Instrument-42 questionnaire to compare baseline to 1 and 3 months of treatment, he found no statistically significant difference between the treatment groups in areas of clarity, expectations, near and far vision, diurnal variation and glare symptoms.

For Your Information:

• Stuart Grant, OD, can be reached at 2001 S. Barrington Ave., Suite 318, Los Angeles, CA 90025; (310) 231-0070; fax: (310) 231-0780; e-mail: sgrant8764@aol.com.
• Jou-Chun (Joyce) Lin, MD, can be reached at the Department of Ophthalmology, Kaohsiung Municipal United Hospital, 976 Chung Hwa 1st Rd., Kaohsiung, Taiwan; 886-7-5552565, ext. 2273; fax: 886-7-5546207, e-mail: k5231@ms24.hinet.net.
• Helen A. Swarbrick, OD, PhD, FAAO, is a senior lecturer (contact lenses) in the School of Optometry and Vision Science, University of New South Wales, Sydney, New South Wales 2052, Australia; 61-2-9385-4373; fax: 61-2-9313-6243; e-mail: h.swarbrick@unsw.edu.au.
• Sarita Soni, OD, is a professor of optometry and vision science, associate dean for research and co-director of the Borish Center for Ophthalmic Research. She can be reached at 800 E. Atwater, Bloomington, IN 47405; (812) 855-7877; fax: (812) 855-7045; e-mail: sonip@indiana.edu.
• Eric R. Ritchey, OD, MS, can be reached at The Ohio State University, 320 W. 10^th Ave., Columbus, OH 43210; (614) 292-2020; fax: (614) 596-7477; e-mail: eritchey@optometry.osu.edu.
• Barbara Anan Kogan, OD, can be reached at 4501 Connecticut Ave. NW, Suite 102, Washington, DC 20008-3711; phone and fax: (202) 244-1324; e-mail: bakogan@mindspring.com.