Brazilian researchers seek to define effects of ortho-k

Washington, DC—Reduction in myopia, central corneal flattening, and improvement of uncorrected visual acuity (UCVA) occur rapidly after the initiation of orthokeratology (ortho-k), but this modality also appears to result in changes in higher-order aberrations and corneal pachymetry, reported Iane G. Stillitano, MD, at the World Cornea Congress V.

"Wavefront analysis of changes in optical aberrations occurring with ortho-k is of utmost importance for understanding how the remodeling of the cornea that occurs with this modality affects the entire optical system," said Dr. Stillitano, who is a PhD candidate in ophthalmology at UNIFESP/EPM Paulist School of Medicine, Federal University of SãPaulo, Brazil.

Dr. Stillitano and colleagues are evaluating topographic, wavefront, and pachymetric changes associated with overnight ortho-k.

The trial was conducted at the Vision Institute of UNIFESP between December 2004 and May 2005. All patients in the study wore the Be Free high dK ortho-k contact lens (BE; Ultravision Pty. Ltd. ) overnight for 8 nights.

The subjects ranged in age from 14 to 52 years and had low to moderate myopia with between –0.75 and –5 D of spherical error.

They were also allowed to have up to –0.75 D of against-the-rule astigmatism or –1.50 D of with-the-rule astigmatism, according to Dr. Stillitano.

Patients were evaluated prior to the fitting, on the first night, and after 8 nights of contact lens wear with evaluations that included visual acuity testing, cycloplegic refraction, topography with the Medmont Corneal Topographer (Art Optical), pachymetry with the Orbscan (Bausch & Lomb), and aberrometry using the LADARWave (Alcon Laboratories).

"For the topography, we obtained differential axial and tangential maps of each eye for analyses of refractive changes and lens position during sleeping, respectively," Dr. Stillitano said.

Prior to the fitting, 73% of patients had UCVA between 20/100 and 20/400, whereas after 8 nights of wearing the ortho-k contact lens, UCVA improved to 20/30 or better in all eyes and was 20/20 in 69%. The change in UCVA accompanied a statistically significant change in mean SE from –2.24 D prefitting to +0.15 D.

Assessing measurement

By the end of the first day, the topography analysis revealed a decrease in the apical radius and a decrease in eccentricity. Further changes in both measurements were observed at the end of the study, and the differences compared with baseline were statistically significant.

Temporal pachymetry increased significantly from the pretest to the first night, but then remained unchanged.

Defocus decreased significantly with a change observed by the first night and a further decrease at the end of the study, while astigmatism remained stable.

Significant changes in total RMS and in higher-order RMS were also observed with total RMS decreasing (due to the reduction in defocus) and higher-order RMS increasing 2.5-fold from about 0.40 to about 1 μm.

Looking at individual higher-order aberrations showed that spherical aberration and coma were particularly affected.

Mean coma nearly doubled and mean spherical aberration increased about 800%.

Horizontal coma also changed significantly, but the direction differed between eyes.

In the right eyes, there was a consistent change in the positive direction while in the left eyes, mean horizontal coma became negative, Dr. Stillitano said.