Orthokeratology: commend or condemn?

Charles Kaiser M.D. wore his CRT lenses overnight. His topography reveals small optical zones, and his wavescan reveals significant spherical aberration. However, in general, he is very happy with his vision
Source: William Trattler, M.D.

Ever since studies on orthokeratology began in the 1960s, support for the ocular version of braces has waxed and waned.
However, a new study, published in the February 2007 issue of Investigative Ophthalmology and Visual Science, suggests that orthokeratology “significantly increases ocular higher-order aberrations and compromises contrast sensitivity function, depending on the amount of myopic correction.”

New problems

In orthokeratology, the lens physically reshapes the corneal epithelium during overnight wear. As a result, patients are able to see well during the day. But orthokeratology has had a rough ride to acceptance. Initially, the procedure was not popular because the lenses were not particularly oxygen permeable, leading to considerable visual danger. But with the advent of hyperoxygen lenses, it began to make its way into the mainstream.
However, the new study, authored by Takahiro Hiraoka, M.D., department of ophthalmology, University of Tsukuba, Japan, suggests that oxygen permeability isn’t the only problem with orthokeratology.
“The present study represents the first reported investigation into the relationship among contrast sensitivity, higher-order aberrations, and myopic correction in subjects undergoing overnight orthokeratology,” Dr. Hiraoka wrote. “Overnight orthokeratology for myopia significantly increases ocular higher-order aberrations, which deteriorate contrast sensitivity function, even after clinically successful orthokeratology.”
Dr. Hiraoka’s study was based on 46 eyes of 23 patients that underwent orthokeratology. The participants used a four-zone reverse geometry lens (Boston XO, Polymer Technology Corp., Wilmington, Mass.) composed of fluorosilicone acrylate; they were fitted according to the manufacturer’s instructions.
Researchers found that treatment significantly increased third-order RMS from 0.074 +/– 0.028 to 0.259 +/– 0.150 microns, fourth-order RMS from 0.038 +/– 0.020 to 0.134 +/– 0.061 microns, and total higher-order RMS from 0.085 +/– 0.032 to 0.297 +/– 0.152 microns.
Contrast sensitivity also decreased significantly at all spatial frequencies from 3 to 18 cyc/deg. Further, the 10% low-contrast visual acuity worsened significantly from 0.02 +/– 0.09 to 0.11 +/– 0.14 logMAR. Letter contrast sensitivity also significantly decreased from 24 +/– 0 to 22.8 +/– 1.9.
But it wasn’t all bad news for orthokeratology. Manifest refraction improved from –2.38 D at baseline to –0.24 D at three months after treatment. Uncorrected visual acuity also improved from 0.77 to –0.03 logMAR. Best-spectacle visual acuity was not significantly changed. Nonetheless, the focus of vision correction is on quality of vision, and these results do not cast a favorable light on this procedure.

Further findings

Dr. Hiraoka noted that the results appear to correlate with some patients’ experience. “In orthokeratology practice, patients sometimes report visual disturbances even though visual acuity is excellent according to high contrast acuity chart testing,” he wrote. “In such patients, it is possible that the quality of vision has deteriorated.” Further, quality-of-vision disturbances appear to correlate with the amount of myopic correction—the more correction, the greater the disturbances.
Dr. Hiraoka pointed out that the study had some limitations. For one, contrast sensitivity was only studied at one time period—three months into treatment. He also noted that some reports indicated that contrast sensitivity can fluctuate over time after LASIK or PRK.
“Further studies are needed to elucidate the relationship between pupil size and contrast sensitivity function after orthokeratology,” he said.

Independent voice

William Trattler, M.D., Miami, said that because orthokeratology lenses work to reshape the cornea by flattening it, if they don’t compensate for the effects of flattening, more higher-order aberrations could develop (see reference).
This is exactly what happened with initial PRK procedures, Dr. Trattler said, adding that many excimer lasers currently have a peripheral blend to smooth things out to try to eliminate higher-order aberrations.
“Maybe down the road they will develop technology to flatten the cornea without inducing higher-order aberrations,” he said. “Maybe there will be a new lens design. So certain orthokeratology technologies may end up causing more higher-order aberrations than others.”
In the meantime, Dr. Trattler isn’t entirely against using orthokeratology. In fact, an ophthalmologist at his practice uses the device three or four times a week to reshape his cornea overnight.

Editors’ note: Dr. Hiraoka reported no financial interests related to his study.

Contact Information
Trattler: 305-598-2020, wtrattler@earthlink.net
Hiraoka: thiraoka@md.tsukuba.ac.jp

Refrences
1. Hiraoka T, Matsumoto Y, Okamoto F, et al. Am. J. Ophthalmol 2005; 139 (3):429-36