Issue: October 2005
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ORTHO-K RESEARCH
Current Research in Corneal Reshaping
An in-depth review of some of the latest research
presented at the Global Orthokeratology Symposium.
By Craig W. Norman, FCLSA
The recent 2005 Global Orthokeratology Symposium in Chicago
was once again a success. A well-known faculty presented numerous presentations
on corneal reshaping topics combined with an international Free Paper section
and 30 scientific posters on this subject.
While
eyecare practitioners are becoming more adept at fitting and managing ortho-k
patients, we still have much to learn as this field is evolving. The high
attendance at the all-day Fundamentals of Orthokeratology was a testament to
practitioners' willingness to learn about this procedure.
Fundamentals
Program
Eef van
der Worp, BSc Optom, reported on patient selection in ortho-k with emphasis on
choosing only patients whose spectacle prescriptions and corneal topography are
well-suited for this procedure. "While there are FDA approvals for up to
–6.00D of myopia, patients will generally have the most predictable results up
to –4.50D," stated Dr. van der Worp. "It's also important to note not
only the amount, but the location of any astigmatism. Today's designs can
correct only corneal astigmatism, primarily with-the-rule (WTR) astigmatism
where the axis is within 30 degrees of the horizontal meridian. Even then, only
a 50 percent reduction in the amount of astigmatism can occur, therefore choose
only patients who have a maximum of –1.50D of WTR astigmatism," he said.
Also, it's helpful to note if the corneal astigmatism is apical only or extends
from limbus to limbus (Figure 1). "Patients who have limbus-to-limbus
astigmatism are difficult to fit due to lens rocking and the tendency for the
lens to position high on the cornea," said Dr. van der Worp.
Randy
Kojima from Vancouver, BC, presented Pre-fitting Corneal Topography for Corneal
Reshaping, pointing out that a real key to successful ortho-k fitting is the
acquisition of accurate corneal maps at pre-fitting and during progress checks.
"The
initial information capture is the most critical," said Kojima.
"Multiple image captures are much more useful than taking just one single
corneal measurement." He continued, "Then, to ensure accuracy, I
review the maps simultaneously to look for symmetry between captures along with
any obvious error. If I'm concerned about the quality and reproducibility of
the images, I take additional maps as necessary."
Kojima
also explained how to use different maps. "The axial map is best for
defining astigmatism and determining the apical radius. Conversely, a
tangential (instantaneous/true) map is best for determining curvature at a
specific point," he said (Figure 2).
The
real key, though, is the use of difference (or subtractive) maps, which plot
the difference between two captures — such as a baseline (pre-fit) measurement
and a post-fit visit. This demonstrates specific changes in radius and
prescription during the fitting process. Subtractive Maps are also quite useful
for evaluating the position and size of the treatment zone, the lens position
(because the lens usually isn't worn into the office) and any surface
aberrations that have occurred secondary to overnight ortho-k wear.
Research
from the General Sessions
Myopia
A real highlight of the symposium was a discussion of myopia and a review of investigations
and studies on contact lens-induced myopia control.
Brien A.
Holden, B App Sc, PhD, from Sydney, Australia led off this discussion with a
review of myopia across the world. "Myopia affects 1.6 billion people
globally and is a serious public health issue," stated Dr. Holden, quoting
studies demonstrating that higher myopic patients (above –6.00D) have an
increased risk of retinal detachment, cataracts and glaucoma. "Uncorrected
myopia decreases quality of life, increases economic cost and reduces learning
opportunities for children," he said.
"While
myopia is viewed as simply a correctable error in the Unites States, there are
countries in Southeast Asia such as Taiwan and Hong Kong where up to 80 percent
of the overall population is myopic," said Dr. Holden. "In fact, the
World Health Organization has identified uncorrected refractive errors as one
of the five priority areas for The Global Initiative to Eliminate Avoidable
Blindness by the Year 2020."
Earl L.
Smith III, OD, PhD, from the University of Houston, in his presentation on the
Mechanisms of Myopia, described many of the longstanding theories regarding the
etiology of myopia, based on both genetic and environmental factors. Quoting a
1977 study by Wiesel & Raviola, he discussed form deprivation myopia (FDM)
where depriving the eye of form vision promotes axial elongation and myopia
(chronic image degradation can cause myopia). Of importance, the ocular changes
in FDM are similar to those associated with juvenile-onset myopia. Therefore,
the potential for a clear retinal image is essential for normal refractive
development.
Dr. Smith
believes that peripheral vision can influence foveal refractive development.
"Peripheral form deprivation can produce axial myopia at the fovea,"
he stated, "and the peripheral retina by itself can regulate
emmetropization." He also discussed how a functioning fovea is not
essential for emmetropizing responses.
"As a
consequence of eye shape and/or aspheric optical surfaces, 'corrected' myopic
eyes often experience significant hyperopic defocus across the visual field.
Under-correction may not be an effective strategy for slowing myopic
progression because small degrees of under-correction aren't likely to
eliminate peripheral hyperopic errors," Dr. Smith stated. "By
increasing the effective curvature of field it would be possible to correct
central errors and either correct peripheral errors or induced peripheral
myopic defocus." This may explain why some of the earlier efforts in
studying myopia control with GP lenses demonstrated results that were less
positive than anecdotal reports have suggested through the years. Such studies
"focused" images clearly on the fovea rather that utilizing a
"defocus" mechanism.
Myopia Control with GP Lenses
Jeffrey J. Walline, OD, PhD, presented the CLAMP Study results. (You can see
these results in detail by turning to Dr. Walline's article "GP Contact
Lenses and Myopia Control: Where Are We Now?" on p. 52.) One of the more
interesting findings of his work was that 80 percent of the myopic children
enrolled in this study initially adapted to GP wear with 70 percent of
successful adapters becoming "lifetime" wearers. Dr. Walline
concluded that GPs slowed myopia progression significantly, but the treatment
effect is likely not permanent with this method.
Pauline
Cho, BOptom, PhD, from Hong Kong Polytechnic University discussed Myopia
Control With Orthokeratology. She set out to determine if ortho-k is effective
for myopia reduction and control through a two-year study that monitored axial
length (AL) and vitreous chamber depth (VCD) changes in children undergoing
ortho-k treatment. She then compared this data to the rates of change in AL
& VCD for 35 children wearing single-vision spectacles from an earlier
study.
The subjects
were children aged 7 to 12 years who had a spherical refractive error of –0.25D
to –4.50D and astigmatism less than –2.00D. Among the many data collected were
vision (logMAR chart), refraction, corneal topography/thickness, AL and VCD.
Of the
initial 43 subjects, 35 (16 male, 19 female) completed the study. Researchers
evaluated eye elongation over the 24-month monitoring period. The mean change
in AL was
Variability
in the AL and VCD changes was quite large — on average, ortho-k lens wear
significantly slowed eye elongation, but some children still showed a large
increase.
In
conclusion, ortho-k was effective for myopic reduction, and the mean eye elongation
in ortho-k children was about half that of spectacle-wearing children. While
this is just one study and has a small study sample, it shows that ortho-k may
prove an option for slowing myopia progression in children.
Optics
of Orthokeratology If orthokeratology may indeed
control myopia progression, what optics of orthokeratology will facilitate
this?
Jennifer
Choo, OD, from Sydney, Australia presented a paper on this topic. Her interest
is to determine the ideal shape of the cornea after ortho-k and to understand
the optical limits to correction with varying pupil sizes and refractive
errors.
Using
software, an optical model of the eye was created with various pupil sizes (
"Optical
modeling provides a useful tool for understanding optical changes and may be
useful to help design lenses to optimize patient vision," said Dr. Choo.
"There is an ideal shape of the cornea after orthokeratology. To correct
vision, the shape will aim to minimize optical defocus and aberrations. In
particular, to decrease myopia progression, it will likely need to incorporate
aberration control."
Ortho-k
vs. Soft Lens Wear Michael J. Lipson, OD, from the
University of Michigan presented Overnight Corneal Reshaping vs. Soft
Disposable Contact Lenses: Vision-Related Quality of Life Differences From a
Randomized Clinical Trial. This study evaluated patients' visual acuity,
symptoms and perceptions of vision-related quality of life in a randomized
crossover clinical trial of overnight corneal reshaping (OCR) using Paragon CRT
lenses and daily wear disposable soft lenses (SCL).
He
evaluated the vision-related quality of life with the NEI RQL-42, which
consists of 42 multiple choice questions that rate many attributes such as
overall clarity, near and far vision, diurnal fluctuations, expectations and
activity limitations, etc.
Researchers
randomly assigned qualified subjects to wear one mode for eight weeks and
complete the RQL-42 at the end of that time. After a washout period, each
subject wore the alternate mode for eight weeks, completed the RQL-42 then
chose which mode they preferred. Of 81 subjects who enrolled, 65 subjects
completed both phases.
Among the
perception of vision-related quality of life were some statistically
significant differences. Activity limitations were better or less limited with
OCR, while glare was less bothersome with SCL. Patients had less dependence on
correction and fewer ocular symptoms with OCR. After completing the study, 67.7
percent of the overall study population preferred to continue with OCR (Figure
4) even though measured VA was better with SCL. Low myopes prefer OCR over SCL
although glare is more of an issue with OCR. While measured acuity was better
with SCL, subjective acuity wasn't significantly different between the two
modes.
When asked
what you can tell your patients about this study, Dr. Lipson responded,
"For patients who have higher amounts of myopia, such as –4.25D, SCL
vision is slightly better and glare will be less of a problem. Conversely, an
SCL patient will be more prone to lens awareness, itching or dryness."
"For
patients who have up to –2.50D correction and who tried CRT and SCL, 85 percent
preferred CRT. Vision with CRT will be comparable, and although patients
will find glare slightly more noticeable, they'll feel less activity-limited,
they'll have less problem with dry eyes, itching or lens awareness during the
day, plus they'll feel less dependent on their correction."
Craig Norman is director of the Contact Lens Section at the South
Bend Clinic in South Bend, Indiana. He is a fellow of the Contact Lens Society
of America and is an advisor to the GP Lens Institute.