BY EEF VAN
DER WORP, BSC, FAAO, FIACLE
many years of experience with orthokeratology behind us now, we can say that it
continues to be a very successful mode of vision correction. We've also learned
that starting with an accurate corneal topography
is a large part of the secret to success.
A Bull's Eye
Ideally a bull's eye pattern is the outcome when you create a
difference topography between the initial corneal topography and the patient's corneal
topography after wearing the orthokeratology lens.
Two other patterns are possible: One occurs when the topographer
overestimates the sagittal height of the cornea, and a central island forms. This
is usually simple to recognize you can see a small area of relative steep
curves (warmer colors) within the optical zone of the eye. Because the topographer
overestimated the sagittal height of the cornea (the cornea is in reality flatter
than calculated), you need a new, flatter lens to resolve this situation.
The other option is a smiley face pattern. The lens is too flat
for the cornea or the lens sag is too low. The cornea is steeper here than the topographer
estimated, so you need a steeper lens fit with a greater sagittal height.
1. A keratoconus suspect. Photo courtesy Marco van Beusekom, Visser Contactlenzen,
The Correct Topography
When fitting orthokeratology it's critical to begin with the correct
baseline corneal topography. For GP wearers, cease lens wear for three to four weeks
before performing topography so the cornea returns to its original state. However,
wait longer for long-term GP lens wearers and patients wearing low-Dk lenses.
PMMA lens wearers with standard GP lenses and then gradually cease lens wear. Also,
be aware that back aspheric multifocal lenses as well as lenses that ride high tend
to alter the shape of the cornea.
Less known than the corneal changes during GP wear, and therefore
more neglected, are topographical changes underneath hydrogel lenses. Advise hydrogel
lens patients to cease lens wear for at least three to four days. However, you might
need longer periods if patients are wearing thick, low-Dk lenses. If you're not
an experienced fitter, avoid these wearers in the beginning.
Another reason for a careful analysis of the cornea is early keratoconus
detection. Normal eyes don't express large differences between the superior and
inferior halves of the corneal surface, but this is often the case in keratoconus.
If the difference is about 1.4D (usually measured by the topographer
at five different points in each half), then the cornea is a keratoconus suspect.
More than 1.9D is considered irregular, and keratoconus is very possible.
Another way of recognizing early keratoconus is to analyze the
axis of the astigmatism. In a normal cornea, the axes of both meridians are roughly
straight lines, but if the angle within one meridian diverges more than 21 degrees
(Figure 1), this is considered keratoconus suspect.
Starting with Success
Don't make a false start when fitting orthokeratology. Excluding
potential problem cases can increase the chances of success.
Dr. van der Worp is a lecturer
at the school of optometry of the Hogeschool Utrecht and a researcher at the University
of Maastricht department of ophthalmology in the Netherlands.
Contact Lens Spectrum, Issue: November 2006