A Good Start
BY EEF VAN DER WORP, BSC, FAAO, FIACLE
With 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.
Figure 1. A keratoconus suspect. Photo courtesy Marco van Beusekom, Visser Contactlenzen, the Netherlands.
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.
Refit 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.