Correcting Residual Astigmatism
for an Irregular Cornea Fit
Gregory W. DeNaeyer
(R)GP contact lenses are often the lens of choice when a patient presents with front surface corneal irregularity in cases such as keratoconus. The firm nature of (R)GPs allows them to hold a tear layer that masks both regular and irregular corneal astigmatism, and most of the time spherical or aspheric lens designs provide patients with their best potential acuity.
However, there are cases in which astigmatism within the eye, such as lenticular astigmatism, is not masked by the (R)GP tear layer. A sphero-cylindrical over-refraction during the diagnostic fitting process will confirm and quantify any residual astigmatism, which will need to be addressed if the amount measured is 0.75D or more. One straightforward and effective way to correct for residual astigmatism is to incorporate the sphero-cylindrical over-refraction into a spectacle prescription that the patient wears over the newly prescribed (R)GP contact lens. Remember, patients with severe corneal irregularity are often very accepting of multiple prescription devices if necessary to achieve best vision.
Another option that is often overlooked is to incorporate front surface toricity into the specialty (R)GP lens design. The additional astigmatic correction can be lathed into the front surface and stabilized with prism, slab-off, or truncation.
Keep in mind that a lens that flexes on the eye can induce astigmatism, and this can be ruled out by doing an over-keratometry measurement. Flexure can be eliminated by simply increasing the center thickness of the lens by 0.1 to 0.2mm.
A patient presented with mild keratoconus in her right eye (Figure above) and was diagnostically fit with a bi-aspheric large diameter (R)GP lens. A diagnostic lens was ordered with the following parameters: diameter of 10.4mm, base curve of 7.34mm and power of -4.00D. Although the lens fit well, it only provided the patient with 20/50 vision.
The result of a sphero-cylindrical over-refraction was +2.00 -1.25 X 105. A front surface toric lens (Figure above) was ordered and dispensed with a power of -2.00 -1.25 X 105 that was stabilized with double slab-off, which gave the patient 20/25 vision. The lens had mild temporal rotation, but the patient was satisfied and no other remakes were ordered.
Keep in mind that it may be necessary to take an extra step to maximize a patient’s vision when using (R)GP lenses to correct for corneal irregularity.