Prescribing for Astigmatism
Managing Scleral Lens Residual Astigmatism
BY KAREN LEE, OD; BROOKE MESSER, OD; & TIMOTHY B. EDRINGTON, OD, MS, FAAO
Fitting guides make prescribing scleral GP contact lenses relatively straightforward; however, they generally do not explain how to manage residual astigmatism, which can result frominternal (lenticular) cylinder or from GP lens flexure.
Strategies to Manage Scleral Lens Residual Astigmatism
If the sphero-cylindrical over-refraction (SCOR) has less than 1.00DC, generally prescribe the equivalent diopter sphere or a scleral lens with aspheric optics. Although the OR residual cylinder does not decrease with an aspheric design, the spherical aberration may be reduced, leading to improved vision.
If there is more than 1.00DC in the SCOR of a spherical scleral lens, over-keratometry or topography sim-Ks can help differentiate between internal astigmatism and lens flexure. Astigmatic over-Ks indicate the presence of lens flexure; spherical over-Ks indicate that the OR cylinder is due to internal astigmatism. The latter can be resolved with a front-surface toric scleral lens.
Eliminate unwanted lens flexure by increasing the lens center thickness (CT) 0.1mm for each diopter of flexure. As the CT increases, the residual cylinder obtained in the SCOR and over-Ks should decrease. However, lens mass will also increase, and the lens may position inferiorly. This lens drop can be problematic, especially if superior limbal or midperipheral clearance is already minimal. To reduce flexure and avoid lens drop, the outer peripheral curves that rest on the sclera can be thickened by 0.1mm without increasing the lens CT (Figure 1). This will stiffen the load-bearing portions of the lens, improve lens stability, and minimize flexure.
Figure 1. Increase peripheral junction thickness to decrease lens flexure.
Another solution to minimize flexure is to decrease the overall diameter (OAD). Lenses with larger OADs tend to exhibit more flexure because toricity may increase in the scleral periphery (van der Worp, 2010). Unfortunately, decreasing the OAD may not always be an option, especially in cases of ocular diseases such as Stevens-Johnson syndrome, in which the larger OAD provides therapeutic protection.
Lastly, a peripheral toric system can be incorporated to decrease flexure, improve scleral alignment, and enhance comfort, especially when both edge lift and impingement are present along the circumference of the landing curves (Visser, 2006). A peripheral toric system will decrease flexure that manifests as with-the-rule residual astigmatism (axis 180º), whereas increasing the CT or junction thickness decreases flexure regardless of axis.
If none of the preceding scleral lens designs are viable options, a pair of overlay spectacles can provide the needed astigmatic correction. This is especially helpful for those needing a multifocal correction as well. Also, there are times where a combination of the aforementioned options may be necessary. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #221.
Dr. Lee is a cornea and contact lens resident at the Southern California College of Optometry at Marshall B. Ketchum University. She received her OD degree from Indiana University. Dr. Messer practices in Minneapolis, Minn. in a private optometry office focused on specialty contact lenses. She is a consultant to Precilens, has received research funding from Alcon and B+L, and has received honoraria from Essilor. Dr. Edrington is a professor at the Southern California College of Optometry at Marshall B. Ketchum University. Drs. Lee and Edrington have received research funding from SynergEyes.