Prescribing for Astigmatism
Finding a Better Lens Option
By Pam Satjawatcharaphong, OD, & Timothy B. Edrington, OD, MS, FAAO
Developments in soft toric designs have resulted in more successful fits for patients who require astigmatism correction. However, non-silicone hydrogel toric lenses are still available. While it is true that most patients can wear these lenses with no difficulty, there are a handful of patients for whom the low Dk is problematic. Often practitioners adopt an “if it ain't broke, don't fix it” mentality that dissuades them from trying what may be a better lens option.
Prism-ballast toric soft lens designs have variable thickness profiles along the vertical meridian, with the thickest portion generally located inferiorly. While non-prism-ballast toric and spherical designs do not show significant Dk/L differences along the vertical meridian, in prism-ballast designs the average Dk/L may be two to three times greater in the superior portion of the lens when compared to a similar inferior location (Eghbali et al, 1996). This suggests that it is important to monitor vessel growth in patients wearing prism-ballasted hydrogel lenses. It is also important to look for corneal vessel growth beneath the upper eyelid regardless of the toric design.
A patient who had a refractive error of OD –9.50 –3.00 x 023 and OS –9.75 –3.50 x 173 presented for a contact lens evaluation. She had a history of wearing soft toric lenses for several years before switching to silicone hydrogel toric lenses. She reported no ocular discomfort while wearing the current lenses, but desired improved vision. Slit lamp examination revealed moderate corneal neovascularization (Figure 1) extending 3mm superiorly onto both corneas.
Figure 1. Corneal neovascularization beneath the upper eyelid.
It is likely that the neovascularization resulted from hypoxia due to the combination of long-term wear of low-Dk non-silicone hydrogel toric lenses, the lens thickness profile, and the anatomical barrier of the upper lid covering the superior corneal region. The neovascularization persisted despite switching to a silicone hydrogel material, and her vision remained unsatisfactory. Considering her prescription, we refit her into GP bitoric lenses in a high-Dk (100) material to optimize oxygen transmissibility and to enhance her vision.
If mentioning “bitoric” caused you to cringe, have no fear. Bitoric fitting guides such as the Mandell-Moore Bitoric Lens Calculator and the Quinn GP Toric Lens Calculator (both at www.gpli.info) will help you empirically design an initial set of lenses based on refraction results and keratometry readings. We recommend using these fitting guides as a starting point and then using the fluorescein pattern and sphero-cylindrical over-refraction to adjust the lens parameters as needed. We hope that with these tools you will be more comfortable performing a bitoric contact lens fit when the occasion calls for it.
Improving Health and Vision
By switching patients to a new lens material or design, you may improve their ocular health and sharpen their vision. So, the next time a patient says he wishes he saw a little bit better, consider all your options, and always take a look under the lid.
Stay tuned for bitoric contact lens fitting tips in our next column. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #191.
Dr. Satjawatcharaphong received her optometry degree from the University of California, Berkeley. She is currently the cornea and contact lens resident at the Southern California College of Optometry. Dr. Edrington is a professor at the Southern California College of Optometry. He has also worked as an advisor to B+L. You can reach him at firstname.lastname@example.org.