prescribing for astigmatism
Bitoric GP Lenses: Double the Toric Fun
BY JULIE A. SCHORNACK, OD, MED
Just yesterday a colleague of mine asked me to take a look at a corneal topography image to rule out keratoconus. The image was a binocular presentation of a topographical map showing a high amount of corneal toricity in both eyes with a beautiful symmetrical pattern and no distortion. The conclusion we came to was that this was just a case of high corneal toricity with little indication of keratoconus.
This 36-year-old patient had more than 7.00D of corneal toricity and was wearing glasses as his only form of visual correction. He had never worn contact lenses and denied that they had ever been suggested to him. I couldn't wait to fit him with a pair of contact lenses and change his life.
This patient's refraction showed that he needed more cylinder correction than the value of his spherical component. Although soft toric lenses are available in parameters that would correct his refractive error, it would not be my first choice of contact lens correction for this patient. If the amount of cylinder a patient requires exceeds the spherical component, visual satisfaction with the performance of a soft toric lens may be more difficult to achieve. Misalignment of the correcting cylinder's orientation on-eye in the soft toric lens would potentially diminish acuity to an unacceptable level.
The customized nature of a bitoric lens design for such a patient would help enhance comfort, and the optics of a GP lens would maximize visual performance. Comfort would be improved because the contour of the base curve more closely approximates the shape of the cornea. Additionally, bitoric peripheral curves more accurately conform to the peripheral corneal, thereby minimizing the lens edge sensation by the lid. This also provides the benefit of more uniform edge lift, which serves to decrease the incidence of 3 o'clock-to-9 o'clock staining.
Fitting Bitoric Lenses
Whether you fit empirically or with trial lenses, either approach will typically result in superior visual performance provided that you collect the data carefully.
An excellent tool to assist you with designing bitoric contact lenses is the Mandell-Moore Bitoric Lens Guide. The Gas Permeable Lens Institute (GPLI) makes this tool available as both an online calculator and as a form suitable for printing as a work-sheet (www.gpli.info — look under the GP Clinical Education section). Either version identifies what patient data are required to calculate the final appropriate base curves and contact lens powers in bitoric contact lens designs. Straightforward and useful directions and tips are available to help you finesse the final lens design.
If you enjoy more of an in-chair experience when fitting high-corneal-toricity patients, then diagnostic contact lens fitting is the way to go. The one limitation in fitting these patients is the parameters of the available diagnostic lenses. Although you could place a spherical diagnostic lens on a 7.00D toric cornea, the comfort would be less than ideal for the patient, the fluorescein pattern would be difficult to read and it's doubtful that the lens would position in a place that would allow you to obtain good over-refraction data.
The bottom line for these patients is that you have an opportunity to fit them in a pair of contact lenses that provide great vision. You could open up their visual possibilities and free them from glasses. With so few opportunities to be a hero in everyday life, take advantage of GP toric technology to maximize the visual benefits in your astigmatic patients. CLS
Dr. Schornack is the associate dean of Clinical Education and serves in the Cornea and Contact Lens Service at the Southern California College of Optometry.