prescribing for astigmatism
The Future is Almost Present
BY TIMOTHY B. EDRINGTON, OD, MS, FAAO
Growing up, I marveled at Dick Tracy's phone watch; no way that could come to fruition in my lifetime. But smart phones and their seemingly unlimited apps have become reality. Toric contact lens technology has improved over the decades, but in incremental — not quantum — leaps. GP lenses are fabricated with computer numeric lathes that allow for expanded design features such as unlimited aspheric and toric surfaces and quadrant specific options. Soft toric lenses have become remarkably reproducible with improved methods of achieving rotational stability. Both soft and GP lenses are available in significantly more permeable materials that enhance corneal physiology.
Aberration-minimizing technology has become commonplace in refractive surgery and intraocular (IOL) lens implantation. It's also successfully utilized in a few mass-marketed soft and GP contact lens designs to minimize the spherical aberration induced by the optical correction of high amounts of myopic and hyperopic refractive errors. There have been a few mass-marketed soft lens designs that used an average aberration-correcting value on all of their lenses. Of course, this type of design tends to overcorrect some patients' aberrations and under-correct others. For this application, a customized approach would seem more appropriate.
Customized correction of aberrations has been attempted with varying success. Customized refractive surgical procedures have increased in recent years. A few soft contact lens manufacturers offer customized lenses that use corneal topography and/or wave-front measurements to neutralize aberrations. However, the customized correction of aberrations with contact lenses is challenging in that vertical, lateral or rotational movement of the lens may cause custom aberration corrections to position inappropriately possibly negating their potential positive effect on vision. Toric soft lenses might be the best design for these custom applications because of their limited on-eye movement and rotational stability. But like the cell phone, anticipate advancements with these lens designs.
There have been small-scale studies investigating the benefit of customized, wavefront-correcting soft contact lenses for mild and moderate keratoconus. The results have been promising. Keratoconus patients exhibit approximately five times the amount of higher-order aberrations when compared to patients who have normal corneas. A reduction in these higher-order aberrations would improve a keratoconus patient's quality of vision, especially in reduced lighting situations.
A confounding variable in correcting nature's ocular aberrations is that aberrations change as we focus and as we age, creating a moving target to adequately correct them. Also, we may have adapted to our natural aberrations, so that changing or reducing them may result in less comfortable vision.
Back to the Future
Perhaps the most important aberrations to correct are the lower-order aberrations of defocus (myopia and hyperopia) and astigmatism. Attention should be given to correcting the refractive error as precisely as possible. As aberration-correcting contact lens technology improves and becomes more widespread in its availability, eyecare practitioners may consider testing for and prescribing smaller refractive increments such as 0.12 diopters. More aggressive correction of astigmatism instead of correcting only the spherical equivalent will also help. Once the sphere and cylinder prescription is optimized, the correction of the higher-order aberrations, such as spherical aberration and coma, should in the not-to-distant future result in enhanced “super” contact lens vision for our patients. CLS
Dr. Edrington is a professor at the Southern California College of Optometry. He has also worked as an advisor to B+L. E-mail him at firstname.lastname@example.org.