Prescribing for Astigmatism
High Definition Vision with Contact Lenses
BY DIANA NGUYEN, OD, & TIMOTHY B. EDRINGTON, OD, MS, FAAO
While watching an old, non-Blu-ray movie on a high-definition (HD) television, a friend remarked on the poor image quality. Practitioners can support their patients’ appetite for HD optics by considering correcting parameters of optical blur beyond sphere and cylinder.
Aberration-minimizing technology has become commonplace in refractive surgery, incorporating wavefront error measurements to minimize the induction of aberrations (wavefront-optimized) and/or to offset existing aberrations in the eye (wavefront-guided). Also, it has been utilized in a few soft and GP lens designs.
Rose K2 (Menicon) is a GP lens designed with an aspheric base curve and a front aspheric surface to minimize aberration. A soft lens example is PureVision2 (Bausch + Lomb), which incorporates an aspheric front surface designed to reduce spherical aberrations induced by the contact lens power; it additionally corrects population-based aberrations.
A few soft contact lens manufacturers offer customized lenses that use corneal topography and/or wavefront error mapping to measure the aberration of an individual eye and compensate for that error in a customized lens. However, this is complicated by the movement of the lens on the eye, which may inappropriately position the custom aberration corrections, possibly negating their potential vision enhancement. Toric soft lenses and scleral GP lenses may be better designs for these applications because of their stability on the eye. With renewed interest in prescribing scleral GP lenses, studies show promising results on the use of wavefront-guided optics on scleral lenses (Sabesan et al, 2013).
Keep in mind that ocular aberrations are not static; they change with age, pupil size, and even tear film stability, making it difficult to fully correct them. We also may have adapted to these natural aberrations; thus, correcting them may not improve vision performance and may even make it worse for some.
Forward to the Future
Several lab-based studies have investigated the use of customized wavefront-guided soft contact lenses for mild and moderate keratoconus patients, who exhibit approximately 5.5 times more higher-order aberrations compared to those who have normal corneas. Reducing these higher-order aberrations would improve their vision quality and would be especially beneficial to those who have GP lens intolerance.
Crawl Before You Walk
Before we can correct the higher-order aberrations (spherical aberration, coma, trefoil, etc.), we must first correct the lower-order aberrations of defocus (myopia and hyperopia) and astigmatism as accurately as possible. Because sphere and cylinder account for about 93% of ocular aberrations (Porter, 2001), most patients can achieve excellent vision with their sphero-cylindrical corrections based on subjective refraction and are not affected by higher-order aberrations. Therefore, it is important to optimize the sphere and cylinder prescription before considering wavefront-guided corrections that reduce both lower- and higher-order aberrations.
As aberration-correcting contact lens technology improves and becomes more widespread, these patient-specific corrections can add to our armamentarium and provide our patients with improved visual performance. CLS
For references, please visit www.clspectrum.com/references and click on document #223.
Dr. Nguyen is a cornea and contact lens resident at the Southern California College of Optometry at Marshall B. Ketchum University (MBKU). Dr. Nguyen received her OD degree from University of California, Berkeley School of Optometry. Dr. Edrington is a professor of optometry at MBKU. He has also received research funding from SynergEyes. You can reach him at firstname.lastname@example.org.