What Type of Peripheral Defocus Are You Prescribing?
BY DAVID A. BERNTSEN, OD, PHD, FAAO
Each day, you prescribe glasses and/or contact lenses to correct your patients’ refractive error and meet their visual needs. Many factors go into the correction that you recommend to each patient. For example, you may prescribe a particular contact lens material or modality based on a patient’s symptoms of dryness, or because of heavy lens deposition.
While many considerations go into the correction that you prescribe, do you ever consider the effect that your selected correction has on a patient’s peripheral defocus?
Why It’s Worth Considering
Evidence continues to mount demonstrating that myopic peripheral defocus slows eye growth while hyperopic peripheral defocus accelerates eye growth in animal models of myopia (Benavente-Perez et al, 2014; Smith, 2011). While studies of myopic children have not found meaningful associations between relative peripheral refraction (RPR) of the uncorrected eye (a surrogate for eye shape) and myopia onset or progression (Mutti et al, 2011; Sng et al, 2011), a large amount of optically induced myopic defocus on the peripheral retina is associated with slower axial eye growth (Berntsen et al, 2013). This may explain the treatment effects reported in studies evaluating soft multifocal contact lens designs (Anstice and Phillips, 2011; Walline et al, 2013; Lam et al, 2014). These results suggest that we not only need to ensure that foveal vision is optimized, we need to understand the effect that spectacles and contact lenses have on peripheral defocus.
So, what do we currently know about the factors that influence the peripheral defocus ultimately experienced by the eye?
What Is the Eye’s Starting Point?
One of the challenges with trying to intentionally change peripheral defocus using optical designs is that not all eyes are created equal in terms of initial ocular shape. On average, myopic eyes typically have hyperopic RPR in the horizontal ocular meridian (Figure 1), while hyperopic eyes typically have myopic RPR (Figure 2) in the horizontal ocular meridian (Seidemann et al, 2002). However, because of variability in shape from eye to eye, a lens that causes myopic defocus in the periphery of one eye may not yield the same result on another eye.
Figure 1. Hyperopic RPR typically experienced in the horizontal meridian of myopic eyes and when myopic eyes wear most standard optical corrections.
Figure 2. Myopic RPR typically experienced in the horizontal meridian of hyperopic eyes. Myopic peripheral defocus is also hypothesized to slow myopia progression.
Unfortunately, there are no commercially available instruments that allow for a quick assessment of eye shape or the peripheral defocus experienced by the eye when wearing correction. Most research studies utilize open-field autorefractors to measure peripheral refraction, which typically requires patients to look at multiple targets while measurements are made at various peripheral locations of the eye. Some have reported asymmetries within eyes that yield slightly different amounts of peripheral defocus nasally and temporally on the retina, in addition to in the horizontal versus vertical meridian of the eye (Atchison et al, 2006; Ehsaei et al, 2011). Thus, we cannot simply measure defocus in one area of the periphery and assume that eye shape and defocus are the same throughout the peripheral retina. Until a rapid, commercial instrument is available for measuring peripheral refraction, eyecare providers will have to make an educated guess regarding the exact type and amount of defocus that their patients experience.
So, what do we know about the change in peripheral defocus caused by the optical corrections that we typically prescribe?
Change in Peripheral Defocus with Standard Corrections
As you would expect, designs currently being investigated for myopia control cause significant myopic shifts in peripheral defocus. Orthokeratology has been shown to slow axial eye growth and is known to cause a significant myopic shift nasally and temporally on the retina (Kang and Swarbrick, 2013), similar to the profile demonstrated in Figure 2. Center-distance multifocal contact lenses also cause a significant myopic shift in defocus, although some designs are more effective compared to others at causing nasal and temporal myopic defocus (Kang et al, 2013; Berntsen and Kramer, 2013). While the changes caused by these lenses should come as no surprise, what about changes caused by standard spectacle and soft lens designs?
Minus-power single-vision spectacles can increase the amount of peripheral hyperopic defocus (Lin et al, 2010; Berntsen et al, 2013). This additional hyperopic shift is not desirable.
Likewise, recently published off-eye measurements of spherical soft contact lenses demonstrated negative spherical aberration in most of the commercially available lenses tested (i.e., increased minus power toward the periphery of the optic zone; Wagner et al, 2014). This increase in peripheral minus power has the effect of increasing peripheral hyperopic defocus. However, one lens type evaluated in their study (lotrafilcon A, Air Optix Night & Day Aqua, Alcon) had a mostly flat power profile, indicating that the lens would not be expected to increase or decrease peripheral defocus when worn on the eye.
Although off-eye power profiles provide us with very useful information about lens optics, the tear lens and lens flexure also interact when a contact lens is worn on eye (Kollbaum et al, 2013), making it important to measure peripheral defocus when a contact lens is worn.
Several studies have investigated the on-eye effects of commercially available spherical soft contact lenses; the changes in peripheral defocus vary with the soft lens used. Most studies have found that wearing spherical soft contact lenses results in either peripheral hyperopic defocus or increased peripheral hyperopia in myopic eyes (Kang et al, 2012; Shen et al, 2010; Berntsen and Kramer, 2013).
Interestingly, in agreement with the off-eye power profiles described earlier by Wagner et al, another recent study found a small myopic shift in peripheral defocus of up to 0.50D on the temporal retina when lotrafilcon A lenses were measured on eye (de la Jara et al, 2014). The same study reported peripheral hyperopic increases of similar magnitude with the other three lenses tested in the study.
While the changes in peripheral defocus caused by spherical soft lenses tend to be clinically small compared to the changes caused by multifocal lenses, these findings indicate that the distribution of power within the optic zone of a soft lens has a measurable influence on peripheral defocus. It remains to be seen whether the differences among spherical soft lenses are large enough to have a meaningful effect on myopia progression.
All Corrections Impact Peripheral Defocus
In the end, even when selecting a non-bifocal correction, the lens you choose has an effect on peripheral defocus. The resulting defocus profile of any contact lens that you prescribe depends on multiple factors that include the eye’s shape, the power profile of the contact lens, consistency of the lens power from lot to lot, lens decentration, and interactions between the contact lens and the eye (such as flexure).
One day, we may be able to rapidly measure the amount of peripheral defocus present when our patients wear a contact lens. Until that day comes, it is important to recognize that differences in design among spherical contact lenses can influence peripheral defocus. CLS
To obtain references for this article, please visit http://www.clspectrum.com/references and click on document #228.
Dr. Berntsen is an assistant professor at the University of Houston College of Optometry. He has received research funding from the Johnson & Johnson Vision Care Institute and Alcon.