Principle of pinhole optics works toward optimal correction of presbyopia
Leveraging pupil size to expand the depth of field is emerging as a high-interest approach for improving near vision.
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Although presbyopia is an inevitable part of the aging process, its adverse impact on individuals’ activities of daily living seems to be more prevalent today than ever before.
This is due in large part to the ubiquitous nature of digital media and communication. For most people, what is most troublesome about near vision loss is the associated difficulty reading.
Adults in this country spend close to half the day watching, reading, listening to or simply interacting with media. No matter the age group, texting is the preferred method of communication — even baby boomers are seven times more likely to text vs. talk in person. Moreover, U.S. adults spend just shy of 4 hours a day using computers, tablets and smartphones. These numbers add up to a constant struggle for the 128 million U.S. presbyopes who are losing their accommodative ability and experiencing frustration each time they check their phones, to the tune of 96 times per day.
Correction options and limitations
Patients have an array of both nonsurgical and surgical options to mitigate the effects of presbyopia, ranging from bifocal and multifocal (progressive) glasses to laser procedures to shape-changing IOLs and everything in between. Corneal interventions include a variety of excimer and femtosecond laser procedures, conductive keratoplasty and inlays/onlays. The femtosecond laser can also be used to perform lens-based presbyopia correction by softening the crystalline lens. IOL choices are myriad with diffractive technology that includes bifocals, trifocals and extended depth of focus implants. Other IOLs include nondiffractive extended depth of focus products as well as accommodating, light adjustable and femtosecond laser-induced shape-changing technologies.
Depending on the approach, a variety of optical principles can be leveraged to achieve pseudoaccommodation, allowing patients to regain near vision function.
The mechanisms of pseudoaccommodation are higher-order aberrations (eg, spherical aberration, coma), corneal asphericity or Q-value, residual astigmatism, corneal-scleral biomechanics, diffractive/nondiffractive IOL technologies, refractive indexing change and the pinhole effect, also known as small-aperture optics. These various mechanisms increase depth of focus independent of ciliary muscle action, using the static optical properties of the eye.
There are limitations to treating the true cause of presbyopia, which is the loss of accommodative ability of the lens to dynamically change focus power. The array of corneal presbyopia-correcting procedures, from presbyLASIK to multifocal and aspheric lenses, may compromise distance vision and degrade binocularity and stereopsis. Plus, surgical intervention carries additional risks for regression, scarring and dysphotopsias. Essentially, these treatments seek to enhance pseudoaccommodation by facilitating an extended depth of focus to achieve enhanced vision rather than restoring a combination of true accommodation and pseudoaccommodation.
Enter small-aperture optics
Specific surgical and pharmaceutical small-aperture strategies are in various stages of development for treating presbyopia. One example of lens-based technology that applies the pinhole concept is the IC-8 IOL from AcuFocus, currently under investigation in the United States and CE marked for use elsewhere.
During miosis, the pupil shrinks, expanding the depth of field. The pupil’s diameter, therefore, plays a role in presbyopia, as a smaller aperture, or pinhole, that can improve the depth of field. Where the pinhole is placed is key in terms of maintaining the peripheral field along with increasing the depth of focus for reading, for example. If the pinhole is in the corneal plane, the visual field is constricted. A small aperture in or near the iris plane can provide increased depth of field without impairing the visual field (Figure 1).
Source: Karolinne M. Rocha, MD, PhD
There is no single pupil size or measurement that creates optimal constriction to enhance the depth of field while not affecting the peripheral vision. The necessary reduction in pupil size for optimal vision, rather, is based on multiple factors including the individual pupil and the lighting conditions encountered. A given optimal pupil size, therefore, is really a range that will allow for maximum image quality for both near and distance vision.
Conclusion
The demand for improved presbyopia-correcting solutions that can enhance reading vision without compromising the visual field will only rise as our dependence on digital media increases and the presbyopia population grows toward an estimated 1.8 billion people worldwide by 2050. Using the old principle of pinhole optics to create a new way of achieving youthful vision is a promising strategy that may be applied using a variety of methods. Until we can prevent the onset of presbyopia altogether, work toward an optimal corrective strategy will continue apace.
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- For more information:
- Karolinne M. Rocha, MD, PhD, can be reached at Storm Eye Institute, Medical University of South Carolina, 167 Ashley Ave., Charleston, SC 29425; email: karolinnemaia@gmail.com.
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