Small-diameter aperture optics to join armamentarium for eye care practitioners
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Small-diameter aperture optics, or as many call it, the pinhole, can increase depth of focus, enhance quality of vision at distance in select patients and mask small refractive errors, whether myopia, hyperopia, astigmatism or presbyopia.
Widely used in the camera industry, small-diameter aperture optics (SDAO) is rapidly evolving as a therapeutic modality in ophthalmology and optometry. I have participated in several projects in this field over the last 20 years and serve on the board of directors, medical advisory board or as a consultant to several companies applying SDAO to unmet needs in eye care.
SDAO is used widely in the camera industry. A prime example is the multiple disposable cameras widely available and used globally many million times a year. Using a small-diameter f-stop in the camera and setting the optics for intermediate focus allows a clear image and subsequent photograph from infinity to near. This optical principle, when discussed in the camera industry, is called “hyperfocality.” I like this term, as it describes increased depth of focus without any camera zoom lens or, for the eye, accommodation involved. This is the exact principle used by AcuFocus in the FDA-approved Kamra inlay (CorneaGen) and the IC-8 IOL, which is approved in Europe and Australia/New Zealand and has just completed phase 3 clinical trials in the United States.
Significant bench and clinical data show if the human eye is set at mild myopia, –0.75 D to –1 D, clear vision can be obtained over a 2.5 D range with the Kamra inlay or IC-8 IOL. In addition, low levels of astigmatism up to 1.5 D can be masked with no concern about IOL orientation to any specific axis/meridian. Small levels of myopia and hyperopia are also masked, and the increased depth of focus allows relatively seamless vision far to near.
Most important to me as a corneal and refractive surgeon, SDAO, like those present in the IC-8, also mask the visual degradation caused by irregular corneal astigmatism or significant corneal higher-order aberrations. We have known for years, when measuring higher-order aberrations with wavefront aberrometry or the surface regularity index (SRI) on corneal topography or tomography, that larger pupils manifest more higher-order aberrations (spherical aberration, coma, trefoil, etc) and smaller pupils manifest lower higher-order aberrations. According to many experts, higher-order aberrations or SRI over 0.6 results in meaningful degradation of image quality.
Studies and surveys by AcuFocus suggest that as many as 12% of patients presenting for cataract surgery have visually significant irregular corneal astigmatism that would benefit from SDAO. Common causes include congenital causes, keratoconus, previous incisional or laser corneal refractive surgery, trauma, corneal degenerations such as epithelial basement membrane dystrophy, Salzmann’s, pterygium and even dry eye disease, to name a few. I believe the IC-8 will find broad acceptance and use by the cataract surgeon in the U.S. once it is FDA approved. This may well be the first premium channel cash-pay IOL that is not only an elective option, but actually indicated and recommended for a significant cohort of patients.
SDAO can also be induced at the level of the pupil with surgical or laser pupilloplasty and, of course, miotic eye drops. The Agarwal approach to surgical pupilloplasty is well described in the accompanying cover story. Multiple companies are pursuing pharmacologic approaches to pupil miosis to treat presbyopia. I believe there will be a market for short-duration and longer-duration miotic drops for the temporary enhancement of near vision. Less well-appreciated, mild miosis also enhances distance vision by masking small levels of myopia, hyperopia and astigmatism and reducing the impact of higher-order aberrations, which are present at some level in all corneas.
I can see many patients benefiting from enhanced distance vision, including those engaged in visually demanding sports such as baseball or tennis. The recreational golfer will enjoy better distance vision, allowing them to see the ball down on longer shots and then benefit from the increased depth of focus when recording their score.
I expect the pharmacologic treatment of refractive errors, including atropine for progressive myopia and miotics for enhancing quality of vision and increased depth of focus for presbyopes, to be a significant new treatment adopted by all eye care practitioners in the next decade. As always, there is no free lunch, and as the aperture allowing light to enter the eye is reduced, mesopic and scotopic visual function and the ability to examine and treat the retina can be compromised.
The ideal target pupil size that maximizes depth of focus, retains good mesopic and scotopic vision, and allows retinal examination and treatment appears to be in the range of 1.3 mm to 2.1 mm, depending on the vertex distance of the aperture to the retina. SDAO at the corneal plane should be larger, and at the IOL plane, smaller. For a miotic drop or surgical pupilloplasty, a 1.5- to 2-mm pupil is a good target.
One benefit of shorter-acting miotic drops will be that their effect will wear off in 3 to 4 hours. This will allow an individual to place a drop in their eye before an evening dinner to read the menu in a restaurant and then have the miotic effect wear off in time to drive home in the dark. For as-needed use to enhance distance and near vision during a sporting event, 2 to 4 hours also seems a good target to me. If longer duration of therapy is desired, a second or even third drop can be placed.
SDAO in the form of corneal inlays, laser and suture pupilloplasty, miotic drops and IOLs such as the IC-8 will be a part of every eye care practitioner’s practice in the future. We have much more to learn, but the age-old pinhole will be a growing part of our medical and surgical therapeutic toolbox.
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