Diagnostic and therapeutic applications exist for small-aperture optics
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We ophthalmologists are all introduced to pinhole or small-aperture optics on the first day of our residency when we are taught how to measure visual acuity and refract a patient.
As a diagnostic tool, we learn that a pinhole visual acuity will give us important information about a patient’s potential visual acuity in a high luminance environment by neutralizing lower-order (myopia, hyperopia, astigmatism) and higher-order (irregular astigmatism, spherical aberration, coma, trefoil) optical aberrations. Two other valuable examinations are a well-done refraction and, when irregular astigmatism is present, a hard contact lens over-refraction. A pinhole visual acuity after a refraction or contact lens over-refraction also gives us insight into a patient’s macular health and visual potential even in the face of a media opacity such as cataract. This finding led to the development of the super pinhole test for macular function. A pinhole is also one of the best ways to test for ocular dominance. Thus, the pinhole is firmly established as a critically important diagnostic test for every eye care practitioner.
All of us have heard patients say when testing their vision with a pinhole in a bright room with a high-contrast target, “Doctor, that is perfect. Please give me that vision!” Today, we are entering a new era in which small-aperture optics (SAO) are being applied as a therapeutic, especially for the management of the 120 million in the United States and 2 billion in the world with presbyopia. Several other subsets of patients may also benefit, including those with irregular astigmatism, the so-called “complex cornea patient,” and a group with unwanted night vision symptoms including halo, glare and light scatter.
I first became interested in SAO in 1982 when I was hired as a consultant for Iolab, which was developing a center-surround refractive bifocal IOL. We needed to know the ideal size for the center near optic, and after much optical bench testing, we settled on 1.5 mm to 2 mm in diameter. Later, I worked with 3M developing the first diffractive bifocal IOL, and again we wanted to know the ideal size for the central clear zone. For the last 20 years, I have been an adviser to AcuFocus as it developed the FDA-approved Kamra inlay (now CorneaGen) for presbyopia and soon to be available IC-8 extended depth of focus (EDOF) IOL. For the last decade, I have worked with several companies, especially Orasis, in the development of miotic drops to treat presbyopia. First, I disclose that I consult widely in the field of SAO for therapeutic purposes. Next, let me share a few of my learnings.
The natural pupil size for a healthy middle-aged adult in fluorescent light is approximately 3.5 mm. This pupil size evolved over several millennia as ideal for quality distance and near vision in a human emmetrope with good accommodation. For this average person, when subjected to a very bright light, the pupil constricts to about 2 mm, and in dim light, the pupil expands to 7 mm to 8 mm. Therefore, luminance is critically important when evaluating the ideal pupil size for best vision. The Achilles’ heel of SAO when applied to a therapeutic setting is low luminance.
The second challenge when applying SAO as a therapeutic, even in a bright light environment, is the negative effect of diffraction on image quality. Image degradation from diffraction begins at an aperture diameter of 1.8 mm, and pupil diameters smaller than this can also reduce one’s visual field, especially in a low luminance environment. These findings lead me to conclude that in a daytime environment, the pupil size target for a therapeutic such as a miotic drop is in the 1.8- to 2.2-mm range.
A significant benefit of SAO is increased depth of field, and every 0.5 mm reduction in pupil size from the base of 3.5 mm increases depth of focus approximately 0.5 D. To make it easy to understand for the clinician, a 0.5 mm decrease in pupil size from 3.5 mm is equivalent to a 0.5 D reading add. Reducing the pupil size from 3.5 mm to 2 mm improves depth of focus about 1.5 D and improves near vision like a +1.5 D near add.
We now have one miotic drop, Vuity (pilocarpine HCl ophthalmic solution 1.25%) from Allergan, that is FDA approved for the treatment of presbyopia. The presbyopic patients it will help the most are those who would benefit from a +1 D to +1.5 D reading add. We also have the FDA-approved Kamra inlay and will soon have the IC-8 EDOF IOL. The therapeutic applications of SAO are happening now and expanding rapidly.
Our learnings about SAO have been expanded by the camera industry, in which increasing F stops resulting in smaller diameter apertures have been applied to enhance focus, image and clarity and increase depth of field for decades. The current disposable cameras and the camera in your digital phone apply SAO to allow quality photographic images with no need to adjust the focus. I measure the aperture size on my iPhone at about 1 mm. With this small aperture size, the camera can be optically focused on an intermediate distance using a single focus plus lens, and the increased depth of focus from the small aperture allows a clear picture at near, intermediate and distance in a high luminance environment. In the camera industry, this is called “hyperfocality.” I like this descriptor, and we have learned the same principle applies in the human eye.
In both optical bench testing and human clinical trials, we have learned that a target of mild myopia, –0.5 D to –1 D, is ideal for the patient implanted with a Kamra inlay or IC-8 EDOF IOL. Near vision is enhanced with a mildly myopic target, and distance vision is not sacrificed as the usual loss of distance vision associated with myopia is compensated for by the pinhole effect. Up to 1.5 D of regular astigmatism is also masked by SAO. Hyperfocality is an important principle for maximizing the benefit of SAO in a camera and in our patients implanted with an IC-8 EDOF IOL.
Another important application of SAO is to enhance vision for the patient with a complex cornea. In 2020, I participated in a global consensus panel on the complex cornea patient. The global consensus was that complex corneas can be naturally occurring, secondary to disease or dystrophy, surgically induced or caused by trauma. In the patient presenting for cataract surgery, 12% to 14% were found to have a complex cornea in the comprehensive ophthalmologist’s practice and twice that number in the cornea subspecialist’s practice. Useful diagnostic tests when evaluating the complex cornea patient include post-refraction vision improvement with a pinhole or hard contact lens over-refraction, a surface regularity index of 0.5 or more on topography or higher-order aberrations of 0.5 or more on wavefront testing.
In my practice, common causes of a complex cornea are untreated dry eye disease; corneal dystrophies/degenerations including keratoconus, epithelial basement membrane dystrophy or Salzmann’s nodular degeneration; prior corneal refractive surgery, especially incisional keratotomy and early-generation excimer laser treatment; and trauma. These patients can benefit significantly from SAO, and I have helped many with miotic drops. Soon, I believe patients with a complex cornea and cataract will benefit from the IC-8 EDOF IOL. A third common clinical application of SAO will be in phakic and pseudophakic patients who have disabling unwanted night vision symptoms.
In closing, the pinhole or SAO is both a highly relevant diagnostic tool and a beneficial therapeutic tool. Refractive optics, diffractive optics and small-aperture optics are all valuable tools as we strive to restore and enhance the vision of our patients.
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