Understanding the eye’s optics is key to assessing, improving quality of vision

This first installment of a 10-part series on Quality of Vision explores the optics that affect quality of vision and determine what tools can be utilized to accurately assess the function of the eye.

For clinicians and researchers to offer patients the best visual outcomes with today’s advanced refractive procedures, they must first attain a clear understanding of the mechanisms behind the optics of the eye.

“Everyone working with human vision should understand about the optics of the eye, how it works, how it relates to vision and how to accurately measure the quality of vision,” said Jack T. Holladay, MD, MSEE, FACS, Optics, Refraction and Contact Lenses Section Editor of Ocular Surgery News and a clinical professor of ophthalmology at Baylor College of Medicine in Houston. Dr. Holladay presented an overview of the physiologic optics of the human eye and how its inner workings affect quality of vision at the biannual meeting of the International Intraocular Implant Club in Aruba.

He defined quality of vision as a multifactorial measurement that includes visual acuity, contrast sensitivity and wavefront aberrometry.

“Quality of vision is a new comprehensive set of measures that determines visual performance at near, far and in variable light conditions,” Dr. Holladay said. “Good vision is no longer defined by 20/20 vision.”

The necessity of evaluating vision beyond Snellen acuity has led officials at the Food and Drug Administration to utilize contrast sensitivity and wavefront aberrometry in most clinical trials related to vision, he said.

“Such requirements will help us to better quantitatively measure the performance of new devices and surgical procedures,” Dr. Holladay said. However, before clinicians can accurately assess contrast sensitivity and quality of vision, they must first understand the complexities of the human eye that lead to poor quality of vision.

In this first installment of a 10-part series on Quality of Vision, Ocular Surgery News explores the anatomy of the normal human eye through interviews with Dr. Holladay. This article reviews the unique physiological properties of the eye that not only limit vision but also improve it. Clinicians who are interested in improving their surgical outcomes will discover the intricacies of the eye’s visual pathways and learn how to assess the performance of these systems by using Snellen tests, reading rates, contrast sensitivity tests and wavefront aberrometry.

Angle kappa not dependable

Traditionally when assessing ocular anatomy and setting measurements for refractive surgery, clinicians measure angle kappa, the 2.6° horizontal angle between the center of the pupil and the visual axis (vertical angle kappa is about one-fourth of this amount). Physicians determine angle kappa by locating the center of the pupil and the light reflex, which is usually near the visual axis at the corneal plane, and measuring the distance between these two points.

Dr. Holladay said he dissuades surgeons from relying solely on angle kappa for their measurements because pupil location is unpredictable and differs from patient to patient.

“The center of the pupil is quite variable. It can be located on the visual axis, on the optical axis or, most of the time, about halfway between the two,” Dr. Holladay said. “For that reason, centering your measurements on the pupil is probably not a good idea. It’s very unreliable.”

He noted that while most people have pupils that are decentered nasally at angle kappa, the variability that exists prevents surgical outcomes from being reproducible from patient to patient. Because measurements related to refraction, topography and other tests are made along the visual axis, centering on the pupil would require an “entirely different set of values” to be correct, Dr. Holladay said.

Angle alpha reliable, consistent

Angle alpha, a measurement that relates to the overall tilt of the globe as measured from the nodal point of the eye, about 7.2 to 7.6 mm posterior to the corneal vertex, is a more reliable measurement, Dr. Holladay said. “The angle alpha is the degree of anatomical tilt of the eye relative to the optical axis. To get an image on the fovea, the eye is tilted 5.2° out horizontally and 1° up vertically from the optical axis,” he explained.

The optical axis extends from the anterior to the posterior pole of the eye and is defined by the geometric centers of the crystalline lens and the cornea. The visual axis is defined from the fovea, through the nodal point and out to an object, a point (such as a star). The angle between the visual axis and the optical axis is angle alpha.

“So the normal eye is not just tilted 2.6° at angle kappa. It’s tilted 5.2° horizontally and 1° vertical and is named angle alpha,” Dr. Holladay said. He said that angle alpha has the least amount of inter-patient variability and that it is the most reliable benchmark for refractive surgery.

Limitations of tilt, pupil size

“Angle alpha is the leading cause of why the human eye only functions at 40% when compared to a diffraction-limited camera with the same focal length and aperture size,” Dr. Holladay said. If the eye were designed like a camera, the “aperture” or pupil would be aligned with the “lenses,” the cornea, crystalline lens and fovea on the optical axis, and visual function would be as much as 2.5 times better than currently achievable, he noted.

“Because the eye is normally tilted, we see all sorts of distortions like astigmatism, coma and other higher-order aberrations,” Dr. Holliday said. He said that the images a camera produces would be similarly poor, functioning at only 40% of its potential, if the camera were tilted obliquely like the eye.

In addition, quality of vision decreases further at nighttime and in low-light settings, Dr. Holladay said. “Any time you make the aperture of the pupil larger in an optical system, the aberrations get worse,” he said.

At night, as the pupil dilates to allow more light to fall on the retina, aberrations in the periphery of the cornea and crystalline lens are exposed. Vision quality worsens, marked by starbursts, halos and glare that are caused by higher-order aberrations such as coma, trefoil and spherical aberration. The Stiles-Crawford effect, which weighs peripheral rays less than central rays, reduces the effect of these aberrations, but aberrations still increase as the pupil gets larger.

As a result, Dr. Holladay said, the smaller the diameter of a pupil, the better the quality of vision, until a significant diffraction effect is reached, which is about 2 mm. In fact, researchers have shown that a 3- to 3.2-mm pupil is the optimal size for achieving best uncorrected vision in a normal emmetropic human eye balancing diffraction against aberrations, he said.

“When your pupil dilates above 4 or 5 mm, visual acuity begins to decline due to aberrations, and diffraction is no longer significant,” Dr. Holladay said. Unlike expensive cameras with aspheric optics and many lenses in which optical quality typically improves with the widening of the aperture, the visual quality of the eye reaches its maximum with a 3-mm aperture and then declines as the pupil widens, Dr. Holladay said.

Based on these limitations, in theory a camera may appear to be a more technically advanced instrument than the human eye. However, the human eye has evolved and adapted for more than 150,000 years, to become an infinitely complex organ with secrets that are still being discovered.

Stiles-Crawford effect

Two physiological properties of the eye are sophisticated beyond the capabilities of a camera were discovered in the last century and in the last decade. The Stiles-Crawford effect and the anti-chromatic effect of higher-order aberrations were found to compensate for some of the eye’s anatomical limitations. These properties tend to improve vision, Dr. Holladay said.

The Stiles-Crawford effect, discovered by scientists in 1933, allows retinal photoreceptors to reduce the effect of the rays of light that enter through the periphery of the pupil and strike the retina obliquely. As a result, the corneal and lenticular aberrations in the periphery that would normally be more significant are minimized.

“When your pupil dilates, you’ve got light coming in through the center of the pupil and from the periphery of the pupil,” Dr. Holladay explained. “The Stiles-Crawford effect causes photoreceptors to rely heavily on that central light coming in and, unlike a camera, rely less heavily on the peripheral rays.”

Consequently, he said, the quality of an image of the human eye is far better than would be expected with large pupils than it would be if the Stiles-Crawford effect did not exist.

Higher-order aberrations, chroma

A second physiological property that improves the quality of vision in the eye is the presence of higher-order aberrations that balance out chromatic aberrations, Dr. Holladay said.

A study by James S. McLellan, PhD, and colleagues found that higher-order aberrations may exist to offset chromatic aberrations.

“Researchers found that higher-order aberrations balance out chromatic aberrations so that we don’t see chromatic rainbows around objects or light,” Dr. Holladay said.

Chroma or “rainbows” present when white light is prismatically dispersed. The normal human eye has approximately 1.5 D of clinical chromatic aberration, between red and blue. However, the negative effects of chroma are typically not experienced because higher-order aberrations, ocular imperfections that are traditionally thought to cause vision problems, cancel out the chromatic blur.

“It’s important for clinicians and researchers to understand that removing higher-order aberrations will cause chromatic aberrations to become more significant,” Dr. Holladay said.

Binocularity

Because so much of the eye’s inner workings remain a mystery to science, aspects of the eye that appear to be “limitations,” such as aberrations or angle alpha, may actually exist to provide benefit, Dr. Holladay said.

“The way that our optical system is set up might actually be the optimal solution for all environments, like night and day, light and dark,” he said. “We just don’t know.”

Binocular vision and stereoacuity should also be considered when trying to modify the optics of the human eye because the two eyes work together synergistically to create binocular vision. Consequently, Dr. Holladay said, clinicians should strive to improve outcomes based on binocular vision, rather than correcting each eye as an individual unit.

“When we do things to optimize systems for a single eye, it may not necessarily be a good thing for both eyes,” Dr. Holladay said. “There are things that we still don’t know about the eye, like binocularity, that come into play.”

For example, the horizontal tilt of the eye (angle alpha of 5.2°) causes coma, a comet-shaped distortion. Because coma exists in both eyes, the distortion is duplicated as a mirror image in each eye, Dr. Holladay said. “The brain knows the image should be a point, because the tail of the coma is in the opposite direction for each eye. The brain can eliminate this tail and still have a depth perception queue, just like Panum’s area for binocular fusion,” he explained. Reducing or eliminating monocular coma may actually decrease our stereoacuity at night, he said.

He added that surgeons should “think binocularly” when approaching the correction of human vision.

Assessing visual acuity

Surgeons can uncover the secrets of the optical system and the visual system by assessing quality of vision. The first step to evaluate the visual system is to determine the visual acuity or the resolution of the system. Visual acuity, with the theoretical limit of the retina being 20/05 or worse, can be assessed with Snellen eye charts and reading rates.

“A reading rate is the best measure of performance we can take in terms of reading and near vision,” Dr. Holladay said. He said that reading tests are more sensitive than other tests and relate more to tasks that are required in everyday life. He noted that multifocal and accommodating IOLs should be evaluated with reading rates, not near acuity.

“Reading rate testing should be automated and last for about 3 to 4 minutes,” Dr. Holladay said. Technician-dependent reading tests, which can offer patients up to 5 minutes to read a line of letters, are ineffective, unpredictable and unreliable, he said. In many instances, patients are “coached” by the technician or have an extended period of time to fixate and focus, making the performance from different devices more dependent on the technician than the device being tested.

Beyond evaluating patients’ ability to focus at near or their numeric acuity, clinicians must assess “real world” aspects of vision, such as contrast sensitivity, he said.

“Visual acuity is just one piece of the puzzle,” Dr. Holladay said. “It measures our ability to see things in detail at high contrast, but it doesn’t tell us how well a patient can see a gray truck in the fog.”

Evaluating contrast sensitivity

Contrast sensitivity testing gauges the visual system’s ability to see objects of low contrast. Peak contrast sensitivity is usually around the size of a 20/100 to 20/200 (6 to 3 cycles/degree) letter but varies depending on the environment, Dr. Holladay said. He noted that the average patient’s peak threshold for low contrast is at 0.5% at age 20, 1% by age 40 and 1.5% at age 60, primarily due to increased spherical aberration in the crystalline lens with aging.

To feel comfortable when driving at night and to retain one’s independence, Dr. Holladay said it is important to have normal contrast sensitivity. “In terms of driving, walking and getting from place to place, good contrast sensitivity is a necessity. Not being able to see things of low contrast can be very debilitating. It will cause a patient to bump into things, miss steps and can severely affect their nighttime driving abilities,” he explained.

Contrast sensitivity is also variable in mesopic and photopic conditions. A patient who is able to see a high-contrast street sign might not be able to detect a gray truck on the highway on a foggy day, Dr. Holladay said.

Current tests that utilize charts and vertical sinusoidal gratings but require little control of room lighting are variable and incapable of measuring these aspects of contrast sensitivity accurately, he said. As a result, contrast sensitivity testing needs be changed and standardized in order for clinicians to acquire useful information for clinical practice and research, he said.

Vernier displacement

Dr. Holladay noted that the human visual task the eye can perform best is Nonius displacement (vernier alignment) by aligning two vertical lines.

“The eye can perform this subjective task of vernier alignment better than any other,” he said.

As a result, measuring instruments such as calipers, Goldman tonometry, keratometers and lensometers use the alignment of vertical lines as an endpoint of their measurements.

Wavefront diagnostics

Objective methods of testing the human optical system had not made great strides until the application of wavefront technology to the human eye, Dr. Holladay said.

Wavefront technology, an advancement that has improved a surgeon’s ability to measure outcomes for LASIK patients, provides invaluable diagnostic information about the optical system, which was never before achievable on a clinical basis, Dr. Holladay said.

“Wavefront is tremendously valuable diagnostically,” Dr. Holladay said. “You get a map of the optical performance of the entire eye, which includes the cornea and the crystalline lens.” Combining ocular aberrometry with topography allows the clinician to determine exactly where the aberrations lie, in the cornea or the crystalline lens.

“When you look at the wavefront printout, you are looking at the contour of the wave measured at the corneal plane of the eye with the entrance pupil projected on the cornea. Another location to look at the quality of the optics is the spot diagram or point spread function (PSF), which is more intuitive and gives us the quality of the image on the foveola or retinal plane,” Dr. Holladay said.

In the perfect optical system, the PSF should look like a point, such as a star, he said. Mathematically converting the PSF to a modulation transfer function (MTF) allows surgeons to determine the overall performance of the eye as a function of spatial frequency. By comparing the area under the MTF with the area under the contrast sensitivity function (CSF), surgeons can pinpoint the location of the visual deficit to an optical or sensory defect, Dr. Holladay said.

“If the reduction in the CSF is proportional to the reduction in the MTF, the problem is optical. If the MTF is normal and the patient has a reduced CSF, the problem is sensory,” he explained.

Quality of Vision series

By utilizing the tools discussed in this first article on quality of vision, surgeons will be able to get a better grasp on their patients’ visual performance and capabilities and therefore be able to assess a patient’s “real world” quality of vision.

Once the diagnostic measures outlined here have been incorporated in the clinic, surgeons can move one step further to more accurately assess quality of vision by specifically detailing how aberrations such as astigmatism influence visual performance and how modern technologies such as aspheric IOLs and excimer laser surgery affect visual performance.

For Your Information:

• Jack T. Holladay, MD, MSEE, FACS, can be reached at Holladay LASIK Institute, Bellaire Triangle Building, 6802 Mapleridge, Suite 200, Bellaire, TX 77401; 713-668-7337; 713-668-7336; e-mail: holladay@docholladay.com.

Reference:

• McLellan JS, Marcos S, Prieto PM, et al. Imperfect optics may be the eye’s defense against chromatic blurs. Nature. 2002;417(6885):174-176.

• Nicole Nader is an OSN Staff Writer who covers all aspects of ophthalmology, specializing in QOV, pediatrics/strabismus and neuro-ophthalmology.

• Joan-Marie Stiglich, ELS, OSN Editor in Chief, contributed to this article.