Making 20/20 Clear

By Barry Santini, ABOM

“In man’s earliest days, he had little need for glasses…”

That little sentence, residing at the bottom of a reading card used for years in my father’s (also an optician) optical office, seems, even today, to never cease echoing through my mind. The paragraph is rendered in the smallest typeface. Just about every single patient could be counted on to recite this particular line aloud, even while the larger-sized paragraphs above it were reviewed in utter silence.

It was, and is, the “acid” test of their new eyewear’s reading ability. “What’s this line supposed to be, 20/20?” they all declaim, invariably accompanied by a chuckle. As my dad used to say: “Ah, if I only had a nickel…”

DEFINING VISUAL ASSESSMENT

The little vignette I retell here is just one act in a larger dramatic play called visual assessment. The bigger picture is pulled into sharp focus with a simple question: Why bother measuring and quantifying human visual performance in the first place?

For one thing, the story of human evolution and survival is intimately tied to our ancestor’s successful ability to process sensory information from our eyes and it has evolved into a very complex process, fraught with different interpretations. Many of our mating and dating rituals revolve around the evaluation of a mate’s suitability and attractiveness, which we glean from visual cues and psycho-visual sensory information. Recognizing the importance of testing for adequate acuity and good vision is also essential to the study of how early humans survived, formed families, procreated or migrated.

Second, human vision has one fundamental and overriding quality—it is fluid. Our eye’s focus is constantly changing, even to the rhythm of each day. Along with age, other factors that change our vision include diet, genetics, physiological growth, radiation exposure (sunlight), occupational and lifestyle choices, and even the air quality of our local environment (with its impact on tear film stability).

Further, we intuitively understand the reasons for quantifying the vision of people driving cars: it is the only personal-choice activity that harbors potentially harmful consequences for each and every other citizen in our society. Driving, originally considered the province of the wealthy or privileged, is now often taken for granted, even to the point of being seen as some sort of entitlement.

Possessing optimal vision permits us to more fully enjoy ourselves. From the pursuit of hobbies to the daily tasks of our workplace, good vision not only enables us to maximize our efficiency, it also reduces the latent fatigue that can insidiously invade all other aspects of our lives. Assessing vision and acquiring the recommended glasses, sunglasses or contacts, ensures continued independence, overall health and prosperity.

PIONEERING VISUAL ASSESSMENT AND THE 20/20 STANDARD

A few millennia ago, our ancestors discovered that the ability to discern, recognize and categorize star patterns helped them to predict the passage of time and improved the chances of finding food and/or evading predators. Those individuals who possessed sharp vision and recognition skills typically may have survived longer and were revered as essential to the welfare of the tribal community.

Fast forward to the middle 1800s, Dr. F.C. Donders of the Netherlands, a physiology professor, motivated by his own pure curiosity became interested in vision and created the following formula to help in evaluating, quantifying and comparing human visual performance:

Letter Size Seen / Letter Size Seen by a “Standard Eye”= Magnification Need

He defined the baseline for a “standard eye” as being just able to see a letter five arc minutes high from a distance of 20 feet. Shortly after, at Dr. Donders’ request, his co-worker Herman Snellen, created the first prototypes of the standardized measurement tool we still employ today: the eye chart.

Snellen made two important contributions. Instead of simple letters, Snellen employed characters he termed optotypes, because he wanted to ensure that researchers making comparative visual assessments elsewhere would be able to share a common benchmark in analyzing results. Snellen also posited that not all letters were also equally resolvable to a common standard. One-hundred years later, a woman named Louise Sloan created a new set of optotypes, because she felt that not all eye chart letters were equally recognizable. Over time, we have come to employ the term “visual acuity” to mean “sharpness of vision.”

A tremendous amount of significant discoveries, optical advancements and theories proposing how the eye works occurred from the middle 1800s to the early 1900s. This period, with its expansive, intense and expositive research is often referred to as the “golden age of ophthalmology.” Names such as Helmholtz, Abbe, Jaeger, Donders, Tillyer, Snellen, Green and Jackson are to be included in the pioneering attempts to increase our understanding of the workings of the human eye.” Upon these giant’s shoulders, subsequent researchers and vision scientists have helped evolve visual assessment into today’s standardized refraction protocol.

IS 20/20 PERFECT VISION?

As the benchmark distance used in assessing visual acuity, just what is so special about these 20 little feet? Why don’t we use 25 feet or even 30? Is 20 feet equivalent to optical infinity for the human eye? The answer is, “no.” Then why did Snellen use 20 feet as the reference distance for his new optotypes?

One reason is that 20 feet was found to represent the longest dimension commonly found in rooms of this era and Snellen was keen to ensure others would be able to quantify their results to the standard reference. But 20 feet is not equivalent to optical infinity and the dioptric error equivalent is 0.16 diopters; here’s how:

20 feet = 6.1 meters; Diopters = 1/refractive distance (meters)

What is the dioptric value of a lens with a 6.1m focal length?
1/6.1m = +0.16 diopters (residual error compared to optical infinity)

Interestingly, even Snellen knew that 20/20 did not describe the acuity limit of a “normal” human eye. Rather, as contemporaries of Snellen published and exchanged their new acuity assessments, it became clear that normal (perfect) human vision is really closer to 20/13. In fact, it is not until humans reach 60 years of age that normal human vision drops to 20/20.

With this in mind, and because he was primarily interested in facilitating a standard, Snellen’s 20/20 became known as “standard” vision. It is used as a universal benchmark even today, despite the evidence that most people, with well-corrected vision, are capable of seeing one or two lines better than this “standard.”

IS 20/20 VISION PERMANENT?

The most constant quality of human vision is that it is not constant. It is forever responding to changes in atmospheric pollutants, overly dry, air-conditioned office environments, emotions, diet, medication and age. The eye’s cornea and crystalline lens, the only two structures we possess that are transparent, are subject to the ravages of time and change with age, just like our bodies.

Our eyes reach 90 percent of their adult size by about our 10th birthday. Before then, our eyes are dimensionally small and therefore inherently far-sighted. Around age 10, most children are growing out of their infantile hyperopia and growing into their adult myopia, if they’re genetically destined to do so. Their eyes continue to grow and reach 99 percent of their adult size at age 18. The last 1 percent of axial growth continues to our 21st or 22nd birthday. At this age, changes in vision due to the eye’s physiological growth will stop. However, it is in these after-college years that many “normal” sighted individuals may find themselves placed in occupational or post-graduate environments that put abnormal reading or close-focus demands on their vision systems. It is not at all uncommon to see formerly emmetropic candidates for law or computer degrees demonstrate the need for mild myopic and/or astigmatic corrections. For example, sustained reading and computer usage can result in:

• Convergent motor-muscle (bio-mechanical) tension on the eye-globe, which responds by a “stretch” of the soft tissue (controversial).
• The mechanism of emmetropization, which influences the axial growth of the globe to maintain ideal focus.
• Ciliary muscle “spasm,” which can result from the abnormally high amounts of accommodation encountered in sustained, close-focus tasks.

From mid-20’s to early 40’s, the eye’s refractive state is reasonably stable. But as we enter our mid 40’s and 50’s, changes in the crystalline lens influence the fluidity of vision. Surprisingly, the eye’s lens is the only organ of the entire body that does not shed dead cells. From birth until death, layer upon layer of new cells accumulate alongside of the old ones. By mid-life, the lenses’ cellular proteins begin to show the effects of almost half-century of sentry-like duty absorbing harmful UV rays before they could harm the retina. At age seven, most children’s lenses are crystal clear and actually pass UV radiation as low as 350nm to the retina. By age 10, deterioration from UV radiation absorption is first evident from a noticeable turbidity, or cloudiness, that begins to manifest itself in our lens. Surely today, this fact alone will inspire parents to place more importance on the use of good sunwear for all their children.

During the fifth decade, changes related to the lens produce refractive shifts known as presbyopia and hyperopia. From age 60 onward, as the lens proteins continue to harden and deteriorate, significant increases in turbidity occur, which ultimately lead to the diagnosis of a cataract. The word cataract is derived from the Latin word meaning “waterfall.” Researchers in 19th century United Kingdom, when inspecting lenses from cadavers, noted a frothy, layered, milky appearance under their microscopes, which reminded them of running water—hence the term cataract. Besides this cloudiness, cataracts are accompanied by further hardening of the lens, leading to myopic shifts in refractions. Our grand and great grandparents, unaware of this myopic shift, often hailed this myopic shift as “second sight,” as it could offset some of the mid-life hyperopic changes mentioned above.

Also as we age, our pre-corneal tear film becomes less robust and stable. This “dry eye” condition is also known to greatly influence our subjective sense of vision sharpness.

THE POLITICS OF 20/20 Driving & States Rights — Since the birth of our country, celebrating and maintaining independence has been central to the American ethos. During the past 100 years, certainly no activity more embodies our independent spirit than driving. But the wisdom our forefathers showed by granting certain powers to the states in order to balance a strong central government sometimes results in a mixed bag of political parochialism. And nowhere is this more easily demonstrated than in the wildly varied vision standards adopted by each state’s department of motor vehicles (DMV). Let’s see how this is demonstrated by looking at the motor vehicle vision standards of two states: New York and Wisconsin.

In New York, the minimum acuity a driver must demonstrate for the most basic license is 20/40 (and also not have a field-of-vision less than 140 degrees). Yet, when this assessment is made at a local DMV, the test is conducted at a distance of six feet (using proportionally adjusted optotypes). This distance has been chosen to ensure efficient processing of the thousands that often funnel through a DMV office in a single day. But in what way does testing someone’s acuity at six feet represent the type of distances normally encountered during driving?

This test distance is self-defeating, as it will allow a person with a reasonable amount of uncorrected myopia to pass without a corrective restriction applied to their license. Additionally, N.Y.’s DMV only requires that vision be assessed every eight years—a very long time in the context of the changing nature of human vision.

In Wisconsin, the basic thresholds for minimum acuity and testing frequency are the same as in N.Y. (20/40, every eight years). However, Wisconsin grants a special, restricted license for those individuals who “possess visual acuity between 20/100 and 20/200, but not including 20/200 in the better corrected eye, as certified by a vision specialist, shall be restricted to daylight hours of operation only.” So, a citizen of Wisconsin with an assessed visual acuity extremely close to the definition of legal blindness, can drive in a torrential rainstorm, as long as it occurs during daylight hours. The legal precedents that sustain keeping this as part of the Wisconsin driving standard probably stem from laws and regulations made during a bygone era of limited population and underused roadways. These conditions permitted people with poor visual acuities to drive with reduced probabilities of harmful consequences to others. This may not make sense to a vision professional, but it reveals the province of state’s rights to determine and maintain their own driving standards.

How Often? — In the United States, the governing bodies of the American Academy of Ophthalmology, the American Optometric Association and the Vision Council all recommend annual eye exams for American citizens. Ensuring optimal eye health through therapeutic measures and screening for potential eye diseases with an eye toward prevention are amongst the central tenants of these organizations.The insurance cost for annual vision exams is enormous and growing exponentially with our aging population.

In Canada, by contrast, a 2005 study funded by the Canadian Ophthalmological Society investigated the optimal frequency of eye examinations. Applying an analysis based on cost-effectiveness to the benefits accrued to routine eye exam; they assessed the vision and health consequences for asymptomatic, symptomatic and high-risk individuals, and came to these surprising recommendations for eye exam frequency:

It has been speculated that one possible influence for approaching the need for an eye exam using such a strict, cost-benefit analysis is that Canadians fund their health coverage with a nationally based healthcare plan

WHAT IS THE FUTURE OF 20/20?

Despite its fractured history and flawed geometric optics, 20/20 today remains a useful and effective benchmark in evaluating visual acuity. But the ingredients that influence the changes in human vision make keeping blind faith in 20/20 a difficult recipe indeed.