Measuring Vision and Vision Loss

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Colenbrander - Measuring Vision and Vision Loss 2009

Measuring Vision and Vision Loss

August Colenbrander, MD San Francisco

PART 1 INTRODUCTION, ASPECTS OF VISION LOSS

ASPECTS OF VISION LOSS

Anatomical and Structural Changes, Visual Functions, Functional Vision,Societal and Economic Consequences

MEASUREMENT and ASSESSMENT

PART 2 MEASURING VISUAL FUNCTIONS

2A VISUAL ACUITY MEASUREMENTBasic Concepts

Historical Developments

Considerations for Letter Chart Testingcognitive factors, test distance, progression, contrast and illumination,notation, rounding, test symbols, chart layout, summary.

Considerations for Near vision Testing and Readingnear letter acuity, reading acuity, modified Snellen formula, letter size notations,reading fluency, reading patterns, eccentric viewing, scotoma interference,

Testing Infants and Young Children

2B VISUAL FIELD RELATED FUNCTIONAL PROBLEMS

Introduction

Central Field DefectsScanning Laser Ophthalmoscope, macular mapping test, central tangent screen.

Peripheral Visual Field

Mental Model of the Environment

2C CONTRAST SENSITIVITY

Introduction

Contrast DetectionThe Contrast Sensitivity Curve

Measurement methodsdetermining the endpoints, determining intermediate points, determiningthe slope, gratings vs. letters, contrast scales, interpreting CS findings.

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PART 3 ASSESSING FUNCTIONAL VISION and QUALITY of LIFE

3A FUNCTIONAL VISION

Introductionestimating functional vision, direct assessment, classifications

Eligibility Guidelines,based on visual function measurementslegal Blindness,

Estimating a Visual Ability Scalefrom visual function measurementsvisual acuity score, visual field score, functional acuity and functionalfield score, functional vision score, AMA impairment rating.

Direct Assessment of Functional Visionassessment of visual abilities, resource inventory, task inventory,performance ranges, how many ranges, measuring performance,Likert vs. Rasch scores, different settings, rehabilitation outcomes, outlook.

3B QUALITY of LIFE

Introduction, Types of Quality of LifeIndividual Health Care

questionnaires, indirect assessment

Health PolicyQALYs, Time Trade-off, Standard Gamble, Visual Analog Scale,Burden of vision loss,

Closing thoughts

Figures1. Aspects of Visual Functioning

2. Assessment of Visual Functioning

3. Visual Acuity-related Scales of Visual Functioning

4. Franciscus Cornelis Donders

5. Eduard von Jaeger

6. Donders Formula

7. Herman Snellen and his charts

8. Visual Acuity and Age9. John Green and his chart

10. Decimal and other Visual Acuity Notations

11. Landolts broken Ring

12. Sloans Optotypes

13. Bailey-Lovie and ETDRS charts

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14. Low Vision Letter chart with cord for 1 meter

15. Various Letter Size Progressions

16. Visual Acuity and Quality of Life

17. Preferred Numbers

18. Various Equivalent Visual Acuity Notations

19. Rounding of Visual Acuity Values

20. Various Optotypes

21. Letter chart vs. Reading Acuity

22. Modified Snellen formula

23. Magnification Requirement for Letter Chart and for Reading Acuity

24. Variability of Jaeger Numbers

25. Variability of Printers Points

26. Reading Cards with Proportional Paragraphs27. Improved hand-eye coordination

28. Visual Behavioral Milestones

29. Various Childrens Tests

30. Micro-perimetry

31. Macular Mapping Test

32. Tangent Screen with Laser Pointer

33. Optical vs. Retinal Causes of Contrast Sensitivity Loss

34. Measurement Methods for the Contrast Sensitivity Curve

35. Mixed Contrast cards.

36. Measurement units for Letter charts vs. Gratings

37. Various Blindness criteria

38. Functional Vision Score and AMA Impairment Ratings

39. The Visual Field Score

40. Medical vs. Rehabilitative outcomes

41. General Scale of Functioning, applied to Visual Abilities

42. Visual Acuity and Visual Ability Ranges

43. Timed Instrumental Activities of Daily Living

44. Matching Tasks to Abilities

45. Results of the LOVIT study

46. Comprehensive Vision Rehabilitation vs. Low Vision Care

47. Patient-based Time Trade Off Utility values

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Measuring Vision and Vision Loss

August Colenbrander, MD San Francisco

PART 1 INTRODUCTION, ASPECTS OF VISION LOSS

In epidemiology as in individual health care, it is important to know not only the cause andnature of a condition, but also the severity of its impact and its functional consequences. In thecase of ophthalmology the question is how to assess how eye diseases and other conditionsaffect the complex entity that is called vision. This chapter will discuss the accuratemeasurement of various aspects of vision and of visual functioning, with the ultimate objective ofenhancing the patients Quality of Life.

ASPECTS OF VISION LOSS

Vision loss can be observed from many different points of view. Each point of view reveals adifferent aspect. Consider a patient with AMD, who wants to make an appointment. The frontdesk will think about when to schedule her. The doctor will think about which treatment toselect. The office manager may worry whether the insurance will pay. The daughter worrieswhether mother can still drive. These aspects are very different, but they all belong to the sameclinical case. Each of these aspects tells us something about the subject, but also gives usinsight in the point of view of the beholder.

Similarly, when considering visual functioning, we can perceive many different aspects of visionloss, depending on our point of view (Fig. 1).

First we may consider how various causes may result in structural changes, such as scarring,atrophy or loss. Here the focus is on the tissue and we need the pathologist to examine these

changes.

However, the structural changes in its components do not tell us how well the eye as a wholefunctions. We need to widen our view from the tissue to the organ. We need a clinician tomeasure aspects of organ function, such as visual acuity, visual field, contrast sensitivity, etc.

Yet, knowing how the eye functions, does not tell us how the person functions. So we need towiden our perspective again, this time to the person level. We need to consider tasks, such asreading, mobility, face recognition. Here we need various vision rehabilitation professionals towork with the patient.

Beyond that, we need to look at the person in a societal context. Do these changes impact onthe persons participation in society, causing job loss or a reduced quality of life? How can webe sure that the patient is satisfied, which should be the end goal of all our interventions?

It is useful to draw a line in the middle of Fig. 1; on the left side we speak of visual functions,which describe how the eyes and the visual system function; on the right side we speak offunctional visionwhich describes how the person functions. When organ functions arereduced, we speak of impairments. Most common are ocular visual impairmentsdue to oculardisorders; they constitute the traditional domain of ophthalmology. More recently, increasedattention is being asked for cerebral disorders, which may cause cerebral visual impairment. Ininfants and children the cause may be perinatal cerebral ischemia, in adults it may be traumatic

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brain injury, in the elderly it may be the result of a stroke. Cerebral visual impairments maycause abnormal visual functioning, which can be captured under the term of visual dysfunction.

Figure 1 Aspects of Visual Function ing

Visual functioning can be approached from different points of view. Depending on our point ofview we will see different aspects (see text). The arrows indicate relationships between theaspects, but these relationships are not fixed. The oblique arrows indicate that there are otherinfluences (including rehabilitation) that can modify the relationships. Without this flexibility,adaptation and rehabilitation would not be possible.

Various activities may cover more than one aspect. When we consider reading, themeasurement of the threshold print size refers to organ function. Measures like Critical (oroptimal) Print Size [1], reading speed (words per minute) and reading endurance (hours per day)describe abilities of the person. Reading enjoyment, finally, is an aspect of Quality of Life.

Anatomical and Structural Changes

The first column of Fig. 1 describes the underlying disorders or diseases. At the organ level allforms of vision involve three distinct stages.

First is the optical stage, where the optical components of the eye deliver an image of theoutside world to the retina. This stage can be disrupted by opacities or refractive errors.Dealing with these problems by prescribing glasses or by removing cataracts has for centuriesbeen a main stay of ophthalmic practice.

Next is the receptor stage, where receptors in the outer retina transform the image into neuralimpulses. Ophthalmoscopy and slitlamp biomicroscopy have given ophthalmology tools todescribe anatomical changes at this level in more detail than is possible for most other organ

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systems. This stage can be disrupted by various retinal diseases, which today demand anincreasing proportion of the ophthalmologists time and attention.

Third is the neural processing, which starts in the inner retina and proceeds through the visualcortex to higher visual centers and eventually gives rise to functional vision and vision-relatedfunctioning.

Most of the ophthalmic literature, including this textbook, is devoted to the first two stages. Untilrecently, the third stage has often been overlooked. Each of these stages involves differentanatomical structures and different functional mechanisms, which can create different problemsand need different therapeutic approaches

Visual functions

This aspect describes functional changesat the organ level. Here again, ophthalmology hasdeveloped unique tools that can measure visual functionswith great precision. The mostcommonly measured functions that have the greatest impact on general functioning are visualacuityand visual field, followed by contrast sensitivity. Many other functions such as colorvision, stereopsis, light and dark adaptation and their psychophysical and electrophysiologicaltests (ERG, VEP) are discussed in other chapters.

Reflecting the fact that the organ function column is flanked by two other columns, organfunction measurements can be used for two purposes: to assist in diagnosing the underlyingdisorder (to the left in Fig. 1) or to predict the functional consequences (to the right in Fig. 1).Different tests perform differently in this regard. Tests such as ERG and VEP are helpful indiagnosing the underlying condition, but are poor predictors of the functional consequences.Visual acuity loss can have many different causes; this means that it is a good screening test,but adds little to the differential diagnosis. Yet, whatever its cause, it can help in predicting theimpact on Activities of Daily Living (ADL). Similar differences exist for other tests: the Ishiharacolor test is good at diagnosing even minor red-green deficiencies for genetic studies, butoverestimates the functional consequences. The D15 color test on the other hand, was

designed to detect only those defects that might have functional consequences. The discussionin this chapter will be mainly oriented towards functional consequences.

Functional vision

This aspect reaches beyond the description of organ function by describing the skills andabilitiesof the individual. It describes how well the individual is able to perform various

Activities of Daily Living (ADL). This aspect has been described under different names. In thefield of vision, the term functional visionis used. In ICIDH [2] loss (or lack) of ability wasdescribed as dis-ability. Its successor, ICF [3] uses the term activities. Activities and abilitiesare two sides of the same coin, describing one without the other is meaningless.

Again, the columns on either side help to clarify different ways in which this aspect can beapproached. On the left are the available resources (vision, touch, hearing, etc.); they definethe ability aspect and are important when defining the means to be used in a rehabilitation plan.Should we facilitate reading with magnification (vision), with Braille (touch) or with talking books(hearing)? The activity aspect, to the right, relates to the societal goal of the action, toparticipation and Quality of Life. This is important in defining the needs and objectives of therehabilitation plan.

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Societal and Economic Consequences

The last aspect in Fig. 1 describes the societal and economic consequences for the individualcaused by a loss of ability. In ICIDH this aspect was described as handicap and measured interms of loss of independence; in ICF it is described under the heading participation, whichpoints to interdependence in a societal context. Handicaps need not preclude participation.

The story of Helen Keller is one example of how some people can achieve full participation inspite of extraordinary handicaps. This aspect is also described as Quality of Life.

While ability describes what people cando, participation describes whether they actually doit.Participation can be described fairly objectively. Quality of Life, on the other hand, refers to ahighly subjective experience. Quality of Life depends not only on individual factors; it alsodepends on factors in the environment.

Improving the participation and Quality of Life aspect is the ultimate goal of all medical andsocial interventions. There clearly are links between the aspects: disorders may causeimpairments, impairments may cause a loss of abilities, a loss of abilities may cause a lack ofparticipation. However, these links are not rigid. Medical and surgical interventions can reducethe impairment caused by a disorder. Assistive devices and special training may improveabilities in the face of a given impairment. Changes in the human and physical environmentmay increase participation, regardless of reduced abilities. The art of comprehensiverehabilitationis to manipulate each of these links so that a given disorder results in the leastpossible loss of participation and the best possible Quality of Life. Comprehensive VisionRehabilitation Fig. 46 involves much more than traditional Low Vision care. An orthopedicsurgeon could not function without the help of physical therapists, prosthetists and others.Similarly, ophthalmologists must reach out to other professions to offer their patientscomprehensive rehabilitation.

MEASUREMENT AND ASSESSMENT

The different functional aspects are measured and assessed in very different ways.

Under the aspect of visual functions (Fig. 2, left), we measure parameters that define how theeye functions. We do this by varying one parameter at a time in a simplified, artificialenvironment. Consider that the visibility of test objects depends on their size, contrast andillumination. If we vary the size, while keeping contrast and illumination constant, we create aletter chart to measure visual acuity. If we vary the contrast, while keeping size and illuminationconstant, we create a contrast sensitivitytest, like the Pelli-Robson [4] or Mars cards [5]. If wevary the illumination, while keeping size and contrast constant, we perform a dark adaptationtest. Each of these three different tests provides us with a thresholdvalue for the measuredstimulus parameter. The threshold criterion is generally defined as the response level that is50% above guessing. Threshold measurements are chosen, not because threshold

performance is the most relevant performance level for activities of daily living, but because theyenable more precise psychophysical calculations.

Under the aspect of functional vision (Fig. 2, center), we must assess how the person functions.To do this, we must focus our attention on visual skills and abilities, such as reading, Orientationand Mobility (O+M) and Activities of Daily Living (ADL). Such tasks always involve multipleparameters, which can vary independently and which cannot be manipulated separately.Measuring functional vision, therefore, is more complex than measuring visual functions. Wealso notice that we are no longer interested in thresholdperformance, but in sustainable, supra-

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threshold performance. When reading a book, print size, contrast and illumination all need to bewell above threshold, to provide a comfortable performance reserve [6].

Figure 2 Assessment of Visual Functioning

The different aspects of visual functioning require different approaches for their assessment(see text). Their assessment should not be mixed.

For visual functions we measure the variable stimulus needed for a fixed response; forfunctional vision we measure the variable performance for a fixed task, either objectively (timing,error rate) or subjectively (questionnaires).

Finally, we must consider the societal context, also described as Quality of Life(Fig. 2, right).For subjective judgments such as making and keeping friendships, social skills, self confidence,etc. the concept of measurement is even more difficult. When we move a New Yorker to asmall rural community, and a rural farmer to the middle of Manhattan, both will complain thattheir Quality of Life has deteriorated. The ultimate goal can best be described with the wordsatisfaction, which describes the subjective balance between individual achievements andindividual expectations.

Part 2 of this chapter will deal with the measurement of visual functions, (how the eyefunctions); part 3 will deal with the assessment of functional visionand Quality of Life (how theperson functions).

Among visual functions, visual acuity measurement is most prominent. Different visual acuityrelated functions are measured on different scales. This can lead to confusion when each ofthese different scales is labeled with the same undifferentiated term vision.

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Fig. 3 previews and compares various scales and measurement units that will be discussed.There are two ways in which the various scales differ. Some scales measure vision loss; onthese scales higher numbers indicate poorer vision; other scales measure functioning; on thesescales higher numbers indicate better vision. The other difference is between scales with anarithmetic (linear) progression (e.g. 0, 1, 2, 3, 4, 5) and scales with a geometric (logarithmic)progression (e.g. 1, 2, 4, 8, 16, 32). The first type is used to describe the performance of the

EYE (M-units, viewing distance, MAR, VA); the second type is used to estimate theperformance of the PERSON (logMAR, log(VA) = - logMAR, VAS).

Figure 3 Visual Acuity related Scales of Visual Functioning

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Visual acuity measurement starts with defining the stimulus parameters: symbol size andviewing distance. The reference standard for size was defined by Snellen; the name M-unitwas coined by Sloan. The reference standard for distance is the meter and its reciprocal, thediopter.

Given these parameters, we can describe how the eye functions by calculating theMAgnification Requirement(MAR) needed to bring the subject to the performance standard.MAR provides a scale of vision loss that can range up to very high numbers; its reciprocal, thevisual acuityvalue, provides a scale of visual function limited to the range from 0 to 1.0 (thereference standard) and somewhat beyond. The MAR and visual acuity scales are useful forcalculations regarding viewing distance, print size and magnification need.

From these linear measurement of visual function (how the eye functions) we can derive astatistical estimateof the persons visual ability(functional vision, how the person may function).This is done by taking the logarithm of the measurements in accordance with Weber-Fechnerslaw [7], which states that a proportional increase in stimulus intensity produces a linear increasein sensation. Like MAR, logMAR is a scale of vision loss; higher values indicate poorerperformance; like visual acuity, the Visual Acuity Score (VAS) is a scale of visual functioning;

higher values indicate better performance. 0 logMAR = standard vision; 0 VA = blindness.

The statistical ability estimates can be used to average across a population, to determine thedifference between groups or to detect trends. They are also useful for administrative use, butnot for vision rehabilitation, since individual performance may be much better or much worsethan the statistical average.

To determine the need for rehabilitation and to assess its outcomes, direct assessment of actualindividual performance is needed. This can be done through observations or through questionsabout the difficulties that are experienced. At this level, using visual acuity based scales is nolonger meaningful.

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Measuring Vision and Vision LossAugust Colenbrander, MD San Francisco

PART 2 MEASURING VISUAL FUNCTIONS

Visual Functions describe how the eye and the basic visual system function. They aremeasured, one parameter at a time, in an artificial test environment. This chapter will discussthe parameters that are considered most relevant to functioning in the real world: visual acuity,visual field and contrast sensitivity. Other functions, such as color vision, stereopsis, darkadaptation, and electrophysiological functions such as EEG, ERG and VEP are discussed inother chapters.

2 A VISUAL ACUITY MEASUREMENT

BASIC CONCEPTS

The visual function that is measured most easily and most often is visual acuity. This chapterdiscusses the clinical testing of visual acuity, based on letter recognition,using a letter chart orsimilar technique. Other aspects, such as light detection, two-line separation, Vernier acuityand other types of hyper acuity are discussed in other chapters.

Since letter chart acuity is measured so often, a common misconception has taken hold thatvisual acuity would define the quality of vision in general or even the ability to function visually.This is not true. Visual acuity is only one of many parameters that describe aspects of vision.

Letter chart testing compares the smallest line seen by the subject to a reference standard, i.e.the line that can just be seen by a standard person. Letter chart testing thus determines theMAgnification Requirement(MAR) for the recognition of detail; the reciprocal of this value isknown as visual acuity (VA = 1/MAR). A subject, who needs characters or symbols that aretwice as large, is said to have a visual acuity of 1/2 (20/40, 0.5). Conversely, a subject with avisual acuity of 1/5 (20/100, 0.2) needs letters that are 5x larger, etc.

The ability to recognize letters or other symbols depends on the size of their retinal image. Thatsize depends on the ratio of the size of the object to its viewing distance. Visual acuity thereforedefines the visual angle under which an object is seen, not its absolute size. When the size of atest object is made 10x larger, and the viewing distance is also made 10x longer, the visualangle remains the same. When a constant object is brought twice as close, its visual anglebecomes twice as large.

Snellen fractions (Fig. 6) express this as:

reference standard size seen by subjectVA = ---------------------------- or: MAR = ----------------------------

size seen by subject reference standard

He expressed the optotype sizes indirectly as the distance at which they subtend 5 arc min.

Letter recognition is a rather complex function; a normal test result requires proper functioningof all three stages of vision: a healthy optical system to produce a sharp retinal image, healthyretinal receptors to transform that image to neural impulses, a healthy nervous system to

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analyze and recognize the image. Testing also requires the motor ability to respond. Manydifferent disorders can thus result in poor test results. Because of this, visual acuity is a goodscreening test, but it is not a good diagnostic test. Other tests (such as ophthalmoscopy) areneeded for the differential diagnosis. Furthermore, letter chart acuity only informs us about thetiny retinal area where the letter or symbol is projected; it does not tell us anything about thesurrounding or peripheral retina.

We will discuss how the current charts evolved, and then consider the requirements for accuratemeasurement.

HISTORICAL DEVELOPMENTS

Reading tests have been used since before the Middle Ages to test the function of the eye.Major changes started to occur in the middle of the 19thcentury.

1843. In 1843 Kuechler, a German ophthalmologist in Darmstadt, wrote a treatise advocatingthe need for standardized vision tests [7]. He developed a set of three charts, to avoidmemorization. Unfortunately, he was a decade too early. His work was almost completelyforgotten.

1850. Around 1850 started what would later be called the Golden Age of Ophthalmology. In1850, Donders [8], from Utrecht, the Netherlands, [Fig. 4] visited William Bowman [9], ofanatomical and histological fame, at an international conference in London. There he met

Albrecht von Graefe [10], who would become the father of German clinical ophthalmology.Donders and von Graefe became lifelong friends. With Bowman and Hermann von Helmholtz[11], who invented the ophthalmoscope in 1851, they became the foursome that would leadophthalmology to become the first organ-oriented specialty.

1854. Thus, the scene had changed considerably when Eduard von Jaeger [Fig. 5] in Vienna,published a set of reading samples [12], initially as an appendix to his book about CataractandCataract Surgery[13]. He labeled his reading samples with the catalogue numbers from theVienna State Printing House. They became an immediate international success as a means fordocumenting functional vision and will be discussed in detail later.

1861. Meanwhile Donders, was working on his epoch making studies on Refraction andAccommodation [14], in which he clarified the nature of hyperopia as a refractive error, ratherthan as a form of asthenopia (weakness of the eyes), bringing the prescription of glassesfrom trial and error at the county fair to a scientific routine.

For this work, Donders not only needed reading samples for presbyopes, but also distancetargets to determine the refractive error of myopes and hyperopes. He had used some of thelarger type samples from Jaegers publication as a distance target; however, he felt the need for

a more scientific method and for a measurement unit to measure visual function. Using theterm visual acuity to describe the sharpness of vision, he defined it as the ratio between asubjects performance and a reference standard [see basic concepts]. In 1861 [15] he proposedhis formula (Fig. 6) and asked his co-worker and later successor Herman Snellen to devise ameasurement tool.

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Figure 4 Franciscus Cornelis Donders

Franciscus Cornelis Donders (1818 1889) grew up under difficult circumstances (his fatherdied before he was 1 year old) and received his first education in a village school, where thevillage schoolmaster triggered his lifelong intellectual inquisitiveness. He went on to study at theUniversity and at age 29 saw a special chair in Physiology created for him at the Medical Schoolof Utrecht University, the total faculty of which had only four members at the time. Hedeveloped an interest in the physiology of the eye and after the experience quoted below,decided to devote his life to Ophthalmology. He not only was an excellent scientist, he also hada strong social conscience. In 1852, after his return from London he privately founded an EyeInfirmary for the Indigent, which in 1858 became an independent foundation.

Donders most renowned work would become his book on The anomalies of Accommodation

and Refraction, in which he unraveled the difference between asthenopia and hyperopia andput the correction of refractive error on a scientific footing. Donders was a gifted teacher andexplained his topics in ways understandable to practitioners.

In 1850 he visited London, where he met Bowman and von Graefe. He later wrote:I had just seen Jaeger (Friedrich, Eduards father, ed.) performing cataract surgery

alternately with the left and the right hand, when a young man stormed into the room embracinghis preceptor. It was Albrecht von Graefe. Jaeger thought that we would fit well together andwe soon agreed. Those were memorable days. Von Graefe was my guide for all we heard inpractical matters, and in scientific matters he listened eagerly to the smallest detail. We livedtogether for a month to separate as brothers. To have William Bowman and Albrecht vonGraefe as friends became an incredible treasure on my lifes path.

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Figure 5 Eduard von Jaeger

Eduard Jaeger, Ritter von Jaxtthal (1818 1884) was born into a prominent family of Vienneseophthalmologists. His father, Friedrich was one of the most distinguished ophthalmologists ofhis days; Donders met him in London in 1850. In addition to his reading samples, Eduard isknown for an early Atlas of the Ocular Fundus. He was a strong advocate for the use ofHelmholtzs direct ophthalmoscope and spent many hours making very detailed drawings. Hemay have inherited his artistic talent from his mother, who was a recognized artist and daughterof another ophthalmologist.

Since his reading samples had no external standard, except the catalogue of the Vienna State

Printing House, others could only imitate them with locally available fonts. This explains theenormous variability among later imitations (Fig. 23). While Snellens emphasis was onmeasuring visual acuity, Jaeger emphasized reading ability; this may be one of the reasons whyhe steadfastly refused to add Snellens letter size notation to his reading samples.

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Figure 6 Donders Formula

Donders compared the letter size just recognized by the patient to a reference standard, thesize recognized by a standard person. The visual acuity is the reciprocal of this value.

(Angular)Size seen by subject/Size of reference standard = MAgnification Requirement

VisualAcuity = 1 / MAgnification Requirement

MAgnification Requirement: 2x Visual Acuity: 1/2 0.5 20/404x 1/4 0.25 20/80

10x 1/10 0.1 20/200

Note that he reference standard is based on a physical measurement (letter height 5 of arc).This choice was partly inspired by the work of the English astronomer Robert Hooke, who, twocenturies earlier [17], had found that the human eye can separate double stars when they are 1apart. Since Snellen chose an external, physical standard, others could accurately reproducehis charts.

1862. The next year, 1862, Snellen (Fig. 7) published his letter chart [16]. His most significantdecision was not to use existing typefaces, but to design special targets for visual acuitymeasurement, which he called optotypes. He experimented with various targets designed on a5x5 grid (Fig. 7, center). Eventually, he chose letters (Fig. 7, right). Some others also published

charts based on Donders formula, but used existing typefaces rather than optotypes. Snellenschart prevailed and spread quickly around the world. One of the early big orders came from theBritish army, wanting to standardize the testing of recruits.

To implement Donders formula, Snellen defined as the reference standard, the ability torecognize one of his optotypes when it subtended 5 of arc with a stroke width of 1 of arc.Since Snellen chose an external, physical standard, others could accurately reproduce hischarts.

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Figure 7 Herman Snellen and his charts

Donders had chosen Herman Snellen, Sr. (1834 1908) as the co-director for his eye infirmary.Snellen would later become his successor. Donders was a scientist and teacher, Snellen waspractically oriented and was an excellent surgeon.

The center images show two of Snellens experimental charts, preserved in the Museum of theUniversity of Utrecht. On the right is his chart as published in 1862.

Snellen advocated a fractional visual acuity notation V = d / D, in which d = actual viewingdistance in any measurement unit, and D = distance at which the optotype subtends 5 min ofarc. Louise Sloan introduced the name M-unit and changed the formula to V = m / M, to makeuse of the metric system explicit and avoid confusion with D = Diopters. Today, actual Snellenfractions are rarely used and are most often replaced by Snellen equivalents (Fig. 18).

See Fig. 15 to compare the letter size progressions on Snellens charts to those used today.

On his charts, Snellen marked the letter size for each line (the distance at which the optotypessubtend 5 of arc); this is the denominator of the Snellen fraction for that line. He left it to theuser to supply the viewing distance as the numerator (Fig. 18).

Donders and Snellen were well aware that their reference standard represented less thanperfect vision and that normal, healthy eyes could do better. It is wrong therefore, to refer to thereference standard of 20/20 (1.0) as normal vision, let alone as perfect vision. Indeed, theconnection between normal vision and the reference standard is no closer than the connection

between the standard American foot and the average length of normal American feet.From his studies on refraction and accommodation and related topics, Donders knew thatnormal acuity decreases with age. While Snellen was preparing his chart, Donderscommissioned a study by one of his PhD students to document the changes with age [17], usingprototypes of Snellens symbols. The study was published in 1862, the same year that Snellenpublished his chart. The similarity with more recent data (Fig. 8) is remarkable.

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Figure 8 Visual Acuity and Age

This chart compares the relationship between visual acuity and age in various studies. The

horizontal bands represent one-line intervals on a standard chart with logarithmic progression.The dark band represents the reference standard (20/20, 6/6, 1.0).

The markers show the data from de Haans study in1862.

The markers show a recent meta-analysis of healthy subjects participating in variousresearch protocols [20]. The coincidence in the younger age groups is striking. In the oldergroups, the recent data show better visual acuity, because the research protocols rejectedsubjects with any pathology.

The markers refer to a recent study of an unselected older population [21]. The coincidencewith the data from 1862 is, again, striking. There has been no change in the average acuity withage over a century and a half.

The data show that until 60 or 70 years of age normal vision for Caucasians is substantiallybetter than the reference standard. The M and F symbols near the top of the chart indicateeven better visual acuities for male and female Australian aborigines [22].

Since Snellens days few major improvements in visual acuity measurement have been made.Many tried to devise better optotypes, but, as Bennet remarked in an exhaustive review ofhistorical developments [18], while preparing for the British standard of 1968 [19], the road of

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visual acuity measurement is littered with stillborn charts. Some developments, however, areworth mentioning.

1868. John Green of St. Louis had traveled Europe after his training in Boston, and had spentsome time with Donders and Snellen, where he wrote a small paper about the measurement of

astigmatism. In 1868 he presented a new chart to the American Ophthalmological Society [20],modifying a prior proposal from 1867 (Fig. 9).

Greens chart featured sans-serif letters (Snellen used letters with serifs), proportional spacingof the characters and a geometric progression of letter sizes (10 steps = 10x); on the smallerlines he used a fixed number of letters per line, all features that are now part of standardizedletter chart design (Fig. 9). He was a century too early; his proposals gained little acceptance.Green went back to letters with serifs, because letters without serifs were said to lookunfinished. A century later, the British standard would choose sans-serif letters, becauseletters with serifs look old fashioned.

Figure 9 John Greens and his chart

The lower lines of Greens chart are shown. Note that he combined sans-serif letters andproportional spacing with a geometric progression, principles that a century later would beincorporated in international standards.

Green 1868 Current Standard (ETDRS)

Proportional spacing Proportional spacingGeometric progression (1010) Geometric progression (1010)Sans-serif letters Sans-serif lettersUp to 11 letters / line 5 letters each line

Too early forgotten International standard

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1875. Snellen originally calibrated his charts in Parisian feet. In the 1860s there were sometwenty different measurement systems used in Europe. It is not surprising that the uniformMetric system [21] was gaining ground. When the Treaty of the Meter was signed in 1875,Snellen changed the distance for his charts to 5 meters, for adherents of the decimal system, orto 6 meters for those who wanted to stay closer to 20 Parisian feet. Today, the 20 ft distanceprevails in the U.S.A., 6 meters prevails in Britain, and a 5-meter distance is commonly used in

continental Europe.

1875. Since comparing the visual acuity values for different distances is awkward, when usingSnellens fractional notation, Felix Monoyer of Lyons, France, proposed replacing the fractionalnotation with its decimal equivalent. (E.g. 20/40 = 0.5, 6/12 = 0.5, 5/10 = 0.5) [22]. His decimalnotation (Fig. 10) made it simple to compare visual acuity values, regardless of the originalmeasurement distance and is commonly used in Europe.

Figure 10 Decimal and other Visual Acuity Notations

In true Snellen fractionsthe numerator indicates the test distance. 5m is commonly used inEurope, 6m in Britain and 20ft in the USA. For patients with low vision a 1 meter distance is

recommended to extend the measurement range.

When Snellen equivalents are used, the numerator is a standard value, regardless of the testdistance. Decimal equivalents are common in Europe, 20/20 equivalents in the USA.

See Fig. 18 for a more extensive listing.

1888. Edmund Landolt had worked with Snellen in Utrecht and later became professor ofophthalmology in Paris. In 1874 Snellen and Landolt had cooperated in publishing a majorchapter on Optometrology [23], the science of measuring vision. Recognizing that not all ofSnellens optotypes were equally recognizable, Landolt, in 1888, proposed his broken ring

symbol, that has only one element of detail and varies only in its orientation [24]. Landolts Cs(Fig. 11) would become the preferred visual acuity measurement symbol for laboratoryexperiments, but gained only limited acceptance in clinical use.

Relatively little happened in the period that followed. Efforts at standardization were made,such as a standard proclaimed by the International Council of Ophthalmology in 1909 [25], butsuch documents were filed and never gained a wide following. That clinicians did not feel anurgent need for standardization can be explained by the fact that the most common letter chartuses do not require it. For refractive correction any set of targets will do, since the only question

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is better or worse? For screening the distinction between within normal limits and not withinnormal limits is the most important. Snellens reference standard at the lower limit of normalvision is well positioned for screening purposes. For screening, the difference between 20/100(0.2), 20/200 (0.1)and 20/400 (0.05)is unimportant; they all indicate a marked vision loss.

Figure 11 Landolts broken ring

Landolts C or broken ring is designed on Snellens 5x5 grid and has only one element ofdetail, the gap, which is 1 unit wide. It can be presented in 4 or in 8 positions.

Low Vision RehabilitationAfter World War II the interest in Low Vision rehabilitation was gaining ground. It wasrecognized that the majority of those considered industrially blind actually had some level ofuseable vision. In 1953 the first Low Vision services were opened in New York at the IndustrialHome for the Blind and at the New York Lighthouse. For rehabilitation purposes the differencebetween 20/100, 20/200 and 20/400, which was unimportant for screening, became veryimportant, since the patient with 20/200 needs twice as much magnification as the patient with20/100 and the patient with 20/400 needs twice as much again. It is not surprising then, thatmajor refinements in clinical visual acuity measurement came from clinicians involved in LowVision rehabilitation.

1959. In 1959 Louise Sloan, the founder of the Low Vision service at the Wilmer Eye Institute ofJohns Hopkins University designed a new optotype set of 10 letters [26] (Fig. 12). She chosesans-serif letters, while maintaining Snellens 5x5 grid. She recognized that not all letters wereequally recognizable and therefore proposed to use all ten letters on each line. This causeslong lines, with many letter sizes requiring more than one physical line.

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Figure 12 - Sloans optotypes

Sloan designed a series of letters without serifs that are widely used in the U.S. They weredesigned on Snellens 5x5 grid. Although the difficulty of individual letters varies, the averagedifficulty approximates that of Landolt Cs.

Sloan also introduced the term M-unit. Snellen had defined visual acuity as:

V = d / D where d = test distance and D = distance at which the letter subtends 5 or arc.

To make this definition less verbose and to avoid confusion with D = diopters, Sloan proposed:

V = m / M where m = test distance in meters and M = letter size in M-units.

1 M-unit subtends 5 at 1 m (1.454 mm, about 1/16 inch). Sloans M is thus equivalent toSnellens D, provided that the measurements are made in meters.

1974. In the 1960s the WHO had surveyed national definitions of legal blindness and foundthat 65 countries used as many different definitions. In 1974 the World Health Assemblyapproved the 9thRevision of the International Classification of Diseases (ICD-9) [27]. In it, theold dichotomy between legally sighted and legally blind was abandoned for a series ofranges of vision loss. In the same year, the International Council of Ophthalmology (ICO) [28]adopted the same ranges, extended them to include normal vision, and used the named rangesused in this chapter and in ICD-9-CM [29] (the US extension of the WHOs ICD-9) (Fig. 18).

1976. In 1976, Ian Bailey andJan Lovie(then at the Kooyong Low Vision Service inMelbourne) published a new chart [30], featuring a novel layout with five letters on each row andspacing between letters and rows equal to the letter size. This layout standardized the crowdingeffect and the number of errors that could be made on each line. Thus, the letter size becamethe only variable between the acuity levels. Their charts have the shape of an inverted triangle

and are much wider at the top than the traditional rectangular charts. Like Sloan, they followeda geometric progression of letter sizes.

That same year, Hugh Taylor, also in Melbourne, used these design principles for an illiterate Echart [22], used to study the visual acuity of Australian Aborigines. He found that, as a group,

Australian Aborigenes had significantly better visual acuity than Europeans (Fig. 8). This isanother reason not to regard 20/20 visual acuity as normal or as perfect vision.

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1982. Based on the above work, the National Eye Institutechose the Bailey-Lovie layout,implemented with Sloan letters, to establish a standardized method of visual acuitymeasurement for the Early Treatment of Diabetic Retinopathy Study (ETDRS) [31]. Thesecharts were used in all subsequent clinical studies, and did much to familiarize the professionwith the new layout and with the logarithmic progression (Fig. 13). Data from the ETDRS werelater used for a revised set of charts where all lines have the same average difficulty [32] . Since

the Sloan letters (designed on Snellens 5x5 grid) are wider than the British letters (designed ona 4x5 grid) used by Bailey and Lovie, the ETDRS chart was designed for a 4m distance, not the6m used by Bailey and Lovie.

Figure 13 Bailey-Lovie and ETDRS charts

The ETDRS chart (right) implemented the layout of the Baily-Lovie chart (left) implemented withSloan letters (Fig. 11). The Bayley-Lovie chart has 4x5 letters and goes to 60M for use at 6 m;the ETDRS chart has 5x5 Sloan letters and goes to 40M for use at 4 m. Both follow the samelogarithmic progression (Fig. 8).

1984. The International Council o f Ophthalmology(ICO) approved a Visual AcuityMeasurement Standard, which also incorporates the above features [33].

2002. The International Council of Ophthalmology(ICO) explicitly recommends the ETDRSprotocol as the international standard in a report on Aspects and Ranges of Vision Loss thatalso asks attention to the aspects of vision loss beyond visual acuity [34].

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CONSIDERATIONS FOR LETTER CHART TESTING

Having discussed the historical developments, we will now discuss several aspects that need tobe considered for letter chart design. This discussion will not be strictly limited to letters or tocharts, but will consider all methods for presenting letters or other symbols at distances of 1meter (3 ft) or more.

Letter recognition, upon which clinical visual acuity measurement is based, is a rather complexfunction, which involves optical, retinal, neural and cognitive factors. The functional significanceof visual acuity is best understood as the reciprocal of the MAgnification Requirement (MAR).The term MAR is best known for its logarithm (logMAR), which will be discussed later. Theacronym MAR can be explained in different ways. It is often interpreted as MinimumAngle ofResolution. This term, although widely used, has two problems. Resolution refers mainly to theclarity of the retinal image, i.e. to the optical part of the process, while the reference to angle ismore applicable to gratings than to letters, where shape, rather than just stroke width affects therecognizability. Our interpretation of MAR as MAgnification Requirement emphasizes thefunctional significance, whether the cause is optical, retinal or cerebral.

The MAgnification Requirement simply is the ratio of the smallest line the subject can see to the

smallest line a standard person can see (i.e. the 20/20 line). When making a measurement, itis important to keep all other, possibly confounding stimulus parameters well above threshold.We will discuss cognitive factors, the choice of test distance, letter size progression, successcriterion, contrast and illumination, visual acuity notation, and the choice of test symbols.

Cognitive factors

The effect of cognitive factors should be minimized. For most adults letter recognition is triviallyeasy. In cases where it is not (infants, pre-school children, illiterate adults, stroke victims), weneed to employ other methods. We may consider grating detection or picture recognition. It isimportant to realize that these are different tasks, which may have different magnificationrequirements. Finally, the ability to respond may play a role. For some developmentally

delayed individuals the motor concept of directionality that is required to respond to tumbling Esmay be a limiting factor. Comparing the results of tests with different modalities, may providesome insight into such confounding factors. In elderly patients with a stroke and maculardegeneration, the question may arise whether inability to read is the result of the maculardegeneration or of the stroke. Better performance with larger print may point to an optical orretinal cause; if the confusion persists, this may point to the stroke. In the following discussionsit will be assumed that cognitive and motor factors are indeed trivial.

Choice of test distance for general practice

The vast majority of patients seen in ordinary practice has visual acuities in the range fromnormal to moderately impaired vision (MAR 1/5, >20/100). Traditional chart designs

reflect this emphasis on screeningand on refractive use. In the near-normal range the stepsbetween letter sizes are small, for lower acuities they become larger; for acuities worse than20/200 (0.1)vague statements such as count fingers and hand motions are used.

Since the magnification requirement is an angular measure, it can be measured at any distance,provided that the viewing distance is properly accounted for in the Snellen formula.

Snellen originally chose a test distance of 20 Parisian feet. Distance testing was chosenbecause it relaxes accommodation and because the optical difference with infinity may be

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ignored. Remember that the stimulus for the development of the letter chart came fromDonders work on refraction. A long viewing distance also reduces the effect of any movementsthe subject may make. After the introduction of the metric system, many went to 5 meter(convenient with the decimal system), others chose 6 meterbecause it is close to 20 ft.

In 1973 Hofstetter proposed a 4-metertest distance [35] for use in smaller rooms. Sloan liked

the 4-m distance because it made for easy conversion to a 40-cm reading distance. TheETDRS charts adopted it because charts with the Bailey-Lovie layout would have to besubstantially wider if designed for 5 m or 6 m. For visual acuity measurement a 4 meterdistance is as valid as any other distance. However, at 4 meters the accommodative demand is0.25 diopters and can no longer be ignored. The alternative for small rooms is the use ofmirrors.

For young children, a test distance of 3 meters(10 ft) is often recommended, because it iseasier for holding their attention. For this group acuity measurement is the most importantapplication; because of their ample accommodative ability, cyclopegic retinoscopy is thepreferred mode for accurate refraction.

The above distances hold for printer charts, which have to be designed for a specific distance.The widespread use of projector charts, which can be adjusted for any distance, has been astrong factor for changing from the use of true Snellen fractions to Snellen equivalents (Fig. 18).

Choice of test distance for Low visionA much smaller group of patients has visual acuities in the Low Vision range (MAR >3, VA


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Figure 14 Low Vision Letter chart wi th cord for 1 meter

This chart, designed for measurement at 1 meter, allows accurate measurement of visualacuities from 1/50 (20/1000, 0.02) to 1/1 (20/20, 1.0). This is not only important for Low Visionpatients, but also for follow-up of various conditions; vague statements, such as Count Fingersor Hand Movements are no longer needed. A 1-meter cord is attached to accurately maintainthe viewing distance.

Choice of Letter size progression

Snellens original charts (Fig. 7) had small steps for the normal range and larger steps for thelower ranges. Introduction of the decimal acuity notation [27] led to charts with visual acuitysteps in 0.1 increments. On these charts the steps at the top of the scale (bottom of the chart),such as 0.9 1.0 1.1, are too small to be practical; at the other end, the reverse is true. Onlya logarithmic scale (Fig. 15) span the full range is evenly.

Snellen advocated reducing the viewing distance to improve measurement for lower acuities.Use of a logarithmic scale, which maintains the same accuracy at all levels, was first proposed

by Green (1868) [25]; it was advocated by many subsequent researchers, including Sloan andBailey-Lovie, but did not gain wider recognition until its adoption in the ETDRS protocol, whichhas become the de-facto world-wide standard.

Use of a logarithmic (geometric) progression of stimuli is in accordance with Weber-Fechnerslaw [7], which states that geometric (proportional) increments in stimulus give rise to linearincrements in sensation. Westheimer [39] has shown that this also holds for visual acuity.Massof [40] showed that it also applies to the relation between visual acuity and visual (dis)ability(Fig. 16).

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Figure 15 Various Letter Size Progressions

On a geometric (often called logarithmic) scale each step represents the same ratio (e.g. 2 4 8 16 32); on a linear scale each step represents the same increment (e.g. 2 4 6 8 10). Only a geometric progression can span a wide range of values with equal stepsthroughout. The logarithms of a geometric scale yield a linear scale with equal incrementsthroughout. Examples are the log(MAR) scale and the VAS scale (Fig. 18).

Figure 16 Visual Acuity and Quality of Life

The diagram demonstrates that there is a linear relationship between Functional Vision (NEI-VFQ after Rash analysis) and visual acuity on a logarithmic scale. The diagram also shows thatthere is no natural breakpoint in this relationship and that the spread of Functional Vision ratingsis much larger than the amount explained by the acuity loss alone.

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Various geometric progressions are possible. The one that fits best with the decimal system isone in which 10 steps equal 10x. This is the sequence used by Green and by Sloan, Bailey-Lovie and the ETDRS. It is known as the Preferred Numbers series (Fig. 17).

Figure 17 Preferred Numbers

Many different geometric progressions are possible. The one that fits best with the decimalsystem is know as the Preferred Numbers series. Its step size is 1010, so that 10 steps equalexactly 10x. Thus, when the series is extended from 1 to 10 to 100 to 1000 etc., the same digitsrepeat in each of these intervals with only a shift in the decimal marker.

The series is widely used in industrial standards and is the subject of an international standarditself [46]; it also is the series used by Green, by Sloan and for the ETDRS charts. For generaluse, the Preferred Numbers series is advantageous because with minimal rounding (


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(20% Michelson). When contrast is reduced to a level where it does affect visual acuity, the testbecomes a contrast sensitivity test, which is discussed later in this chapter.

Front lightingis the most common way to illuminate printed charts. Care should be taken thatthere is no glare and that there are no bright spots or dim spots on the chart. For most patientsa high illumination level is advantageous. However, for patients with conditions such as

albinism or rod dystrophy, it should be possible to reduce the illumination, which may result in asignificant increase in visual acuity.

Back lightingof a translucent chart on a light box gives the most even and most reproducibleillumination. The usual backlit ETDRS chart has an illumination level of about 200 cd/m2. Formore portable applications, as in school testing, smaller rear illumination cabinets are availablealso.

Projector chartsin a dim or darkened room are the preferred presentation mode in professionaloffices. For these charts, stray room light may significantly affect the test. If that is the case,measurements made in different rooms with different amounts of stray light may no longer becomparable. In the U.S. the average projector chart has a luminance of about 85 cd/m2;European charts are generally brighter, up to 300 cd/m2. The lower luminance has theadvantage that the pupil may be wider, so that refractive errors may be more obvious; thebrighter charts have the advantage that they suffer less from stray light. In most situations thedifference is negligible. The ICO Visual Acuity Measurement Standard [38] recommends arange, which includes both the lower and the higher values.

Projector charts are convenient for routine eye exams; however for vision rehabilitation, wherethe goal is to predict the everyday performance of patients, a lighted printed chartin a lightedroom is preferred, since it is closer to normal conditions.

Presentation on a computer screenis gaining ground. This method is less sensitive to straylight and allows easy switching between different optotypes as well as control over parameterssuch as crowding, contrast and brightness. The earliest implementations were the E-ETDRSsystem [41] and the E-HOTV system [42]. Both use single letter presentation with crowding bars.Newer systems offer more varied presentation modes.

Choice of visual acuity notation

The result of the visual acuity measurement may be recorded in a variety of ways. It should benoted here that although some may refer to the Snellen system as being different from theETDRS or logMAR system, the two measure exactly the same visual acuity value, but report iton a different scale. This can be compared to the Fahrenheit and Centigrade scales, whichmeasure the same temperature, but report it on different scales.

True Snellen f ractionsSnellen preferred a true Snellen fraction, in which the numerator indicates the actual testdistance and the denominator indicates the actual size of the letter seen. The advantage of this

notation is that it indicates the actual test conditions. The disadvantage is that it becomesawkward to compare visual acuity values, measured under different conditions. This isespecially true since the advent of projector charts where the viewing distance is dictated by thegeometry of the room and the projector magnification is adjusted to accommodate fractionalviewing distances.

Snellen equivalentsTo overcome this difficulty, Snellen equivalentsare used. In Europe, the decimal equivalent[27] of the Snellen value is used most often. This notation is clear, because there is no

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numerator or denominator. The notation becomes confusing when the decimal notation isconverted back to a pseudo-Snellen fraction. E.g. 5/250.22/10; the 2/10 fraction wouldsuggest that the subject saw a 10 M letter at 2 meter, instead of a 25 M letter at 5 meter.

The U.S.notationuses a 20 ft. fraction as a Snellen equivalent. If two examination lanes haveviewing distances of 18 ft. and 21 ft. respectively, the true Snellen fractions for standard acuity

would be 18/18 or 21/21. Instead the visual acuity is recorded as 20/20 in both cases. Thus,seeing 20 as the numerator of a visual acuity fraction rarely implies that the actualmeasurement was made at 20 ft.

In Britainand related countries, the 6/6 notation is similarly used as a Snellen equivalent.

Visual angle notationwas used by Louise Sloan. It refers to the visual angle of the strokewidth of 5x5 letters. This angular value is the same as the MAgnification Requirement (MAR),discussed earlier.

LogMAR notationwas introduced by Bailey [35]. As the name implies, it is the logarithm of theMAR value, thus converting a geometric sequence of letter sizes to a linear scale of perception.It provides a more scientific equivalent for the traditional clinical statement of lines lost or linesgained and has gained widespread use in psychophysical studies and for statistical

calculations.On an ETDRS-type chart each line interval is equivalent to 0.1 logMAR; therefore +1.0 logMARmeans 10 lines lost or 20/200 (0.1)and +2.0 logMAR means 20 lines lost or 20/2000 (0.01).Since higher logMAR values indicate poorer vision, the logMAR notation should be considered anotation of vision loss, rather than a notation of visual acuity. 0 logMAR = no loss = standardvision, while 0 VA = NLP = blindness.

Since Bailey used the logMAR notation with a geometric progression of letter sizes, the termlogMAR chart is often used to imply a geometric progression. However, the MAR and logMARnotations can be used with any letter size progression.

Visual Acuity Score(VAS) [43] and Visual Acuity Rating(VAR) [44] are two names given to amore user-friendly alternative to the logMAR scale. Like the logMAR scale it converts the

geometric progression of visual acuity values to a linear scale. However, the VAS scale is moreintuitive, since it is a scale of visual functioning, where higher numbers indicate better vision.For VAS or VAR the reference standard (20/20, 1.0) is rated as 100, a 10-line loss (20/200,0.1) is rated as 50 and a 20-line loss (20/2000, 0.01) is rated as 0. While this 0 value is nottotal blindness, MAR = 100x, hardly is a measure of detail vision anymore. On ETDRS typecharts, where each line has five letters, the Visual Acuity Score can be interpreted as a count ofthe total number of letters read, starting from 20/2000 (0.01).

Similar letter-count scores are used in many studies and population surveys; however, they areoften anchored arbitrarily; the original ETDRS score was anchored at 40 M at 1 m (20/800) = 0,so that 20/20 = 85. VAS and VAR are anchored so that the reference standard = 100. (Fig. 18)compares these different notations.

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Figure 18 Various equivalent visual acuity notations

The table compares the different visual acuity notations discussed in the text.

Note that the MAR and logMAR scales indicate vision loss (higher values indicate poorervision); the VAS scale indicates functioning (higher values indicate better vision).

The progression of visual acuity values (rows) follows the preferred numbers series (Fig. 17).Note that when the viewing distances (columns) also follow this series (1, 4, 5 or 6.3 m; 20 ft),the required letter sizes (numerator of the Snellen fraction) are also preferred numbers. When

the chart is designed for 6 m (5% less than a preferred number) the required letter sizes alsohave to be reduced by 5% and are no longer preferred numbers.

For chart design, the exact numbers, as shown in this table, should be followed. For clinicalnaming it is acceptable to round 32 to 30, 63 to 60, etc. The error involved is 5% or 1/5 lineinterval, and corresponds to 1 letter seen or not seen on a standard chart, a difference muchsmaller than the measurement accuracy of clinical measurement.

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Which notation to use?

For everyday clinical practice, the direct visual acuity values are most useful (Fig. 3). Theyallow simple calculations about viewing distance, letter size and magnification need. E.g. aneye with 20/20 acuity can, by definition, just see a 1 M letter (1.5 mm) at 1 m; a road sign withletters that are 100x larger (100 M, 150 mm, 6) can therefore be seen at 100x that distance

(100 m, 300 ft). A person with 20/40 (1/2) acuity can see the same letters at half the distance(1/2 x 100 m = 50 m, 150 ft); to read normal print the person with 20/40 acuity needs 2xmagnification.

Today, true Snellen fractions, in which the numerator indicates the actual testing distance, arerarely used anymore. Snellen equivalentnotations are preferred, since they can be comparedregardless of the testing distance used. Decimal notation (Europe), US fractions (20/) andBritish fractions (6/) are the notations most people are familiar with.

For graphical displays with a very limited acuity range, any scale might be used. However, fordisplays that cover a wider range a scale based an the logarithm of the visual acuity (log(VA),logMAR, VAS or lines) is preferable.

For statistical calculationsusing log(VA), logMAR or VAS is the only option. The difference

between using a linear and a geometric scale for calculations can best be shown with anexample:

What is the average of 20/20 and 20/200?Using VAS, the average of VAS 100 (20/20) and VAS 50 (20/200) is VAS 75 (20/63).

Averaging the decimal values: (1.0 and 0.1), the average = 0.55 (20/40 +) would be too high.Averaging the denominators of 20/20 and 20/200, the average = 20/110 would be too low.The table in Fig. 18 clearly shows that the first answer is the most appropriate.

What not to use? In most offices, the viewing distance is never varied. This means that eachline can be labeled with the visual acuity value for that distance. On many charts this has led toomission of the actual letter size, as it was always listed on Snellens charts. When the chart isused at any other distance, determining the acuity value without knowing the letter size involves

some calculations. The lack of letter size designations has led to the habit of using visual acuitynotations to describe letter sizes. This is erroneous, since any letter size can reflect any acuityvalue, depending on the viewing distance (Fig. 22).

Choice of criterion and rounding of values

The recorded visual acuity value can also be influenced by the choice of the completion criterionand by rounding. Most clinicians will record visual acuity in line-incrementsand consider a lineread if more than half of the letters are read correctly (e.g. 3 of 5 on an ETDRS type chart). Asuffix such as 1 or +2 is often added to indicate 1 letter missed or 2 letters read on the nextline. On older charts, where the line intervals and the number of letters per line may vary, these

suffixes have no constant meaning. (At the top of most traditional charts (Fig. 7), +1 would be 3ETDRS lines; for Greens bottom lines of 11 letters (Fig. 9), +5 would still be less than half aline). Thus, when averaging visual acuity values across patients or over time, these suffixesshould be ignored.

On ETDRS-type charts, the line intervals and the number of letters per line are fixed and thevalue of each additional letter read may be assumed to be equal. This provides the opportunityto record the test results in letter-incrementsrather than in line-increments. Such letter-countscores are used in many research studies; on the VAS scale each letter read increases thescore by one point. In this case the suffixes -, - -, + and ++ are recommended [45]. Omitting the

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suffix will automatically round to the appropriate Snellen equivalent; use of these suffixes ispreferred over the use of interpolated values, which convey a sense of spurious accuracy.

Figure 19 Rounding of Visual Acuity Values

The question is sometimes asked is 20/63 worse than 20/60? The answer is that they areequivalent within the measurement error. They differ by only 1 VAS point (one letter), which is5x smaller than the 95% confidence interval for clinical measurement error (one line). To bringthe accuracy down to 1 letter, one would have to average 52= 25 measurements.

The following table represents a small segment of the table in Fig. 18; it is expanded from 1 line(5 letters) per row to 1 VAS point (one letter) per row.

The bold values indicate the line values for a strict logarithmic progression; the horizontal bandsindicate line intervals. Because of the rule that a line is considered read if more than half of thecharacters (3 of 5) are correctly identified, all values within a band should be consideredequivalent for clinical recording and may be rounded to the same value.

Thus, all values from 20/63 ++ to 20/63 (20/57 to 20/70) may be rounded to 20/63; thisincludes the more traditional value 20/60. A rule that says better than 20/60 should bemathematically interpreted as better than 20/56 (> 0.35); a rule that says worse than 20/60should be mathematically interpreted as worse than 20/71 (< 0.28). Fortunately, all traditional

values are close (+/- 1 letter) to a logarithmic value.The only exceptions are 20/70 and the decimal values 0.7 and 0.9, which represent half steps.The 20/70 value is a remnant of Snellens original chart (Fig. 15), which had no lines for 20/60and 20/80, but went from 20/50 to 20/70 to 20/100 and 20/200.

Also note that to indicate letter count steps, the notation with + and suffixes (in the US +/column) is easier to understand than the use of interpolated numerical values, as shown in theUS interpol. column and in the decimal column); they suggest spurious accuracy.

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It should be noted that, although the letter-count score improves the precisionwith which theresult is recorded, it does not improve the accuracyof the underlying measurement method. Onany measurement scale, a 1 point difference may or may not be significant; a change from 2 to3 might indicate an insignificant change from 2.4 to 2.6 or a significant change from 1.6 to 3.4.

Since the accuracy of common one-timeclinical measurements is about 1 line interval, theresults may as well be rounded to the nearest line. The use of a letter count score isadvantageous, however, when sequentialmeasurements are compared to detect a possibletrend.

Another factor that may affect the score is whether subjects are encouraged to guess. Sincedifferent subjects may vary in their willingness to guess, forcing all to guess (a forced choiceparadigm) will produce more homogeneous results.

When a subject cannot read a line on a chart, some clinicians will present an isolated line or anisolated letter. This makes the task easier and can improve the visual acuity score. One shouldbe aware that using different presentation modes at different times reduces the comparability ofthe scores.

To make sure that the actual thresholdis reported, the chart should be presented close enoughso that there is at least one line the subject can see, or far enough so that there is at least oneline the subject cannot see. Terminating testing at the 20/20 level is a mistake, since, Snellenchose the reference value of 20/20 (1.0) so that most healthy eyes can do better (Fig. 8).

Choice of test symbols

Most visual acuity charts utilize letters. For the patient, this choice gives a sense of immediatevalidity, since the primary objective of most patients is to read. For the practitioner, errors areeasy to spot, since most practitioners know their chart by heart. Use of letters, however, iswarranted only if the assumption can be made that letter recognition is trivially easy. TheETDRS charts use the Sloan letter set (Fig. 12), which has made this the preferred set for manystudies. Many other letter sets exist, including sets for non-roman alphabets.

For less literate adults the use of a numberchart may be more appropriate. Number charts canalso be used for deaf patient who use sign language, since they can respond by holding up theappropriate number of fingers.

An alternative is the use of various symbol sets.

Internationally used symbol sets

Landolt Cs[29] (Fig. 11) have become the symbols of choice for many scientificmeasurements. They are much less frequently used in a clinical setting. When used in a chart

format it is harder to detect errors, unless the observer points to the symbol. However, pointing,like single presentation, affects the difficulty of the test.

The 1984 Visual Acuity Measurement Standard of the International Council of Ophthalmology[38] recommended that letter charts in non-Roman alphabets (Cyrillic, Arabic, Hindi, Kanji,Hebrew, etc.) be calibrated against Landolt Cs for equal recognizability. Since the ETDRSchart has become a de-facto standard, calibrating against an ETDRS chart is another option.

Tumbling Esare probably the symbols most often used for the testing of children. They arealso widely used in developing countries and in countries where the Roman alphabet is not

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used. Tumbling Es and Landolt Cs require a sense of laterality, which can be a stumblingblock for young and for developmentally delayed children. They can be presented in a chartformat or as single symbols. When comparing findings, it should be remembered thatpresentation as single symbols is an easier test than presentation in a chart format.

Symbol sets of ten used in the US

The HOTVtest uses the four letters H, O, T and V as symbols with distinctive shapes that canbe recognized even by pre-literate children; these letters were chosen because they do notrequire a sense of laterality [46]. For children who are shy or have difficulty naming symbols,matching cards can be used where the child only needs to point at the matching symbol.

For illiterate patients and pre-school children, picturesmay be used. However, it is difficult tojudge the equivalence of letters and various pictures and a childs performance may depend ontheir familiarity with the objects. Most pictures cannot be designed on Snellens 5x5 grid.

Figure 20 Various Optotypes

This chart depicts a selection of commonly used optotypes.First row:three types of H, Snellen H (with serifs, 5x5), Sloan H (no serifs, 5x5), H on Bailey-Lovie chart (no serifs, 4x5) [35]. Note that the 5x5 non-serif design appears wider than normalprint; the 4x5 design, based on the 1968 British standard [24], more closely resembles normalprint. Yet, the 2003 British standard [58] went back to the 5x5 Snellen design.Second row:Number, tumbling E, Landolt C. These groups have fewer symbols, so that theguessing level is notably higher than with letter charts.

Most optotypes approximate the recognizability of Landolt Cs; they represent 20/20 (1.0) acuity

when their height subtends about 5 [59

].

Patti Picsform a set of five stylized pictures, designed on the 5x5 Snellen grid, that provide amiddle ground between letters and pictures [47]. They are left-right symmetrical and require littlenaming ability. They have been calibrated for equal recognizability with Sloan letters. As aresult, no shift in visual acuity is observed when a child graduates from Patti Pics to letters.They can also be used for adults who are not familiar with the Roman alphabet.

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LEA symbolswere devised by Lea Hyvrinen [48]. The set has only four symbols that havenarrower lines than Snellen symbols. They are widely used for pediatric testing and areavailable in a variety of formats, from game formats for the very young to letter chart formats.However, since they are somewhat easier than Sloan letters, there may be a difference whenchildren make the transition from symbols to letters [49].

Grating acuitypresents a different form of visual acuity measurement. The subject is asked todetect the difference between a fine grating and an even gray surface or is asked to detect theslant of a grating. Grating acuity is expressed in cycles per degree (cpd), where each cycle =one light + one dark bar. Mathematically, 30 cpd = 20/20; however, grating acuity is often betterthan recognition acuity. In young infants, a reason may be that gratings offer a detectiontask,rather than a recognitiontask, and therefore require less cognitive processing [50]. In adults withretinal disease a reason may be that gratings cover a larger retinal area and thus have a betterchance of catching a good area. Teller cards[51] provide a well-known preferential lookingtestfor infants; they have also been used for demented elderly.

Electro-physiologic testsin infants record the VEP response to a visual stimulus. Here too ithas been found that a simple grating stimulus may overestimate later recognition acuity.Vernier acuity has been found to be a better predictor.

Chart layout

Snellens charts (Fig. 7) and most subsequent charts had a rectangular format with few largecharacters at the top and many small ones on the bottom lines, and with spacing dependentupon the available space. Bailey and Lovie [35] introduced a new format (Fig. 13) that combinedthe logarithmic progression with a fixed number of characters per line and with proportionalspacing. This resulted in a large chart in the shape of an inverted triangle, where everycharacter is surrounded by a white space equal to the width of the character. In this format,which was also followed for the ETDRS charts, the crowding effect is the same for all characters

and the only variable from line to line is the letter size.Although the Bailey-Lovie and ETDRS carts are often recognized for their logarithmicprogression, it was their layout that was new, not the logarithmic progression, which had beenused earlier (Fig. 9).

For projector chartsand computer screensthis format is possible only for the smallest lines,since the projection area is not large enough. The logarithmic progression, however, can andshould be maintained [52].

When symbols are presented in isolation, the crowding effect of the surrounding letters is lost.For single letter presentation, the letters are therefore often surrounded by crowding bars. Foradult charts the crowding bars are generally placed at one character width [49]. For children, thespacing is generally 1/2 character width [50], because of the interest in detection of amblyopia.

Summary

The measurement of visual acuity involves determining the MAgnification Requirement (MAR)that allows a subject to reach a standard performance. Visual acuity is defined as the reciprocalof the MAgnification Requirement (VA = 1/MAR). The measurement tool is usually a letter chartor similar instrument. The test is usually a distance measurement. Distance testing was

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chosen because it relaxes accommodation and facilitates refractive measurement, not becausedistance vision is most representative for activities of daily living.

Charts with a logarithmic(geometric) progression of letter sizes are recommended. Wherespace permits, the triangular format of the Bailey-Lovie / ETDRS layoutis preferred. Resultsare best recorded in Snellen equivalents; in the US this means use of the 20/20 notation, in

Britain the 6/6 notation and in Europe decimal notation.For occasional use, printed chartsare the simplest solution. For more standardized testing andschool tests rear illumination cabinetsmay be used. In professional offices projector chartsarepreferred. In the future many of these will probably be replaced by computer displays.

The choice of other test parameters depends on the setting and the purpose for which visualacuity is to be measured. For refraction and testing of mainly normally sighted subjects, testdistances in the range of 5 m to6m(16 to 20 ft) are preferred. When a small room does notallow this distance, use of mirrors is recommended. For children, 3 m(10 ft) is often used. Fortesting in the low vision range, testing at 1 m(40) is recommended. This distance not onlyallows a much wider measurement range, it also is a better average of common distancesinvolved in the performance of activities of daily living.

Since many activities of daily living involve near activities, assessment of near vision is alsoimportant. The next section will discuss near testing.

CONSIDERATIONS for NEAR VISION TESTING AND READING

Although the testing of reading vision predated the development of letter charts to measuredistance vision, the methodology to accurately measure reading acuityhas lagged behind.Several interrelated factors contribute. Firstly, measurement tends to be less accurate, sincerelatively small differences in viewing distance can cause large changes in the measured acuity.Secondly, the prescription of a reading correction for normally sighted individuals is aimed more

at achieving a comfortable, supra-threshold reading performance(functional vision) than ataccurate measurement of the visual acuity threshold (a visual function). Thirdly, there also is alack of accurate measuring tools. Reading distances are more often estimated than measured,while the Jaeger numbers, which are widely used in the U.S., have no numerical meaning.Under these circumstances, it is not surprising that many practitioners believe that readingacuity and distance acuity have little in common. We will show that this is not so.

Near letter-chart acuity

Many eye professionals carry a pocket card with a miniature letter chart. If this chart is properlycalibrated, if the viewing distance is properly measured and if the subject has the properrefractive correction for this viewing distance, there is no reason that the threshold visual angle

measured at a close distance should be any different from that measured at a longer distance.

As noted earlier, letter chart acuity is a good test for the integrity of the optical system of theeye, since defocus at the fovea predicts equal defocus in the periphery of the image. A pocketletter chart is an easy test for presbyopic subjects in whom we have good reason to believe thatthe retina is normal. However, as the population ages and we are confronted with more retinalproblems, the limitations of letter chart acuity have become more obvious, since theperformance of the retinal area where the letter is projected, does not tell us anything about thefunction of other retinal areas.

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Reading acuity

If there is a suspicion that there may be retinal problems, a near letter chart alone is no longeradequate. As a functional test, a reading test is far superior to a test of letter chart acuity.Functional reading requires not only the recognition of individual letters, but of words and

preferably of parts of a sentence. This requires a larger retinal area.

Consequently, making the distinction between distanceand nearacuity is relevant for thediagnosis of presbyopia, but distinguishing between letter-chartand readingacuity, is morerelevant for functional vision. The various combinations are summarized in Fig. 21.

Figure 21 Characteristics of Letter chart vs. Reading acuity

Note that letter chart tests and reading tests do not test the same qualities. They differ in the

scoring criterion, which explains part of the different measurement results. They also differ inthe problems for which they are most useful. See also Fig. 22 and Fig. 23.

Note that the criterion used for letter chart acuity is different from that for reading acuity. Thisdifference is often overlooked and accounts for some of the difference between the twomeasurements. For letter chart acuity we usually press for thresholdperformance andencourage guessing. This is done because threshold (forced choice) performance is bettersuited for psychophysical calculations, not because threshold performance is the most relevantperformance level for activities of daily living. In reading tests we aim for a supra-threshold levelof comfortable and sustainable performance that is more relevant to everyday functioning, but

less precisely defined. Except for Olympic athletes, people rarely perform at the limit of theircapabilities. Reading a book with a marginal print size and marginal contrast under marginalillumination is not an acceptable performance level.

Modified Snellen Formula for near acuity

Before we can discuss reading acuity in more detail, we need an easy way to measure it. As isthe case for distance vision, this requires two variables: letter size and viewing distance. For

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distance vision the viewing distance for a particular office is usually standardized, so that theacuity value for each line can be noted on the chart. For reading tests the viewing distances areless standardized. Today, many clinicians use 40 cm (16, 2.5 D reading add), some use 14(35 cm, 2.75 D add), others use 33 cm (13, 3 D add)or 30 cm (12, 3.25 add) or even 25 cm(10, 4 D add, the reference point for the power of magnifiers). Individuals in the Low Visionrange often need distances that are even shorter and certainly cannot be handled with a one

size fits all distance. We therefore need a formula in which both the letter size and the viewingdistance can be varied easily.

The standard Snellen formula V = viewing distance / letter size becomes awkward to usewhen the numerator (viewing distance in meters) is itself a fraction within a fraction. This can beovercome by using the reciprocal value of the viewing distance, recognizing that the reciprocalof a metric distance is known as the diopter (2 diopters = 1/2 m, 5D = 1/5 m, etc.) [53].

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Measuring Vision and Vision Loss