Diagnostic Procedures in Ophthalmology HV Nema, Nitin Nema
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Visual Acuity1

STEPHEN C HILTON,
LEELA V RAJU,
VK RAJU
Vision is the most important of all senses. Approximately 80% of the information from the outside world is incorporated through the visual pathway. Loss of vision has a profound effect on the quality of life.
The process of vision includes:
  1. Central resolution (visual acuity)
  2. Minimal light sensitivity
  3. Contrast sensitivity
  4. Detection of motion
  5. Color perception
  6. Color contrast
  7. Peripheral vision (spatial, temporal and motion detection).
In the normal clinical settings, we measure only one of these functions – central resolution at high contrast (visual acuity).1
 
Definition and Terminology of Visual Acuity
The most basic form of visual perception is detection of light. Visual acuity is more than just detecting light. It is the measurement of the ability to discriminate two stimuli separated in space at high contrast compared with the background. The minimal angle of resolution that allows a human optic system to identify two points as different stimuli is defined as the threshold of resolution. Visual acuity is the reciprocal of the threshold of resolution.2 Clinically, discriminating letters in a chart determine this, but this task also requires recognition of the form and shape of the letters, which are processes that also involve higher centers of visual perception.
Discrimination at a retinal level may, therefore, be determined by less complex stimuli, such as contrast sensitivity gratings. Theoretically, the maximum resolving power of the human retina could be derived from an estimate of the angle of approximately 20 seconds of arc because this represents the smallest unit distance between two individually stimulated cones. Thus the resolving power of the eye could be much greater than what is measured by visual acuity charts.3
Cones have the highest discriminatory capacity, but rods can also achieve some resolution. The greater the distance from the fovea the level of visual acuity falls off rapidly. At a 5° distance from the foveal center, visual acuity is only one quarter of foveal acuity.4 Luminance of test object, optical aberrations of the eye and the degree of adaptation of the observer also influence the visual acuity.52
Visual thresholds can be broadly classified into three groups:
  1. Light discrimination (minimum visible, minimum perceptible)
  2. Spatial discrimination (minimum separable, minimum discriminable)
  3. Temporal discrimination (perception of transient visual phenomena such as flickering stimuli).
Many clinical tests can assess many visual functions simultaneously. In a healthy observer in best focus, the resolution limit, or as it is usually called, the minimum angle of resolution (MAR), is between 30 seconds of arc and one minute of arc. Clinically, we use Landolt C and Snellen E to assess visual acuity. The minimum discriminable hyper-acuity or vernier-acuity is another example of spatial discrimination. The eye is capable of subtle discrimination in spatial localization, and can detect misalignment of two line segments in a frontal plane if these segments are separated by as little as three to five seconds of arc, considerably less than the diameter of a single foveal cone. The mechanism subserving hyper-acuity is still being investigated.
 
Charts and Scales to Record Visual Acuity
The function of the eye may be evaluated by a number of tests. The cone function of the fovea centralis is assessed mainly by measurement of the form sense, the ability to distinguish the shape of objects. This is designated as central visual acuity. It is measured for both near and far, with and without the best possible correction of any refractive error present. Because only cones are effective in color vision and because they are concentrated in the fovea, the measurement of the ability to recognize colors is also a measurement of foveal function. The function of the peripheral retina which contains mainly rods, may be assessed by peripheral visual field.1
Visual acuity is the first test performed after obtaining a careful history. Measurement of the central visual acuity is essentially an assessment of the function of the fovea centralis. An object must be presented so that each portion of it is separated by a definite interval. Customarily, this interval has become one minute of an arc, and the test object is one that subtends an angle of five minutes of an arc. A variety of test objects has been constructed on this principle, so that an angle of five minutes is at distances varying from a few inches to many feet5 (Figs 1.1 and 1.2). The most familiar examination chart is Snellen chart (Fig. 1.3). Conventionally, reading vision is examined at 40 cm (16 inches). The testing distance of a preferred near distance chart should be observed accurately.
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Fig. 1.1: Snellen letters subtend one minute of arc in each section, the entire letter subtends five minutes of arc
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Fig. 1.2: Each component of Snellen letters subtend one minute of visual angle the entire letter subtends five minutes of visual angle at stated distance
3
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Fig. 1.3: Snellen chart
The Snellen notation is simply an equivalent reduction for near, maintaining the same visual angle. Most of the Snellen-based distance acuity charts are also commercially available as ‘pocket’ charts to check the near acuity at a preferred distance for every patient or at a defined distance for clinical trial purposes including ETDRS (Fig. 1.4) and Snellen letter “E”.
The Jaeger notation is a historic enigma and Jaeger never committed himself to the distance at which the print should be used. The numbers on the Jaeger chart simply refer to the numbers on the boxes in the print shop from which Jaeger selected his type sizes in 1854. They have no biologic or optical foundation. Clinically, Jaeger's charts (Fig. 1.5) are widely used.
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Fig. 1.4: ETDRS chart
Central visual acuity is designated by two numbers. The numerator indicates the distance between the test object and the patient; the denominator indicates the distance at which the test object subtends an angle of five minutes. In the United States these numbers are given in inches or feet, whereas in the Europe the designation is in meters.
The test chart commonly used in the United States has its largest test object one that subtends an angle of five minutes at a distance of 200 feet (6 m). Then there are test objects of 100, 80, 70, 60, 50, 40, 30, 20 and 15 feet. If the individual is unable to recognize the largest test object, then he or she should be brought closer to it, and the distance at which he or she recognizes it should be recorded. Thus, if the individual recognizes the test object that subtends a five minute angle at 200 feet when he or she is at 12 feet, the visual acuity is recorded as 12/200. This is not a fraction but indicates two physical measurements, the test distance and the size of the test object.4
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Fig. 1.5A: Jaeger's type near vision chart
5
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Fig. 1.5B: Near vision chart: Music type and numericals
6
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Fig. 1.6: Broken C, letter E and pictures of familiar objects for testing visual acuity in illiterates and children
The most familiar test objects are letters or numbers. Such tests have the disadvantage of requiring some literacy on the part of the patient. Additionally, there is a variation in their ability to be recognized. “L” is considered the easiest letter in the alphabet to read and “B” is considered the most difficult. To obviate this difficulty, broken rings (Fig. 1.6) have been devised in which the break in the ring subtends one minute angle, and the ring subtends a five minute angle. Similarly, the letter “E” may be arranged so that it faces in different directions (Fig. 1.6). These test objects are easier to see than letters, eliminate some of the difficulties inherent in reading, and can be used in the testing of illiterates and persons not familiar with the English alphabet. A variety of pictures (Fig. 1.6) have also been designed for testing the visual acuity of children.
When a person is unable to read even a top letter, he or she is asked to move toward the chart or a chart can be brought closer. The maximum distance from which he or she recognizes the top letter is noted as the nominator. When visual acuity is less than 1/60, the patient is asked to count fingers from close at hand (CF at 20 cm). When a patient cannot even count fingers, the patient is asked if he or she can see examiner's hand movements (HM positive). When hand movements are not seen we have to record whether the perception of light (LP) is present or absent by asking the patient if he or she sees the light.
Standard illumination should be used for the acuity chart (10 to 20 foot candles for wall charts). When a patient is examined with the Snellen chart in a dark room, the subject sees a high contrast and glare-free target. But in real circumstances, contrast and glare reduce visual acuity, and even more so in a pathological conditions. The contrast sensitivity function of a subject may be affected even when Snellen acuity is normal. The contrast sensitivity tests are more accurate in quantifying the loss of vision in cases of cataracts, corneal edema, neuro-ophthalmic diseases, and retinal disorders. A patient with a low contrast threshold has a high degree of sensitivity; therefore, a healthy young subject may have a threshold of 1%, and a contrast sensitivity of 100% (inversely proportional). It is important to have adequate lighting when testing visual acuity so that it does not become a test of contrast sensitivity.
 
Factors Affecting Visual Acuity
Factors affecting visual acuity may be classified as physical, physiological and psychological. 7Uncorrected refractive error is a common cause of poor acuity.
Physical factors include illumination and contrast. Increased illumination increases visual acuity from threshold to a point at which no further improvement can be elicited. In the clinical situation this is 5–20 foot candles. When contrast is reduced more illumination is required to resolve an object. Beyond a certain point, illumination can create glare. Therefore, visual acuity is recorded under photopic condition and one wants to evaluate best visual acuity at the fovea.
Physiological conditions include pupil size, accommodation, light-dark adaptation and age.2
 
Pupil Size
The pupil size has great influence on visual acuity. Visual acuity decreases if pupils are smaller than 2 mm due to diffraction. Pupil diameters larger than 3.5 mm increase aberration. Variation in pupil size changes acuity by altering illumination, increasing depth of focus, and modifying the diameter of the blur circle on the retina.
 
Accommodation
An accommodation creates miosis, which could account for small hyperopic prescriptions being rejected for distance viewing in younger individuals.
It is worth while to discuss the role of a pinhole in obtaining the best visual acuity in the clinical setting. The optimum pinhole is 2.5 mm in diameter. A pinhole in an occluder (Fig. 1.7) may be introduced in a trial frame with the opposite eye occluded. Single pinhole device is not adequate. The patient must be able to find a hole, therefore, multiple pinholes are preferred. If the patient is older or infirm, or has tremors, he is asked to read only a single letter from each line as we proceed down the chart to record the vision.
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Figs 1.7A and B: Occluder with multiple holes
Many patients have been referred for neuro-ophthalmologic consultation because of painless loss of vision in one eye only. The best visual acuity may be 20/60 in the affected eye but when properly tested with the pinhole, the acuity may improve to 20/20. This indicates that the macula and optic nerve are functioning normally. When the patient's vision is improved with pinhole one knows the problem is a refractive one and simply need the change in glasses. If the patient's vision is less when looking through the pinhole; it indicates that the patient has either an organic lesion at macula, or a central scotoma, or functional amblyopia. A patient with 20/400 vision that improves with pinhole to 20/70 indicates that the improvement is refractive, but some pathology may also be present.8
 
Visual Acuity Testing in Young Children
Early determination of vision loss and refractive error is an essential component of assessing the infant's ultimate visual development potential. The visual acuity of a newborn as measured by preferential looking is in the range of 30 minutes of arc (20/600); acuity rapidly improves to six minutes of arc (20/120) by three months. A steady but modest improvement to approximately three minutes of arc (20/60) occurs by 12 months of age. One minute of arc (20/20) is usually obtained at the age of three to five years.6
The examination is generally performed on the parent's lap. The room should never be totally darkened because this may provoke anxiety. Objective retinoscopy remains the best method of determining a child's refraction.
Other clinical methods involve estimation of fixation and following behavior. A test target should incorporate high contrast edges. For infants younger than six months the best target represents the examiner's face. For the child of six months and older, an interesting toy can be used. After assessment of the binocular fixation pattern, the examiner should direct attention to differences between the two eyes when tested monocularly. Objection to occlusion of one eye may suggest abnormality with the less preferred eye.7
Three common methods are used for determining resolution acuity:
  1. Behavioral technique (preferential looking Fig. 1.8)
  2. Detecting optokinetic nystagmus (OKN Fig. 1.9)
  3. Recording visual evoked potentials (VEP Fig. 1.10).
It is desirable to measure the visual acuity of children sometime during their third year to detect strabismic or sensory amblyopia and to recognize the presence of severe refractive errors. In this group of preschool children, visual acuity testing is easier to perform with the use of the following charts:
  1. Allen and Osterberg charts (Fig. 1.11)
  2. Illiterate E chart
  3. Landolt broken ring.
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Fig. 1.8: Preferential looking test chart
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Fig. 1.9: OKN drum
9
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Fig. 1.10: VEP testing
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Fig. 1.11: Allen and Osterberg chart
 
Contrast Sensitivity
A general definition of spatial contrast is that it is a physical dimension referring to the light-dark transition at a border or an edge of an image that delineates the existence of a pattern or object. Contrast is defined as the ratio of the difference in the luminance of these two adjacent areas to the lower or higher of these luminance values. The amount of contrast a person needs to see a target is called contrast threshold.
The contrast sensitivity is assessed by using the contrast sensitivity chart. It has 5–8 different sizes of letters in six or more shades of gray. Some contrast sensitivity charts contain a series of alternating black and white bars; 100 line pairs per mm is equivalent to space of one minute between two black lines. The alternating bar pattern is described as spatial frequency. The contrast sensitivity is measured in units of cycles per degrees (CPD). A cycle is a black bar and white spaces. To convert Snellen units to units of cycles per degree, divide 180 by Snellen denominator. Contrast sensitivity measurements differ from acuity measurements; acuity is a measure of the spatial resolving ability of the visual system under conditions of very high contrast, whereas contrast sensitivity is a measure of the threshold contrast for seeing a target.8
 
Visual Acuity in Low Vision Patients
Individual near acuity needs are different among different population groups. For low vision patients these differences are magnified. Two persons with the same severe visual impairment may exhibit marked differences in their ability to cope with the demands of daily living. Visual acuity loss, therefore, is the aspect that must be addressed in individual rehabilitation plans. Colenbrander9 subdivides several components of visual loss into impairment aspects (how the eye functions), visual ability (how the person functions in daily living), and social/economic aspects (how the person functions in society (Table 1.1).10
TABLE 1.1   RANGES AND ASPECTS OF VISION LOSS
Impairment aspects (how the eye function)
Visual ability aspects/functional vision (how the person functions-daily living skills)
Social and economic aspects (how the person functions in society)
Ranges (ICD-9-CM)
Visual acuity
Newsprint (1 M)
Statistical estimate of reading ability
Visual aids
VAS
Comments
Normal vision
20/12.5
20/16
20/20
20/25
63in
50in
40in
32in
Normal reading speed Normal reading distance Reserve capacity for small print
None
110
105
100
95
Note that normal adult vision is better than 20/20
Vlild vision loss
20/32
20/40
20/50
20/63
25in
20in
16in
12.5in
Normal reading speed Reduced reading distance No reserve for small
90
85
80
75
Many functional criteria (whether for a driver's license or for cataract surgery) fall within the range
Moderate vision loss
20/80
20/100
20/125
20/160
10in
8in
6in
5in
Near-normal with appropriate reading aids Low-power magnifiers and large-print books
Vision enhancements aids
70
65
60
55
In the United States, children in this range qualify for special educational assistance
Severe vision Loss
20/200
20/250
20/320
20/400
4in
3in
2.5in
2in
Slower than normal with reading aids High-power magnifiers (restricted field)
50
45
40
35
In the United States, persons in this range are considered legally blind and qualify for tax-break disability benefits.
Profound vision loss
20/500
20/630
20/800
20/1000
1.6in
1.2in
1in
Marginal with aids Uses magnifiers for spot reading, but may prefer talking books for leisure
30
25
20
15
In the EU, many benefits start at this level. The WHO includes this range in its blindness category.
Near-blindness
20/1250
20/1600
20/2000
1cm
1cm
1cm
No visual reading must rely on talking books or other
Vision substitution aids
10
5
0
In this range, residual vision tends to become unreliable, though it nonvisual sources may still be used as an adjunct to vision substitution skills.
Total Blindness
NLP
(From Colenbrander A. Preservation of vision or prevention of blindness [editorial]? Am J Ophth almo 2002;133:2. p. 264.)
11
 
Summary
Both distance and near visual acuities are recorded for each eye with and without spectacles. Distance visual acuity is recorded at a distance of 20 feet or in a room of at least 10 feet using mirrors and projected charts. Near visual acuity can be recorded using reduced Snellen or equivalent cards at 40 cm. Acuity performance, like any other human performance, is subject to impairment depending on ocular and general health, emotional stress, boredom, and a variety of drugs acting both peripherally and centrally. The examiner must provide encouragement and must have patience.
For clinical studies the ETDRS charts are recommended because near vision is often more important in the daily life of older or infirm patients. Reading charts or other near vision testing charts should be used as part of the routine assessment of the visual acuity. Visual acuity measurement is often taken for granted. Many pitfalls make this most important assessment subject to variability.10 Ambient illumination, aging bulbs, dirty charts or slides, small pupils, and poorly standardized charts are just some of the factors that can lead to erroneous results. A little care in ensuring the proper environment for testing can significantly improve accuracy.
References
  1. Newell FW. Ophthalmology Principles and Concepts. Mosby,  St Louis,  1969.
  1. Moses RA (Ed). Adlers Physiology of the Eye. Mosby,  St Louis,  1970.
  1. Scheie H. Textbook of Ophthalmology. WB Saunders,  Philadelphia,  1977.
  1. Duane TD. Clinical Ophthalmology. Harper and Row,  New York,  1981.
  1. Michaels DD. Visual Optics and Refraction. Mosby,  St Louis,  1985.
  1. Vander J. Ophthalmology Secrets. Hanley and Belfus. 
  1. Borish I. Clinical Refraction. Professional Publisher,  1970.
  1. Owsley C. Contrast Sensitivity. Ophthalmic Clinics of North America 2003; 16:173.
  1. Colebrander A. Preservation of Vision or Prevention of Blindness? Am J Ophthalmol 2003; 133:263.
  1. Kniestedt, Stamper RL. Visual Acuity and its Measurements. Ophthalmic Clinics of North America 2003; 16:155.