Diagnostic Procedures in Ophthalmology HV Nema, Nitin Nema
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Assessment of Visual AcuityChapter 1

Nitin Nema
Visual acuity (VA) is a measure of spatial resolution of the visual processing system. The minimal angle of resolution (MAR), that allows a human optical system to identify two points as different stimuli, is defined as the threshold of resolution. The MAR lies between 30 seconds and 1 minute of arc. Visual acuity is reciprocal of the threshold of resolution.1 It is the most important criterion for testing the functional integrity of eye and visual pathway. It is maximal at fovea where cones are closely packed and highly differentiated.
In 1854, Eduard von Jaeger published a set of reading charts in German, French and English languages to record the VA. Donders first time, coined the term visual acuity to describe the sharpness of vision and defined it as a ratio between a patient's VA and a standard VA. Hermann Snellen published his classical chart for recording the VA in 1862. Unlike Jaeger, he did not used the fonts from printing house but designed special letters, based on 5 × 5 grid, which he called optotypes.
The Snellen test-types are constructed on the principle that two distant points can be visible as separate only when the minimum angle subtended by them at the nodal point of the eye is 1 minute. This forms the standard of normal visual acuity. The visual acuity depends upon the resolving power of the eye and varies with the wavelength of the light and size of the pupil.
The Snellen test-types consist of letters arranged in lines (Fig. 1.1). The size of the letters gradually diminishes from above downward. Each letter is so designed that it fits in a square, the sides of which are five times the breadth of the constituent lines.
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Fig. 1.1: Snellen chart
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Fig. 1.2: The Snellen letters subtend five minutes of visual angle and each component subtends one minute of the angle
Therefore, at a given distance, the letter subtends an angle of 5 minutes at the nodal point of the eye (Fig. 1.2). The top letter of the chart subtends a 5-minute angle at the nodal point of the eye from a distance of 60 meters. The letters in the subsequent lines subtend same angle if they are 36, 24, 18, 12, 9 and 6 meters away from the eye. The chart should be well-illuminated (20 foot candles) for recording the vision correctly.
For recording the visual acuity, the patient should be seated at a distance of 6 meters from the chart as the rays of light are practically parallel from this distance and accommodation is negligible. When the space in the room is limited, the test-types may be seen after being reflected from a plane mirror kept at a distance of 3 meters from the patient. The patient is asked to read the test-types after occluding one eye either by an occluder or by palm of the hand. The visual acuity is expressed as a fraction, the numerator of which is the distance of the chart from the patient (6 meters) and the denominator is the numerical number written underneath the line up to which the patient can read. For example, if a patient can read only the top letter, his visual acuity is recorded as 6/60. In fact, a normal person ought to have read the letter from a distance of 60 meters. When patient reads the second, third, fourth, fifth, sixth and seventh lines the visual acuity of the patient is recorded as 6/36, 6/24, 6/18, 6/12, 6/9 and 6/6, respectively. When foot is used as a unit of measurement (as commonly done in US), VA is expressed as fraction 20/200, 20/125, 20/80, 20/40, 20/32 and 20/20. Another commonly used scale is logMAR, which expresses the logarithm of minimum angle of resolution. Visual acuity scales in feet, meters, decimal and logMAR are shown in Table 1.1.
Normally, a person can read the line marked 6 and the visual acuity is expressed as 6/6 or 20/20. When the top letter cannot be read, the patient is asked to move towards the chart and if he or she reads the top letter from 3 meters distance, the visual acuity is recorded as 3/60. If the patient cannot appreciate the top letter even from a distance of 1/2 meter then the distance in feet or inches is recorded at which finger counting (FC) is possible. When the patient fails to count the fingers, see whether he appreciates the movements of the hand (HM). In absence of recognition of hand movements, perception of light (PL) should be tested. The examination should be repeated for the fellow eye.
TABLE 1.1   Visval acuity in feet, meters, decimal and logMAR
VA in feet
VA in meters
VA in decimal
VA in logMAR
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Fig. 1.3: E chart
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Fig. 1.4: Landolt chart
In ill literate persons ‘E’ test-types (Fig. 1.3) or Landolt broken rings (Fig. 1.4) should be used. The patient is directed to point his finger in the same direction as bars of E or open end of C.
The conventional Snellen charts are not accurate as there is no equal progression of size of letters from one line to another.3
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Fig. 1.5: Bailey-Lovie chart
There are some letters that can be recognized easily (letter L) while others are difficult (letter B) to recognize. Therefore, Bailey-Lovie chart (Fig. 1.5) is developed. The chart has standard spacing between letters on each line and between lines of print. There is a geometric progression of size of letters in each line. The chart contains 10 lines and each line has 5 letters. It is more useful for recording VA in low vision patients and, therefore, was preferred in the Early Treatment Diabetic Retinopathy Study (ETDRS).
Normal visual acuity is often considered as 20/20 (in feet), 6/6 (in meters), 1.00 decimal or 0.0 logMAR. The 20/20 standard should be considered as the lower limit of normal or as a screening cutoff. Normal acuity in healthy adults is one or two lines better than that. Average acuity in a population sample does not drop to the 20/20 level until the age of 60 or 70 years. This explains the existence of the two lines smaller 4than 20/20: 20/15 and 20/10. Super acuity and hyperacuity are the terms used for vernier acuity that measures the ability to align two line segments. The vernier acuity is about 0.13 arc minutes, well below normal visual acuity (which is about 0.6 arc minutes); therefore, it is presumed to be function of visual cortex rather than retina.
Stereoscopic acuity is the ability to detect tiny differences in depth with the two eyes. For more complex targets, stereoacuity is similar to the normal monocular visual acuity. Some people may have very poor or even absent stereoacuity with normal monocular acuities. This represents abnormal visual development in young age.
The near visual acuity is usually tested with the help of Jaeger test-types (Fig. 1.6A) or Roman test-types. The Snellens notation maintaining the same visual angle may also serve the purpose. In Jaeger test types, a series of different sizes of print types are arranged in increasing order and arbitrarily marked 1, 2, 3, 4, 5, 6 and 7. Rosenbaum Pocker Vision Screener or Lebensohn charts can also be used. The near vision is also tested by music-types, numericals or tables (Fig. 1.6B). It is usually tested in good illumination preferably in day light. The patient is asked to read the chart kept at a distance of 16 inches or 40 cm. Generally, a person with normal vision and accommodation reads the smallest types easily. If unable to read the smallest types on the chart, the smallest type, which the patient can read should be noted. Patients with high hyperopia, presbyopia or anomalies of accommodation have defective near vision.
Physical, physiological and psychological factors usually affect visual acuity.2,3 Physical factors include illumination and contrast. Physiological conditions comprise pupil size, accommodation, light-dark adaptation and age.
Refractive Errors
Uncorrected refractive error affects the VA maximally and is a common cause of poor VA. A young hyperope can compensate the small error by accommodation. However, high refractive errors especially hyperopia and astigmatism are known to cause amblyopia if not corrected.
Increased illumination increases visual acuity from threshold to a point at which no further improvement can be elicited. In a clinical situation, this is 5–20 foot candles. Beyond a certain point, illumination can create glare. Therefore, visual acuity is recorded under photopic condition and evaluated at the fovea.
Contrast Sensitivity
In routine practice, vision is recorded with Snellen chart in high contrast. When contrast is reduced more illumination is required to resolve an object. Contrast sensitivity is reduced even when Snellen acuity is normal. Contrast sensitivity tests are more accurate in quantifying vision, especially in diseased states such as cataract, corneal edema, and disorders of retina and central nervous system. Contrast sensitivity is assessed by using contrast sensitivity chart (Fig. 1.7). It is measured in units of cycles per degree (cpd).
Pupil Size
The pupil size has great influence on visual acuity. Visual acuity decreases if pupils are smaller than 1 mm due to diffraction and reduced retinal illumination. Pupil diameter larger than 6 mm increases aberration, scattering of light on retina and reduces VA. Variation in pupil size changes acuity by altering illumination, increasing depth of focus, and modifying the diameter of circle of blur on the retina.
Retinal Eccentricity
VA is maximal at the center of fovea. About 60% reduction in VA occurs just at one degree away from the center.4 The reduction in VA is due to farther separation of cones as the distance from the fovea increases.
Accommodation creates miosis, which accounts for small hyperopic prescriptions being rejected for distance viewing in younger individuals. Spasm of accommodation induces myopia and decreases VA.
Normal VA depends on the integrity of the eye and visual pathway. It is seen that after the age of 40 years about 50% of eyes show aging changes and deterioration in vision.
Mental Status
The assessment of VA will not be correct if patient is feeling sleepy, intoxicated, or having any disease that can alter the patient's consciousness or his mental status.5
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Fig. 1.6A: Jaeger type near vision chart
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Fig. 1.6B: Mixed near vision chart showing music types, tables, cross-words, print types and playing-cards
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Fig. 1.7: Contrast sensitivity chart
Visual acuity measurement should be performed at the earliest possible age that is practical (usually at approximately 3 years of age). Early detection and prompt treatment of ocular disorders in children is important to avoid permanent visual impairment. All infants who are premature or have family history of congenital cataract, metabolic or genetic diseases must be screened as early as possible. Children usually do not complain of visual deficit, therefore, first visual acuity testing must be done in all children at the age of 3 years.
Accurate testing of VA is difficult in infants; some sophisticated tests are used for recording the vision. Assessment of the visual acuity (vision screening) represents one of the most sensitive techniques for the detection of eye abnormalities in children. The American Academy of Pediatrics Section on Ophthalmology,5 in cooperation with the American Association for Pediatric Ophthalmology and Strabismus and the American Academy of Ophthalmology, has developed certain guidelines to be followed by ophthalmologists and opticians for the assessment of VA in children.
Visual Acuity Measurement: Age 0 to 3 Years
Before the assessment of vision, it must be ensured that the child is awake and alert. Vision in children younger than 3 years is assessed by evaluating the child's ability to fix and follow objects.6,7 Initially, it should be determined binocularly that eyes fixate on an object, maintain fixation, and follow the object in all directions of gaze. Then it should be tested monocularly. If the eyes fail to perform these maneuvers, a significant bilateral eye or brain abnormality is suspected.8-10
Besides ocular motility, following procedures can roughly assess the vision in infants and preschool children:
  • Optokinetic nystagmus (OKN)
  • Electroretinogram (ERG)
  • Visual evoked response (VEP)
  • Preferential looking test
  • Flooks' symbols or other picture matching charts.
Optokinetic Nystagmus Test
A rotating pediatric OKN drum with eight alternating black and white stripes is used (Fig. 1.8). Involuntary pursuit response to the stripes is observed. A positive nystagmus response indicates that the infant has at least counting fingers vision between 3 and 5 feet.
The electroretinogram (ERG) consists of 4 waves: an initial a-wave, large positive b-wave, a mild deflection c-wave and an off response d-wave. Both cone (photopic) and rod (scotopic) responses are obtained. The b-wave response arises from the bipolar cells of retina and normally is about 150 mv. In degenerative conditions of the retina and cone dystrophy, it is often subnormal or extinguished. Focal ERG provides information about macular function.
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Fig. 1.8: Optokinetic nystagmus drum
Visual Evoked Potential
The visual evoked potential (VEP) can be used to test the visual acuity in infants (Fig. 1.9) and children especially in whom other vision tests, like the preferential looking test, are giving ambiguous and unreliable information. It measures the acuity by assessing the response of the brain to alternating black and white stripes or checks. The three metal electrodes on the head are placed and connected to a computer. The child is then positioned in front of a TV screen displaying alternating black and white checkerboard patterns. As the child views the stripes, a signal is relayed from the eyes to the brain (the visual cortex). This signal is detected by the electrodes. Often the vision of each eye is tested separately.
Preferential Looking Test
The preferential looking test is used to test the visual acuity in infants and young children who are unable to identify pictures or letters. The child is presented two types of fields, one with stripes and the other homogeneous (Fig. 1.10). Both fields have same luminance. The location of the stripes is randomly alternated. Generally, infants and children prefer to look at the more interesting stripes rather than at the blank field. The smallest stripe width that the child has preferred to see is considered to be his resolution threshold or visual acuity. Teller acuity cards (Fig. 1.11) may also be used to check the visual attention of infants. The card presents vertical and horizontal bars on one side and the other side has blank page.
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Fig. 1.9: Visual evoked potential testing(Courtsey: Dr E Vaithalingam)
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Fig. 1.10: Preferential looking test
The visual development in a newborn was measured by the preferential looking test. It was found 20/600 at birth, and reached 20/120 by 3 months. A modest improvement of 20/60 was seen by 12 months. VA becomes 6/6 by the age of 3 to 5 years.
Fflooks' Symbols
The Fflooks' test uses simple symbols like squares, circles and triangles (Fig. 1.12). The child is shown a figure of circle or triangle from a distance of 3 meters and asked to match it from the figures kept in front of him.
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Fig. 1.11: Teller acuity chart
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Fig. 1.12: Fflooks symbols
Matching toy test and other familiar picture test are useful in assessing VA in children who are 2 to 3 years' old.
Bruckner Test
Bruckner test can provide a rough estimate of the presence of refractive error, strabismus and medial opacity. Bruckner test should be performed in a dark room. The ophthalmoscope is focused on each pupil individually approximately 12 to 18 inches away from the eye, and then both eyes are viewed simultaneously from a distance of about 3 feet. Generally, a bright red reflex is seen in each eye. The presence of black spots in the red reflex, a dull red reflex, or a white reflex indicates a diseased eye. Photo screening examination11,12 of pupils (size, shape and reactions of both pupils), and binocular red reflex provide a wealth of information.
The parents should be informed about the disability of the child who fails in vision screening test and must consult a pediatric ophthalmologist trained in treating the condition.
It is often helpful and convenient to explain the examination procedure to the parent of the child. The child should comfortably sit on the parent's lap. Many parents assist by demonstrating the anticipated testing procedures to the child. Before the measurement of VA, it is essential to examine both the eyes to exclude ocular structural abnormalities, such as cataract, corneal opacities, and ptosis, which are known to result in poor vision. The child should be seated at a distance of 3 meters or 10 feet except for the Allen card test that is done at a farther distance.
Vision in each eye should be tested separately however, most children dislike occlusion of the non-testing eye. Occlusion must be perfect and it should not allow peeking. Occluding patches are commercially available. They provide complete occlusion.
Visual Acuity Measurement: Age Older than 3 Years
Tests are available for measuring the visual acuity in older children. Wall charts, such as Snellen letters, Snellen numbers, the tumbling E test, and the HOTV test (a letter-matching test involving these 4 letters), can be used to measure VA in children older than 4 years.10 Picture tests, like LEA symbols and Allen cards, are used for children between 2 and 4 years of age.
Snellen Acuity Chart
While testing the child's right eye occlude the left. If the child uses corrective eyeglasses, the vision should be tested with them. Ask the child to read the letter or indicate the direction of tumbling E. If he fails to recognize, then move him towards the chart to enable him to correctly identify at least two symbols. The child must correctly identify at least 4 of the 6 symbols in the line. Repeat the procedure covering the right eye.
Tumbling E
The tumbling E (Fig. 1.13) is for children who do not cooperate in vision testing by letters and numbers.
HOTV test is an excellent test for 3 to 5 years old children who do not recognize alphabet. The test comprises a wall chart composed only of Hs, Os, Ts, and Vs (Fig. 1.14). The child is provided a board on which the alphabets H, O, T, and V are written with bold letters. The examiner points to a letter on the wall chart, and asks the child to match the correct letter on the board.
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Fig. 1.13: Tumbling E chart
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Fig. 1.14: HOTV test
Allen Cards
The Allen card test consists of 4 flash cards containing seven figures: a truck, a house, a birthday cake, a bear, a telephone, a horse, and a tree (Fig. 1.15). When viewed at 20 feet distance, these figures represent 20/30 vision. It must be confirmed from the child before testing that he is going to identify verbally or by matching all the seven pictures. Initially, cards should be shown to the child with both eyes open from a distance of 2 to 3 feet and ensure that he understands the procedure. Present 1 or 2 figures and begin walking backward 2 to 3 feet at a time, as long as the child correctly calls out or matches the figures presented. If a child is able to identify pictures accurately at 15 feet, the visual acuity would be recorded as 15/30. This is equivalent to 20/40 or 10/20. To perform this test, like HOTV testing, a matching panel should be provided to the child.
LH Symbols
The LH symbols are made up of flash cards held together by a spiral binding. The flash cards of this picture test contain a house, an apple, a circle, and a square. These shapes are usually known to a child. Unlike the Allen cards, the LH symbol test (or Lea test) contains flash cards with more than one figure per card and with smaller figure sizes so that testing may be performed at 10 feet. Recorded on each card is the symbol size and visual acuity value for a 10-foot testing distance. If the child is able to identify the 10/15 symbol at 10 feet, the child's visual acuity is 10/15 or 20/30. If testing is not possible at 10 feet, move closer to the child until he or she correctly identifies the largest symbol but the vision is recorded and calculated as the smallest symbol identified.
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Fig. 1.15: Allen cards
Near visual acuity in children can be tested with the Allen picture cards or Lighthouse picture cards.
Assessment of visual acuity is the first and essential step in the examination of eye. Measurement of VA in infants and children needs a lot of time, and patience. A little care in ensuring the proper environment for testing can significantly improve the accuracy. Preferential looking test and HOTV/ Allen card test are often used for recording vision in infants and young children, respectively. Vision disorders have an impact on the quality of life of patients of all age groups. Reading charts or other near vision testing charts should be used as part of the routine assessment of the visual acuity. Insufficient illumination, aging bulbs, soiled vision charts, small pupils, and non-standardized charts may lead to errors in the assessment of visual acuity.
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