Primary Eye Examination: A Comprehensive Guide to Diagnosis

© Springer Nature Singapore Pte Ltd. 2019

Jong-Soo Lee (ed.)Primary Eye Examinationhttps://doi.org/10.1007/978-981-10-6940-6_1

1. Visual Acuity Test

Ji-Eun Lee¹  

(1)

Department of Ophthalmology, Pusan National University School of Medicine, Busan, South Korea

Ji-Eun Lee

The visual acuity test is the best way to show the status of eye. It includes uncorrected and corrected visual acuity, distant and near visual acuity, binocular and monocular visual acuity, decimal and fractional visual acuity, visual acuity measured with one and parallel object, logMAR (minimal angle of resolution) visual acuity, and central and out-of-central visual acuity. In general, visual acuity is the central visual acuity when viewed from the foveola. The visual acuity in ophthalmology refers to the corrected visual acuity which is the corrected visual acuity measured by refraction. If the uncorrected visual acuity is poor and the corrected visual acuity is good, the refractive error should be considered. If the corrected visual acuity is bad, a pathology from the eye to the central nervous system should be considered. The appropriate visual acuity test should be selected and recorded according to the subject and purpose. If the patient has the history of using contact lenses, cataract surgery, or LASIK surgery, record them on your visual acuity chart.

1.1 Distant Vision Test

1.1.1 Indication

It is possible to examine at 3 years of age or older. At 3–8 years of age, the visual acuity is measured higher when using single object chart than the parallel chart. That is, all persons who can read the target in the visual acuity chart are all candidates.

1.1.2 Purpose

The causes of visual impairment include refractive errors and eye diseases involving visual pathway. The refractive error can be improved by correction, but for the cases of eye diseases, it is difficult to improve the visual acuity only by correcting the refractive error. Therefore, visual acuity test can be a great help in determining the cause of vision impairment.

1.1.3 Method

1.1.3.1 How to Represent Visual Acuity

The minimum separable visual acuity represents the ability to distinguish between two points or two lines. The angle formed by these two points or two lines on the eye is called minimum visual angle and is indicated by visual acuity. In other words, the angle formed by the two straight lines from the two points of the object under consideration to the nodal point of the eye is referred to as the visual angle, and the size of the retinal image is determined thereby. The closer the distance of the same size object is from the eye, the closer the visual angle becomes. The larger the size of the object at the same distance, the larger is the visual angle. Visual acuity is represented by the reciprocal of visual angle, visual acuity = 1/visual angle (minute) (Fig. 1.1).

Fig. 1.1

The reciprocal of the minimum visual angle is visual acuity. If the visual angle Q is 2 minutes, the visual acuity is 0.5

1.1.3.2 Visual Acuity Chart

The visual acuity chart includes a decimal visual acuity chart, a fractional visual acuity chart (Snellen visual acuity chart), and a logMAR chart. The devices of the visual acuity chart include reflection type, transmission type, and projection type.

The decimal visual acuity is 0.1 intervals from 0.1 to 1.0 and 1.2, 1.5, and 2.0 afterward. Because it is expressed as a reciprocal of the minimum visual angle, the arrangement does not show the same interval. 0.1 and 0.2 and 0.9 and 1.0 are both one step, but the visual angle is 10 minutes and 5 minutes in the former and 1.1 minutes and 1 minute in the latter. The former is twice the difference, and the latter is 1.1 times, which does not represent the same difference. Therefore, if 0.125 and 0.16 are inserted between 0.1 and 0.2, 0.25 is inserted between 0.2 and 0.3, and 0.7 and 0.9 are subtracted, the arrangement of decimal visual acuity chart becomes almost equal.

The fractional visual acuity (Snellen method) has the same arrangement of visual acuity, and unlike the decimal visual acuity charts, it is possible to indicate by two-step visual acuity increase or decrease.

The logarithmic visual acuity chart displays the visual acuity in logMAR, which is the logarithm of the minimal angle of resolution (MAR). Each step of the visual acuity chart is at the same interval, so an arithmetic average is possible to be calculated.

The logarithmic visual acuity chart has five characters per line, and the left and right character spacing is a spacing of the size of each character, and the size ratio of the large character and the lower character is kept at a constant ratio of about 5:4, respectively. The spacing between the upper and lower lines is the size of the character in the lower row. They are arranged at the same interval at which the index is reduced by 0.1 by one logMAR at the top. This visual acuity chart is used at a distance of 4 m. It can measure up to 20/957, that is, 0.02 at 1 m, which is useful for patients with low vision.

Decimal Visual Acuity

This is the method of displaying the reciprocal of the minimum visual angle as a decimal. That is, when the minimum visual angle is 1 minute, it is displayed as 1.0, and the index number (0.1, 0.15, 0.2, etc.) on the visual acuity chart is displayed as it is.

Fractional Visual Acuity

It is widely used in Europe. Write the test distance on the numerator and the test number on the denominator. The character width of the number 20 will have a visual angle of 1 minute at 20 feet, and if you look at it at 20 feet, your visual acuity will be 20/20, and you will have normal vision. The test distance is usually 20 feet or 6 m. 20/40 is equivalent to 0.5, but does not convert the fraction to a decimal. It is an old custom to use fractional visual acuity in Europe.

Algebraic Visual Acuity

In the United States, logMAR is used to display. Decimal visual acuity (VA) is VA = 1/MAR in which logMAR is the logarithm of MAR. In the reciprocal relationship, the absolute value is the same, but – is added so that logVA = −logMAR.

The logarithm is the logarithm of 10, and the decimal visual acuity comes from 10−logMAR. The minimum visual angle of 1 min (1/60°) is 1.0 in decimal visual acuity and 0 in logMAR visual acuity from log 1. Decimal visual acuity, fractional visual acuity, and logMAR conversion are shown in Table 1.1.

Table 1.1

The comparison of distant visual acuity

ETDRS (diabetic retinopathy study) visual acuity chart by Ferris has been used recently. ETDRS (logMAR indication) visual acuity chart is carried out as follows.

Sit at a distance of 1 m or 4 m to shield the left eye. (Corrected visual acuity is measured after correcting refractive error using chart R). Have an eye chart for the right eye (chart 1), one line at a time, from top to bottom. For example, let’s read from left to right and then right to left. Calculate the number of correct answers. In case of low vision, test is performed at 1 m. Place the patient in a 1 m position and insert a +0.75D spherical lens. As in the case of 4 m, calculate the number of character by reading the chart from the top by one line. The visual acuity chart for the left eye (chart 2) is used to perform the same visual acuity test as the right eye (Fig. 1.2).

Fig. 1.2

ETDRS visual acuity chart

Unlike the decimal visual acuity test, if the patient reads four or more characters on one line, the next line is checked. If you only read three or less characters, the patient is stopped on that line. The result is not determined from the line read, but logMAR is calculated from the number of characters the patient reads. The difference between each line in the ETDRS chart is 0.1 logMAR, and there are five characters in each line, so one character corresponds to 0.02 logMAR. Therefore, it is possible to evaluate the visual acuity five times as detail as the conventional visual acuity chart and is also useful for measurement of low vision. At the time of 4 m measurement, it is obtained by subtracting the number of character or the number × 0.02. For example, if five of five characters of +0.4 (decimal visual acuity of 0.4) line can be read and two out of five characters at +0.3 (decimal visual acuity of 0.5) line can be read, logMAR is +0.4 − (0.02 × 2) = +0.36.

For low vision, test can be made at 3.2, 2.5, 2.0, 1.6, 1.3, and 1 m, and the measured values ​​at these distances can be added to the logMAR values at 4 m by adding 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 respectively. For example, when 1 m is measured, it is obtained in the same manner as the 4 m measurement, and 0.6 is added to the value. When logMAR is +1.0 at 1 m, +1.0 + 0.6 = +1.6.

For the results, it is recommended to use a dedicated result sheet. Check the unread character with an X mark, and calculate the logMAR with the number of read marks. It is general to record the visual acuity results numerically, but sometimes the number of characters read is recorded together. The test distance of ETDRS chart is usually 4 m, but one character is added when reading the largest character at 1 m. Therefore, if 4 m is used, add 30 characters because reading at 4 m is the same as reading 30 characters.

1.1.3.3 Standard Visual Acuity Measurement

Type of Visual Target

At the 1909 International Eye Society, the Landolt ring was defined as a standard character. The International Organization for Standardization (ISO) agreements between 1980 and 1981 require the use of the Landolt ring as a standard target (Fig. 1.3).

Fig. 1.3

Snellen E and Landolt ring

The 1.5 mm wide grating of the Landolt ring with a diameter of 7.5 mm and a width of 1.5 mm is made to be 1 minute at a test distance of 5 m. The visual acuity at this time is 1.0. Snellen E is also designed with the same principle. However, the character visual acuity chart is used mainly for the purpose of quick examination of clinical charts. This was determined by a comparative experiment with the Landolt ring, which can be used in semi-standard test equipment.

Illuminance of Visual Acuity Chart

In general, the luminous intensity of the visual acuity chart is 500 ± 125 lux. If the illumination of the visual acuity chart is adjusted to about 500–1000 lux, it is good as a semi-standard chart apparatus.

Test Distance

The distance vision test is performed at a distance of 3–6 m. If a 5 m visual acuity chart is installed, it is accommodated to 0.2 diopter in emmetropic eye. Therefore, myopia of −0.2D is judged on emmetropia. The test distance in foreign countries is 6 m or 20 feet. At this time, the patient faces the chart, and the height of the visual acuity chart is such that the index of 1.0 is the height of the patient’s eye.

Indoor Lighting

Indoor illumination affects vision. For example, if only the visual acuity chart is illuminated and when the surroundings are dark, night vision will occur, and the visual acuity will be decreased, and the influence of the pupil should be considered. The vision test is best measured in its natural state and should therefore be done in bright places in principle. The lighting should be evenly brighter than 50 lux.

1.1.3.4 Measurement of Visual Acuity at 5 m

Measure one eye at a distance of 5 m from the chart. When a target of 0.1 is not visible at a distance of 5 m, the target is approached to the position where the target of 0.1 can be seen. If the distance from the target is X m, the visual acuity is 0.1 × X/5. For example, if you read a 0.1 target at 2 m, your visual acuity will be 0.1 × 2/5 = 0.04. If the target of 0.1 is not readable at a distance of 1 m, the number of the target is set by the examiner’s finger in front of the eye. For example, if the number of fingers is adjusted to 30 cm, the finger is counted as FC (finger count)/30 cm. If you cannot count your fingers, but you can only see the motion of your hand shaking in front of your eyes, then your vision is recorded as hand movement (HM). When the patient do not know the movement of your hand and notices the light at darkroom, you should write light perception (LP). The light is projected to the eye in the up, down, left, and right directions, and it is asked about the projection direction. If you answer this accurately, it is said that light projection is good. When the light direction is clear, it can be assumed that there is no significant change in the retina. If there is no light perception, the visual acuity is written as zero. This is a strict definition of blindness.

1.1.4 Interpretation

The subject’s visual acuity is used as the visual acuity number when reading more than half of the acuity charts. If the number is less than half, it means that the visual acuity is partially shown, but the visual acuity value of p cannot be applied when writing in the paper. According to ISO, if you fail to read more than two, the visual acuity of the preceding step is taken as the visual acuity of the eye (Table 1.2).

Table 1.2

International visual acuity chart

You must check the printability of the target, the brightness of the visual acuity chart, and the standard of indoor illumination. Be careful not to squeeze the patient’s eyes when the patient covers the eyes. If the patient has strong light stimuli, check after about 5 minutes. During the test, it is recommended to keep the patient eyes open so that you do not frown.

When the pupil enlargement is performed, a 3 mm pinhole plate can be used. If the pinhole is used, the depth of focus is deepened to increase the visual acuity. If the pinhole visual acuity is decreased, the corneal opacity, cataract, vitreous opacity, or macular abnormality is suspected.

Children under 8 years old, especially children under 6 years old, are precisely measured with using a stand-alone basis visual acuity chart. When observing the progress of visual acuity change, it should be done under the same conditions as before.

1.1.5 Record of Visual Acuity

Uncorrected visual acuity refers to visual acuity without wearing glasses or contact lenses, but radial keratotomy, PRK (photorefractive keratectomy), laser in situ keratomileusis (LASIK) and laser epithelial keratomileusis (LASEK), pseudophakia with intraocular lens implantation, or phakic IOL insertion should be recorded.

The right eye is recorded as VOD, RV, or vd, and the left eye is recorded as VOS, LV, or vs. Both eyes are recorded as OU.

VOD: 0.8 × IOL (1.2 × −1.50 D): 0.8 with pseudophakia and 1.2 with adding glasses

VOS: 0.5 × LASIK (1.0 × −0.5 D): 0.5 with LASIK surgery and 1.0 with adding glasses

Contact lenses, RK, PRK, LASEK, and phakic IOL are also recorded in the same manner. Vcc or VAcc is used for recording the corrected visual acuity, and VAsc is used for recording the uncorrected visual acuity.

1.1.6 Pinhole (PH) Visual Acuity

It is used for the purpose of determining whether vision loss is correctable by glasses. Focal depth is deepened and retina image is cleared by using pinhole. If the patient’s visual acuity improves more than two lines, there is a high possibility of refractive error. When using glasses and having visual acuity of 20/30 or less, the pinhole visual acuity test method should be performed and is as follows. One eye is covered, and a pinhole plate with a diameter of less than 2.4 mm is placed in front of the other eye to measure the distant visual acuity. Record the visual acuity of pinhole after corrected distant visual acuity. For example:

1.2 Near Visual Acuity

1.2.1 Indications and Purposes

1.2.1.1 Accommodation Disturbance due to Presbyopia

Near visual acuity should be tested for the purpose of measuring the degree of near vision disturbace, determining whether the near vision glasses are suitable, and prescribing the lens power suitable for the near vision glasses.

1.2.1.2 Hyperopia

In case of latent hyperopia having good distant visual acuity, it is easy to think that prescription of glasses is unnecessary. However, the patient needs great accommodation effort for near target and does not like reading or complains of asthenopia. In the case of mild hyperopia, the spectacle lens can cause near visual impairment when prescribing only for the distant vision. It is wise to decide the lens power that focuses on near vision even if it is a little overcorrected at a distance.

1.2.1.3 Patients in Bed

If the patient is unable to perform a distant visual acuity test, the visual acuity can be roughly determined by measuring the near visual acuity in a lying state.

1.2.1.4 Amblyopia

According to the Moore-Johnson method or the near penalization method, atropine is instilled in the normal eye, and the near vision is made into the amblyopia to optically shield the normal eye at near vision. At this time, check that the degree of near vision in the normal eye is suppressed enough and worse than the near visual acuity of amblyopic eye.

1.2.1.5 Assessment of Visual Acuity for visually handicapped

It is based on the reading and writing ability, and it is the evidence data to choose the public school, the school for amblyopia, or the school for blindness.

1.2.2 Method

Using a near vision chart, measure visual acuity at the distance of 30 cm. The bright indoor level is enough to be tested, and the illuminance should be 400–800 lux. If the largest target cannot be read, the test distance should be shorter. For example, if a target of 0.1 is read at 15 cm, the visual acuity becomes 0.1 × 15/30 = 0.05, which is not a near visual acuity of the original meaning but a visual acuity indicating at least how many characters can be read. At this time, the record should be written in 0.1 (15 cm). A large target of 0.1 or less should be prepared as the maximum target.

1.2.3 Interpretation

When reading 3/4 or 4/5 or more characters, it is regarded as a visual acuity (Table 1.3).

Table 1.3

The comparison of near visual acuity

It is important to strictly set the test distance to 30 cm, and it is necessary to define the distance between the eyes and the target in a ruler or a thread. The chart made in Europe and United States should be careful because the test distance is set to 1/3 m (33 cm). Generally, the visual acuity chart printed on ordinary paper has a deterioration of contrast sensitivity due to discoloration due to years, so it is necessary to change it to a new one every 3 years. The standard should have a contrast sensitivity of 85% or more.

1.3 Contrast Sensitivity Test

An object does not have the same brightness. This change of brightness is expressed as contrast. Contrast sensitivity refers to how to distinguish objects with different contrasts from one another. For example, there is a strong contrast between the face and hair, but there is less contrast between the nose and mouth. The visual acuity is related to the sensitivity of the retina. The amount of light and the background play a more important role than the size of the object. That is, you can see stars in the middle of the night, but you cannot see them during the day.

Snellen acuity has 100% contrast sensitivity and is completely black on white paper. Visual acuity measures the ability of the eye to resolve the fine objects, but does not adequately represent the ability to see objects with low contrast. In visual abnormalities such as cerebral abnormalities, optic neuritis associated with multiple sclerosis, glaucoma, diabetic retinopathy, and amblyopia, visual acuity is close to normal, but contrast sensitivity decreases. For example, the VCTS, CSV-1000, Regan letter chart, Vistest picture test, and Pelli-Robson letter sensitivity chart are examples of devices that can test the spatial frequency.

1.3.1 Indication and Purpose

The purpose is to find out the causes of visual deterioration that cannot be found in visual acuity test. When the contrast sensitivity of a normal person is measured, the contrast becomes higher as the spatial frequency increases, highest as the spatial frequency reaches the middle, and lower as the spatial frequency increases again. This spatial frequency is established under the optical system of the eye, the intraretinal processing system, the neurotransmission system, and the information processing system of the visual cortex, so the specific pattern of contrast sensitivity represents the area where the defect happened. For example, a decrease in the contrast sensitivity in the high frequency means a decrease in the visual acuity itself, and a decrease in the maximum height means a decrease in the retinal inhibitory effect. If the visual acuity and fundus examination are both normal and the contrast sensitivity of low frequency is decreased, a lesion that gives pressure on visual pathway such as optic neuritis, multiple sclerosis, or Parkinson’s disease can be considered. However, spatial frequency does not have diagnostic value to accurately determine the area of ​​the defect.

1.3.2 Method

The VCTS has a distance (3 m) and near test charts which have approximately 1.5° size of the circle with 8 horizontally, 5 vertically, and 5 blank vertical targets and a total of 45 targets. The target is arranged such that the contrast is decreased from left to right or the spatial frequency is increased from the top to the bottom.

High-contrast examples are shown for the purpose of indicating that there are three kinds of stripes with vertical, 10° right angled and 10° left angled (Fig. 1.4).

Fig. 1.4

VCTS chart

There is a set containing one check sheet for each of distant and near and a set containing three sheets for each. In the 3-sheet set, the arrangement of the patterns that can be placed on the 40 marks of each test chart is changed so that the patient cannot memorize the direction of the target. Show the patient a sample of the mark on the bottom of the checklist, and make a full understanding what the check is. Begin the examination from the top most target with the smallest spatial frequency. First, ask the patient whether the direction of the stripe is vertical or whether it is tilted slightly to the left or right. If the answer is correct, ask the direction of the target with low contrast on the right side, and repeat this until the patient cannot answer. Perform this procedure for the next four groups. Write the contrast sensitivity at each spatial frequency on the recording paper (Fig. 1.5).

Fig. 1.5

VCTS recording paper

1.3.3 Interpretation

The normal value displayed on the attached paper is compared with the recorded result to determine whether the contrast sensitivity of low frequency area, the highest value area, or high frequency area is decreased.

1.4 Glare Test

1.4.1 Indication

It is necessary to determine the surgical indication of a case with media opacity such as cataract, after cataract, or corneal opacity or to observe after the corneal refractive surgery.

1.4.2 Purpose

Glare is the decrease of contrast sensitivity on the retinal image due to scattering of light in the eye. When there is opacity, the light is sometimes scattered when the light passes, so the patient sometimes complains of photopia, and the visual acuity in the strong and bright light is worse than the visual acuity measured by the normal visual acuity test.

The glare test measures the degree of glare that cannot be checked by conventional visual acuity test, and it is the main purpose to determine the indication of cataract surgery. This is the example of the test using the Miller-Nadler glare tester.

1.4.3 Method

The Miller-Nadler glare tester is a tabletop-shaped instrument that uses a projector to illuminate the slides of the charts attached to the background (Fig. 1.6). The target is a Landolt ring of 20/400 and is displayed in a 4 cm diameter circular background. It consists of 19 slides with a different contrast between the target and the background from 80% (No. 1 slide) to 2.5% (No. 19 slide). The target is displayed in the center of the square screen of the background light of 3200 lumens.

Fig. 1.6

Miller-Nadler glare tester

Fix the patient’s head at a distance of 36 cm from the projection plane. After correcting the refractive error so that the patient can see well at this distance, the target slides attached to the background are projected onto the light source from the No. 1 slide. Repeat the above procedure by sequentially presenting slides with different contrast between the target and the background. Obtain a glare disorder from the contrast difference of the final slide that was readable.

1.4.4 Interpretation

The lower contrast slide (higher number slide) is readable, the less glare disorder is. The glare disorder is expressed by the contrast value (%). For example, if you read contrast value 80%, the glare disorder is expressed as 80%. If the screen illuminating the slide becomes dirty, the contrast of the target will change, which can affect the test results.

1.5 Pediatric Visual Acuity

1.5.1 Indication and Purpose

Pediatric patients can be tested with using Landolt ring from the age of 3 years old. In recent years, screenings have been conducted in kindergartens, so that visual development disorder in children can be detected in early stage.

The characteristics of visual acuity in children is better visual acuity because of using the Landolt ring instead of the character, the single object chart instead of parallel object chart, and the near distance instead of 5 m distance. There is also a problem that it is difficult to interpret the result only by a single test.

Children cannot express their thoughts unlike adults, so the cognitive ability of children can be tested through physical tests or electrocephalogram. There are a number of tests that can be used to determine the development of visual acuity in children.

1.5.2 Method

Under 3 years old, it is possible to evaluate the visual acuity through qualitative tests which include the reponse of the child, gaze, pursuit movement, eyelid reflex and optokinetic nystagmus (OKN), and quantitative tests which include OKN, visual evoked potential, preferential looking test, and grating acuity card. Landolt ring single object visual acuity test can be used for 3- to 7-year-old children, and Landolt ring parallel visual acuity test for children over 8 years old.

1.5.2.1 Response of Child

When a child is suspected to have a visual disturbance, attention should be paid the child’s expression, attitude, or behavior. Check whether the child is looking the eyes or the head by moving the milk bottle or the red target of the toys. Although there is a device such as Dayton that uses nystagmus, OKN drum (Fig. 1.7a) or cloth pocket strip (Fig. 1.7b) is widely used. If there is no response or no eye contact or no response, check the eyelid blinking motion or the light response by illuminating the pupil with a pen light or strong light coming from the side.

Fig. 1.7

The equipment of measuring optokinetic nystagmus (OKN). (a) OKN inducing shaking drum. (b) OKN inducing cloth pocket strip

1.5.2.2 Optokinetic Nystagmus OKN

It is a qualitative test that records the phenomenon of nystagmus. This can record visual movement response even in children who are not sure to watch or respond. Rotate the belt with various frequency bands (stripes) to cause nystagmus, and record the highest frequency band as a visual acuity.

1.5.2.3 Visual Evoked Potential

It is a method to see the reaction of the electroencephalogram by the stimulus coming from the eye by putting the electrode on the back part of the head of the child, that is, the part where the visual cortex is located. The visual stimulus uses western chessboard patterns, stripes, or stereoscopic signs. When the pattern is seen, the brain wave is generated, and the size, thickness, or contrast of the pattern becomes smaller. When the child cannot see, the brain wave is not generated. Therefore, the visual acuity and stereoscopic vision of the child are indirectly measured (see Chapter Electrophysiologic Test).

1.5.2.4 Preferential Looking (PL) Test

In 1962, Frantz performed the first visual acuity test for child, which used child’s interest in striped pictures. The device is a rear-projecting slide Awaya-Mohindra visual acuity test with two translucent circular windows with a diameter of 11° in the center of the screen, at a distance of 50 cm, projecting a stripe pattern and a uniform image from the back to the projector. The width of the stripe pattern varies from 30.0 to 0.25 mm, the illuminance is about 40 cd/m², and the contrast sensitivity is about 85–95%.

There are two methods, forced-choice and pointing. The forced-choice method shows a black-and-white stripe pattern and a grayish-white uniform color to the child. The examiner looks into the hole in the middle of the two circular targets. This test is simpler than OKN and VEP and is for infant aged between 2–3 months and younger than 1 year.

If the child is able to talk, the examiner first shows a striped pattern and then displays a pattern less pattern. Make a child point in the direction of the screen by pointing the finger on the screen.

If the child is pointing at the target, the examiner presents the target with the same width once again. If the target is wrong, return the target with bigger size. If the correct rate is over 75%, the target value is regarded as a child’s PL visual acuity. If it is fully skilled, it takes about 5–10 minutes. Using the forced-choice method and pointing method, it is possible to measure the visual acuity from 2 to 3 months infant to 3-year-old child.

1.5.2.5 Grating Acuity Card

In principle, it is the same as the preferential looking test. It is simpler and it is possible to perform visual acuity test for premature infants and newborn babies. Teller acuity cards (TAC), which are available from Vistech, are commonly used as test cards (Fig. 1.8).

Fig. 1.8

Preference looking test using TAC

This shows the visual acuity card at a distance of 38 cm and judges which side the patient is looking at by eye movement. The narrowest stripe target is regarded as a visual acuity. After confirming whether the child is staring at the card, the examiner can move the card about 15 cm from