VISUAL ACUITY MEASUREMENT IN

PEDIATRICS

Hira Nath Dahal

PRESENTATION LAYOUT

• Introduction to Visual acuity and its types

• Visual development

• Normal visual response

• Different methods of visual acuity assessment

• Visual acuity assessment in• Non & Preverbal children• Verbal children

• Conclusion

VISUAL ACUITY

• measure of the spatial resolution of the visual processing system

• Expresses the angular size of the detail that can just be resolved by the observer

• Visual acuity testing is a clinical procedure to access the ability of an individual to discriminate detail and distinguish form.

• Four types of Visual Acuity

• Detection acuity: estimates the minimum size visible i.e. the actual presence or absence of target is determined e.g. Catford drum, stycar balls

• Resolution acuity: minimum separation which allows discrimination i.e. the minimum separation detected between the elements of gratings or checkerboard is determined e.g. preferential looking charts, Lea gratings

• Recognition acuity: the minimum size, which facilitates identification i.e. the letters or pictures or orientation of symbols are identified e.g. letters or pictures

• Localization acuity (Vernier Acuity): estimates ability to distinguish when two lines placed end-to-end is displaced laterally i.e. the differences in spatial position of a test target is determined

• Normal development of visual function starts with the detection acuity acquired at birth

undergoes a rapid development into higher acuity function such as resolution, developing later on.

• There are many techniques of visual acuity assessment available to quantify the different level of visual function

• Each visual acuity function requires different level of cortical functioning, therefore may result in different acuity measurements for given individual, if measured by different techniques.

• In a child, level of recognition, resolution and detection acuities differ; with recognition acuity measured lower than resolution, which is still lower than detection acuity.

Detection acuity of same value is not equal to other forms of acuities; similarly the same value of resolution acuity is not equal to the recognition acuity.

WHY TO ASSESS VISUAL ACUITY ??

• Normal acuity in infants and young children can help the clinician r/o a no. of disorders, including significant refractive error, amblyopia, and ocular disease.

• VA is routinely utilized to initially set the direction for rest of the examination. For e.g. a unilateral reduction in visual acuity should alert a clinician for different problems including optical disturbances, optic nerve or chiasmal lesion, optic neuropathy, macular disorders or functional amblyopia.

• To know if visual development is normal.

• Helps to decide eligibility for low vision and rehabilitation services.

Development and

Maturation of Vision

In order for visual system to develop normally, several components are required such as

• Normal anatomical structures

• Two eyes must be positioned correctly and have clear media.

• Neurological connections of visual pathway to visual cortex must also be functional.

• Visual acuity improves rapidly during the first year of life and then matures more gradually to adult levels at approximately 5-6 years of age.

• Structural development is largely completed by 2-3 yrs. of life but functional changes continues throughout life.

• Although the central cones function by birth, acuity as measured by the different techniques does not approach 20/20 (6/6) until from 6 to 30months (depending upon the examination technique used).

• Reasons for this delay include the incomplete • development and specialization of

photoreceptors,• immaturation of synapses in the inner retinal

layers,• Incomplete myelination of the upper visual

pathways.

• Foveal cones do not attain adult appearance until 4 months after term birth, and visual pathway myelination continues until 2 years of age

VISUAL MILESTONES :

• Very soon after birth - Can fix and follow a light

source, face or large, colourful toy.

• 1 months - Fixation is central, steady and

maintained, can follow a slow target, and converge,

preference of looking at face.

• 3 months - binocular vision and eye coordination,

eyes follow a moving light or face, responsive smile.

• 6 months - Reaches out accurately for toys.

• 9 months – look for hidden toys.

• 2 years - Picture matching

• 3 years - Letter matching of single letters (e.g., Sheridan Gardiner)

• 5 years - Snellen chart by matching or naming

Age Reflex

Birth Blinking (to light stimulus)

1 week Vestibulo-ocular

2 weeks Small saccades

2 months Large saccades, pursuit, bifoveal fixation, convergence

3 months Uniocular fixation

4 months Fusional vergence, sensory fusion, stereopsis

6 months Accomodation

Other function to correlate with vision

DEVELOPMENT AND MATURATION OF VISUAL ACUITY :

NORMAL VISUAL DEVELOPMENT• Pupillary light reaction : 30 weeks gestation

• Blink response to visual stimuli : 2-5 months

• Fixation - 2 months

• Smooth pursuit : 6-8 weeks

• Saccades : 1-3 months

• Optokinetic nystagmus(OKN): Developed by 2-4 month

• Ocular Alignment : 1 month

• Foveal maturation : 4 months

• Optic nerve myelination : 7 months to 2 yrs

• The vision of new born is quite blurry, indistinct and shadowy, which improves significantly over the next 6 months, by that time a child recognize face of the mother, which further improves in another 6 months period

NORMAL VISUAL RESPONSEAge Visual response

Newborn Light perception

4-7 weeks Eye contact with mother

4-12 weeks Fixates and follows interesting bright coloured objects

3 months Change expression smiles and cries

3-4 months Reach objects using vision

6-9 months Crawling and later walking avoiding objects

Gwiazda et al 1980

VISUAL ACUITY DEVELOPMENT IN HUMAN

• Develops & matures from birth to 6 years

• Newborn: quite a blurry vision : Indistinct & Shadowy

• Improves significantly over the next 6 months: Can recognize the faces • At birth VA = 6/240 (20/800) • 1 month VA = 6/120 (20/400)• 6 months VA = 6/30 (20/200)• 1 year VA = 6/15 (20/50) (Gwiazda et al 1980)

• Marg et al 1976 estimated the resolution acuity

VA of infants by Visually evoked potential (VEP) and preferentially looking chart as follows:

With VEP the measured VA was: at 3 months VA = 6/18 (20/60)

by the end of 6 months VA = 6/6 (20/20) With Preferential looking chart, an

accurate response by a normal child as like an adult is possible by the age of 1 year.

• The different available techniques for the assessment of visual acuity function are:

1. Detection acuity: It can be • Non quantifiable or• Quantifiable by eliciting voluntary visual

response• Quantifiable by eliciting involuntary visual

response

NON-QUANTIFIABLE DETECTION ACUITY

It is difficult to quantify the visual acuity of children who are non-verbal under 1 year of age.

They don't show visual response to any object or pattern, eye care practitioner cannot quantify the VA accurately, so we should closely explore the visual responses expected at that age by using various techniques appropriate

LIGHT, FIXATION AND FORM PERCEPTION EVALUATION

A. Illumination: A child of 4-7 weeks has expected normal VA of light perception. So a light stimulus (torch light, Ophthalmoscope light) can be used to elicit the fixation reflex, facial or postural changes or an enhanced level of alertness when lighting is altered.

Children who don’t respond to simple illumination, visual stimuli may be demonstrated by colorful flickering lights (most preferably red light which is most effective)

B. Blink reflex: A hand colorful object is placed in front of the child face of age 4-7 weeks, a consistent response, such as blink reflex, closing eyes or avoidance behavior is a sign of visual discrimination using form perception.

C. Bright coloured lights or objects: A child of 4-12 weeks can follow and fixate the bright coloured objects(toys) or light. Where as a child of 3-4 months will reach for the bright toys using his hands.

D. Pupillary response: the presence of pupillary contraction to light and near object indicates a visual response. But this can give false impression in the case of cortical blindness, where normal pupillary response is obtained.

E. 10 PD fixation test:

This test is an indirect technique to assess possible difference in acuity between two eyes in infants and preverbal children.

A 10 PD vertical prism is placed in front of one eye, if both eyes have equal visual acuity; the child will be switching the fixation back and forth from right to the left eye. But if one eye only takes the fixation all the time, the non-fixating eye indicates of having poor vision

QUANTIFIABLE DETECTION ACUITY BY ELICITING VOLUNTARY VISUAL RESPONSE

• Children who cannot response verbally but active enough to response visually to the three dimensional acuity targets (1 to 3 years of age) are subjected to detection acuity measurement by presenting interesting three dimensions targets which elicit voluntary visual response.

• Some of the techniques are:• Ivory balls • Use of candy beads

IVORY BALLS

Uses white Styrofoam spheres of various diameters that are rolled perpendicular to the patient’s line of sight individually along the black cloth strip.

The speed, distance and the side of roll is varied and a record is made of the smallest sphere that the child follows at a set of distance and corresponding acuity levels is calculated

USE OF CANDY BEADS

Children of less than 3 years of age need more effective motivator to co-operate to the practitioner during the visual acuity assessment. Chocolate coated candy beads can be the best

possible option.

Candy bead is placed in one hands of the practitioner presenting both hands to the child. The visual response is determined by observing the fixation pattern, gesture or child’s hand reaching for the candy.

This test is a gross acuity assessment technique because it does not arrive to the threshold acuity value. Approximately, 1mm bead located at 33cm represents detection acuity of 6/60 (20/200). Also, it provides valuable information to the practitioner that the child is able to see at a normal working distance.

QUANTIFIABLE DETECTION ACUITY BY ELICITING INVOLUNTARY VISUAL

RESPONSE

• Visual acuity in non-verbal and preverbal children who don't response voluntarily can be obtained by using techniques which elicit involuntary visual responses. Few techniques are:

a. Optokinetic Nystagmus (OKN)a. Catford drumb. OKN drum

b. Visual Evoked Potential

OPTOKINETIC NYSTAGMUS

• OKN can be elicited at birth but has poor directed saccade, which can be reasonably accurate by the age of 3 months.

• OKN is a series of repetitive eye movements, consisting of a slow pursuit phase, during which a moving target is smoothly tracked, followed by the fast saccade, allowing refixation when eye meets its limit of movement in the direction of the pursuit.

• Theoretically if an individual can visually discriminate the series of bars movement across the visual field, as the target gratings rotate, OKN is observed. Clinically, a Catford drum or an OKN drum is available to measure visual acuity

CATFORD DRUMPrinciple

• The Catford drum uses oscillating black dots on a white ground, and is based on the principle that a child’s attention is drawn to a moving target

• The observer can use the corresponding oscillatory eye movement as confirmation that the child sees the target. The oscillatory movement is generated by the pursuit system.

• This consists of a white drum marked around its circumference with black dots corresponding in size to Snellen letters if viewed from a distance of 60 cm. the size of dots ranges from 6/60 to 6/6. each dot is displayed singly in the rectangular aperture of the screen which covers the other dots.

• Once the child’s cooperation has been assessed, the drum is held at a distance of 60 cm, one eye is covered and dots of decreasing size are exposed in the aperture until the minimal visible has been estimated. The other eye is then tested

• It has been shown to overestimate visual acuity by the factor of four (Atkinson et al 1981)

OKN DRUM

Principle• Strips which move across

the fields of vision elicit an observable eye movement, comprising a following movement as the subject fixates on one stripe(pursuit) and a rapid movement in the opposite direction to fixate the next stripe(saccade).

• The stripes are best presented as optical gratings using black and white stripes of equal width. To hold the infants attention a large part of his field must be filled by the stripes

• Stripes can be presented in various ways: rotating drum or electronically generated stripes displayed on a television screen

• An eye movement reponse indicates that a grating can be seen, the movement can be directly observed or more accurately, electro-oculography can be used to trace and record eye movement

Method:

• The stripes should be vertically positioned and moved horizontally at both slow (3 stripes/s) and fast (30 stripes/s) speeds. The slower rate stimulates the smooth pursuit and saccadic systems, while the faster speed appears to involve a more primitive optokinetic system

• Stripes of increasing spatial frequency are presented until a frequency is reached which fails to elicit an optokinetic movement. The highest spatial frequency which produces OKN at a slow speed, is the measure of visual acuity

VISUAL EVOKED POTENTIAL

• It is an electro diagnostic test that provides the fast, objective assessment of visual function and visual acuity of infant and young children.

• Stimuli, such as gratings or flickering checkerboard is presented before the child, an the occipital cortical response in the form of change in electrical activity to visual response is detected by placing electrodes on the scalp surface, overlying the occipital cortex.

• The cortical electrical responses are analysed in the form of wave front and amplitude by a computer system estimating the visual acuity. The VEP mostly measures the macular function but doesn't estimate higher visual function. So, there exists difference between estimated visual acuity and overall visual functioning of the child.

2. RESOLUTION ACUITIES

• Resolution acuity is measured by using the preferential looking (PL) technique in the form of Teller, Keller cards or Lea grating cards.

• This technique determines the objective information of visual acuity in nonverbal children. This is based on the research that infants, when simultaneously presented with a patterned stimulus and a homogenous field will preferentially view the pattern stimulus.

• It involves the presentation of the two stimuli, one black and white gratings pattern and the other unpatterned grey field of equal size and luminance

• The child preference is observed by viewing the pattern and length of fixation while subsequent presenting the cards with higher grating frequency, till the child show no preference

• At this time, the child is unable to discriminate the black and white pattern and the resolution acuity is noted in the form of cycle per degree of the highest frequency that was preferred by the child

TELLER ACUITY CARDS

• A set of 16 cards is available containing a uniform grey background on one side and other side, containing a square on the right or left side of which is printed a square-wave grating of known spatial frequency. The card is presented to the patient through a rectangular grating.

PROCEDURE

• The child is held in front of a grey cardboard screen that shields his or her view of the room . For infants the 38cm testing distance is recommended, whereas the 55cm distance is recommended for the toddlers

• To test the acuity, the examiner displays a series of cards, each containing a black and white grating of different spatial frequency, located to the left or right of the central peephole

• It is best to begin with the stripe width that is wider than the threshold predicted for the age of the infants. If it is clear that infants see the stripes, the examiner presents the card with the next finest stripe width.

CONTD…

• Testing continues until the examiner is confident enough about he child’s responses to make a judgement concerning the finest grating that the child can detect.

• The spatial frequency of this grating is taken as an estimate of that child’s visual acuity

• The results can be expressed in octaves or converted to equivalent Snellen values, min of arc or cycles per degree

• The testing time ranges from 6-10 min

• To eliminate the possibility of a side preference for a particular child, the examiner should position the cards so that the side with the stripes varies from right to left on a random fashion

KEELER CARDS

• These cards are printed with a circular patch to avoid identification of the grating by its edge

• They also have an ‘empty’ circle printed on the other side: this leads to a different visual response whereby the infant may look from one circle to the other before a definite fixation preference is made

Clinical problems arise if there is :

• Nystagmus, which may make it difficult to assess when the patient is looking towards the grating, especially if there is a compensatory head posture. Testing in the vertical plane can be helpful in this situation

• Large angle alternate esotropia with crossed fixation, when it can be difficult to know to which side the child is looking when both eyes are open

• Loss of interest and fatigue

LEA GRATINGS CARDS

• The test can be used at different distances and with two different presentation techniques:

1. By lifting the grey and the striped stimulus simultaneously in front of the child and keeping them there without moving them.

2. By hiding the striped pattern behind the grey surface and sliding the two surfaces apart with the same speed in opposite directions.

• Normally the child will follow the movement of the striped pattern if (s)he sees it. If the child has problems in seeing visual information in motion there will be no following movement

The result is reported as “responded to ___ cpcm grating at a distance of ___ cm/inches”.

3. RECOGNITION ACUITY

• Recognition acuity can be attempted to a child of 3 years or more.

• The standard subjective visual acuity testing requires verbal communication, sustained attention and concentration. So, cannot be used with non-verbal and preverbal children and is difficult to use with preschool children

• Even if the child doesn't give verbal communication, matching activities can elicit useful information about recognition acuity.

• Standard Snellen acuity chart is applicable to the children of age 6 or more, but for the younger children various two dimensional symbols, matching a puzzles are used in shorter testing distance (usually 3m ) to assess the recognition acuity

BROKEN WHEEL TEST

• Uses a clinical approach of testing for visual acuity by incorporating the Landolt C.

• The Broken Wheel Acuity Test utilizes cards that have a familiar, non-threatening symbol (car), presented in a forced choice response. The simple recognition of the gap in a Landolt Ring is the critical feature. 

• Testing distance: 3mThe acuity level in this test has been established to be equivalent to the Snellen Letter optotype

PRE SCHOOL (ALLEN) PICTURE TEST

The Allen chart includes easily recognized pictures, including a cake, hand, bird, horse, and telephone.

• This test is recorded in terms of a 30-foot denominator. It is intended for preschool children and has given reliable results from the age of two years and up.

• Method: Pictures are shown to the seated child at close range with both eyes open and the child is asked to give a name to each picture. One eye is then covered and the examiner presents the pictures in sequence while backing away from the child. The greatest distance at which three of the pictures are consistently recognized by each eye is then recorded as the numerator of a 30 foot denominator .

• For e.g.• Right eye maximum distance =15 feet / VA: 15/30• Left eye maximum distance= 10 feet / VA: 10/30

KAY PICTURE TEST

• The Kay Picture Test books are all designed to make testing young children a fun, quick and easy process.

• Can be used quickly, easily and accurately from as young as 18 months

• All the acuity sizes are together in one book and there is a choice of three or four pictures at each acuity size. This variety keeps a child interested during the test and allows a different selection to be shown when testing each eye in turn.

TESTING WITH KAY PICTURE TEST

• First, child is asked to name each picture and accept what they say, repeating all plausible names back as confirmation. “ cup of tea, fishy, house, welly” etc.

• then move to the correct testing distance (3 metres or 10 feet).

Don’t forget to tell the child how clever they are to know all those pictures, then say something like “we are going to play a game to see how clever your eyes are at seeing all the tiny little pictures in my book”

• Single Kay Picture Test

• Crowded Kay Picture Test

Repeated with smaller sizes until child’s threshold acuity is reached. At this point child is asked to name all the pictures at that acuity level plus one size above and below if possible.

TUMBLING E

• For children who may be unable to perform vision testing by letters and numbers, the tumbling E test may be used

• A chart consists of letter E in different orientations (up, down, right and left) and sizes. Children are tested by asking what orientation or direction the letter E is in at each letter size.

STYCAR LETTERS

• The letters, which are based on square, circle and triangular shapes – the first to be recognized and copied by young children are presented in three groups.• Five letters, VTOHX, for normal up to and including 3

year olds• Seven letters, adding A and U, for 4 year old children• Nine letters, and L and C, for older children

SHERIDAN GARDINER CHART

• This test uses the seven letters STYCAR test and key card. The letters can be viewed singly, using flip-over cards which range from 6/60 to 6/3

• The test is easily understood by normal 3 year olds and by some intelligent younger children

• The number of letters is sufficient to eliminate guessing and the test is quick and accurate

HOTV• This test is similar to Sheridan-Gardiner but uses only

four letters

• This test consists of a wall chart composed only of Hs, Os, Ts, and Vs. The child is provided a board containing a large H, O, T, and V.

• The examiner points to a letter on the wall chart, and the child points to (matches) the correct letter on the testing board. This can be especially useful in the 3-to 5-year-old who is unfamiliar with the alphabet.

F FOOK’S TEST

• This test uses the basic shapes of a square, circle and triangle presented singly in sizes ranging from 6/60 to 6/6, one on each face of a cube, or as a chart

• The child performs the test by picking up or pointing to a black plastic replica of the shape he sees.

LEA SYMBOL

• This test consists of four optotypes (test symbols): the outlines of an apple, a pentagon, a square, and a circle. Because these four symbols can be named and easily identified as everyday, concrete objects ("apple", "house", "window", and "ring"), they can be recognized at an earlier age than abstract letters or numbers can be

• The Lea Symbols Chart consists of lines of four different symbols, arranged in combinations of five symbols per line. The symbols on each line are smaller than those on the line above.

• The child views at distant chart and matches symbols of different size presented on it to similar symbols on a key cards.

VISUAL ACUITY ASSESSMENT

IN NON & PRE VERBAL

CHILDREN

1. Tests for indirect assessment of vision

a) Historical and observational tests,

b) Binocular fixation preference and fixation targets,

c) CSM method.

2. Tests for recognition acuity :

a) Dot visual acuity,

b) Coin test

c) Miniature toy test

d) Marble game test

e) Sheridans ball test

f) Bock’s candy test (100’s and 1000’s test)

g) Worth ivory ball test

3) Tests for resolution acuity :

a) Optokinetic nystagmus,

b) Preferential looking test,

c) Cardiff acuity cards,

d) Visual evoked potentials

TESTS FOR INDIRECT ASSESSMENT OF VISION.

• Historical and observational tests

• Binocular fixation preference and fixation targets

• CSM method

HISTORICAL AND OBSERVATIONAL TECHNIQUES :

• Parents or caretakers are asked routinely whether the child responds to a silent smile, enjoys silent mobiles, and follows objects around the environment.

• Pertinent observations include strabismus, nystagmus, persistent staring, and inattention to objects

• For example, when a unilateral, constant strabismus is present, visual acuity is presumed to be reduced in the strabismic eye.

• In the presence of a constant, alternating strabismus, visual acuity is likely to be normal in both eyes.

• Another behaviour that is unique to babies is “eye popping”. Sometimes, for a variety of reasons, very young infants don't show any distinguishable visual behaviour at all. In this case, the eye popping reflex indicates at least the infant’s ability to detect changes in room illumination.

• When the room lights are suddenly dimmed, the baby's upper eye lids should pop open wide for a moment. The baby will often close its eyes when the lights are brought back up, but will again pop its eyes open when the lights are dimmed. This behaviour is documented as "positive eye popping".

FIXATION TARGETS (FIX AND FOLLOW) :

• If appropriate targets are used, this reflex can be demonstrated by about 6 weeks of age.

• The test is performed by seating the child comfortably in the caretaker's lap. The object of visual interest, usually a bright-coloured toy, is slowly moved to the right and to the left. The examiner observes whether the infant's eyes turn toward the object and follow its movements (fix and follow behaviour) . The examiner can use a thumb to occlude one of the infant's eyes in order to test each eye separately.

• If the child has a f/f behaviour then it is assumed that the patient could see a small target or toy in a normally illuminated room.

Binocular fixation preference :

Behavioural evidence of decreased vision in right eye. (A) A small toy is used to get the child’s attention, and the examiner covers the right eye to monitor fixation of the left eye. The child fixates on the toy without objecting. (B) When the left eye is covered, the child objects and tries to move the examiner’s hand. (C) When the right eye is covered, the child does not object and tracks the object.

Some children object to having either eye covered, simply because they do not like having the examiner’s hand near their face. If this is the case, this test cannot accurately determine whether there is a difference in vision between the eyes.

CSM METHOD :

• It is done with an eye fixating on an accommodative target held at 40cm

• ‘C’ refers to the location of corneal light reflex as the patient fixates the examiner’s light under monocular conditions. Normally reflected light from cornea in near the centre of the cornea and it should be positioned symmetrically in both eyes. If fixation target is viewed eccentrically, fixation is termed uncentral.

• ‘S’ refers to steadiness of fixation on examiners light as it is held motionless and also as it is slowly moved about.

• ‘M’ refers to the ability of the patient to maintain alignment first with one eye, then with the other, as the opposite eye in uncovered. Maintenance of fixation is evaluation under binocular conditions. Inability to maintain fixation with either eye, with opposite eye uncovered is presumptive evidence of a difference in acuity between the two eyes.

Evaluation :

• CSM – 6/9 – 6/6

• CSNM –6/36 – 6/60

• Unsteady central fixation < 6/60

TESTS FOR RECOGNITION ACUITY

Dot visual acuity

Coin test

Miniature toy test

Marble game test

Worth ivory ball test

Bock’s candy test

Kay pictures

LEA symbols

F-fooks symbols

Sheridan Gardiner single letter optotypes

• Dot visual acuity test : child is shown an illuminated box with black dots of different sizes printed on it. The smallest dot identified denotes the visual acuity of the child.

• Coin test : Child is asked to identify two faces of coins of different size held at different distance.

• Miniature toy test :Child is shown a miniature toy from a distance of 10 feet and asked to name / pick the pair from assortment.

• Marble game test : The child is asked to place marbles in holes of a card or in a box. It compares the functioning of the child’s eye when one or the other is closed and vision is noted as useful or less useful.

• Worth Ivory ball tests : Ivory balls 0.5 to 2.5" in diameter are rolled on the floor in front of the child and he is asked to retrieve each. Acuity is estimated on the basis of smallest size for the test distance.

• Bock’s candy bead test : Snellen equivalent of 6/60 is estimated by this method. The child is asked to match pick up beads 1mm size at 40 cm.

Examples of recognition acuity. A. Kay pictures B. LEA symbols.

Tests for resolution acuity

Optokinetic nystagmus

Preferential looking test

Cardiff acuity cards

Visual evoked potentials

OPTICOKINETIC NYSTAGMUS :

• Evaluation of the presence or absence of optokinetic nystagmus was the first “technologic” approach to acuity measurement in preverbal children.

FORCED CHOICE PREFERENTIAL LOOKING :

• The FPL technique was conceived by David Teller.

• This testing technique is based on the observation that infants demonstrate a greater tendency to fix a pattern stimulus than a homogeneous field.

• They measure resolution acuity, using either a grating target as with the Teller cards or the vanishing optotype principle, as with the more recently Developed Cardiff Acuity Cards.

• Preferential looking involves showing the infant two stimuli, a grating composed of black and white stripes (or other quantitated patterns), and a grey screen of equal space-average luminance.

• An observer, unaware of the location of the patterned stimuli, is positioned behind a peephole located centrally between the grating and the homogeneous field.

• The observer monitors the direction of the child’s eyes and head during stimulus presentation. The position and width of the stripes are varied on each trial.

• Acuity is estimated by determining the smallest striped width to which the infant will show differential fixation of the grating as opposed to the homogeneous field i.e. The frequency of the line spacing determines the visual acuity.

• The threshold is usually defined as when the observer is correct 75% of the time.

• This technique becomes a “forced choice” method when the observer has to decide, based on their observation of the child’s head and eye movements, where the stimulus is located.

CARDIFF ACUITY CARD

• Each Cardiff card presents a line drawing of the object

• The picture is formed by a line that consists of a central white line with finer black flanking lines on either side. The luminance averaged across the black-white-black line matches the luminance of grey background

• The Cardiff Test is good for slightly older children (18 - 60 months). It consists of different cards, which are held in front of the child.

• Each has a picture in the upper or the lower part of the card. If the child looks towards the picture on the card, you note the size as detected.

• Consequently, when the line are too fine to be individually resolved, they become indistinguishable from the grey of the background

• The clinician determines the finest line drawing that still attracts the child’s attention.

VISUAL EVOKED POTENTIALS :

VISUAL EVOKED POTENTIAL

• Visual evoked potentials (VEPs) are electrical brain responses that are triggered by the presentation of a visual stimulus. VEPs are distinguished from the spontaneous electroencephalogram (EEG) due to their consistent time of occurrence after the presentation of the stimulus (time-locking).

• The surface-recorded VEP reflects the activity of cortical visual areas, with contributions from subcortical generators being apparent only under highly specialized recording conditions

• Types :

1. Flash VEPs

2. Pattern reversal VEPs

3. Sweep VEPs

SWEEP-VEP

• The S-VEP employs vertical stripes for testing and the feature size is simply the width of the stripe.

• The inbuilt S-VEP program computes the acuity from the VEP data. This represents a great advantage over other methods such as preferential looking and the routine pattern Visually Evoked Potentials (p-VEPs).

• In Sweep VEP, the spatial frequencies are varied very quickly over time and the amplitudes are immediately plotted with respect to spatial frequency (or time). • For example, to measure VA, the spatial

frequency changes from low to high in about 10-20 seconds. The regression line of the response amplitude is extrapolated to zero, which gives a measure of the VA.

SELECTING THE APPROPRIATE CLINICAL TEST :

•

Because a child can vary significantly from expected age norms, it is important not to rely solely upon chronological age when choosing testing procedures. Appropriate test procedures need to be based on the child's developmental age and specific capability.

Age Suitable visual acuity test

<18 months Response to occlusionBock candy beads (100’s and 1000’s)Keeler acuity cards(FPL test)Stycar graded balls testCardiff acuity cards

18 mths-3yrs Keeler acuity cardsCardiff acuity cardsKay picture testsSheridan-Gardiner test

VISUAL ACUITY OF INFANT EYES

Test 2Months 4Months 6Months 1Year Attainment (months)

Optokinetic nystagmus test

20/400 20/400 20/200 20/80 24–30

Forced choice preferential looking test

20/400 20/200 20/200 20/50 18–24

Visual evoked response test

20/200 20/80 20/60–20/20

20/40–20/20

6–12

VISUAL ACUITY ASSESSMENT IN

VERBAL CHILDREN

• Tumbling E chart

• HOTV chart

• Snellen’s chart

• Bailey-Lovie chart

SNELLEN’S CHART

• Visual acuity expresses the angular size of the SMALLEST target that can just be resolved by the patient

• Snellen Fraction is an expression of angular size of an optotype at the eye

• Snellen Fraction is the most common notation of acuity for children >5 yrs of age

SNELLEN’S FRACTION

VA =Testing Distance

Distance at which letter subtends 5 min of arc

BAILEY LOVIE CHART

Characteristics :

• Logarithmic size progression

• Same no. of letters at each level

• Spacing between the letters and between rows that is proportional to the letter size

• Equal (or similar) average legibility for the optotypes at each size level

• The particular visual acuity test selected by an optometrists to assess any specific child will depend upon factors such as test availability, age of the child and responsiveness of the child.

• Generally once the child reaches the developmental age of 6 to 7, standard Snellen acuity charts can be utilized

FINALLY,

Assessment of visual acuity in Pediatrics is a Challenging Task

• Immature Visual system

• Poor Cognitive ability

• Communication barrier

• Rapid development of visual system

HOW TO OVERCOME ??

• Selection of age appropriate technique • Understanding the development of visual

system• Accessibility/availability of VA accessories

RECOMMENDATIONS

• VA assessment – challenging task

• It should be given a great importance

• VA measurement should be repeated in each visit

• The best , realiable , reproducible & age appropriate techniques should be choosen.

CONTD…• Child friendly environment

• Mother’s lap is the best couch

• Interest creating surrounding

• Better to avoid VA when the child is in stress (cry, hunger)

• Don’t take VA when the child is fatigue