Visual acuity

VISUAL ACUITYCONTRAST SENSITIVITYTESTS FOR POTENTIAL VISION

Dr. Pooja Bhatlavande

VISUAL ACUITY• Visual perception/ Vision – complex

integration of light sense + form sense + contrast sense + colour sense• Visual acuity – measure of form

sense• Defined as reciprocal of the

minimum resolvable visual angle in minutes of arc

Nodal point / axial point

• One of the two points in a compound optical system, located so that a light ray directed through the first point will leave the system through the second point, parallel to its original direction

• Visual angle – angle subtended at nodal point of eye by physical dimensions of an object in the visual field

• 2 adjacent points can be seen discretely and clearly

if visual angle is >= 1 minute , but depends on size and distance

If size of retinal image is >4.5 microns (1stimulated cone 1.5microns + 1unstimulated cone 1.5microns + 1stimulated cone 1.5microns)

Components• V. A - measures threshold of

discrimination of 2 spatially separated targets – function of fovea centralis

1. Minimum visible / detectable2. Resolution3. Recognition4. Minimum discriminable

Minimum visible / detectable• Ability to detect whether or not an object is

present• Depends on size, shape, illumination of stimulusi. Black dot – white background – if diam. >=

30sec of arcii. Black square – white background – if

l(diagonal) >= 30 sec of arciii. Extended line 0.5sec thick – subthreshold

signals converge to give suprathreshold of V. A.iv. Illuminated object – dark background –

depends only on intensity

Resolution / Ordinary V. A.• Discrimination of 2 spatially separated targets• Minimum separation that can be

discriminated = minimum resolvable• Function of fovea centralis• Angular threshold at nodal point = 30-60 sec

or arc = Minimum Angle of Resolution (MAR)• Snellen’s charts• Landolt’s rings

Recoginition

• Identify patterns from past experience• Spatial resolution + cognitive

components• E.g. Identification of faces

Minimum discriminable / Hyperacuity

• Vernier acuity – threshold < ordinary visual acuity

• 2-10 sec of arc

Factors affecting V. A.• Stimulus-related factors :i. Luminance of test objectii. Geometrical configurationiii. Contrast from surroundingiv. Wavelengthv. Exposure durationvi. Interaction effects of two targets

• Observer-related factors :i. Retinal locus of stimulationii. Pupil sizeiii. Accomodationiv. Effect of eye movementsv. Meridional variationvi. Optical elementsvii. Developmental aspects

MeasurementClinical tests measure minimum resolvable

1. Detection acuity tests : detect smallest stimulus

i. Dot visual acuity testii. Catford drum testiii. Boek candy bead testiv. STYCAR graded ball’s testv. Schwarting metronome test

Dot visual acuity test

• Black dots on white background• Smallest dot child touches is

approximately the visual acuity• Test distance – 25m

STYCAR test(Screening Test for Young Children and Retarded)

2. Recognition acuity tests : recognize and distinguish stimulus

A. Direction identification tests-i. Snellen’s E-chart testii. Landolt’s C-chart testiii. Sjogren’s hand testiv. Arrows test

B. Letter-identification tests-i. Snellen’s letter chart testii. Sheridan’s letter testiii. Flook’s symbol testiv. Lipman’s HOTV test

C. Picture-identification tests-i. Allen’s picture card testsii. Beale Collins picture chart testsiii. Domino cards testiv. Lighthouse testv. Miniature toy test of Sheridan

D. Tests based on picture identification on behavioural pattern-

vi. Cardiff acuity card testsvii. Bailey Hall cereal test

3. Resolution acuity tests :i. Optokinectic Nystagmus (OKN) testii. Preferential looking test

a. two-alternate forced choice testb. Operant variation looking testc. Teller acuity card tests

iii. Visually evoked response

Measurement in school children (>5yrs) and adults

• Snellen’s test types :– Distant central visual acuity– Series of black capital letters on white board, in

lines, progressively decreasing in size– Breadth of each line will subtend an angle of 1min

at nodal point– Each letter fits in square whose sides are 5X the

breadth of the constituent line– So each letter subtends an angle of 5min– Starting from top, letters should be read clearly at

60,36,24,18,12,9,6,5,4 metres

Snellen chart

Landolt’s test types• Each broken ring subtends an angle of 5min• Detection of orientation of the breakpoint in

the circle

Procedure• Patient at 6mtrs – light rays practically parallel &

minimal accomodation• Illumination >= 20 footcandle• Read with each eye separately• Numerator = distance = 6m• Denominator = smallest letters read accurately• If unable to read top line at 6m, patient is asked to

walk towards chart & distance at which patient reads is noted – 5/60, 4/60 so on

• Unable at 1m – counting fingers• Hand movements• Perception of light (PL)

Measurement of visual acuityin 3-5years

• E- cutout test – Child given an E cutout & asked to match orientation with various Es on the chart

• Tumbling E-pad test – large E 20/200 on one side and series of five 20/20 tumbling Es on the other – caliberated to 20ft.

• Isolated hand-figure test – E replaced with hand

• Sheridan – Gardiner HOTV test – child handed out card with HOTV and asked to match letters

Snellens equivalent of 6/6-6/60 can be estimated

• Pictorial vision charts – – Kay picture test– Allen preschool test

• Broken wheel test – – Pair of cars in progressively smaller sizes one of

which has a broken wheel

• Boek’s candy bead test – child asked to match beads at 40cm. Snellen ‘s equivalent 20/200 can be estimated

• Light home picture cards – at 10ft – 20/200 to 20/10

Measurement of visual acuityin 2-3years

• Dot visual acuity test• Coin test – identify two faces of coins at

different distances• Miniature toy test – identify and match

toy at 10ft

Measurement of visual acuityin 1-2years

• Marble game test – place marbles in holes on cards – to find is ‘useful’ or ‘less useful’

• Sheridans ball test – balls of progressively smaller size rolled at 10ft against white background – smallest size the infant can fixate

Measurement of visual acuityin infants

1. Optokinetic Nystagmus test (OKN) – succession of black&white stripes elicit nystagmus – visual angle subtended by smallest stripe which elicits nystagmus is measure of acuity

6/120 – in newborns6/60 – at 2months6/36 – at 6months6/6 – at 20-30months

2. Preferential Looking Test (PLT) – 2adjacent stimulus fields – on homogenous and one striped- examiner notes head movements from behind the screen through a hole- infant tends to look at striped pattern for greater portion of time-upto 4months- 6/240 – newborns 6/60 – 3months 6/6 – 36months

3. Visual Evoked Response (VER) – EEG recording from occipital lobe in response to visual stimuli- clinically objective- functional state beyond retinal ganglion cells

• Flash VER – integrity of macular and visual pathway

• Pattern reversal VER – checkerboard stimulus reversed with same illumination

• 6/120 at 1month, 6/60 at 2months, 6/6-6/12 at 6-12months

4. Catford drum test5. Cardiff acuity cards6. Indirect assessment –

i. Blink reflex in response to light since birthii. Menace reflex – reflex closure of eyes on

approach of object since 5 monthsiii. Fixation reflex – fixation behaviour test,

binocular fixation pattern, central steady mantained (CSM) monocular fixation

Measurement of visual acuity for near

• Jaeger’s charts – prints marked 1-7 – acuity J1-J7• Roman test types – Times Roman font with

standard spacing – N5,N6,N8,N10,N12,N18,N36,N48

• Snellen’s near vision test types – graded thickness of letters is 1/17th of the distant-vision chart letters by photographic reduction – so letters equivalent to 6/6 line subtend 5min at avg. reading dist. 35cm/14inches

CONTRAST SENSITIVITY

• Ability to perceive slight changes in luminance between regions that are not separated by definite borders

• First measured by Schade• Types –– Spatial– Temporal

• Spatial – detection of striped patterns at various levels of contrast and spatial frequencies – Arden gratings – sine wave gratings of light and dark bands & minimum contrast at which bars can be seen at each frequency is measured

• Spatial frequency – number of pairs of light and dark bands subtending angle of 1degree

• High = narrow bars, Low = wide bars• Temporal – time-related processing by

presentating a uniform target field modulated sinusoidal in time

Measurement• Presented with grating frequencies• Resolution below which contrast is not

possible is threshold level• Reciprocal of threshold is contrast sensitivity• L – luminance recorded by photocells• Contrast sensitivity =

(Lmax–Lmin)/(Lmax+Lmin)

Methods

• Arden gratings – 1 screening & 6 diagnostic plates– Contrast changes from top to bottom covering

1.76 log units– Studied at 57cm– Spatial freq. Increases from 0.2cycles/deg

to6.4cycles/deg– Score – 1-20/plate, sum of 6plates = upper limit 82

in normal, interocular diff. <12

• Cambridge low-contrast gratings – 10 plates at 6m in order of descending contrast, each paired with a blank page of same reflectance 7 patient has to identify page with gratings

• Conversion table with scores – plate 10 = score 11

• Pelli – Robson contrast sensitivity chart – letters subtend angle of 3degrees at 1m– Letters arranged as triplets– Contrast decreases – log contrast sens from 0.00

to 2.25– Luminance of white areas 60-120cd/m

• Vistech chart – sine wave gratings at 3m – identify orientation of grating

Neural mechanisms

• Campbell and Green – different visual channels handle different spatial frequencies

• Fovea – high acuity & high freq.• Peripheral retina – low freq.

• Factors affecting contrast sensitivity – – Refractive errors – high freq.– Age – decreases with age 10% per decade from

20’s onwards– Lens – low freq.– Ocular and systemic diseases

TESTS FOR POTENTIAL VISION

• To check whether significant cause of visual impairment is cataract or associated retinal pathology

INTERFEROMETRY

• Estimation of VA through mild to moderate media opacification by projection a resolution target on macula

• Set of interference fringes of light and dark bands produced on retina by waves from 2 coherent light beams each < 0.1mm in diam.

• Depends only on ability of retina to conduct signals from photoreceptors to nervous system

Types

• LASER – – 2 point light sources from Helium-Neon gas laser

(632.8nm)– Focussed red light penetrates through opaque

media• White light – – Polychromatic white light from incandescent bulb– Contrast of gratings may be reduced by chromatic

abberations

OPTICS• 2 periodic waves go in-phase and out-of-phase• Maxima – points on retina where both are in-

phase – bright white bars• Minima – points on the retina where both are

out-of-phase – black bars• Spacing (fringe pitch) – function of separation of

pinpoint beam areas (grating angle)• Increased separation – finer fringe – greater

macular resolution

• Space is adjusted till patient cannot identify orientation – last perceived grating value converted to Snellen’s potential

• Thiry-three maxima per degree of visual angle corresponds to Snellen’s equivalent of 6/6

Technique• Explain patient – orientation of band patterns,

ignore scotomas• Pupil dilated, interferometer mounted on slit-

lamp• Retroillumination, beam passed through area

of maximum transparency of media• Pupil diam. 1.5mm + steps of 0.1mm• Patient indicates direction of fringe

Interpretation

• Normal – alternate dark and light stripes• Media opacity – shooting stars, moving

worms, jumble but can identify stripes• Very dense opacity – no pattern• False postive – tilted retinal receptors (Stiles-

Crawford phenomenon), healthy receptors in CME, parafoveal stimulation

• False negative – poor pupillary dilatation, very dense cataract, VH

Potential Acuity Meter

• Guyton & Minkowski• Small device on slit-lamp• Projects image of Snellen’s chart through

0.15mm diam aperture• Slide scale from +13D to -10D• 20/20 to 20/400• Pupil dilated, best refractive correction, beam

focussed, reads charts

• Factors affecting accuracy of PAM & LI

– Severity of cataract– Type of cataract– Preoperative visual acuity < 20/200

Comparison

• Moderate cataracts – both useful• Severe cataracts – PAM underestimates• Retinal disorders – LI overestimates• PSCs – both underestimate

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