visual aquity and contrast sensitivity

HOPE YOU DIWALI WAS AS

SATISFYING AS JENNIFER

ANISTON’S

Visual acuity and contrast senstivity

Presentor - Dr Vishy SrivastavaModerator - Dr Anubha Mittal

VISUAL ACUITY

Visual assessment in adults

Visual assessment in children

CONTRAST SENSTIVITY

1. Examination of Vision

Assessment of visual function

Forms of visual perception are form

sense , the field of vision, the light

sense and the colour sense .

Dark adaptation – measurement of least luminance required to produce a visual sensation

Contrast sensitivity – is measurement of the smallest distinguishable contrast ,it is assessment of quality of vision

Colour vision –with lantern test (Edridgegreen lantern) and Isochromatic charts

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Visual Acuity

The principal of assessment and measurement of spatial resolution

of the eye i.e. an estimation of ability of eye to discriminate between

two points.

DISTANCE VISION

Two distance point can be visible as separate

only when they subtend an angle of 1 minute

at the nodal point of eye.

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Visual Acuity

-Measure of the smallest retinal image which can be appreciated with reference to its shape and size .

- Measure of form sense.

- Subjective examination

- Central or direct vision

- Distance vision - Snellen test type

- Near vision - Snellen test type or Jaeger’s

test type

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Snellen chartPrinciple

6 / 60

Numerator-Distancefrom the patient

Denominator-Distance at which letter is seen clearly

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Procedure of testing• Patient seated at 6 meters distance from Snellen’s chart

assuming that the rays are almost parallel and patient

exert minimum accommodation)

• Illumination- 20 feet candles.

• Patient made to wear trial frame(adjusted according to

inter pupillary distance).

Patient asked to read from 6 mtsdistance

Depending upon the distance recorded as 6/60, 6/36, 6/24, 6/18, 6/9,6/6

If not able to read from 6 mts, patient asked to move 1 mt close to chart and recorded as 5/60, 4/60 etc

If not able to read from 1 mt, asked to count fingers and recorded as CF3FT, CF2FT and so on

If finger counting absent than wave hand in front of eyes and recorded as Hand movement(HM) + or _

If HM -, Perception of light(PL)tested

If PL + than tested from all 4 quadrants(dark room)

Designated as Absent or Defective

4 November 2016Dr Sanjay Shrivastava

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English, other language charts

Landolt ring chart -

broken rings, with each

gap subtending an angle of 1 minute at nodal

point at a given distance.

Used in illiterate patients.

E-chart – used in illiterate patients

Simple picture charts for children.

LOGMAR CHART

-Used Logarithmic scale

-Spacing between the two letters is the width of 2 letters

-Height between the two rows is the height of the letter of lower row

- Size of letter increase by 25%

- Use 5 letters in each row

PRINCIPLES

- Results obtained in terms of LOGMAR score i.e log of minimum angle

- As each letter size changes by 0.1logmar units per row and there are 5 letters in each row hence each letter in that paricularis assigned value of 0.02

- Recording visual acuity using the LogMAR chart. ... LogMARVA = 0.1 + LogMAR value of the best line read – 0.02 X (number of letters read)

CATRACT is the 2nd most frequently spoken term in an Eye Hospital

1st is still “BABA BADE AANKH KHOLOAMMA LINE MAIN LAGO”

Pin hole testInterpretation

- Improved with pin hole – Refractive error.

- If static acuity - Structural or Organic cause.

- If reduced –Macular pathology(papillary area),Corneal

opacity or lenticular opacity

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Near vision

Charts for testing near vision are

1) Snellen near vision chart -

2) Jaeger chart

3) Roman test type

Ask the patient to sit with his back to the light

If the patient is using glasses for distance same no put on trail frame

same.Occlude other eye

Hold the near vision by his right hand at a distance of 25 to 33 cms.

Note the near vision as per the letter read

Repeat the test for the other eye.

Visual Assessment in Children

Preliminary Tests :

Preliminary Tests Begin with general observation of the child.

Hirschberg’s Test: Gross check of ocular alignment.

Pupils: size, shape, reaction.

Ocular motility.

Fixation pattern.

MEASURMENT OF VISUAL ACUITY IN INFANTS

1- Prefrential looking test

Child presented with two visual field : Striped and plain background

Child look at the striped pattern for longer time

Location of strip moved from right to left

Fineness of strip reduced till child can no longer differentiate between the two

2- Optokinetic Nystagmus

Nystagmus is elicited by passing a drum with

black and white stripes. If normal vision- rail-road nystagmus

(involuntary, rapid, rhythmic of eye ball)

3- Visually Evoked Response:

-In infants.

- Electroencephalographic recording from the occipital in response to visual stimuli.

-Determines the integrity of macular and visual pathway functions.

-Pattern depends on form sense

4-Indirect assessment of VA: Blink reflex in response to sound.

Menace reflex i.e. closure of eyes on approaching of if vision is normal.

VA measurements (1-2 yrs) :

1- Marble game test: Place marbles in the holes of the box.

Intended to monocular visual functioning.

Vision is noted being ‘useful’ or ‘less useful’.

2 - Sheridan’s ball test

Balls progressively smaller sizes used

Balls rolled on grey or white background

smallest size which the child picks

gives the estimation of vision

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VA measurements (3-5 yrs) :1 - Tumbling E test:

Child asked to identify the correct direction.

2 - Sjogren’s hand test:

Consists of picture of hand in 4 different directions.

3 - Landolt’s C test / Broken wheel test:

Pair of cars of progressively smaller sizes, one of it with a broken

wheel is shown. Child asked to

identify the one with broken wheel.

VA measurements (3-5 yrs) :4- Sheridan-Gardiner HOTV test:

Child is handed a card of HOTV.

Asked to match the letters on the chart.

Snellen's equivalent 6/60 – 6/6 can be

estimated.

5- Boek candy bead test:

Child asked to match beads at 40 cms.

Snellen’s VA of 20/200 can be

estimated

6- Light home picture cards:

- Performed at 10 feet distance.

- Contains apple, house and umbrella arranged in Snellen's

equivalent 20/200 – 20/10.

- Child asked to identify pictures in each line.

7- Kay picture tests :

- Child is given set of pictures in hand.

- Similar pictures shown at 10 feet distance on a log MAR chart.

- Snellen’s equivalent 20/200 – 20/20 can be measured.

Cardiff acquity test

Teller acuity test

- Combines prefentialsequencing and vanishing optotype

- Child will fix his vision to a picture rather than plain stimulus

- Child fixes his vision to stripped background rather than plain

Width of strips gradually reduced

Point where child stops identifying the stripped pattern is recorded

LEA SYMBOLS TEST

- Combine Optotype and pictures

- Task is to match point out of distant target

- 10 ft distance

- Pictures consist of circle, square, apple, house.

CONTRAST

SENSITIVITYTHE SURGICAL STRIKE

Definition:

Contrast

degree of blackness to whiteness of target.

Contrast threshold

smallest amount of contrast required to be able to see the target.

Contrast sensitivity

reciprocal of the contrast threshold.

CONTRAST SENSITIVITY

The ability to perceive slight changes in luminance between

regions which are not separated by definite borders and is just as

important as the ability to perceive sharp outlines of relatively

small objects.

SNELLENS VISUAL ACUITY VERSUS

CONTRAST SENSITIVITY

Person may have good VA in snellen chart due to high

contrast but low contrast senstivity in day to day work

Vision drops in the situation of low contrast

Hence contrast senstivity is the better predictor of vision

CONTRAST SENSITIVITY FUNCTION

- Subjective measurement

- Ability to detect a low contrast pattern stimuli.

- Give a more accurate representation of the eyes' visual performance.

- Depends on spatial frequency

MEASURMENT OF CONTRAST SENSITIVITY

1- MICHELSON FORMULA:

L max- Lmin/Lmax+Lmin

Lmax = Luminance on the lighter surface

Lmin = Luminance on the darker surface

2- WEBERS FORMULA:

Defined as ( Lb – Lt) /(Lb + Lt)

Lb-luminance of the background. Lt- luminance of target

Weber contrast used in calculating the contrast of letters.

TYPES OF CONTRAST SENSITIVITY:

1. Spatial contrast sensitivity

2. Temporal contrast sensitivity

SPATIAL CONTRAST SENSITIVITY

- Detection of striped pattern at various levels of

contrast and spatial frequency.

- Sine wave gratings of parallel light and dark bands.

- Width of the bar is defined as spatial frequency

- High spatial frequency- narrow bars.

- low spatial frequency- wide bars.

TEMPORAL CONTRAST SENSTIVITY

Contrast sensitivity function is generated for the time

related processing in the visual system by presenting

a uniform target field modulated sinusoidal in time.

The shape of the contrast sensitivity function varies with factors:

Luminance

Target size

Grating motion

Grating shape

Neural mechanism of contrast sensitivity:

Campbell and Green gave the concept for handling information

about bands of spatial frequencies.

Concept indicates that the retina is not uniform.

Fovea is for high acuity i.e high spatial frequencies

Retinal periphery, only low frequency channels

VARIABLES

Amount of light reflected

Degree of blackness in relation to white background

Distance between the grating period of cycle per degree of visual angle

METHODS FOR MEASURMENT

PelliRobson Test

BalieyLovieChart

Vision contrast grading chart

Cambridge low contrast grading

Regan Chart

FACT charts

PELLI ROBSON CONTRAST SENSITIVITY CHARTS-

Letters of the same size but with reducing contrast

- Each chart has 6 letters in each row organized into two triplets of varying contrast.

- Illumination- 85 cd/mm2

- 3 degree at 1 meter distance

- The score recorded by the faintest triplet out of which at least 2 letters are correctly identified.

CAMBRIDGE LOW CONTRAST GRADINGS

- Performed at 6mt distance

- Comprise 12 pairs of plate with varying contrast

- 1st for proper demonstration, rest numbered from 1-10

- Plates changed sequentially starting from plate 1. Results interpreted from table

The plates are changed sequentially starting from plate

1 till the patient fails to respond.

Then a new series is begun starting 4 plates prior to

where the patient failed to respond.

Four such series are completed and the score of each

series is noted (numbered as per the number of plate

read) and added.

The final total value is converted into contrast

sensitivity from the provided table.

BAILEY LOVIE CHART

Low contrast acuity testing in presence of glare.

Each row has same no. of symbols & constant spacing is used between rows & letters.

Chart is based on visual acuity of minimum angle of resolution on logMAR.

- Comprises of a chart with sine-wave gratings of varying frequencies

- The chart tests five spatial frequencies (sizes) and nine levels of contrast

- The Contrast varies in a row, decrease from left to right

FUNCTIONAL ACUITY CONTRAST TESTING(FACT)

- Spatial frequencies increase as one move down the various columns from top to bottom.

- Performed at a distance of 10 feet.

- The patient determines the last grating seen for each row (A, B, C, D and E) and reports the orientation of the grating: right, up or left.

- Readings plotted on a contrast sensitivity curve.

THE ARDEN PLATE TEST

- Introduced in 1978.

- Consists of a booklet containing several sine wave gratings patterns

- Each grating is oriented vertically

- The contrast varies from the top to the bottom of the grating, lowest at the top

and highest at the bottom.

VISTECH CONTRAST SENSITIVITY TEST

These charts consist of sine wave gratings. Each chart contains five

rows and nine columns of circular photographic plate

Contrast within the row reduces from left to right.

The gratings are presented in three orientations: vertical 90 degrees,

15 degrees clockwise or anticlockwise.

REGAN LOW CONTRAST LETTER CHARTS

Consists of three letter charts, printed on white cardboard having

contrast of 97%, 7% and 4%

3 meter with eight letters in each line

Patients are instructed to start at the top and to continue reading

until they can correctly identify no letters on a line.

For one chart letter size goes on reducing and contrast remains

constant

1. Refractive errors.

2. Age

3.Cataract

4.Contact lenses

5.Refractive surgery.

6.Glaucoma.

7.Diabetic retinopathy.

8.Optic neruopathies

9.Pituitary adenoma.

10.Drugs.

11.Toxic chemicals

FACTORS AFFECTING CONTRAST SENSTIVITY

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