visual acuity by pd

VISUAL ACUITY,CONTRAST SENSITIVITY

PRESENTED BY DR.PUSHKAR DHIRMODERATED BY DR.WANGCHUK

DOMA

CONCEPT OF VISUAL ACUITY

• It is quantified by the minimum dimension that an object must have in order for the observer to be able to identify, distinguish or simply detect it.

Visual Acuity is the capacity to detect, resolve or recognize the details of objects

Weymouth Classification of Visual Acuity Measurements

• MINIMUM DISTINGUISHABLE (OR MINIMUM VISIBLE)• The observer should be able to decide on the presence or absence of an object in their field of vision• Quantified by the angle that is subtended by the object that is perceived by the observer

• MINIMUM SEPARABLE• The ability to determine a group of points or lines as separate and distinct• Quantified by the minimum angle of separation that should exist between the two objects so that they can be perceived as separate by the observer

• MINIMUM RECOGNIZABLE • The ability to detect form sense (e.g., Landolt C) or minimum legible (e.g., letters or numbers)• Quantified by the inverse of the angle that subtends the object to be recognized.

TASK OF DETECTION OF THE MINIMUM

VISIBLE

TASK OF RESOLUTION OR MINIMUM

SEPARABLE

TASK OF RECOGNITION OR

MINIMUM RECOGNIZABLE

TASK OF LOCALIZATION

Types of test:Luminous points on black backgroundBlack point on luminous background

Type of test:Points or lines separated by a variable distance, which are clearly distinguishable if they are presented individually

Types of test (Optotypes):LettersNumbersSnellen “E”Landolt Rings

Types of test:

- Lines that shift until the observer sees them aligned

History

• Early astronomers found two stars resolvable as separate if the distance between the stars subtend an angle of at least 1 minute of arc

• 1861 FRANCISCUS DONDERS coined the term visual acuity to describe the “sharpness of vision” and defined it as the ratio between a subject's VA and a standard VA.

• 1862 HERMANN SNELLEN published his famous letter chart.

• Laid out font in 5x5 grid pattern & k/a OPTOTYPES.(PIC)*

• Standardize the font/chart -> easily reproducable.

• Defined “standard vision” as the ability to recognize one of his optotypes from distance of 20 feet when it subtended 5 minutes of arc.

• Thus the optotype can be recognized only if the person viewing it can discriminate a spatial pattern separated by a visual angle of 1 minute of arc (one element of the grid).

Typical Snellen Optotype

1 degree =60 minutes1minutes = 1/60degree

• Choice of 20 ft as measurement distance was arbitrary.

• 20/20 is NOT perfect human vision rather it is the near average for adults in their 60’s.

• 20/20 should be interpreted as limit of normal vision.

• Snellen used 9optotypes intially

C,D,E,F,P,O,Z,T,L.

• In 1875 Snellen created new set & used 6 metres .

• So now in England 6metres & USA 20ft n hence our HMS casses

Variations on the Snellens Chart

• In 1959, Louise Sloan designed a new set of 10 optotypes nonserifletters.

• Standardized effect of crowding btw.letters & avoided problem of some letters being more prominent

SLOAN LETTERS

The Bailey-Lovie logMAR chart• Principle:- Used logarithmic scale. Each step indicates increase of 25% in letter size. Letter sizes ratio as we move up is a constant value of 1.26(0.1log unit steps). Incorporated 5 letters in every row. Spacing b/w 2 adjacent letters = width of 1 letter. Spacing b/w 2 hz rows = height of the letter on lower row.

• Results of this chart were obtained in terms of logMAR score i.e log of minimum angle of resolution.

• As each letter size changes by 0.1logMAR units per row & there are 5 letters on each row ,therefore each letter can be assigned value of 0.02.

• Thus final logMAR takes account of every letter that has been correctly read.

• In 1982, the National Eye Institute combined the Sloan letter set with the Bailey-Lovie layout to produce charts for use in the Early Treatment of Diabetic Retinopathy Study.

• These charts became known as the ETDRS charts.

5.8 cm

4.8cm

Snellens Chart

• Irregular progression of letter size

• Variable number of letters in

each line

• Variable legibility (difficulty) of test letters

• Distance between each letter is not uniform

• SHERRIFS FONT

logMAR Chart

• Uniform progression of letter size

• Same number of letters in each line

• All letters with similar legibility

• The distance b/w each letter is equal to the width of the letter

• NO SHERIFFS FONT

Chart in work up Area says Read from 3m

& 6m respectively

Chart in Ward Area

Says read from 4m

???? What to be Followed???

Tan 5 is the rule

Tan 5`=x(letter lenghth)/y

Tan 5`= x/400cm.00145*400=x

0.58cm=x

Should be 4metres but its 374 approx.!!!!

Snellen fraction(20ft)

Snellen fraction(6m)

Snellen Acuity(Decimal)

Minimal Angle of Resolution(inverse of

sn.frac)

Log-MAR

20/200 6/60 0.10 10 1

20/160 0.12 8 0.9

20/125 ~6/36 0.8

20/100 0.20 5 0.7

20/80 6/24 0.25 4 0.6

20/63 ~6/18 0.5

20/50 0.40 0.4

20/40 6/12 0.50 2 0.3

20/32 0.67 0.2

20/25 0.80 0.1

20/20 6/6 1 1 0

20/16 -0.1

20/12.5 -0.2

20/10 -0.3

Alternative Approaches For VA

Tumbling E

Near Vision Visual Acuity Assesment

Measurement of VA for near

• Near vision is tested by asking the pt. to read a near vision chart kept at a distance of 25cm to 35cm.

• Each eye should be tested separately.

• The near vision is recorded as the smallest type which the pt. can read comfortably.

Commonly used near vision charts are

1) Jaeger’s charts

• Consist of ordinary printer’s fonts of varying sizes.

• Prints are marked from 1 to 7 and accordingly pt’s acuity is labelled as J1 to J7 depending upon the print he can read

(2) Roman test types

• Consists of Times Roman fonts with standard spacing.

• -The near vision is recorded as N5, N6, N8, N10, N12, N18, N36 and N48.

(3) Snellen’s near vision test types

• Constructed on the same principles as of the distant types.

• The graded thickness of the letters is about 1/17 of the distant vision chart letter.

• The letter equivalent to 6/6 line subtend an angle of 5 min. at he average reading distance.

Meri Refraction kon karega

PEDIATRIC REFRACTION

Age Approx. Visual Acuity

Birth 6/300

1month 6/200-6/90

3months 6/90-6/60

6months 6/36-6/30

9months 6/24

1year 6/18

18months 6/12

2years 6/12-6/9

3years 6/9-6/6(single)

4years 6/9-6/6(crowded)

5years 6/6-6/5

Infants (Birth-14 months)

Toddlers ( 14 months – 21/2

years)Preschoolers

(21/2 years – 5years)School going children

(5years- 15 years)

1.Fixation maintainence

2.Fixation preference

3.Optokinetic Nystagmus (OKN)

4.Force Choice Preferential looking test (FCPL)

5.Visual Evoked Potential (VEP)

1.Hundred & thousand sweet test

2. The Cardiff acuity Test

3.Dott Visual Acuity Testing.

1.LEA Symbols

2.Allen Picture Card

3.Broken Wheel Test

4.Tumbling E chart

5.Landolt C chart

6.Sjogren hand test

7.HOTV test

1.Tumbling E chart

2.Landolt C chart

3.Sjogren hand chart

4.Snellen’s chart

5.LogMAR chart.

There are 2 types fixations are present:-1.Centric2.Eccentric

Component of FixationC= CentralS= steadyM= Mantained

Fixation Preferance

• Place a 10D base down prism in front of one eye

• Alternate fixation indicate equal VA in both eyes.

• If fixation does not maintain then preferred eye occluded.

• If fixation is mantained through a blink , then also visual acuity is good.

OPTOKINETIC NYSTAGMUS TEST (OKN)

• Nystagmus is elicited by passing a succession of black and white stripes through the pt’s field of vision.

• The visual angle subtended by the smallest strip which elicits an eye movement is a measure of VA.

• OKN acuity is 6/120 in newborns,6/20 at 2 months,6/6 by 20-30 months.

PREFERENTIAL LOOKING TEST

• Assumes that the child will prefer to look at an area of higher visual interest, rather than a neutral grey field. Child presented with two adjacent stimulus fields, one which is striped and other homogenous.

• Method suitable for infants upto 4 months of age.

• Ex- Lea’s paddles, Teller’s acuity

cards

• Visual acuity ranges from 6/240 in

newborn,6/60 at 3 months and 6/6 at 36 months

VISUAL EVOKED RESPONSE (VER)• Refers to EEG recording made from the occipital lobe in

response to visual stimuli.

• It is useful in assessing visual function in infants.

• Only clinically objective technique available to assess the functional state of visual system beyond the retinal ganglion cells.

• Two types – flash & pattern reversal VER

• Flash VER tells about the integrity of macular and visual pathway

• Pattern reversal VER uses some patterned stimulus like checkerboard

• The pattern of stimulus is changed and so it gives an idea of form sense

• VER studies shown VA in infants to be 6/120 at 1 month,6/60 at 2 months & 6/6 at 1 yr of age

Hundred & Thousand Sweet Test

• Very sweet test.

• if child able to pick up small sweets at 33cm,visual acuity is at least 6/24 or 20/80


years)




THE CARDIFF ACUITY TESTToddlers ( 14 months – 21/2

years)




The Principle of the target design is that of vanishing optotype.

Targets are pictures drawn with a white band , all on a neutral grey background.

Examiner simply observes the child’s fixation.

DOTT VISUAL ACUITY TESTING

• Black dots on white background.

• Touch a black dot

• Test dist.—25cm

• VA—20/800 to 20/20.


years)




Preschoolers (21/2 years – 5years)

1.LEA Symbols


3.Broken Wheel Test

4.Tumbling E chart

5.Landolt C chart

6.Sjogren hand test

7.HOTV test

Preschoolers (21/2 years –

5years)

1.LEA Symbols

2.Light House Flash & Card testing


4.Broken Wheel Test

5.Tumbling E chart

6.Landolt C chart

7.Sjogren hand test

8.HOTV test

School going children (5years- 15 years)

1.Tumbling E chart

2.Landolt C chart

3.Sjogren hand chart

4.Snellen’s chart

5.LogMAR chart.

CONTRAST SENSITIVITY• The ability to percieve slight

changes in luminance between regions that are not separated by definite borders.

• Provides critical information about the edges, borders and variation in the brightness of two objects.

• Contrast sensitivity is the reciprocal of the contrast at threshold.

The various methods:

1. Arden gratings

2. Pelli Robson test

3. Bailey Lovie chart

4. Vision contrast test system (VCTS) by Vistech

5. Cambridge low contrast grating

6. Regan charts

7. FACT charts

Cambridge low contrast gratings• Rapid and simple screening test performed at a distance of 6m.

• 12 pair of plates consisting of stripes of varying contrast. First one is for demonstration and rest are for the proper testing and are numbered from 1-10. 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 he number of plate read) and added.

• The final total value is converted into contrast sensitivity from the provided table.

PELLI ROBSON contrast sensitivity charts

• Two charts and two scoring pads.

• Each chart has 6 letters in a row - two triplets of varying contrast.

• The test is carried at a distance of 1 meter with patient wearing the best correction.

• The patient reads from left hand corner, when he fails to respond, the score of the test is recorded by the faintest triplet out of which at least 2 letters are correctly identified.

• The log CS value for this triplet is given by the number on the scoring pad nearest to the triplet, either on the left or the right side.

• Each eye is tested separately and then the both eyes together.

Helpful links

• http://www.icoph.org/downloads/visualstandardsreport.pdf

• http://books.google.co.in/books?id=lt7fAH0s7QYC&pg=PA468&lpg=PA468&dq=.00145+%3D+how+many+tan?&source=bl&ots=sKtytRDOoJ&sig=LM-pcUl1IcnLNTTdFEHYUna8WEI&hl=en&sa=X&ei=3RRfU-zTO8qyuATc34GwDg&ved=0CCoQ6AEwAA#v=onepage&q=.00145%20%3D%20how%20many%20tan%3F&f=false

• http://www.acuity.de/history/index.php• http://www.icoph.org/dynamic/attachments/resources/ic

ovisualacuity1984.pdf• http://www.eboptometry.com/content/optometry/ebp-re

source-step-4-apply/practitioners-students-teachers/development-and-adoption-visual-acuity-charts-based-%E2%80%9Clogmar%E2%80%9D-principles

http://www.icoph.org/downloads/visualstandardsreport.pdf

http://www.icoph.org/downloads/visualstandardsreport.pdf

http://www.acuity.de/history/index.php

http://www.icoph.org/dynamic/attachments/resources/icovisualacuity1984.pdf

http://www.icoph.org/dynamic/attachments/resources/icovisualacuity1984.pdf

http://www.eboptometry.com/content/optometry/ebp-resource-step-4-apply/practitioners-students-teachers/development-and-adoption-visual-acuity-charts-based-%E2%80%9Clogmar%E2%80%9D-principles




THANK YOU EVERYONE FOR PATIENTLY LISTENING TO THIS SEMINAR.

For feedbacks & brickbats plz mail [email protected]./[email protected]

NO MORE SEMI--NAR KYUNKI ABKI BAR ……..

visual acuity by pd

Education