VISUAL ACUITY MEASUREMENT, CONTRAST SENSITIVITY
PRESENTER : DR.OM PATELMODERATOR : DR.ALKA
VISUAL ACUITY PRINCIPLES
• The visual acuity is determined by the smallest retinal image the form of which can be appreciated
• For discriminating the form of an object its parts must be differentiated
• It is necessary that two individual cones must be stimulated with one between them remaining unstimulated
VISUAL ANGLE
• It is found that the object must subtend a visual angle of 1 minute at the nodal point of the eye
• VA test types consists of a series of letters of diminishing size.
• Each letter is shaped such that it can be placed in a square, the sides of which are five times the breadth of the constituent lines. Hence the whole letter will subtend an angle of 5 min. at the nodal point of the eye at the given distance.
COMPONENTS OF VISUAL ACUITY
• Detection or visibility
• Resolution
• Recognition
1. Detection or Visibility
• Ability to determine whether or not an object is present in an otherwise empty visual field is termed visibility.
• This depends upon the specification of stimulus such as size, shape, & illumination.
• A black dot against a white background can be detected if its diameter is of the order of 30 sec or more
2. Resolution (Ordinary VA)
• Discrimination of two spatially separated targets is termed resolution
• It is essentially an assessment of function of the fovea centralis
• This component of VA is measured clinically using Snellen’s or other test types
3. Recognition
• Virtue by which an individual identifies the test patterns with which he had some experience
• It involves cognitive component in addition to spatial resolution
• E.g. Identification of faces
MEASUREMENT OF VISUAL ACUITY
• The visual acuity is a highly complex function
• In clinical practice, VA is considered synonymous with the measurement of minimum resolvable only
• Hence, examination with various VA charts is quite satisfactory, although incomplete
MEASUREMENT OF VISUAL ACUITY IN ADULTS
• The distant central VA in adults is tested by :
• Snellen’s test types• Landolt’s C test types• E chart
SNELLEN’S TEST TYPE
• Basis of the test : Two distant points are visible as separate only when they subtend an angle of 1 min at the nodal point of eye
• Black capital letters on white board, arranged in lines, each progressively diminishing in size
• Each letter of the chart is so designed that it fits in a square,the sides of which are 5 times the breadth of constituent lines
• The line comprising the letters have such a breadth that they will subtend an angle of 1 min at the nodal point
METHOD
• Patient seated at 6 meters from the chart• Illumination is 20 foot candles • Each eye is tested separately.• VA is recorded as a fraction (6/60, 6/36,
6/24,6/12,6/9, 6/6) Numerator: distance of the pt. from the chart Denominator: smallest letters accurately read
• If pt. cannot see the top line from 6m, he is asked to slowly move towards the chart till he can read the top line(5/60, 4/60, 3/60, 2/60, 1/60)
• If the pt. is unable to see even from 1m, he is asked to count fingers of the examiner
• When the pt. fails to count fingers, the examiner moves his hands close to the pt’s face & asks whether he could appreciate the movements or not. (HM +/-)
• When the patient cannot appreciate hand movements, perception to light is noted.
(PL +/-) with projection of rays in four quadrants.
LANDOLT’S TEST TYPES
• Similar to Snellen’s test types
• Instead of letters, broken rings are used & the patient is asked to detect the direction of the break in the circle
• Each broken ring subtends an angle of 5 min. at nodal point
E CHART :-
• Similar to snellen’s and landolt’s the difference is that in this chart E of different sizes are arranged
• Pt. is asked to tell the direction towards which the arms of the E are pointing
SNELLEN’S EQUIVALENT
• In U.S., the metric system is not usually employed & the values are converted to feet• ( 6m = 20 feet)
VA 6/6 = 20/20 VA 6/60 = 20/200 VA 3/60 = 20/400
Decimal acuity:-
• In this system, the Snellen’s fraction is reduced to a decimal no.
• Higher VA is represented by a numerically larger number, which is reverse in the Snellen’s grading
• Ex- 6/6 = 1.0 6/9 = 0.67 6/60 = 0.10
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
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
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
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
Measurement of VA for Near
• Near vision is tested by asking the patient 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 patient can read comfortably
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 labeled 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.
VISUAL ACUITY MEASUREMENTS IN CHILDREN
• OBJECTIVE TESTS Preverbal children ( < 2 ½ yrs )
• SUBJECTIVE TESTS Verbal children ( > 2 ½ yrs )
OBJECTIVE TESTS
• Fixation & following behaviour
• Preferential looking test ( PLT)
• Optokinetic Nystagmus (OKN)
• Visual evoked potential (VEP)
1) FIXATION & FOLLOWING• Bright colored objects with
high contrast edges are used.• Best target however is the
human face.• Binocular fixation is assesed
first.• Monocular fixation – reveals
the defective vision in one eye.
• Infant may not fix with the defective eye and objects to occlusion of the better eye
• Quality of fixation behaviour – C S M
C – Central – foveal fixation S - Steady – no nystagmus M – Maintained – fixation after a blink
QUANTITATIVE METHODS
• Methods to detect the resolution acuity• More sophisticated method of visual assesment
than mere fixation assesment.• Include -
PREFERENTIAL LOOKING TEST OPTOKINETIC NYSTAGMUS VISUAL EVOKED POTENTIAL
(2) 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
Procedure
• On a screen homogenous surface is projected on one side & black and white strips on the other
• These two stimuli are alternated randomly
• The eyes of the infant are observed and the movements recorded
PROCEDURE
• Gradually the fineness of stripes is reduced unless there is no longer correlation between direction of gaze & location of the striped pattern.
• Visual acuity ranges from 6/240 in newborn,6/60 at 3 months and 6/6 at 36 months
(3) Optokinetic Nystagmus Test (OKN)
• Nystagmus is elicited by passing a succession of black and white stripes through the patient’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
(4) 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
Visual evoked response (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.
SUBJECTIVE TESTS
• Optotype- symbol whose identification implies VA
• Eg; Lea’s symbols ,HOTV, Snellen’s chart, Landolt-C ,E chart.
• These tests are employed in verbal children.
Lea’s symbols
• Pt. is asked to match the picture shown to the hand held cards
• 4 shapes are included in this : Circle, Square, Hut & Apple
• Done at 3m
HOVT /SHERIDAN GARDINER TEST
• It is done at a distance of 6 meters from the child
• It includes simple alphabets such as H,O,T,V,X,A,U
• The patient matches the letter being displayed with the hand held cards
Snellen’s,E Chart,Landolt’s C
IN SUMMARY
• INFANTS -
Fixation methods
Preferential looking test
Optokinetic nystagmus
Visual evoked potential
• TODDLERS - LEA’S SYMBOLS HOTV TEST
• CHILDREN > 5 YRS & ADULTS- SNELLEN’S CHART LANDOLT’S C CHART E – CHART ETDRS
THANK YOU
