Evaluation of visual functions in low vision patients involves assessing visual acuity, visual fields, color vision, contrast sensitivity, and glare tolerance. Visual acuity is measured using charts with logarithmically progressing optotypes at both distance and near. Other tests include Ishihara plates for color vision, Pelli-Robson chart for contrast sensitivity, and brightness acuity testers for glare sensitivity. Together these evaluations help identify the extent and nature of vision loss to guide low vision rehabilitation.

1. Evaluation of visual functions
in low vision patient
Presenter
Prakash Jha
2nd Year
B. Optometry

2. Objectives
To describe normal visual functions
To explain and record distance and near acuities in low vision.
To describe the low vision refraction.
To explain the kinetic visual field in low vision patient.
To explain the color vision test used in low vision.
To state the contrast sensitivity in low vision patient.

3. Visual Function
 Visual function: A person’s ability to integrate the components of
vision effectively to accomplish a task.
Category of visual functions
• Visual acuity
• Visual field
• Color vision
• Contrast sensitivity
• Glare

4. Low Vision
WHO Definition
 The impairment of visual functioning even
after treatment and /or standard refractive
correction and has
• a visual acuity of less than 6/18 to light
perception or
• a visual field of less than 10 degree from
the point of fixation, but who uses or
• is potentially able to use vision for the
planning and /or execution of a task.

5. Visual function in Low Vision
Vision loss may not be corrected by glasses, contact lenses, surgery,
or medical treatment.
Cause of vision loss
• Loss of central vision (e.g., AMD, diabetic retinopathy)
• Loss of peripheral vision (e.g., glaucoma, stroke, retinitis pigmentosa)
• Blindness (e.g., various acquired and hereditary eye conditions)
• Blurred or hazy vision (e.g., corneal disorders)
Assessing the visual function will help
• identify the extent of vision impairment and
• make the best use of low vision patient’s remaining vision to regain or retain
their independence and quality of lives

6. Visual Acuity
Visual Acuity : Measurement of threshold of discrimination of two
spatially separated targets.
Measures minimum angular size of detail that can just be resolved.
Testing distance is 6 m for distance and 40 cm for near.
Increase the size of target acuity (e.g., 6/6) using magnifying devices
to the patient’s threshold acuity (e.g., 6/60)

7. Objectives of VA Evaluation in Low Vision
Provides the examiner with baseline information from which the
course of pathology may be monitored.
Essential for calculating a patient’s device magnification.
Provides the patient with an appreciation for residual vision.
Documents a level of VA that may establish eligibility for services ,
benefits and even driving privileges.

8. WHO Classification
S.N
.
Visual Acuity (Snellen
Notation)
Classification Grading
1 ≥6/18 Normal Normal
2 <6/18-6/60 Moderately VI Low Vision
3 <6/60-3/60 Severely VI Low Vision
4 <3/60-PL Legally Blind Blindness
5 NPL Totally Blind Blindness

9. Types of Distance VA Charts
 Feinbloom distance test chart
 Ferris-bailey ETDRS chart
 Sloan distance acuity chart
 Bailey-lovie chart

10. Feinbloom Distance Test Chart
 Consists of numeric optotypes
 Calibrated for 20 ft but may be used at any distance
 As the size of optotypes decreases , more numbers
are added to each row
 Because of the reduced number of optotypes ,
numerals are easier to guess than letters

11. Ferris-Bailey ETDRS Chart
 Consistent number of letters i.e 5 in each row
 Seperation between optotypes and between
rows of optotypes are proportional to the size of
the optotypes
 Results in smaller spacing in the higher visual
acuity levels , giving the chart its characteristics
triangular configuration.
 Geometric (log MAR) progression of size
difference between lines.

12. Ferris-Bailey ETDRS Chart
Optotypes on each line are 0.1 log unit or 25% larger than the preceding
line
Logarithmic progression and proportional spacing of optotypes allows for
consistent and accurate evaluation of visual acuity levels
May be used at any distance , but testing distance are typically 4m or 2m
Available in Landolt C configuration , LEA symbol test system , HOTV chart
for pediatric low vision patient

13. Metric Conversion
of VA

14. Types of Near Acuity Charts
Charts using M notation
 Reduced snellen’s chart
 Sloan M series charts
 Reduced Ferris Bailey ETDRS chart
 Lighthouse game card
 Lighthouse continuous text card
 MNREAD card
Charts using N notation
N system chart

15. Sloan M series charts
 A 1 M optotype will subtend 5’ of arc at 1 m.
 A 1M letter viewed at 1m may be equated to
snellen acuity in the following manner.
 1.00/1M = 20/20 snellen equivalent
 Because near acuity is frequently measured at
40cm , 1M is equivalent to 20/50 at 40 cm.
 Testing may occur at any distance.
 Recorded as Testing distance (meters)/ M notation
 For example, 3M at 25 cm would be recorded as
0.25/3M

16. Reduced snellen’s chart
 Designed such that a 20/20 letter
subtend a 5’ angle at a given
distance (typically 40cm).
 As in standard Snellen distance
charts, the levels of acuities are
limited.

17. N system chart
 An 8-point optotype (N8) subtends 5’ of arc at 1m viewing distance.
 N notation may be converted to M notation by dividing by 8.
 E.g N4 is equivalent of 0.5M print.

18. Reduced Ferris Bailey ETDRS Chart
 Maintains a constant number of letters in each row.
 There is geometric progression of size differences between lines.
 Consistent and accurate evaluation of VA levels.
 May be used at any distance.
 Provides Snellen equivalent acuities for 40 cm and 20 cm.

19. Lighthouse Game Card
 Uses the same geometric progression as the single
letter chart with each three row progression
representing a half or double of VA levels at any
viewing distance.
 May be used at any distance , but the snellen
equivalent provided are calculated at 40cm.

20. Lighthouse Continuous Text Card
 Follows same geometric progression as
the lighthouse game card.
 Recorded VA may be poorer than single
letter VA , especially in cases in which
central scotomas are present.
 Snellen equivalent noted on the testing
card are calculated for 40cm.

21. MN Read Card
 Combines a quick reading performance
assessment with a reading acuity assessment.
 Reading passages are printed in decreasing M
sizes in logarithmic progression from 8.0M to
below 0.2M.
 Enables the examiner to determine optimal
print size for fluent reading tasks.

23. Refraction in Low Vision
 Patient with low vision may have high prevalence of refractive error,
along with unclear optical media
• Multitude of refraction techniques should be used
• Assess VA with multiple pinhole to see any improvement
• Perform radical retinoscopy for unclear media
• Stenopaic slit refraction for astigmatism

24. Refraction in Low Vision
Objective retinoscopy with pupil dilation (if media is not clear) shall
be followed by subjective refraction (with normal pupil)
Common techniques of subjective refraction
• Bracketing
• Stenopaic slit refraction

25. Bracketing
 It involves changing from presenting high minus lens and then high
plus lens.
 Large and equal steps of dioptric changes are made.
 Then reducing the size for the dioptric changes.
 Until the finest and just detectable blur is induced by equal steps
above and below the refractive errors.

26. Just Noticeable Difference
 The amount of spherical lens power needed to elicit an appreciable
change in clarity or blur is called the just noticeable difference.
 The lower the acuity , the larger the JND.
 The denominator of the 20 foot snellen acuity is a good rule-of-thumb
estimator of the JND for a given eye.
 For example, a 20/200 eye will be sensitive to a lens change of
approximately 2.00 diopters using this rule.

27. Stenopaic Slit Refraction
A slit aperture of 0.5 to 2.0 mm can be introduced.
With slit in one orientation, sphere that gives clearest vision is
determined.
This provides the power correction for the meridian corresponding to
the slit orientation.
The slit is then rotated 90 degree and spherical lenses are introduced
until the lens giving clearest vision is determined.
This gives power correction required for second meridian.

28. Visual field
Visual field: That portion of space in which objects are visible at the
same moment during steady fixation of gaze in one direction.
Regular visual field testing can help to monitor the potential vision
loss over time.

29. Normal Visual Field
• Low vision - <10 degree in either side of fixation

30. Uses of visual field in low vision
 To document visual field parameter for legal blindness.
 To fulfill the eligibility criteria for those states which require a minimum
visual field.
 Orientation and Mobility: learn skills and influence plan for rehabilitation.
 To follow disease progression.
 It helps to characterized the patient vision loss and design devices and
strategies to allow the patients to achieve maximum potential.

31. Consequences of Visual field defects in low vision
Central field defect
• Partial perception of people and
objects
• Cannot discriminate fine details
• Poor color vision
Peripheral field defect
• Mobility problems
• Slow dark adaptation
• Glare and photophobia
• Poor near vision
Hemianopic field defect
• Distorted sight
• Double vision
• Visual hallucination

32. Measuring VF in Low Vision Patient
1.Amsler Grid
2. Tangent Screen
3. Bernell Disc Perimetry
4. Confrontation Method
5. Goldmann Perimeter
6. Automated Perimeter

33. Amsler Grid
 Measures central 20 degree.
 Look for distortion.
 Uses reading glasses.
 It consists of seven grid like charts
each with slightly different patterns

35. Tangent Screen
 Measures central 30 degree.
 Tests at 30 cm distance.
 Vary target size based on patients
visual sensitivity/acuity typically
beginning with a 3 mm white
target.

36. Bernell Disc Perimetry
A kinetic perimeter ; uses plastic disc
formed in a semicircle.
To test the nasal and temporal boundary -
disc oriented horizontally
To test superior and inferior boundary – disc
oriented vertically
A stimulus is mechanically moved across the
arc from non seeing area to seeing area.

37. Goldmann Perimetry
 Evaluate both the central and
peripheral visual field.
 Standard test performed by most
perimetrists.
 Test targets: dots of varying size and
illumination.

38. Goldmann Perimetry : Target Size
 Sizes of stimuli (0 to V scale)
 Each size increment equals: a two fold increase in diameter and a
fourfold increase in area.

39. Automated Visual Field
 Light stimuli is presented in a fixed
position in the VF.
 The size of stimuli is constant and varied
in intensity until patient is just able to
detect it.

40. Confrontation Visual Field Exam
 A simple and preliminary test.
 Extent of VF tested by this method is
120 degree
 Examiner separated with the patient
by a distance of about 60cm and
asked to cover one eye and stare at
the examiner.

41. Confrontation Visual Field Exam
 When the patient covers their right eye, the examiner covers their left
eye, and vice versa.
 The examiner will then move his hand from a position as far as possible
from the line of sight inward until patient reports seeing it.
 This process should be repeated in each of the four quadrants.
 The target should be moved in a plane equidistant from the examiner
so that the examiner may compare the patient’s VF with his/her own
VF.

42. Color Vision
Color vision is the perception of color induced by different
wavelength of visible spectrum.
Color vision testing helps to monitor the progression of a disease.
It helps to monitor the level of difficulty a patient may have
performing tasks that require processing of colour information.

43. Tools for Measuring Color Vision
 Ishihara Pseudoisochromatic Plates
 Fransworth D-15 Dichotomous Test (low vision version)
 Anomaloscope

44. Ishihara Plates
 Ideal for screening.
 Detection of presence of protan/deutan.
Currently available editions are- 38, 24 and
16 plate version.
 For detection of color vision defect 38 plate
edition is generally used.

45. Types of plates
1.Demonstration plates
2.Transformation plates
3.Vanishing plates
4.Hidden digit plates
5.Diagnostic plates

46. Diagnostic marker
38 plate edition
• 4 or less - normal
• 8 or more – CV defect
24 plate edition
• 2 or less – normal
• 6 or more – CV defect

47. Fransworth Dichotomous D-15 test
 A set of 16 different colored caps contained
in a tray.
 Because of large differences in color of
adjacent caps it evaluates major color
confusion of severe R-G or B-Y defects.
 Reference cap is fixed while others are
available.

49. Anomaloscope
Patient looks into the anomaloscope via
eyepiece to view a bipartite color field.
 The observer is advised to mix red and
green colors in such a way that the
mixture should match the yellow color
disc.

50. Contrast sensitivity
The ability to perceive slight changes in luminance between regions
which are not separated by definite borders.
It is just as important as the ability to perceive sharp outlines of
relatively small objects.

51. Why CS in low vision
 To detect abnormal visual performance.
 To predict performance on specific visual task.
• Driving ability
• Mobility
• Face recognition
• Sports
 Contrast provides critical information about
• Edges
• Borders and
• Variation in brightness.
 To diagnose ocular disease and monitor its states.

52. Implication in Low Vision
Loss of high spatial frequency contrast usually indicates problem with
near point and reading task.
Loss of low spatial frequency contrast usually indicates problem with
orientation, mobility and night time travel.

53. Contrast sensitivity charts
 Pelli-Robson chart
 Bailey Lovie low contrasat chart
 Regan Low contrast chart
 Lea low contrast chart
 University of waterloo Low contrast chart
 Cambridge Grating
 Arden grating

54. Contrast sensitivity charts
 Vistech chart
 Melbourne Edge Test
 Hieding –Hiedi
 Multivision contrast tester
 Terry vision analyser

55. Pelli-Robson Chart
 Score 2.0 = normal
 Score < 1.5 = visual impairment
 Score < 1 =visual disability

56. Hiding Heidi
Determines the level of
contrast that an infant can
detect.
Four cards printed on both
sides in the following contrast
levels: black, 25%, 10%, 5%,
2.5% and 1.25%.

57. Glare
The presence of one or more areas in the field of vision that are of
sufficient brightness to cause discomfort in vision.
Glare sensitivity has been shown to be an important factor in hindering
the mobility of low vision patients.
Tools for glare testing
• Brightness Acuity Tester
• Optec 1500 Glare tester
• Miller-Nadler Glare tester
• Terry Vision Analyzer

58. Summary
VA < 6/18 or VF of less than 10 degree from either side of fixation is
considered as low vision.
 chart following logarithmic progression is used for distance VA
assessment.
Fransworth Dichotomous D-15 (Low vision version) is used for color
vision assessment.
Pelli – Robson chart is used for contrast sensitivity assessment.
Low vision can have a significant impact on daily life.
Understanding the changes in visual functions, can provide better
support and care for those with low vision.

59. References