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Low Vision Evaluation. Ms MB JAN- 24/01/2012. Content. The difference between a low vision exam and a regular exam The Case History Evaluating visual performance. 4. Evaluating visual performance Visual acuity Visual field evaluation Contrast sensitivity Colour vision

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low vision evaluation
Low Vision Evaluation

Ms MB JAN- 24/01/2012

  • The difference between a low vision exam and a regular exam
  • The Case History
  • Evaluating visual performance
4. Evaluating visual performance
  • Visual acuity
  • Visual field evaluation
  • Contrast sensitivity
  • Colour vision

5. Objective refraction

6. Subjective refraction

7. Ocular health evaluation


At the end of this lecture, learners should be able to:

Discuss the importance of a case history specifically for a low vision patient
  • Discuss specific questions that will be asked to a low vision patient
  • Describe the different techniques and charts used to measure a low vision VA (near, distance, aided, unaided etc.)
Discuss the need for evaluating visual fields in a LV patient
  • Discuss methods and techniques for evaluating visual fields in a LV patient
  • Discuss the need for evaluating contrast sensitivity in a LV patient
Discuss the need for evaluating colour vision in a LV patient
  • Discuss the methods and techniques used for evaluating colour vision in a LV patient
  • Discuss the objective refraction techniques available to the low vision optometrist
  • Discuss the technique and implications of radical retinoscopy
Discuss the method used for refracting a low vision patient
  • Analyze a low vision case based on a history, and then decide on and describe the most appropriate evaluation routine for a specific patient
  • Distinguish between a low vision refraction routine and a normal refraction routine
  • Explain the concept of JND (just noticeable difference) and be able to use it to test a low vision patient
the pre evaluation information sheet
The pre-evaluation information sheet
  • It sets clear boundaries on what you will be able to do
  • Draw up your own sheet in practice
information sheet
Information sheet
  • The appointment duration
  • Schedule appointment around a time when patient’s vision is stable
  • Bring with old glasses, magnifiers – even if not usable anymore
Think about specific problems the patient is experiencing
  • Start thinking in terms of goals – write down what you would like to achieve
Bring along special materials he/she want to be able to use (E.g. books)
  • Bring along a report from the ophthalmologist
Follow-up visits or training sessions with equipment may be necessary
  • State that there are no miracles, we will use your remaining vision effectively
the difference between a low vision examination and a regular exam
The difference between a low vision examination and a regular exam

Give the differences and explain

each point given

case history

Case History

NB. Very important, It has to be even more detailed

the patient interview
The patient interview
  • The successful patient interview has 3 functions (Cohen-Cole)
    • Gathering data to learn about the patient’s problem
    • Developing rapport, and responding to the patient’s emotions
    • Educating patients about their problems, and motivating them to adhere to the prescribed treatment
interview techniques
Interview techniques
  • Both parties should be seated at eye-to-eye height
  • Seating should be comfortable
  • Control lighting – not too dim or bright
  • Carefully observe the patient
Use both open-ended and specific questions
  • May be emotionally charged
  • Note taking should be done subtly
Be alert to inconsistencies
  • Take sufficient time that patient doesn’t feel rushed
  • BUT keep it brief – old people tire more easily
  • Use positive language
  • Question in a friendly, enthusiastic manner
Adjust pace to that of patient.
  • Don’t use medical jargon, explain patient’s condition if they do not understand it
  • Never give false reassurances
  • Primary aim is to help patient – don’t fear to be inquisitive – but respect privacy too!
the purpose of the case history
The purpose of the case history

Why is it important to take LV

case history?

the real questions you want answered are
The real questions you want answered are:
  • What does the patient want?
  • What does the patient need?
  • What is the real reason for the patient’s visit?
information required
Information required
  • Basic identifying information
    • Name, address etc
  • Who accompanied the patient?
    • Support system / self-sufficient?
    • Relative, friend, counselor, teacher etc
    • Contact person
    • Provide insight into history
Referral source
    • Send thank you note
    • Reports
  • Diagnosis of eye condition
    • In patient’s own words
    • See if patient understands condition
    • Begin with patient education on problems.
Visual history
  • Duration
  • Previous care
  • Nature of vision loss (congenital or acquired? Stable or progressive?)
  • Fluctuation of vision
Problems with color vision
  • Is there a preferred eye?
  • Problem with glare or lighting?
  • Current glasses / low vision aid
  • Current visual capability (specific task-related questions)
    • Smallest print read?
    • Newsprint
    • Headlines
    • Large print
Able to watch television?
    • What viewing distance?
    • Size screen?
    • Can you recognize faces at a distance?
    • Can you see well enough to get around?
  • Family visual history
5. Medical history
  • Undergoing treatment for medical condition?
  • Does the patient have a disease with known ocular implications?
  • Is there medical problems that might affect the use of a LVA? (stroke)
  • Family history
  • Allergies and drug sensitivities
  • Medications (many systemic drugs have ocular side-effects)
6. Employment or school history
    • Investigate the effect the visual loss has on the work/school performance
    • Investigate the use of appropriate devices to alleviate problems
    • Some older people might want to continue their education
  • Avocations
    • Hobbies or activities
8. Social assessment
  • Does the patient live alone or with family?
  • How is daily life affected by the vision problem?
  • Does the patient have a support network?
  • Is the patient’s independence threatened?
10. General appearance of patient
  • Well groomed, clean or untidy?
  • Food stains – cannot see that level of detail
  • Poor grooming - emotional disorders such as depression
  • Walk without assistance?
  • Mobility
  • Does the patient look ill?
Patient goals (Chief complaint)
  • Possibly the most important part of the case history
  • Allow a full elaboration of the visual disabilities
  • Patient’s new problem should be fully investigated
After the patient has completed a list of complaints, several issues should be addressed regardless of the patient’s failure to mention them
    • Distance vision
    • Near vision
    • Orientation and mobility skills
    • Glare
    • Lifestyle
external evaluation
External evaluation
  • Some do this just after VA’s, but depends on circumstances. Give an example
  • Brief look into the eyes, do not shine bright lights into the eye
Note the following about the eyes:
    • Position of eyes (strabismus)
    • Pupil – size, reaction to light, appearance,
    • Cornea – opacities: size, density, position
    • Lens – opacities, position (especially IOL)
    • Motility – strabismus, nystagmus, restrictions
  • Binocular dysfunction is usually of secondary importance
  • Compare with normal performance, or accepted standard (eg driving regulations)
  • Set a baseline for monitoring the condition
  • Quantify the patient’s own subjective impression of visual performance
Early detection and diagnosis of (other) visual disorders
  • Assessment of the benefits of an intervention (medical, surgical, rehabilitation) program
  • Predicting visual function in every day tasks
visual acuity1
Visual acuity

1.Why do we want to accurately measure acuity?

2. Limitations of VA measurement

3. Factors affecting VA measurements

4. Distance Visual Acuity

5. Near Visual Acuity

why do we want to accurately measure acuity
Why do we want to accurately measure acuity?
  • It establishes a baseline from which to monitor pathology
  • Used to predict the magnification level of the optical devices that will be required to achieve the patient’s goals
  • Often requested by other agencies to establish legal blindness, driving privileges, job eligibility etc.
limitations of va measurement
Limitations of VA measurement
  • The clinical acuitydoes notgive an accurate indication of the functional acuity. Explain
  • Clinical measure of person’s ability to read letters under controlled circumstances
  • It doesn’t always correlate with daily activities
Function can be influenced by differences in contrast sensitivity, glare sensitivity, motivation and numerous other factors
  • VA can vary due to test setting, illumination, doctor-patient relationship and target contrast
how does each of the following factors affect va measurement
How does each of the following factors affect VA measurement?
  • Lighting
  • Optotype
  • Mental state of the patient
  • Instructions to patient/attitude / encouragement
  • Glare recovery
  • Educational level
  • Recognition/memory/speech
  • Motivation
distance visual acuity
Distance Visual Acuity
  • VA Notations
  • Acuity chart design
  • Currently used charts
  • Measuring distance VA
va notations
VA Notations
  • Snellen
    • Either metric or imperial
    • We use imperial (feet)
  • LogMar (logarithm of the minimum angle of resolution)
  • Decimal: Snellen fraction
  • Angular (specified in minutes of arc)
    • Not used clinically
acuity chart design
Acuity chart design

The following aspects of chart design can be considered

  • Optotype –
    • style of print and selection of letters
    • Should yield equivalent results to Landolt C
  • Number of letters per row
    • Equivalent – equal task progression
    • 5 good clinically
Sequence of Letters
    • not form words/part of words
  • Optotype Size
    • 0.1 logarithmic progression of character size
    • Accurate measurements at both standard and non-standard test distance
  • Letter spacing
    • systematic
commonly used charts
Commonly used charts
  • Feinbloom Number Chart

Refer to your notes for advantages and disadvantages

Of this chart



  • logMar format
  • Equal number of letters at each line
  • Can be used at any test distance
Projected cards
  • Other
    • Lighthouse distance acuity card (available in our clinic)
    • Lighthouse symbol cards
    • Designs for vision pediatric picture chart
    • University of Waterloo Chart
    • ETDRS chart
measuring visual acuity
Measuring Visual Acuity
  • Use special low vision charts
  • Use a 10feet / 3 m working distance, or less
  • Emphasize residual vision
  • Offer encouragement and realistic feedback
Watch for and encourage eccentric viewing
  • Let the patient attempt to read all letters on the chart, and look for scotomas
  • Record as Snellen fraction, e.g 10/700
  • To convert between feet and meter, divide by 3.25 (feet to meter) or multiply by 0.3 (meter to feet)
  • Always measure the acuity correctly: “less than 6/60” is unacceptable
recording va measurements
Recording VA Measurements
  • Can have a measurement recorded as BEO (both eyes open) – distinguish from OU
  • Record the fractions read:
  • 10/240 + 2 of 10/200 + 1 of 10 / 180
near va
Near VA
  • The measurement of Near VA is a very important part of low vision
  • Most low vision patients struggle with reading, so magnification for near tasks is vital.
near va1
Near VA
  • Specification of Nearpoint acuity
  • Measuring near acuity with the M system
M notation
    • Method of choice
    • Metric notation
    • Represents the distance in meters at which the target subtends an angle of 5’ of arc
    • 1.00M subtends 5’ at 1m
    • Consistent, meaningful, flexible testing distance
N notation
    • Point size of lower case Time Roman print
    • Standardized so that each point is 0.18 mm on the printed page
    • N10 is twice N5
    • Quite valid
    • Necessary to specify both test distance and target size
Point type
    • Actual print size in printers point notation
    • Size of slug, but not actual print size
    • Not a very good system
  • Reduced Snellen
    • Characters subtend the same angle indicated by the designated fraction at 20 feet
    • Specified test distance
    • Not 20 units, not a standard angle at 20 distance units
    • Cannot be used at any other distance
    • Useless - inflexible
visual field evaluation
Visual field evaluation
  • This another important aspect in low vision patient
  • Desirable to test all patient’s fields, but not always possible or practical
instruments and techniques
Instruments and techniques
  • Confrontation test
    • Only a gross estimate of the peripheral field
    • Screening method
    • Use light as a target
What is it?
    • Hand-held chart used to evaluate central 20° of vision
    • Can identify early changes like metamorphopsia or small central scotoma
  • What does it look like?
    • 20 blocks x 0.5mm each
How does it work?
      • “Place a finely quared chart before an eye suffering from an affection of the central region of the retina, and the patient will immediately point out spots and distortions which affect his/her vision”
      • Measures the central 20° of vision if the chart is held 28-30cms from the eye
types of charts
Types of charts
  • Standard chart *
      • Every case, and usually sufficient
  • Diagonal lines*
      • Use with central scotoma
  • Red on black standard chart
      • Colour scotoma
  • Spots only
      • Reveals scotoma (no lines to be distorted)
Parallel lines
      • Use horizontally and vertically
      • Shows metamorphopsia
  • Parallel lines for reading
      • Allows a more minute evaluation of reading area
  • Standard block with smaller reading area
      • Minute examination of juxta-central area
      • Rectangle shows limit of fovea
general method
General method
  • Testing distance
  • Optimal refraction
  • Clean, clear, well-lit chart
  • No ophthalmoscopy etc prior to evaluation
  • Do monocularly and then BEO to check for interference/suppression
What chart?
    • Start with grid
    • Then use lines and spots
  • Do monocularly and then BEO to check for interference/suppression
questions asked
Questions asked
  • Do you see the white spot in the centre of the squared chart?
  • 4 corners? 4 sides? Whole of the square?
  • Network intact?
  • Lines straight + parallel?
  • Anything else?
  • Plotting the distortions?
colour vision
Colour vision
  • Pathological conditions like glaucoma and ARMD can cause changes in colour vision, so it is necessary to evaluate this.
    • City University (not available in our clinic )
    • Isihara
    • Farnsworth D15
  • Tests for colour deficiency of congenital origin
  • Limited value in LV
farnsworth d15
Farnsworth D15
  • Available in our clinic
  • Check functional tests notes


Always obtain the best possible refraction with the best possible VA – to give the lowest magnification, why?

objective refraction
Objective refraction
  • Autorefractors
    • Limited use, due to media problems or eccentric viewing (off axis fixation)
  • Previous glasses
    • Can be a good starting point
    • Just make sure patient is using own Rx!
    • Patient might have had ocular surgery since glasses were prescribed
    • Useful with astigmatism – amount and orientation of cyl
    • Patient may have difficulty fixating
    • Can be helpful in detecting irregular corneal surfaces or irregular astigmatism
  • Retinoscopy
    • Very useful, especially if patient is a poor responder
    • May be necessary to use radical retinoscopy
  • Always do the ret in a trial frame
  • If there is no initial response or no reflex is seen, try using very large lens changes like +/- 5D, +/- 10D, +/- 20D
radical retinoscopy
Radical retinoscopy
  • Radical retinoscopy means that the working distance is drastically reduced (as close as 10cm)
  • Radical retinoscopy can also mean deliberate off-axis scoping to use any visible reflex – this will induce unwanted cylinder, but the results can be potentially valuable
subjective refraction
Subjective refraction

1 General conditions

2 The trial frame

3 The JND (just noticeable difference)

4 Spherical refraction

5 Cylindrical refraction

general conditions
General conditions
  • Use a 10feet or less working distance
  • Use full illumination unless otherwise indicated (e.g. patient with achromatopsia)
  • Use the low vision chart in subjective refraction
  • Always do a trial frame refraction
the trial frame
The trial frame

Why do we use it?

the refraction itself

The refraction itself

Use standard methods and background knowledge to refine cylinder axis, power and sphere power

the jnd just noticeable difference
The JND (just noticeable difference)
  • Essential concept
  • The smallest dioptric step that a patient is able to discriminate
  • It is senseless and frustrating to use 0.25D steps when (because of the visual impairment) the patient can only notice a 1.00D change
  • Use the 10-feet equivalent as a rough starting point for JND
spherical refraction
Spherical refraction
  • Use the tentative result from your objective refraction as a starting point
  • Determine the JND-lens, and check the sphere value with that
  • “Better with the lens, or without it”, not “one or two”
  • Patients may have a poor, slow, variable response – could be due to pathology
cylindrical refraction
Cylindrical refraction
  • Check the axis using a hand-held Jackson Cross-cylinder of +/- 0.50D or +/- 1.00D if possible
  • You can also use rotation to blur/clear and let the patient rotate the axis her/himself
Cylinder power is checked in the normal way
  • Double check cyl power with direct comparison (with or without) – if no subjective or objective improvement, it is not necessary to prescribe
  • Finally, double check the spherical component again – use bracketing (eg +0.50 and -0.50 should blur equally)
ocular health evaluation
Ocular health evaluation


  • Ophthalmoscopy
  • Keratometry
  • Tonometry
  • Slitlamp
  • Von Herick
  • Dilated fundus exam
  • Binocular indirect ophthalmoscopy