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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

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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

  • It sets clear boundaries on what you will be able to do

  • Draw up your own sheet in practice

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

Give the differences and explain

each point given

Disadvantages of using phoropter

  • Why not phoropter?

Case History

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

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

  • 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

Why is it important to take LV

case history?

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

  • 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

    • 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

    Evaluating visual performance


    • 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 acuity

    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?

    • 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

    • 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

    Factors affecting VA measurements

    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

    • VA Notations

    • Acuity chart design

    • Currently used charts

    • Measuring distance VA

    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

    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

    • Feinbloom Number Chart

      Refer to your notes for advantages and disadvantages

      Of this chart

    • Bailey-Lovie


    • 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

    • 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

    • 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

    If the patient is unable to identify any optotypes, which designations are you going to use?

    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 VA

    • Specification of Nearpoint acuity

    • Measuring near acuity with the M system

    Specification of Nearpoint acuity

    • 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

    • This another important aspect in low vision patient

    • Desirable to test all patient’s fields, but not always possible or practical

    Instruments and techniques

    • Confrontation test

      • Only a gross estimate of the peripheral field

      • Screening method

      • Use light as a target

    2. Amsler grid

    • 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

    • 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

    • 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

    • 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

    • 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

    • 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

    • 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

    • Keratometry

      • 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 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

    1 General conditions

    2 The trial frame

    3 The JND (just noticeable difference)

    4 Spherical refraction

    5 Cylindrical refraction

    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

    Why do we use it?

    The refraction itself

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

    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

    • If the best VA is 10/100, the JND will be 1.00D

    • 10/50 = JND of 0.50D

    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

    • 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


    • Ophthalmoscopy

    • Keratometry

    • Tonometry

    • Slitlamp

    • Von Herick

    • Dilated fundus exam

    • Binocular indirect ophthalmoscopy

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