Low Vision Assessment

measuring visual function in visually impaired patients

why is it important to measure visual acuity?

• to compare with ‘normal’ performance

• to set a baseline for monitoring

• quantify subjective impressions of visual performance

• early detection + diagnosis

• assess benefits of optical devices

• refraction procedures and decision making

• social-legal purposes

what should an ideal chart for low vision assessment involve?

• moveable

• variable illumination

• high contrast

• range of sizes

what are the common test charts used in the LVA?

• snellen chart

• logMAR chart

  • bailey lovie distance visual acuity chart

  • early treatment of diabetic retinopathy study (ETDRS)

• keeler A chart

what are the advantages and disadvantages of snellen charts?

advantages:

• cheap and readily available

• measures baseline VA

• predicts + verifies magnification for low vision aids

disadvantages:

• no control over crowding - 6/60 line have one letter therefore no crowding, 6/36 has 2 letter etc.

• non-uniformity in letter size progression

• inability to easily score letter by letter acuity

• due to non-uniform progression it is inaccurate for testing low vision patients

what are the advantages and disadvantages of logMAR charts?

advantages:

• equal number of letters on every line

• logarithmic progression of letter size

• letter by letter acuity

• more popular in research setting

disadvantages:

• large size

• cost

what are the corrections factors for the Bailey Lovie test chart?

• designed to be used at 6m

3m: add 0.30

1.5m: add 0.60

0.75m: add 0.90

1m: add 0.80

what are the correction factors for the ETDRS chart?

• designed to be used at 4m

2m: add 0.30

1m: add 0.60

0.5m: add 0.90

when and how is the Berkeley Rudimentary Vision Test used?

• used when it is not possible to use a letter chart for VA

• tested at 1m (100cm) and 0.25m (25cm)

• consists of three card pairs, each containing 2×25cm square cards hinged together (4 panel faces/card)

  1. single tumbling E: used for measuring VA with a single optotype and px has to identify the direction the legs are facing

  2. grating acuity card pair: px has to identify if the strips are orientated horizontally or vertically

  3. basic vision card test: tests 2 aspects of vision:

     • white field projection: 2 card faces are used (one black with white quadrant and one is divided into black and white halves) + px has to identify the location of the white area

     • black white discrimination: one card face is all black and one is all white and the px has to tell which one is black and which is white

what is the Freiburg Visual Acuity Test?

• BRVT but computerised

• uses Landolt’s C

how can distance VA be measured remotely?

• using the home acuity test

• developed by MEH

• can be printed out and sent to px or ask them to print it out

• carried out at 150cm/190cm - send a string to the px that is of this length

what is contrast threshold?

• the smallest difference in luminance than an observer can detect

• values vary from 0 to 1

  • can be expressed at a percentage

what is contrast sensitivity?

• reciprocal of contrast threshold

what is contrast sensitivity function?

• a plot of contrast sensitivity over a range of spatial frequencies

what are the four contrast sensitivity tests?

• functional acuity contrast test (FACT)

• Pelli-Robson letter chart

• Mars letter contrast sensitivity test

• Regan & Bailey-Lovie low contrast acuity chart

what does the functional acuity contrast chart look like?

• spatial frequency increases as you go down

• contrast decreases as you go across

how is the Pelli-Robson chart scored?

• either record letter by letter - each letter being 0.05

• or record the line if the px gets 2/3 on that line

why is it important to measure contrast sensitivity?

• indicates what everyday tasks a px is likely to find difficult

• correlates better than VA with the performance of daily tasks e.g. reading and mobility

• poor contrast sensitivity means a px will likely not benefit so much from optical aids - electronic aids are more suitable

• to determine whether to prescribe a monocular or binocular low vision aid

what are the three reading charts available and what do they measure?

• they measure near VA and reading speed

  1. Bailey Lovie reading test - unrelated words

  2. MNREAD acuity chart - sentences

  3. IReST - paragraphs

what are the different ways of recording near VA?

• point system - one point is 1/72 inch

• N notation - n is the point size of the print

• sloan M - common in US

• keeler A - A1 subtends an angle of 5 minutes of arc

• jaeger notation - different depending on chart used

what is the MNREAD chart?

• a near chart with each sentence having the same no. of characters and spacing between sentences + words

• hence it can measure speed (in words/min.) and VA (in log)

• done at 40cm

  • add a correction factor if done at a different distance

why is measuring reading ability important?

• reading is the primary rehabilitation goal by most low vision individuals

what affects reading ability?

• acuity reserve

• contrast reserve

• field of view

• central scotoma

how many words per minute are needed for:

• optimum reading

• fluent reading (pleasure reading)

• spot reading

• optimum reading: 300wpm

• fluent reading: 160wpm

• spot reading: 40wpm

what are the requirements for fluent reading:

• acuity reserve

• contrast reserve

• scotoma diameter

• field of view

• acuity reserve: 3:1 (needs to read N4 if wanting to reading N12 comfortably)

• contrast reserve: 10:1

• scotoma: 4 degrees

• FoV: 4-6 characters

what are the requirements for optimum reading:

• acuity reserve

• contrast reserve

• scotoma diameter

• field of view

• acuity reserve: 6:1

• contrast reserve: 30:1

• scotoma: 0 degrees

• FoV: 4-6 characters

what are the requirements for spot reading:

• acuity reserve

• contrast reserve

• scotoma diameter

• field of view

• acuity reserve: 1:1

• contrast reserve: 3:1

• scotoma: 30 degrees

• FoV: 1 character

how are visual fields measured?

• amsler chart for central vision

• arc perimeter

• scanning laser ophthalmoscope

• microperimetry - allows compensation for poor fixation

• automated perimetry

• goldmann manual perimetry

why is measuring visual fields important?

• it is a diagnostic test

• allows for functional assessment of the extent, location and quality of areas of the best vision

• help determine what LVAs and rehabilitation strategies will be useful

• gives us information on how well the px is performing in real life conditions

how do you assess binocular vision?

• cover test

• stereopsis

  • titmus

  • frisby

  • TNO

what is the definition of QoL?

‘An individual’s perception of their position in life in the context of the culture and value systems in which they live and in relation to their goals, expectations, standards and concerns’ - World Health Organisation

what do QoL questionnaires assess?

• functional capacity

• social interactions

• relationships

• wellbeing

what are some examples of low vision QoL questionnaires?

1. Low Vision Quality of Life Questionnaire (LVQOL)

2. National Eye Institute Visual Function Questionnaire (NEI-VFQ)

What observations should be made?

• Postural abnormalities e.g. AHP

• Mobility e.g. long canes, guide dogs

• Appearance

what are halberg clips and when are they used?

Used when doing refraction over the patient’s current spectacles

what should be done if you cannot get a ret value and VA is poor?

• estimate the sphere

• estimate the cyl and cyl axis

• refine using large steps (±2.00DS, ±1.50DC)

if the working distance for subjective is 3m, how would the prescription by adjusted to ensure DV is corrected?

minus around 0.25D

what can be recommended to px’s that don’t benefit from LVAs?

sensory substitutes

how do you measure magnification if a LogMAR chart is used?

(1.25)^n

n = number of steps

e.g. VA is 0.5 and 0.1 is required - this is 4 steps so (1.25)^4 = 2.5X

what are the distance magnifiers available?

telescope

• monocular

• binocular

what are the near magnifiers available?

• spectacle mounted magnifiers

• hand magnifiers

• stand magnifiers

• near vision telescope

• bar magnifiers

what are useful contacts should you make the px aware of?

• RNIB

• Macular degeneration society

• Social services department

• Esme’s umbrella support group - for Charles Bonnet

History and symptoms ocular health questions?

• diagnosis: length of time

• treatment: discharged/active

• spectacles

• last eye exam

• general description of how they see and how it affects them day to day

  • vision - can they see bus numbers, street signs, tv, reading etc.

  • mobility - walks alone or accompanied

  • glare - tinted sunglasses

  • activities e.g. cooking

  • flashing light

  • contrast

  • how do they get around the house

  • Charles Bonnet - do you ever notice objects that may not be there

History and symptoms general health questions?

• co-morbidities

• medications

• how they manage other health conditions e.g. how they measure their glucose

• any falls in the last 12 months

• how they are in themselves

History and symptoms social questions?

• what support is available at home

• hobbies, activities

• what they do for work or education

  • work requirements

• does anyone depend on them e.g. children

• registered as sight impaired

• do they drive

• have social services rehabilitation service visited them or offered any help

• what is their accommodation like

• are they managing their correspondence

History and symptoms aid questions?

• what magnifiers, assistive tech and non optical aids do they already have

• are they helpful

• what do they use them for

• are they in good working condition

• what are they hoping we can help with