Functional Vision Assessment
Presented in Dublin on 11 March 2010
Teacher for the Visually Impaired
Why do teachers of the visually impaired need to assess vision?
Clinically done by ophthalmologists, optometrists and orthoptists
TVIs are looking for different information - "The purpose of all assessment should be to obtain information which can be used to plan services and develop appropriate activities. For example, these may relate to the curriculum and may also refer to the need for training in effective use of residual vision." ("Vision for the future; a framework for minimum standards in visual impairment education" RNIB, 1995)
Differences between clinical and functional vision assessment?
Functional assessment looks at HOW the student uses vision in real-life situation.
Functional assessment includes:
- pertinent info obtained from clinical reports, parents, pupil, class teacher(s)
- observations by TVI, plus orientation and mobility specialist (if appropriate).
Notes: At home, in school, testing in non-clinical conditions as this is what real life is like. Clinical assessments look at thresholds and parameters of visual function, more quantitative, functional more qualitative
"While an optometric or ophthalmological exam in and eye doctor's office can yield vital information about the student's eye condition, the results may not be readily transferable to another setting.
Unless care is taken to observe the student's visual performance in real-life environments where lighting and contrast cannot always be controlled, a true picture of a student's functional vision cannot be obtained." (Anthony, 2000, p32)
Taking a History
Why is this important?
- Helps to build a profile of each individual child.
- Can provide info relating to preferred learning style.
What to consider:
- If pupil now blind, did he/she previously have vision?
- Look for changes between recent and older reports (has visual functioning increased or decreased?).
- Is eye condition stable or degenerative?
- Is VI secondary or other conditions?
- Congenital or acute (accident or illness?)
Notes: In transitions may also seek input from ed psych for social and emotional component
"Functional vision is most often assessed through observation rather than formal testing" (Goodman and Wittenstein, 2003)
May take place in the home. Especially helpful if:
- parents have concerns about daily living skills;
- shows families you have real concern for needs of individual child;
- allows opportunities for discussion with parents which can provide invaluable information.
Mostly done in school and involves consultation with class teacher
Notes: Because TVI is only there part time and may miss something, best to timetable yourself for whole or half day observation if possible.
What to look for
Child's daily routine:
- Move rooms?
- Change teachers?
- Does he/she have support assistant? If yes, what type of support is offered? How well is it received?
- Number of students in class
- Pupil's regular seating position
Now you're ready for the tools!
Notes: Obvious - DOB, stage, other agencies involved
Assessing Visual Acuity
- VA = measure of the eye's ability to distinguish object details and shape
- Assessed by the smallest identifiable letter/number/picture that can be seen at a specified distance.
- "Distance" = 6m
- "Near" = 0.33m
LogMAR Acuity (aka Glasgow Acuity ) Cards
Cambridge Crowding Cards
- Results are recorded as a fraction.
- Numerator equates to the distance (in metres) at which the test chart was presented, usually 6m.
- Denominator identifies the position on the chart of the smallest line read by the 'patient'.
- "Normal" visual acuity usually considered to be the ability to recognise an optotype when it subtends 5 minutes of arc, that is, 6/6 on Snellen chart.
(As with all tests, remember to record whether or not glasses worn during test.)
- LogMAR scale converts geometric sequence of a traditional chart to a linear scale.
- Each line represents a change of 0.1 log units
- Each letter has a score value of 0.02 log units; 5 letters per line
- Scores closer to 0 = better VA, closer to 1 = poorer VA
- Minimum Angle of Resolution for a 6/6 letter is 1 second of arc.
-the logarithm of the MAR for a 6/6 size letter is 0
- Equal number of letter per line
- Regular spacing between lines and letters
- Uniform progression in letter size
- Final score based precisely on total of all letters read
- Finer grading scale allows greater accuracy and improved test/retest reliability
Near Visual Acuity
- The ability to detect objects of low contrast is an important component of the visual system
- Relates to real life experiences where things seldom purely black and white
- Determining the levels of contrast that a child can detect informs plans for intervention and provides a baseline to evaluate future changes
TEST Hiding Heidi (preferential looking)
Present cards in order of decreasing contrast 100%, 25%, 10%, 5%, 2.5% and 1.25%
Why test visual fields?
- Visual field results can be used to monitor treatment for eye and brain disorders.
- Even when the diagnosis is known and the condition is stable without treatment, the visual field informs us of important effects on vision that are relevant to everyday functioning.
Methods of assessing VF
- Clinically, 2 approaches:
Static (Humphrey perimeter)
Kinetic (Goldmann perimeter)
- For FVA, approximation of perimeter of visual field by confrontation
- Objective method based on interpretation of responses in relation to assessor’s own VF
- Difficult to accurately identify or measure blind spot(s)
Notes: for children many studies have proven automated techniques unreliable (too many false positives)
- Color Vision Test Plates for Infants (Handaya);
- City University Colour Vision Test
Equipment: RAF rule; fixation stick or other small target
Method: (objective) encourage child to keep looking at target as you bring it close to eyes
Results: most children should have a near point of 6cm, watch for point where one eye diverges or "drifts off"
Implications: reduced convergence may cause discomfort, blurring or double vision after prolonged close work
Equipment: RAF rule; fixation stick or other small detailed
target (equivalent to near acuity)
Method: (subjective) encourage child to keep target clear as you bring it close to eyes
Results: average near point depends on age, child reports point where target starts to blur
Implications: reduced accommodation may cause blurring or discomfort, eg; headache, after prolonged close work
Reduction in accommodation relative to age
Stereopsis (depth perception): Lang, Frisby
- Examples of continuous text in variety of print styles, sizes and layouts – pupils can state preferences; TVI can observe working distance
- Selection of toys, threading beads, books (print and tactile)
- Coloured cloths for changing surfaces, texture of background, patterns/different colour for assessing figure/ground
Goodman, S A and Wittenstein, S H (2003) Collaborative Assessment, Working with Students who are Blind of Visually Impaired including those with Additional Difficulties. AFB Press, New York
Rowe, F (2004) Clinical Orthoptics, Blackwell Publishing Ltd.
Vision for the Future: a framework for minimum standards in visual impairment education (1995) The Scottish National Steering Committee for the Provision of Educational Standards for Children and Young People with Visual Impairment, RNIB