Functional Assessment of Vision
Presented 19-20 March 2008
Do The Figures Add Up?
Clinical Assessment of Vision
Dr Jennifer Skillen, PhD
VI Scotland, Edinburgh
Specialist Orthoptist – NHS Fife
Structure of Functional Vision Assessment
- Get background information
- Structured functional history
- Observe behaviour
- Structured Vision Assessment
- Discuss & write report
1. Does your child recognise you before you speak?
2. Does your child recognise other family members?
3. Does your child recognise friends?
4. Does your child recognise people from photographs?
5. Can your child identify him/herself from photographs?
6. Can your child recognise shapes?
7. Can your child recognise objects?
8. Can your child name colours?
9. Can your child match colours?
10. Can your child find his/her way around the house?
11. How often does he/she ask for directions around the home?
12. Does he/she lose objects around the house?
13. Can your child find his/her way around new surroundings?
14. How often does he/she ask for directions in new surroundings?
15. Does he/she have difficulty reaching out for and grasping objects?
16. Does your child have difficulty distinguishing a line from a step?
17. Is your child able to see moving objects or are they seen only when they are stationary?
18. Does your child have difficulty seeing objects when he/she is moving quickly him/herself?
19. Can your child find objects on a patterned carpet?
20. Can your child find objects in complex pictures?
21. Does your child eat food from only one part of the plate and ignore the rest?
22 Does he/she misjudge going through doorways or along corridors?
Structured Vision Assessment
The Visual Pathway
– Snellen Acuity
- First developed in 1862
- Still the most widely used clinical test to measure visual acuity (at current time)
- Measures VISUAL ACUITY using RECOGNITION
Figures from Snellen Chart
What does 6/60 actually mean?
6 = The distance between the test and the patient.
The size of letter which is visible to a normal eye at 60 metres
Tests based on Snellen Principle
Letter recognition/letter matching
Why change now? – Snellen Design Flaws
- Some letters are easier to read than others – a O is easier to discern than a H or a X
- Step size from 6/60
to 6/24 differs from 6/9 to 6/6
implications for monitoring treatment
- Task differs from reading 6/60 line
when compared to 6/6
Degree of Crowding variable
Step size change – inconsistent
- Single optotypes versus linear
- Recognised that its easier to see one letter in isolation when compared to when in a cluster.
Cambridge Crowding Cards – contour interaction
Next Generation Vision Test - LogMAR
- Basis in vision research labs 1980's
- Equal step size
- Equal legibility
- Equal crowding
Statistical analysis of changes in VA
Now becoming commonplace in hospital eye departments
LogMAR Vision Charts
- New design
- New recording system
- The log of the Minimum angle of resolution
Minimum Angle of Resolution
Note that to 'see' the E requires all of the limbs of the E to be resolved
Log of 1 = O SN 6/6 = LogMAR 0
Log of 10 = 1 SN 6/60 = LogMAR 1
SAME Principle of LMAR and SN
High numbers equate to Poorer VA
TRAP - Tendency is to think that a score of zero means no vision /poor vision.
- Regular progression of letter size
- The ratio difference between one line and the next is 0.1 log units
- Equates to a factor difference of 1.26
- Vary test distances and accurately change the scoring systems
that is, by halving the distance you double the score value (closer to the target the worse the vision and the higher the score)
- The numbers of letters on each line remain constant with each letter identified correctly added to the score
- If 5 letters on a line each letter will get 0.020 score
- If 4 letters each letter will equate to 0.025
- 0.200 equates to 6/9.5 (all letters)
- 0.325 equates to 6/12 (3 letters from 4)
- 0.850 equates to 6/38 (2letters from 4)
- Notice one letter correct on the line will give a higher value = lower vision than reading all 4 correct
- Note – 6/38 is not a Snellen values as no such thing as 6/38 on a Snellen chart.
Portable Vision Assessment using LogMAR
Testing Plates (x2)
Suitable adults and children
Tests based on LogMAR - Kay Picture tests
What if they cant letter match or picture match?
What is detectable?
Preferential Looking: Gratings
- Measured in cycles per cm (cpcm)
- Or one cycle per degree
- One cycle is a black and a white stripe
- 4 stripes per cm
- 2 cycles per cm
- Light rays are focussed at back of eye
- Light rays focussed at a point prior to back of eye
- Light rays focussed at a point behind the back of the eye
Refraction – Streak Retinoscope
What other figures do we need to understand?
R = +3.50/-1.50 x 75
L = +2.75/-1.00 x 80
+ values correct longsightedness
- values correct shortsightedness
- This is a measure of how well a child can see shades of grey against white
- Rarely performed in routine clinical practice
- Relatively easy to perform
Contrast Sensitivity Function
Why is it important?
- Communication, Facial expressions
- Orientation and Mobility, Kerbs
- Accessing information, Reading Text
- Activities of daily living, Eating
Conditions with loss of contrast
- Optic Atrophy
- Retinal Dystrophies
- Corneal conditions
There are many ways to measure Contrast Sensitivity
- Hiding Heidi
- Lea Symbols
- Pelli-Robson chart
Hiding Heidi- Preferential looking test
What are Visual Fields?
The Visual Fields are a measure of the area from which you are able to perceive visual signals, when your eyes are in a stationary position and looking straight ahead.
Normal Adult dimensions
50-60 deg superiorly
70- 75 deg inferiorly
60 deg nasally
90-100 deg temporally
Why measure fields?
- Understanding of functional abilities
- Help diagnose a vision/brain condition
- Monitor treatment/progression of condition
Common behaviours observed in children with field defects
- Tilting and turning head
- Bumping into objects
- Losing items easily
- Hesitates when using stairs and tips head down
- Difficulty locating food on a plate
Goldmann Perimetry – Kinetic testing
- Light stimulus is moved from outside of bowl towards the centre
- Manually operated
- Light source can be varied in size and brightness
- Start with I4E brightest and smallest light
- And then reduced brightness of target to I3E and then I4E – checking sensitivity
- If peripheral field reduced – increase size of target III4E until using the largest target V4E
- Result is isopters
Humphrey -Static Perimetry
- Computerised/automated test
- Light stimulus is randomly flashed within a number of ‘static’ positions
- If light is not seen – it will be presented again at a higher intensity
- Light source can vary in brightness
- Calculation on age matched normal data is carried out
Sample field plots - static
- High number = good ( good sensitivity to seeing light) - Dark/grey area corresponds to area with reduced sensitivity to light target
- Darker area = absolute scotoma/Lighter area = relative scotoma
- Related to age related norm – in terms of SD
What are we measuring?
- The integrity of the visual information – starting at the retina and reaching the visual cortex (located at the back of the brain within the occipital lobe)
- Looking at the pattern of field loss – we can get an idea of where a problem may be occurring within the eyes and/or the brain.
See also confrontation