University of Edinburgh
 

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

Visual acuity
Contrast sensitivity
Visual field

The Visual Pathway

the visual pathway

Visual Acuity

– 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

snellen chart

Figures from Snellen Chart

6/60 20/200

6/36 20/100
6/24 20/50
6/18 20/40
6/12 20/30
6/9 20/25
6/6 20/20
6/5 20/15

What does 6/60 actually mean?

6/60
6 = The distance between the test and the patient.

6/60
The size of letter which is visible to a normal eye at 60 metres

Tests based on Snellen Principle

Letter recognition/letter matching

letter matching

Picture Recognition/matching

picture 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

step size change

Variable Crowding

  • Single optotypes versus linear
  • Recognised that its easier to see one letter in isolation when compared to when in a cluster.

variable crowding

Cambridge Crowding Cards – contour interaction

cambridge crowding

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

logmar chart

Minimum Angle of Resolution

Note that to 'see' the E requires all of the limbs of the E to be resolved

angle of resolution


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.

Advantages?

  • 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)

Scoring System

  • 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

Dual record

  • 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

Gold Standard
Testing Plates (x2)
Screening plates

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

Refractive Error

  • 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

refractive error

Refraction – Streak Retinoscope

streak retinoscope

What other figures do we need to understand?

Glasses Prescriptions

R = +3.50/-1.50 x 75
L = +2.75/-1.00 x 80

+ values correct longsightedness
- values correct shortsightedness

Contrast Sensitivity

  • 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
  • Important

Contrast Sensitivity Function

contrast sensitivity

function chart

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
  • Cataract

There are many ways to measure Contrast Sensitivity

  • Hiding Heidi
  • Lea Symbols
  • Pelli-Robson chart

Hiding Heidi- Preferential looking test

hiding heidi

Lea Symbols

lea symbols

Visual Field

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.

visual fields

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

Visual Field
Common behaviours observed in children with field defects

  • Tilting and turning head
  • Bumping into objects
  • Losing items easily
  • Clumsy
  • 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

goldmann perimetry

Goldmann

  • 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

humphrey perimetry

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

field plots

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