University of Edinburgh
 

The Assessment of Visual Functions in Children with Visual Impairment

Presented on Saturday, 11 December 2010

Assessment of Vision in Children

Prof A J Jackson PhD, MCOptom, FAAO, FBCLA

Purposes for measuring visual functions

  1. Screening
    Looking for signs of unsuspected disorders or losses
  2. Investigation
    Pursuing diagnoses and testing suspicions of disorders or losses
  3. Monitoring
    Following stability or instability of known disorder or loss
  4. Predicting functional visual abilities
    • Assessing potential optical /visual and non-visual interventions
    • Advising on capacities and limitations
  5. Socio-legal
    • determinining eligibilities (benefits, compensation, privileges, classify, categorize)

Methods of Assessment

  • Visual Acuity (Distance Vision)
  • Near Acuity (Reading Acuity)
  • Contrast Sensitivity (Shades of Grey)
  • Visual Fields (Peripheral Vision)
  • Colour Vision

Visual Acuity (Distance)

  • The assessment of the eyes' ability to resolve spatial detail.
  • The assessment of the eyes central visual status using optotypes.
  • A measure of the angular size of detail that is just resolvable by the eye.

Unaided, Best Corrected, Habitual

Limitations of the Visual System

  • Optical
  • Diffraction = Blur Circle Diameter
  • Optical Aberrations
  • Media Clarity
  • Incident light wavelength

pupil size

Neural

  • Retinal photoreceptor layout
  • Cortical wiring

Minimal Distinguishable Difference 
a) Point 10-30" of Arc 
b) Lines 0.5-5" of Arc

Minimal Separable 
a) Grating acuity 50-65" of Arc 
b) Vernier acuity 3-10" of Arc (Hyper Acuity)

Minimal Recognisable (Legible)
Clinical Test of Visual Resolution using Optotypes

Landolt C's Illiterate E's Snellen Letters
- (Font Styles Font Layout)

Visual Acuity Test Charts

test charts

Visual Acuity Expression

VA= Test Distance
Distance at which the smallest letter read subtends 5" of Arc

Snellen Fraction (metric(m)/imperial(ft)/decimal notation/Log format)

6/6= 20/20 =1.0 =LogMAR 0.0

test distance

Design Features

  • Symbol selection UK vs USA
  • Row progression, Size & Spacing
  • Test distance 6M vs 4M
  • Lighting Internal vx External

LogMar

Visual Acuity Test Charts (Children)

children's tests

So what difference does the test chart really make?

Test Retest on a series of 9 conventional LogMAR based externally illuminated acuity charts and 4 flip charts, under standardised testing conditions (Good-lite, Precision & Hong Kong). Presentation orders were randomised. 22 Normally Sighted Individuals with current Rx.

Test Chart Design: Optotype layout

optotype layout

  • Sloan Letters ETDRS (Precision)
  • Landolt Cs (Precision)
  • Tumbling Es (Precision)
  • HOTV (Precision)
  • Numbers
  • Bailey Lovie (UCB) BS Letters
  • PV Numbers (Precision)
  • Lea Numbers ( Good-Lite)
  • Patti Pics (Precision)
  • Lea Symbols (Good-Lite)

Visul Acuity charts

visual acuity

Overall Mean VAR = 105.7 + 3.2 Population profile Age 32.6+ 12.2 (Range 18-66yrs M:8 F:14)

Visual Acuity Beyond Snellen

visual acuity

Visual Acuity Classification

clinical outpatients

Visual acuities recorded from 37 eyes (using Sonksen Silver Optotype test charts at appropriate working distances) initially classified within an outpatients setting as having a visual acuity of count fingers.

b & ps register

Visual acuities, both corrected and uncorrected, as specified on A655 Blind and Partially Sighted registration returns relating to the years 2001-2004 (n=1941).

Berkeley Rudimentary Vision Test 1

  • Screening Card (S)
  • Detection of Light Card (LD)
  • Projection of Light Card (LP)
  • Grating Acuity Card (GA)

Visual Acuity (Near)

  • Letter / Optotype Acuity ( T Z 1 2 + # )
  • Word Acuity (Random allocation)
  • Text Acuity (Age Matched)
  • Reading Fluency
  • Reading Comprehension

Near Acuity Test Charts

near acuity test

Range of adult and children letter, word and text reading charts

Study Methodology

  • To assess reading speed and strategy under standardised conditions.
  • 20 Normally Sighted Individuals (Best Corrected/Binocular)
  • Low Vision consulting room (UCB)
  • Supplementary Lighting
  • Fixed distance- Reading stand with head rest
  • Bailey Lovie near word & text charts
  • Variable distance- Support stand
  • Video record time and distance data
  • Process all data

reading results

Contrast Sensitivity

  • Clinical significance apparent since the 1960s (Campbell F et al J Physiol Vol 181)
  • Initially measured using computer generated square and sine wave gratings (Time consuming)
  • Contrast Sensitivity Function
    Reciprocal of Contrast Detection Threshold Peaks at 3-5 cpd (6/30 LogMAR 0.8)
    "Cut-off spatial frequency" = VA VA 6/6 LogMAR 0.0 = 30cpd
  • CS loss in LV is a predictor of problems experienced with mobility, orientation, reading, face recognition and postural stability (RCOphth 2008)

contrast sensitivity

Contrast Sensitivity and Low Contrast Acuity Tests

contrast sensitivity

Berkeley Computerised Contrast Sensitivity Test

Selectable Modes

Detection- Blinking, Jumping

Randomly assigned to 1 of 4 quadrants

  • 40mm squares (4.5 deg at 50cm)
  • 24 contrast levels (0.0 to 2.3 logCS Weber)
  • 6 sequences of 8 targets, in 0.3 unit steps
  • 6 sec presentations, 0.5 sec pause (<2 minutes)
  • Ends with 2 consecutive misses

Search- Letters or Numbers

Randomly scattered (n=8 per screen)

  • Target size may be varied
  • 24 contrast levels over 6 presentations
  • Presentation time 20 secs per screen

Selectable Blink rate, Movement rate, Exposure time
Hardware CRT Touch Screen
Software Java
Output Response time vs Contrast graph

contrast sensitivity

(Weber : Lmax-Lmin/Lmax)

Target Presentations

search task

Search Task

blinking task

Blinking Task

Berkley Blinking Squares
Male, age 49, Low Myope, VAR 104 (20/16) Normal

blinking squares

Female, age 23, Low Myope, VAR 85 (20/40) RP U23  

blinking squares

Whats New in Contrast Sensitivity ?

Berkeley Discs

berkley discs

Mixed Contrast Cards

mixed contrast

Visual Fields

That proportion of space in which objects are simultaneously visible to the steadily fixating eye Normal-Gross Fields (75 Inf, 60 Sup, 60 Nas, 100 Temp)

  • Central vs Peripheral Fields
  • Threshold vs Suprathreshold
  • Static vs Kinetic Black & White vs Colour
  • Automated vs Manual Normal

Normal Visual Field

Visual Field

Influencing Factors

  • Physical Factors Anatomy
    (monocular/binocular)
  • Physiological Factors
    - Refractive Error
    - Pupil Size
    - Media Clarity
  • Psychological Factors
    - Response Style
    - Fatigue

Purpose of test dictates test parameters

  • Screening - for unsuspected disease
    moderate visibility target
    demonstrate blind spot
    diagonal sweeps or large zig-zag
  • Investigation - for suspected disease
    test as sensitive as possible
    target just visible in region being investigated
  • Monitoring - for change in known disease
    use same conditions as last time
    if previous conditions were not appropriate then do second field test with optimal conditions
    this becomes the new baseline
  • Functional fields - for predicting practical problems
    Large targets (and very large)
    Might change illumination conditions

Colour Vision

  • Hue, Saturation, Brightness
  • Congenital (Boys 1;8, Girls 1;200) Bilateral & Stable, VA normal)
  • Acquired (Pathology, Assymetric, Progressive, Chromatopsia and colour confusion, Visual Functions abnormal)
  • Red/Green (Protan vs Deutan)
  • Blue/Yellow (Tritan rare)