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
- Screening
Looking for signs of unsuspected disorders or losses - Investigation
Pursuing diagnoses and testing suspicions of disorders or losses - Monitoring
Following stability or instability of known disorder or loss - Predicting functional visual abilities
- Assessing potential optical /visual and non-visual interventions
- Advising on capacities and limitations
- 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
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
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
Design Features
- Symbol selection UK vs USA
- Row progression, Size & Spacing
- Test distance 6M vs 4M
- Lighting Internal vx External
Visual Acuity Test Charts (Children)
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
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Visul Acuity charts
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 Classification
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.
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
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
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 and Low Contrast Acuity Tests
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
(Weber : Lmax-Lmin/Lmax)
Target Presentations
Search Task
Blinking Task
Berkley Blinking Squares
Male, age 49, Low Myope, VAR 104 (20/16) Normal
Female, age 23, Low Myope, VAR 85 (20/40) RP U23
Whats New in Contrast Sensitivity ?
Berkeley Discs
Mixed Contrast Cards
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
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)