Week 2: Spatial Vision, Distance/Near Visual Acuity Flashcards
What is visual acuity and list reasons why we would do it.
- Most commonly used clinical measure of visual function
- Measure of ability to resolve detail, discriminate fine details of a target
Reasons:
- Compare against age-norms
- Monitor disease progression via changes in acuity
- Assess treatment success
- Legally define disabilities
- Used medico-legally
What is the visual acuity measured in?
- Snellen acuity
- LogMAR
What are some special cases for tests if px cannot do/see the largest letter on the chart?
- Count fingers
- Hand movements = HM
- Light perception = LP
- No light perception = NLP
What are the different types of visual acuity tasks?
- Detection
- Resolution/Grating Acuity
- Recognition Acuity
- Localisation Visual Acuity
Explain detection acuity and give an example
- An object/target present or not
- Applicable to real life situations
E.g. visual field tests (detection tasks based on change in brightness between object & its background)
What are some tests conducted for detection acuity in children?
- Candy Test / Coin Test / Bead Test
- Forced Preferential Looking (FPL)
- STYCAR: Screening Test for Young Children & Retards
Explain resolution/grating acuity and give an example
- Identification of critical feature
- Discrimination of two or more stimuli
- E.g. resolution of the gap in a Landolt C
Explain LEA grating test
- It is a resolution/grating acuity test
- Black on white grating which requires discrimination of direction of long lines
- Simultaneous presentation: similar to Forced-choice Preferential-Looking
- Reveal pattern: if can resolve pattern, eyes follow pattern
Explain recognition acuity and give an example
- Ability to recognise an object (usually letters)
- Most common measure of visual acuity
- Adult charts generally use letter naming (e.g. Snellen, LogMAR, Bailey-Lovie, ETDRS)
Explain localisation visual acuity and give an example
- Discrimination of object location in relation to other objects
- Detection of misalignment
- E.g. “Is the picture straight on the wall?”,
Explain what the numerator and denominator are for Snellen measurement. Snellen = d/D
- d = Numerator = test distance (in metres)
- D = Denominator = distance at which the detail would subtend one minute of arc
Threshold for resolving a sinusoidal grating is affected by?
- Spatial Frequency
- Luminance Contrast
High spatial frequency vs. low spatial frequency
- Loss of contrast in retinal image is greater at high spatial frequencies than at low reducing recognition
- Contrast sensitivity reaches a maximum at 4-6 cycles/degree
Explain point of spread function of the eye?
- Optics of the eye spread light so point of light forms normal distribution on retina
What is Rayleigh’s Criteria formula and explain each unit?
aradians = 1.22 λ/d
Where; λ = wavelength (in metre)
d = pupil diameter (in metre)
Explain the relationship between small pupils and visual acuity
- When pupil is less than 1-2 mm diffraction occurs
- Light waves spread out and create rings or spikes around the object
- Small pupils: diffraction limited
- Large pupils: aberration limited
Differences between Snellen & LogMAR Chart
Snellen:
N.o letter = varies
Line spacing = varies
Inter-letter spacing = varies
Letter sizing progression = variable progression
Advantages = familiarity, incorporates test distance, fraction
Disadvantages = variables
LogMAR:
N.o letter = 5
Line spacing = consistent 0.1 log unit
Inter-letter spacing = consistent progression
Letter sizing progression = consistent progression
Advantages = statistically easy to work with, easy to score
Disadvantages = not consistently used clinically
What is the patten of LogMAR from negative and larger numbers
- Negative/lower numbers = better vision
- Larger/positive numbers = worse than 6/6 acuity
What is visual acuity rating (VAR). List the formula as well.
- VAR = 100 – (50 x logMAR)
- Designed to be used with logMAR charts
- Better vision = larger value
Explain the process of performing measurement of visual acuity
Starting Visual Acuity:
- Start with unaided then move onto aided
- Start testing monocularly, starting with right eye, left eye, then binocularly
- Cover one eye with cover paddle (clinician should be covering NOT patient)
- Ask the patient to read letters
When to Stop:
- Patient makes more than 4 or more mistakes on a line
Record in Snellen Notation (Snellen Fraction):
- Account for partial line reads: e.g. 6/9++
What does pinhole provides?
- Improves vision up to a point
- Creates ‘blur circle’ on retina
- Pinholes eliminate all but straight line beams of light which falls directly on retina giving a clear image
List the different types of near vision chart
- Snellen
- LogMAR
- Jaeger numbers
- M Point
- N-Point
Explain Jaeger numbers for near vision
- No numerical meaning
- Recorded with a J followed by the ‘size’
- Smaller numbers = smaller print sizes (J1 – J20)
Explain M Point for near vision
- Best option for easier conversion
- Recorded as Snellen fraction
Explain N Point for near vision
- Letter size measured in points where 1 point = 0.35 mm
- Measured from top to bottom or bottom to top
- Letters in Times Roman
How do you take measurements for near visual acuity?
- Visual acuity measured within arm’s length/working distance
- Must specify chart type & test distance and smallest line/ letter read
- Patient holds near reading card at habitual reading distance
- Record distance using tape measure
- Children: clinician holds reading card for them
Advantages & Disadvantages of Near Charts (refer to summary notes)
Advantages:
Bailey-lovie = LogMAR, words
Snellen = Can use matching tests with children
EDTRS = LogMAR
Continuous Text = Replicates reading, can
convert N and distance
to M and Snellen
Disadvantages:
Bailey-lovie = 25 cm only, unrelated
words
Snellen = 40 cm only, letters only
EDTRS = 40 cm only, letters only
Continuous Text = May not reflect acuity, crowding
Ranges of sizes tested:
Bailey-lovie = LogMAR 0 to 1.6 (6/240)
Snellen = 6/6 to 6/60
EDTRS = -0.3 – 1.3
Continuous Text = 0.32 to 3.2 M
