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assessment of vision 1 > Week 2: Spatial Vision, Distance/Near Visual Acuity > Flashcards

Week 2: Spatial Vision, Distance/Near Visual Acuity Flashcards

1
Q

What is visual acuity and list reasons why we would do it.

A
  • 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

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2
Q

What is the visual acuity measured in?

A
  • Snellen acuity
  • LogMAR
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3
Q

What are some special cases for tests if px cannot do/see the largest letter on the chart?

A
  • Count fingers
  • Hand movements = HM
  • Light perception = LP
  • No light perception = NLP
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4
Q

What are the different types of visual acuity tasks?

A
  1. Detection
  2. Resolution/Grating Acuity
  3. Recognition Acuity
  4. Localisation Visual Acuity
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5
Q

Explain detection acuity and give an example

A
  • 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)

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6
Q

What are some tests conducted for detection acuity in children?

A
  • Candy Test / Coin Test / Bead Test
  • Forced Preferential Looking (FPL)
  • STYCAR: Screening Test for Young Children & Retards
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7
Q

Explain resolution/grating acuity and give an example

A
  • Identification of critical feature
  • Discrimination of two or more stimuli
  • E.g. resolution of the gap in a Landolt C
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8
Q

Explain LEA grating test

A
  • It is a resolution/grating acuity test
  • Black on white grating which requires discrimination of direction of long lines
  1. Simultaneous presentation: similar to Forced-choice Preferential-Looking
  2. Reveal pattern: if can resolve pattern, eyes follow pattern
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9
Q

Explain recognition acuity and give an example

A
  • 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)
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10
Q

Explain localisation visual acuity and give an example

A
  • Discrimination of object location in relation to other objects
  • Detection of misalignment
  • E.g. “Is the picture straight on the wall?”,
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11
Q

Explain what the numerator and denominator are for Snellen measurement. Snellen = d/D

A
  • d = Numerator = test distance (in metres)
  • D = Denominator = distance at which the detail would subtend one minute of arc
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12
Q

Threshold for resolving a sinusoidal grating is affected by?

A
  1. Spatial Frequency
  2. Luminance Contrast
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13
Q

High spatial frequency vs. low spatial frequency

A
  1. Loss of contrast in retinal image is greater at high spatial frequencies than at low reducing recognition
  2. Contrast sensitivity reaches a maximum at 4-6 cycles/degree
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14
Q

Explain point of spread function of the eye?

A
  • Optics of the eye spread light so point of light forms normal distribution on retina
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15
Q

What is Rayleigh’s Criteria formula and explain each unit?

A

aradians = 1.22 λ/d

Where; λ = wavelength (in metre)
d = pupil diameter (in metre)

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16
Q

Explain the relationship between small pupils and visual acuity

A
  • 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
17
Q

Differences between Snellen & LogMAR Chart

A

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

18
Q

What is the patten of LogMAR from negative and larger numbers

A
  • Negative/lower numbers = better vision
  • Larger/positive numbers = worse than 6/6 acuity
19
Q

What is visual acuity rating (VAR). List the formula as well.

A
  • VAR = 100 – (50 x logMAR)
  • Designed to be used with logMAR charts
  • Better vision = larger value
20
Q

Explain the process of performing measurement of visual acuity

A

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++

21
Q

What does pinhole provides?

A
  • 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
22
Q

List the different types of near vision chart

A
  1. Snellen
  2. LogMAR
  3. Jaeger numbers
  4. M Point
  5. N-Point
23
Q

Explain Jaeger numbers for near vision

A
  • No numerical meaning
  • Recorded with a J followed by the ‘size’
  • Smaller numbers = smaller print sizes (J1 – J20)
24
Q

Explain M Point for near vision

A
  • Best option for easier conversion
  • Recorded as Snellen fraction
25
Q

Explain N Point for near vision

A
  • Letter size measured in points where 1 point = 0.35 mm
  • Measured from top to bottom or bottom to top
  • Letters in Times Roman
26
Q

How do you take measurements for near visual acuity?

A
  • Visual acuity measured within arm’s length/working distance
  • Must specify chart type & test distance and smallest line/ letter read
  1. Patient holds near reading card at habitual reading distance
  2. Record distance using tape measure
  3. Children: clinician holds reading card for them
27
Q

Advantages & Disadvantages of Near Charts (refer to summary notes)

A

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

assessment of vision 1 (10 decks)

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