W2 Distance visual acuity

Question 1

WHY DO WE MEASURE VISUAL ACUITY?

Answer 1

Visual acuity: most commonly used clinical measure of visual function, can be used in
the clinical assessment and ongoing management of patients to:
o Compare against age-norms/previous measurements
o Monitor disease progression via changes in acuity
o Assess treatment success via stabilisation or improvement in acuity
o Legally define disabilities e.g. low vision and blindness
o Used medico-legally e.g. driving standard, occupations

Question 2

What are the different types of visual acuity tasks?

Answer 2

1. Detection – is an object present or not, e.g. Visual field test
2. Resolution – identification of critical feature, e.g. direction of gap in Landolt C, Tumbling E
3. Recognition – ability to recognize an object (usually letter), may not be related to acuity (agnosia/prosopagnosia), e.g. Snellen test
4. Localisation – discrimination of object location in relation to other objects, e.g. stereopsis, vernier acuity, and other hyperacuities (bisection etc.)

Question 3

What is detection acuity?

Answer 3

o Presence /absence of a target
o Applicable to real life situations e.g. is there something on the road
o Visual field tests – light on or off in peripheral vision
o These are detection tasks – (based on change in brightness (luminance) between object and its background) and are not true spatial recognition tasks

o Candy Test /Coin Test /Bead Test: pick up smallest seen
o Forced Preferential Looking (FPL)
o STYCAR: Screening Test for Young Children and Retards: balls rolled across contrasting
floor, pursuit eye movements indicate object seen

Question 4

What is Resolution Acuity?

Answer 4

Also known as Grating Acuity
o Discrimination of two or more stimuli
o Resolution of the gap in a Landolt C
o Contrast Sensitivity Function high frequency cut-off, from 40 to 60 cycles/degree in young healthy adult- is an example of grating acuity

Other examples are checkerboard, orientation of limbs of Tumbling E

Question 5

What is a resolution acuity chart?

Answer 5

o LEA Grating Test – black on white grating which requires discrimination of direction of long lines
1. Simultaneous presentation (like Forced-choice Preferential-Looking, FPL): which one do they look at
2. Reveal pattern: if can resolve pattern, eyes follow pattern
o Presented at different distances to determine distance at which direction of lines can be perceived = Cycles per degree (CPD)

Question 6

What is recognition acuity test?

Answer 6

oSnellen: introduced in 1862 by a Dutch ophthalmologist, Herman
Snellen
oMost common measure of visual acuity
oAlso has elements of resolution acuity
oAdult charts generally use letter naming
oSnellen, Bailey-Lovie chart, Early Treatment of Diabetic Retinopathy (ETDRS) chart
oRestricted number of letters: usually 10 (Sloan letters: have more or less equal recognition difficulty)

Question 7

What is a letter construction: snellen chart?

Answer 7

o Letters constructed on a 5x5 or 5x4 grid
o Each limb is one fifth of the letter height
o Size of Snellen letters is designated by Snellen
notation
o Numerator = test distance (in metres)
o Denominator = distance at which the detail
(limb width or gap width) would subtend one
minute of arc

Question 8

snellen acuity theory:

Answer 8

o Is a fraction. Recorded VA = test distance/distance at which test detail subtends 1’ arc
o Snellen acuity: Numerator (above the line) = test distance (in metres)
o Denominator = distance at which the detail (limb width or gap width) would subtend one minute of arc

Question 9

SPATIAL FREQUENCY & LUMINANCE CONTRAST

Answer 9

The threshold for resolving a sinusoidal grating is affected by
1. Spatial Frequency
2. Luminance Contrast
o High spatial frequency vs. low spatial frequency
1. The loss of contrast in the retinal image is greater at high spatial frequencies
than at low frequencies reducing recognition (see the right side of the graph below)
2. Contrast sensitivity reaches a maximum at 4-6 cycles/degree

Question 10

What is a LOCALIZATION VISUAL ACUITY

Answer 10

o Detection of misalignment
o Is the picture straight on the wall?
o Is the upper line to the left or right of the bottom line?
o Ask patient to detect differences in location of features
o Resolution greater than that calculated from photoreceptor spacing/separation: processed at level of the visual cortex
o E.g. vernier acuity, stereopsis (depth
perception)

Question 11

Snellen vs LogMar

Answer 11

o Snellen:
o Different number of letters per row (10 on 6/6 line, 3 on 6/24 line)
o Uneven increase in letter size from one line to the next
o 6/6 to 6/7.5 = 25% change, 6/7.5 to 6/9 = 20% change
o 2-line increase may be 50% (6/ 9 to 6/6) or 80% (6/9 to 6/5).
o More widely understood
o Usually ends at 6/5
o Is a fraction, so easy to work with in some ways

LogMAR/EDTRS:
o Same number of letters per row (five letters per row)
o Equal spacing of the rows on a log scale (the rows are separated by
0.1 log unit)
o Equal spacing of the letters on a log scale
o Individual rows balanced for letter difficulty
o Easier to work with statistically
o Better repeatability, and sensitivity to interocular differences

Question 12

DIFFERENCES BETWEEN SNELLEN & LOGMAR CHARTS