Visual Acuity Flashcards

Question

What is LogMAR?

Answer 1

Bailey and Lovie expressed VA in terms of the logarithm of the angular limb width in minutes of arc at 6m. Gold Standard method for assessing Visual Acuity

Answer 2

1 minute of Arc

Answer 3

6/12 20/40 = Subtends 5 minutes of Arc = 2 minutes Arc = log 2 = 0.3 = 0.50 6/12 letter subtends 5 minutes of arc, which equates to a MAR of 2 minutes of Arc and a logMAR of 0.3.

Answer 4

5 letters on each line This ensures that the task is equivalent for each row and helps to ensure equal contour interaction. Spacing between each letter and each row is related to the width and height of the letters respectively Constant progression of letter sizes ratio 10√10 VA is logarithm of the angular limb width of smallest letters recognised at 6m Logarithmic Progression in 0.1 LogMAR intervals so equal scale. After LogMAR 0 there are 3 lines of smaller size, having negative LogMAR values (-0.1, -0.2 and -0.3) – the angular sub tense is less than 1 minute of arc The MAR is taken as the stroke width of the letters, which is 1/5th of their vertical angular subtense. Provides more letters for patients with poor VA.

Answer 5

Log10 of the Minimum Angle of Resolution

Answer 6

Test charts illuminated at a level where Acuity does not alter greatly with change in illumination. To avoid glare, surrounding illumination should be at a similar level. Room lights on. VA affected by poor contrast and poor illumination. Contrast on charts should be a minimum of 90% International standard ISO 8596 – standard test type, Landolt C in a logMar progression Full illumination in the room (chart luminance between 80 to 320 cd/m2)

Answer 7

1) Ensure the chart is at the appropriate distance and is calibrated correctly. 2) Leave room lights on 3) Full illumination in the room (chart luminance between 80 to 320 cd/m2) 4) Seat the patient comfortably with an unobstructed view of the test chart. 5) Sit in front (and off to the side) of the patient in order to monitor facial expressions and reactions. 6) If you are going to measure both vision and habitual VA, measure vision first to avoid memorisation. 7) Measure the VA of the poorer eye first. 8) Occlude left eye with occluder (measure RE VA first). 9) Explain what measurement you are about to take. This can be as simple as “Now we shall find out what you can see in the distance.” 10) Instruct patient: “Please read the smallest line of letters that you can see on the chart.” 11) Note down value and swap occluder

Answer 8

Monitor patients facial expressions. Encourage patient to read a little more.

Answer 9

Do not get patient to use their hand to cover their eye. Do not allow the patient to peep around the occluder. Do not allow your patient to screw up their eye or squint as this can invalidate results.

Answer 10

1) If less than 6/60, record at 3m. If less than 3/60, record at 1m Get the patient to move 1m nearer to the chart, then even closer if letters are still not visible. 2) Count Fingers held at some specified distance. 3) If less than 1/60, record Hand movements (HM): Move a hand at measured distance (record distance) 4) Light Projection (Lproj): Pen light is held at 50cm away from 8 different directions. Ask the patient to report the direction of light and record the areas of the visual field that a patient has light perception. 5) Light Perception (LP): - If the pen light is perceived, but not the direction, note down LP. - If the light is not perceived, then note down NLP.

Answer 11

A unit of angular measurement equal to 1 degree. Since one degree is of a turn, one minute of arc is of a turn.

Answer 12

UK - Metric System | USA - Imperial System

Answer 13

1. Vernier Acuity 2. Stereo Acuity 3. Children's Visual Acuity Charts

Answer 14

d / D Exemplar -- Snellen 6 / 12 = Decimal 0.5

Answer 15

d / Decimal Acuity d = Testing Distance Exemplar -- Decimal 0.5 = 6 ÷ 0.5 = 12 Snellen = 6/12

Answer 16

Log10 (D ÷ d) If Snellen = 6/12 LogMAR = Log10 (12/6) = 0.30log units

Answer 17

Each individual letter is assigned a score value of 0.02log units. 5 letters per line Total score for a line on the LogMAR chart represents a change of 0.1 log units

Answer 18

lowest line completed correctly - (number of letters read on line below x 0.02)

Answer 19

Conversion factor for testing distance that is not 6m New d = Log10(6/d)

Answer 20

d / D x New distance

Answer 21

1) They might be unable to recognise letters 2) They might be unable to verbalise what they see 3) They might lack concentration 4) They might dislike occluding of one eye

Answer 22

For young children who are unfamiliar to letters - | Ages 2 1/2 to 5yrs

Answer 23

Optotypes/Shapes which are considered to be easily recognisable by this age group. Recognition booklet provided to help determine how child interprets shapes and to encourage familiarisation.

Answer 24

6m Does NOT work well with mirror

Answer 25

Single Optoytype test and therefore there are no crowding effects.

Answer 26

Figures or letters of different sizes used in testing the acuity of vision.

Answer 27

Grating preferential looking test

Answer 28

Sheridan-Gardener Test Revised Version

Answer 29

The patterns used are Square wave gratings in terms of their Spatial Frequency

Answer 30

The patterns used are Square wave gratings in terms of their Spatial Frequency

Answer 31

Number of black and white pairs in each degree of Visual Angle. The higher Spatial Frquency, The finer the grating

Answer 32

3m No mirror required

Answer 33

- Multi-Optotype Crowded and Single Optotype Uncrowded version - Portable - Useful for slightly older children - External Illumination

Answer 34

Snellen Modified LogMAR

Answer 35

Children aged 18 months +

Answer 36

* The symbols are uniform in detail, line width and overall size. * The test is based on 4 symbols: house, heart, circle and square which blur equally so all symbols are equally sensitive to blur and equally difficult to distinguish. This means when blurred all the symbols appear to the observer to be circles. * The patient can still perceive answering correctly, while the examiner can easily detect the VA threshold without revealing to the child the failure of not recognizing the symbol.

Answer 37

Designed to measure VA in toddlers aged 1-3years. Can be used in older children and adults with Intellectual Impairments.

Answer 38

Vanishing Optotypes

Answer 39

White band boarded by two black bands, each half the width of the white band on a neutral grey background. Average luminance of target is equal to that of the grey background.

Answer 40

If the target lies beyond the patients Acuity limit, it merges with the grey background and becomes invisible. Targets are all of the same size but decrease in width of the black and white bands. The acuity is given by the narrowest white band for which the target is visible (see back of card). If a non-verbal patient is being tested, the practitioner must observe the fixation.

Answer 41

1) The examiner holds the card in front of the child (1 m or 50 cm) and is unaware of the location of the target (bottom or top). 2) The examiner judges where in which direction the child is looking (up or down). 3) If the direction of gaze of the child and the position of the target are the same, another card with finer lines is presented.

Answer 42

Preferential looking

Answer 43

An infant will choose to look towards a target rather than towards a plain stimulus when they are visually attentive.

Answer 44

'minimum recognisable ability'

Answer 45

'minimum resolvable ability'

Answer 46

As we get older, the crystalline lens within the eye stiffens up so its ability to move and focus at nearer distances becomes harder.

Answer 47

Near VA measurements do not measure an acuity threshold. The measurements can be approximately related to Snellen distance acuity, but this is difficult in reading-test types. Reading of a near chart involves different perceptual processes than the recognition of single letters.

Answer 48

The measurement is quicker than a threshold measurement.

Answer 49

In the printing industry, the size of a typeface is specified by ‘points’. One point is 1/72 of an inch The height of a typeface is known, and hence the angular subtense at a working distance can be calculated. An approximation to Snellen Acuity can be made.

Answer 50

* These letter sizes were denoted by a number indicating the point size and prefixed by an ‘N’ * Starts at N5 up to N48 * N8 is approximately equal to 6/15, generally the size of a tabloid newspaper - benchmark

Answer 51

* These letter sizes were denoted by a number indicating the point size and prefixed by an ‘N’ * Starts at N5 up to N48 • N8 is approximately equal to 6/15, generally the size of a tabloid newspaper - benchmark

Answer 52

• The distance in m at which it subtends an angle of 5 minutes of arc. • This is the same as the ‘D’ in the Snellen fraction So if the height of the 6m letters on a standard Snellen chart is 8.73mm, then the notation is 6M 1M is therefore 8.73/6 or 1.45 mm. • This is very near the N8 size – 1M is therefore approximately equal to N8

Answer 53

Bailey and Lovie designed a logMAR Near chart that used Times Roman typeface. There are 17 sizes from 14.5mm (N80) to 0.36mm (N2) There are unconnected words instead of continuous reading material.

Answer 54

* Long paragraphs, N5 to N48 * Do not progress logarithmically * No recommended distance

Answer 55

* The most common ‘N’ notation chart * Book * Should be held at 35cm

Answer 56

Designed especially for children at different reading ages. Each age has been giving a grade, numbered 1 - 7, so that if a child's reading ability is not equal to the average age, it can still be recorded by the grade number. Within each grade, specimens of printing sizes N5, N6, N8, N10, N12, N14, N18, N24, N36 and N48 have been used. Differentiates between children's ability to see and their ability to read. The test is based upon the Ladybird Keywords reading scheme using school script, which is a simplified type written without serifs.

Answer 57

* Unaided near VA (near vision) * Habitual near VA (with the patient’s own spectacles) * Optimal near VA (with best refractive correction)

Answer 58

* At least two of these should be performed at every eye examination for each eye and then binocularly. * The N notation must be recorded together with a reading distance e.g N8 at 35cm R and L

Answer 59

1. Room lights on and explain: “Now we shall find out what you can see at close distances” 2. Measure RE first (or ‘poorer’ eye if known) 3. Instruct Px: “please place the chart at your normal reading distance” 4. Hold the occluder in place and ask Px: “please read the smallest paragraph that you can see” 5. If appropriate, prompt Px: “Try and read some of the words on the smaller paragraph” 6. Repeat for LE and binocularly

Answer 60

1. Room lights on and explain: “Now we shall find out what you can see at close distances” 2. Measure RE first (or ‘poorer’ eye if known) 3. Instruct Px: “please place the chart at your normal reading distance” 4. Hold the occluder in place and ask Px: “please read the smallest paragraph that you can see” 5. If appropriate, prompt Px: “Try and read some of the words on the smaller paragraph” 6. Repeat for LE and binocularly

Answer 61

You are measuring ‘near vision adequacy’ rather than a true near VA.

Answer 62

separate contours that are approximetly 1.75mm apart.

Answer 63

The numerator, 6 is the distance in metres between the patient and the testing chart. The denominator, x is the distance in metres at which a patient with 6/6 Acuity would discern the same optotype.

Answer 64

A person with 6/6 vision would discern the same optotype from 12 metres away (i.e. at twice the distance). This is equivalent to saying that with 6/12 vision, the person possesses half the spatial resolution and needs twice the size to discern the optotype.

Answer 65

The angle a viewed object subtends at the eye in degress of arc.

Answer 66

Visual Performance

Answer 67

Defines the ability of the eye to visually discern fine detail in a moving object.

Answer 68

This measures the ability to discern a misalignment among 2 line segments /gratings.

Answer 69

Is the smallest detectable depth difference that can be seen in BV.

Answer 70

1. Presence of objects 2. Observation 3. Selection 4. Organisation 5. Interpretation 6. Response

Answer 71

Acuity declines towards the periphery in an inverse-linear fashion (i.e. the decline follows approximately a hyperbola).

Answer 72

Writing system used for Russian language and various langauges across Eurasia.

Answer 73

N = [50 × W × Tan(D / 360)] / 0.1875

Answer 74

D = 360 × Arctan[(0.1875 × N) / (50 × W)]

Answer 75

This test involves the letter C positioned in either up, down, right or left and thus the patient has to indicate which one it is.

Answer 76

- Does not demand literacy | - Avoids the difficulties of unsuitable alphabets

Answer 77

If uncorrected Astigmatism is present, recognition of the gap is easier sometimes.

Answer 78

This test involves the letter E positioned in either up, down, right or left and thus the patient has to indicate which one it is.

Answer 79

The test has 9 letters: A,C,H,L,O,T,U,V and X; these letters were chosen because they can be easily copied by young children. The child is given a key cad with the appropriate choice of letters.

Answer 80

This test uses 3 symbols free from directional bias: a square, a circle and an equilateral triangle. The presentation is by means of a book with two to four symbols on each page. Cut out symbols are given to the child.

Answer 81

A hyperopic shift is common in older patients and some varifocal wearers adapt to this change by habitually raising their chin to improve distance vision by viewing through the additional plus power in the intermediate section of the lens. This can be seen when assessing habitual VA as these varifocal wearers will raise their chin during measurements to improve VA. Vas should be measured through the distance portion of the lens.

Answer 82

1. Light perception e.g. the threshold of vision in the normal eye 2. Discrimination: ability of the visual system to distinguish an object from its background 3. Form vision and recognition: ability to identify letters and words 4. Resolution: ability to see in detail, the smallest interval measurable 5. Localisation: object situation compared to other objects 6. Higher tasks: stimulation of other responses e.g. motor response in handling something

Answer 83

1) Target Detection: The presence or absence of a stimuli 2) Target Localisation: Discriminating differences in the spatial position of segments of a test object, such as a break or discontinuity in contour. 3) Target Recognition: The recognition or naming of a target

Answer 84

* An image of a letter in focus on the retina has sharp lines * An image of a letter out of focus forms a blur circle for each point of the object

Answer 85

A trial case accessory

Answer 86

Used to determine whether reduced VA is due to refractive error or ocular disease.

Answer 87

The pinhole reduces the effect of ametropic blur by reducing effective pupil size

Answer 88

then the reduced VA is due to uncorrected refractive error

Answer 89

If the retinal image is out of focus PH will reduce the size of the blur circle.

Answer 90

A trail case accessory

Answer 91

Can be used to locate the principle meridians in the astigmatic eye of a patient with marked lens opacities; provided that the individual blurs composing the retinal image are sufficiently elongated, rotation of the slit will enable a ‘best position’. - Reduces effective pupil size in the meridian that is perpendicular to the slit. - Place slit in a correctly centred trial frame and rotate until the patient identifies the orientation that gives rise to maximum clarity on the letter chart.

Answer 92

An opaque disc with two pinholes separated by a distance less than the pupil diameter e.g. 2-4mm apart.

Answer 93

1. The disc is placed close to the eye and the patient looks at a small distant spotlight 2. The view that the patient sees i.e. the 2 separated images can indicate refractive error

Answer 94

- If the image is doubled, the patient sees 2 separate images, then indicates myopia/hyperopia - If the 2 images seen appear reversed/crossed this indicates hyperopia - If uncrossed indicates myopia

Answer 95

The central bright circular region of the pattern produced by light diffracted when passing through a small circular aperture. The central disk is surrounded by less intense concentric rings, so light intensity takes local maxima and mimina while it decreases away from the center.

Visual Acuity Flashcards

(119 cards)