Visual acuity Flashcards
What are 5 stages of assessing visual acuity?
- check DISTANCE unaided
- chest DISTANCE with distance prescription
- check DISTANCE with pinhole (if <0.2 logmar, <6/9.5)
- check NEAR unaided
- check NEAR with near prescription
What are 2 options for distnace vision testing for literate adults?
- Snellen
- Logmar
What are 2 options for near visual acuity testing for literate adults?
- Test type N chart
- Logmar
What are 3 distance visual acuity tests for illiterate adults?
- Keeler Logmar
- Sheridan Gardiner
- Kay picture test
What is 1 near visual acuity tests for illiterate adults?
- Reduced Sheridan Gardiner
What are 3 distance visual acuity tests for children age 3 years and older?
- LogMAR
- Keeler LogMAR
- Kay picture test
What are 2 tests for distance visual acuity for children age 2 years and above?
- Kay picture test (single optotype)
- Cardiff cards
What is 1 test that can be used for near visual acuity testing for children 2 years and older?
Reduced Kay picture test
What are 3 types of tests which can be used for visual acuity testing in babies/infants?
- preferential looking tests - Keeler, Teller, cardiff cards
- Clinical tests - fixing + following, objection to occlusion, picking up fine objects
- Electrodiagnostic tests (EDTs): VEPs, alternating chequerboard of varying frequency
What are 3 preferential looking tests that can be used in babies/infants?
- Keeler
- Teller
- Cardiff cards
What are 3 clinical tests of visual acuity in babies/infants?
- fixing and following
- objection to occlusion
- picking up fine objects
What are 2 electrodiagnostic tests that can be used for babies/infants VA testing?
- visual evoked potential (VEP)
- response to alternating chequerboard of varying frequency
What distance of arc is subtended by the optotypes (letters) of Snellen charts when read at the distance for the line?
5min of arc (each component of letter subtends 1min)
What is normal Snellen acuity in the young eye?
1min of arc or 6/6
What may Vernier acuity be up to in the young eye?
5s of arc
What change in Snellen acuity should be regarded as significant?
change of 2 lines
What is decimal acuity from a Snellen chart?
the numerator divided by the denominator - not be confused with LogMAR notation (lower value = better acuity, reverse of decimal notation)
What do LogMAR charts record?
the logaritm of the minimum angle of resolution
What type of chart is the LogMAR chart based on and how many letters are there per row?
Bailey-Lovie LogMAR chart - 5
Which LogMAR chart is in common usage?
Ferris modification, known as ETDRS chart
What are 3 advantages of LogMAR chart over Snellen?
- All letters are equally legible
- Controls crowding phenomenon with 5 letters on each line and appropriate separation
- There is logical geometric progression of resolution
What do the lines of the logMAR chart score?
- starting with 1.0 line at the top (snellen 6/60), each letter read and scored
- each correct line (worth 0.1U) or each correct letter (worth 0.02U) is subtracted from 1.0 to give final score
- 6/6 corresponds with 0.0 score
At what distance is the logMAR chart typically used?
4m
What is the logmar to Snellen conversion?
What is crowding?
phenomenon where neigbouring targets interfere as proximity increases
Which patients are particularly susceptible to crowding and what can be done to help this?
Amblyopic patients; single optotype tests e.g. Sheridan Gardiner rather than multiple test (e.g. Snellen)
How much refractive error can be neutralised by a pinhole?
up to 3DS of refractive error
What is the structure of a pinhole?
stenopaeic aperture, typically 1.2mm in diameter
When is the use of a pinhole test useful?
to determine whether lower than expected VA is due to optical, rather than patholoical, cause
How is near (reading) acuity usually tested?
various charts available - paragraphs of text read at pt usual reading distance (30-40cm)
How is near visual acuity recorded and what is the normal range?
N notation - corresponds to point size of text being read, typical range from N5 - N48 (M notation used in USA - 1M = N48)
What does a text size of N5 correspond to in terms of visual acuity?
6/12 when read at 30cm
What are 2 types of near acuity units in the USA?
- M notation
- J point (Jaeger) - print size for same J point different between charts
How should vision be tested if it is <6/60?
- walk pt metre by metre towards chart (or chart to patient).
- if <0.5/60, test ability to detect hand movements
- if less than this test light perception (PL) with bright light
- if PL present try all 4 quadrants and ask pt to point to which quadrant light is perceived as arising from (PL with accurate projection)
Why should going straight from <6/60 to counting fingers be avoided?
big spectrum between 6/60 to 0.5/60, makes real difference to patients, no adequately represented by CF
What do Cardiff cards look like?
What do Kay picture cards look like?
What do Keeler cards look like?
What do Sheridan Gardiner cards look like?
Waht do Teller cards look like?
