Lecture 6 - Acuity charts and Optical low vision aids Flashcards
Snellen chart advantages and disadvantages:
Advantages
• Well known, commonly used standard clinical measure of VA & visio
- Very sensitive to blur and uncorrected refractive error
• Good size (portability)
- Easy to move closer to patient
• Has letters ‘O’ suitable for x-cyl
Disadvantages
• Unequal numbers of letters on each line (crowding not constant)
• No relationship between line size & number of letters - from 1 to B
• Poor control of ‘contour interaction’
• Unequal letter size progression- 1.2X increase from 6/5 to 6/6, 1.67 increase from 6/36 to 6/60
• Designed to measure ‘normal’ acuity
• Scale intervals change at non-standard distances (crowding inconsis
Bailey-Lovie (logMAR) advantages and disadvantages:
Advantages
• Size, letter and line spacing equivalent throughout chart
• Equal numbers of letters per row (5)
Sensible progression of letter size (0.1 log or 1.25X)
• All letters have equal legibility
• Constant crowding for all VA levels
• Final score takes into account all letters that have been read successfully
Disadvantages
• LogMAR not used routinely as a clinical measure of VA
• Scoring and conversion not as easy as Snellen
• there’s no O letter for X-cyl
• A bit on the big side and may be hard to illuminate
Whats the size difference between 2 lines on logMAR chart?
• The size difference between 2 lines on a logMAR chart is 0.1 log unit so it follows: 10 0.1 = 1.25 -> the size difference between the lines is equal to factor 1.25
• NB. This only works for the viewing distance the chart is designed for, any other distance requires a conversion factor.
What is MAR:
MAR refers to an angle (minimum angle of resolution) and not letter size.
• Log score decreases with improving VA.
• Size change from line to line = factor 1.25
- 3 lines - approximate doubling of VA (1.25 x 1.25 x 1.25 = 1.95->~ 2)
• Letter-by-letter scoring
- each line = 0.1 log units & 5 letters per line so each letter = 0.02 log (5 x 0.02 = 0.1 )
• Viewing distance not part of notation as opposed to Snellen!
• Because logMAR defines an angle the acuity score must be adjusted for the viewing distance if the chart is used at a different distance from the one it is designed for.
Using LogMAR charts at reduced viewing distance:
Rule of thumb:
Every time the viewing distance is halved add 0.3 to the logMAR -score.
Example (logMAR chart designed for 6 m)
- Px views chart at 6 m
- Reads the 0.8 line + 2 letters on the line below
- logMAR VA is 0.8 - 0.04 = 0.76
Px views chart at 3 m
- Px now reads 0.5 line + 2 letters on the line below
- logMAR VA is 0.5 - 0.04 = 0.46 + correction factor of 0.3 for moving from 6 to 3 metres -> 0.46 + 0.3 = 0.76
What correction needs to be made when viewing distance is reduced by factor 1.25x?
• i.e 6m (1.25x) = 4.8m
0.1 logmar needs to be added for each 1.25x viewing distance reduced
What are other distance acuity tests we may use?
• Sheridan-Gardener
• Glasgow Acuity Cards
• Kay picture test
Function of Near vision charts
To distinguish ‘functional reading acuity’, the ability to read paragraphs of text, from NVA (threshold acuity), the ability to resolve letters or words. Different near charts are designed to assess different aspects of ‘reading’ or ‘VA’.
Faculty of opthalmologists Times new Roman near chart:
• N-Point Notation
•Standard reading chart widely used
• Based on printing standards
• 1 point is 1/72”
• Top of ascending to bottom of descending limb of letter
• Times Roman type face
•Easy to understand and convert e.g N24 = twice size of N12
• Blocks of text
Bailey-Lovie word-reading chart
• LOgMAR chart
• 2 to 6 words on a line
• Unrelated words, no guessing
• Limited number of words on line - good test of reading
• Difficult for children, those with poor English or poor cognition
Keeler A system
• Log scale
• A1 to A20 (A7 = newsprint)
• A1 = 5 min arc @ 25cm (or 1’ gap size)
• Smaller print is more widely spaced
•Estimates the magnification assuming Pt wants to read A7
Institute of Optometry Near Chart (IOO CHART)
• Rapid assessment of acuity
• Text chart - random order words
• Words children can recognise
• Approximately log change in letter size
Practical near acuity chart (PNAC)
• Designed to establish low vision patients near acuity rapidly
• Large range of print sizes; N80 to N5
• Uses ‘N° notation & as logMAR
•Regular progression of print size, line spacing 0.1 log MAR
• Equal numbers of words & letters
• Words related therefore makes sense & practical test, easy for children (4years+) to recognise
MNREAD (Minnesota Low-Vision Reading Test)
• Continuous-text reading-acuity charts
• Measures reading speed and accuracy as a function of print size
• Clinician can establish the magnification that will allow the highest reading speed
