Advanced Visual Acuity
Improve your understanding of visual acuity measurement and find how simple it is to use LogMAR or VAS as the preferred method of VA scoring. Sure 20/20 and 6/7.5+2 are still commonly accepted, but read below to find out why the way VA is scored is changing.
There are many ways of measuring visual acuity (VA) and the Spectrum Eyecare software currently features eight internationally recognized standards. These are as follows:-
• Snellen (6/6)
• US Feet (20/20)
• UK Metric (6/6)
• EU Metric (5/5)
• Decimal (1.0)
• MAR (1.0)
• LogMAR (0)
• VAS (100)
The true ETDRS charts use a metre notation, where 4 meters was used as the testing distance; hence the numerator four being used. e.g. 4/4 vision or 4/8. For research purposes of consistency the LogMAR notation is used.
LogMAR is an abbreviation for “Logarithmic Minimum angle of resolution”.
The logic behind the LogMAR form of measurement is essentially that if a patient has 20/20 vision, or 6/6, what it means in terms of LogMAR, is that they got none wrong and hence would have a score of “0”.
Where each incorrect letter represents an additional score of 0.02, a VA score of 6/7.5 on the regular chart (meaning not seeing any on the 20/20 or 6/6 line) would result in a true LogMAR value of 0.1, (5 letters wrong * 0.02 per letter) The LogMAR score of 0.1 in simple terms would mean “1 lines worse than “0”.
The LogMAR scale scores what the patient “GETS WRONG”, rather than what the patient gets right as is the case with most other notations. If the VA scored is better than 6/6 vision, e.g. 6/4.8, they would score -0.1 on the LogMAR scale. This essentially equates to 1 line better than “0”.
The notation gets more involved, and a bit confusing when the patient gets a few letters right on the next line down. We have to start thinking rather than “gets how many letters right”, but rather “gets how many letters wrong”. For example; we no longer score a patient who has 6/9+2 in this way, but rather as 0.16. This is essentially calculated as 1 line worse than “0”, giving us the 0.1, and then for every letter MISSED on next line UP, (in this case it would be 3 letters on the 6/7.5 line), we add 0.02 for every letter they get wrong , hence the 0.06 added to the 0.1. If this patient had got 4 letters correct on the 6/7.5 line it would have been scored as 0.12, (as they only missed 1),and similarly if they got all the letters on the 6/9 and 1 extra letter on the 6/7.5 then the LogMAR score would be 0.18.
A score of 0.2 would indicate “no letters correct on the 6/7.5 line. (This would equate to 6/9 on the Snellen scale)
For LogMAR, think in terms of letters “MISSED” rather than letters “CORRECT”. Work from the bottom up, rather than the top down.
Practical exercise 1
Calculate the LogMAR score for a patient who achieves a VA of 6/12-2
They got 2 of the 5 letters wrong on the 6/12 line (LogMAR 0.3)
So his score is going to be somewhere between 6/12 (LogMAR 0.3), and 6/15 (LogMAR 0.4)
The 2 wrong is 2/5ths = 0.04 which we add to the LogMAR 0.3
So our final score becomes: 0.34
Practical exercise 2
Calculate the LogMAR score for a patient who achieves a VA of 6/24+2
They got 3 of the 5 letters wrong on the 6/19 line (LogMAR 0.5)
So his score is going to be somewhere between 6/19, (LogMAR 0.5) and 6/24 (LogMAR 0.6)
The wrong is 3/5ths = 0.06 which we add to the LogMAR 0.5
So our final score becomes: 0.56
Note: Convert to the negative. A score of 6/12+1, it is better to score as 6/9.5-4 as it will make the LogMAR values easier to understand.
VAS is probably the simplest and easiest method of VA scoring, hence is fast becoming the preferred method of scoring globally. VAS is an acronym for “Visual Acuity Score”, also known as letter count.
The VAS notation offers a “SCORE PER LETTER read CORRECTLY”, so a score of 100 essentially means that the patient got all the letters up to and including the 6/6 line correct. For every letter they get wrong as on moves up each line of the chart, they lose 5 letters and the score drops by 5 per line. For example if the patient achieves 6/12+2 on the normal notation, the VAS score would be 87. (85 for the 6/12 line and 2 points for the 2 letters on the 6/9.5 line)
Practical exercise 1
Calculate the VAS score for a patient who achieves a VA of 6/12-2
Start with the fully correct line where they got all the letters (6/15) VAS score 80
Add the number of letters they got correct on the next line down (6/12-2 is the same 6/15+3)
Therefore they got 3 of the letters correct so add 3
So his score is going to be VAS 80 + 3
So our final score becomes: 83
VA Notation Table
• In the case of the Snellen scoring system, the Numerator always indicates the testing distance.
• In the case of the LogMAR and the VAS system, the chart has 5 letters per line
• Select the Options | Settings from the top Spectrum toolbar to customize the LHS and RHS VA notations