C L I N I C A L
A N D
E X P E R I M E N T A L
OPTOMETRY EDITORIAL
Measuring visual acuity is not as simple as it seems Clin Exp Optom 2014; 97: 1–2
Barry L Cole AO PhD MAppSc BSc LOSc FAAO Chairman Editorial Board Clinical and Experimental Optometry E-mail: [email protected]
Measuring visual acuity is the most ubiquitous and useful test in the optometric armamentarium and one of the easiest tests to do. It is a good predictor of the presence or absence of refractive error and of a good number of sight-threatening ocular diseases. Visual acuity is vitally important for monitoring the progression of ocular disease. Reduced visual acuity after refraction signals the need to find an explanation. Visual acuity is a good measure of visual capability in everyday life and best of all, it is measured on an easy to understand scale. It is surprising that it took so long for a test of visual acuity to be devised. Glasses were invented nearly 800 years ago and for the next 700 years, spectacle makers and opticians worked industriously to improve the quality of spectacles and of course, continue to do so. The science of measuring refractive error did not flower until the 19th century, so it was not until 1862 that Hermann Snellen developed his visual acuity chart. The Spanish friar Benito Daze de Valdes, who is credited with writing the first optometric textbook, The Use of Glasses, in 1623 saw the need for a measure of visual acuity.1 He used a visual acuity test that involved counting mustard seeds but the idea did not catch on. Eduard von Jaeger developed his standard reading cards and his J scale a few years before Snellen but it was Snellen’s chart and notation that brought about a standard universal language for expressing visual acuity. A problem soon emerged: the design of the Snellen letter chart was not standardised. Aspects of Snellen’s letter chart were respected over the ensuring years, such as a 5 × 5 or 5 × 4 matrix for letters, limb width one-fifth of the letter height and the Snellen notation but there was no standardi-
DOI:10.1111/cxo.12123
sation of letter size sequences, chart layout and optotype design.2 Snellen charts varied wildly for the next 100 years. They were useful to individual practitioners but by no means an international metric. Numerous attempts were made to rectify this problem. Several international ophthalmological congresses from 1872 to 1922 attempted to do so.3 The American ophthalmologist, John Green in 1867 advocated use of non-serif letters, 11 letters per line and logarithmic progression of size, but the world was not yet ready for this. Louise Sloan in 1959 found that not all letters were of equal legibility and proposed a set of 10 equi-legible letters. The British Standards Institute issued a standard for visual acuity charts in 1968 but still there was great variability in acuity chart design. It is a matter of national pride that it was two Australian optometrists, Ian Bailey and Jan Lovie, who had the right idea at the right time. Working in the National Vision Research Institute of Australia, they developed the Bailey–Lovie logMAR visual acuity chart in 1976.4 It took the ophthalmic world by storm. Their principles for the design of a visual acuity chart covered all the bases so that letter size is the only significant variable in measuring visual acuity. The principles included non-serif letters on a 5 × 4 matrix, a limited family of equi-legible letters, the same number of letters in each line, a logarithmic progression of letter size in 0.1 log steps (constant ratio of size), proportional spacing of lines and letters to hold contour interaction constant and letter-by-letter scoring. Their ideas had something of a setback because the US National Academy of Science National Research Council Working Group 39 produced its report on the measurement of visual acuity a few years later, in 1980, without reference to the Bailey and Lovie paper. The Working Group got some things right but by no means all; however, good ideas always triumph. About 1978, the
© 2013 The Author Clinical and Experimental Optometry © 2013 Optometrists Association Australia
Early Treatment of Diabetic Retinopathy Study (ETDRS) Group was planning a multicentre research study and saw the need for a standard visual acuity chart in each participating centre. It adopted all of the Bailey– Lovie principles in devising the ETDRS logMAR chart, although there are some differences from the Bailey–Lovie chart (for example, four metre test distance, 5 × 5 letter matrix instead of 5 × 4, Sloan letters instead of BSI letters). The ETDRS chart is probably the best known logMAR chart and has firmly ensconced the Bailey–Lovie principles. The principles were also adopted by the International Council of Ophthalmology in 1984. The dutiful reader may have got this far, perhaps with some interest in being reminded of the history of measuring visual acuity but might well be asking if there is any practical import to this editorial. Researchers whose experiments have a measurement of visual acuity as a variable have adopted the logMAR principles universally. Many optometrists have logMAR charts in their consulting rooms. Is it all done and dusted? The point of this editorial is to ask practising optometrists to do a self-audit of their measurement of visual acuity. Have you moved with the times, from the 19th to the 21st century or do you still use one of those wildly diverse Snellen charts? If you use a logMAR chart do you use it to its full advantage?
SELF-AUDIT Here is a series of questions for a self-audit: • Do you use a logMAR chart for routine clinical work? If not perhaps you should. • Do you have a standard protocol for testing? Do you point to letters? Do you allow patients to correct their response? Research studies do not allow these. Do you encourage guessing? It is usually encouraged. When do you stop testing? One recommendation for best sensitivity Clinical and Experimental Optometry 97.1 January 2014
1
Editorial Cole
is to stop when four errors are made in a line. • Do you know the luminance of the chart you use? Perhaps it is not a critical factor: in the range of 40 to 600 cd/m2 a doubling of luminance improves visual acuity by one letter in a five-letter line.5 British Standard BS 4274-1 2003 recommends a minimum luminance of 120 cd/m2 but other countries differ (85 cd/m2 to 300 cd/m2); however, make sure that the luminance is bright rather than dull and that it is uniform. • Do you record the number of correct responses in the smallest line read either as number of letters read or by adding 0.02 log units for each letter read? More precise recording of visual acuity gives better sensitivity to change allowing more precise monitoring of visual acuity for patients with progressive ocular disease. If visual acuity is recorded only to the nearest line, the clinician can only be sure (95% confidence) that there is a real difference, if visual acuity has changed by two lines (0.20 log units). Letter-by-letter scoring gives confidence (95%) that a five letter change of visual acuity is a real difference.6 • What do you do when the 1.0 logMAR letters cannot be read? Do you reduce the distance? Just recording ‘< 6/60’ is no longer acceptable practice. • When no letters can be read do you hold up fingers, wave hands and flash lights for eyes? This is very qualitative and imprecise. Have you thought of using the Berkeley Rudimentary Vision test (Precision Vision, USA)? It is a very clever alternative to finger counting and hand waving.7 It is a pretty new test: why not be among the first to adopt it? • Do you use a logMAR reading chart? • Do you use the logMAR system for predicting magnification needed for patients with low vision? A recent paper by Bailey and Lovie8 is well worth reading and helpful if you choose to engage in a self-audit of your methods of measuring visual acuity. It might also be the time to reflect again on the terminology we use to differentiate the various conditions, under which visual ability is measured, as the Editor of this journal enjoined us to do five years ago.9 He deprecates use of the term ‘best corrected visual acuity’ and its terrible acronym BCVA, since visual acuity is by definition the best spatial resolution of the eye and the words Clinical and Experimental Optometry 97.1 January 2014
2
‘best corrected’ are redundant. He advocates use of ‘presenting vision’ and ‘unaided vision’ for the other two common circumstances, in which vision is measured using a visual acuity chart.
2. 3.
Computerised visual acuity charts If you use a computer display for measuring visual acuity, there are some words of caution in the Bailey and Lovie-Kitchin paper8 that should be considered carefully. It is presently impractical to display a full logMAR chart on a computer screen: it would need to be a very large and very high resolution monitor. A solution to this is to display a line at a time or single letters, perhaps surrounded by contour interaction bars. The risk with this is that visual acuity measurement will again descend into such a morass of meaningless diversity as software geeks develop yet another ‘app’ for visual acuity. The result will be that measurement of visual acuity on one chart will not be reproducible with another; however, Bailey and Lovie-Kitchin8 report that the E-ETDRS chart, a one-at-a-time letter computer chart with flanking bars correlates well with the standard ETDRS chart.
4.
5.
6.
7.
8. 9. 10.
English translation of the 1923 translation into modern Spanish by Runge PE is published as part of the series Hischberg J ed. History of Ophthalmology. Belgium: G Schmidt, 2004. Bennett AG. Ophthalmic test types. Br J Physiol Opt 1965; 22: 238–271. The College of Optometrists. Test charts. URL: http://www.college-optometrists.org/en/ knowledgecentre/museyeum/online _exhibitions/optical_instruments/charts.cfm. Bailey IL, Lovie JE. New design principles for visual acuity charts. Am J Optom Physiol Opt 1976; 53: 740– 745. Sheedy JE, Bailey IL, Raasch TW. Visual acuity and chart luminance. Am J Optom Physiol Opt 1985; 61: 595–600. Bailey IL, Bullimore MA, Raasch TW, Taylor HR. Clinical grading and the effect of scaling. Invest Ophthalmol Vis Sci 1991; 32: 422–432. Bailey IL, Jackson AJ, Minto H, Greer RB, Chu MA. The Berkeley Rudimentary Vision Test. Optom Vis Sci 2012; 89: 1257–1264. Bailey IL, Lovie-Kitchin JE, Visual acuity testing: from laboratory to the clinic. Vis Res 2013; 90: 2–9. Collin HB. Is BCVA an invention of ophthalmology? Clin Exp Optom 2008; 91: 425–426. Anstice NS, Thompson B. The measurement of visual acuity in children: an evidence-based update. Clin Exp Optom 2013; doi: 10.1111/cxo.12086.
Visual acuity of young children Thought should also be given to the measurement of visual acuity of young children. Testing visual acuity of children presents its own challenges with issues such as holding the child’s attention, accounting for cognitive immaturity and the lack of comprehensive age norms. The classic tumbling E and Landolt’s ring were devised in the late 19th century so the visual acuity of patients who are illiterate by reason of their very young age, lack of education or intellectual disability can be measured. Since then, quite a number of tests have been especially devised for testing the visual acuity of very young children. The lead paper10 in this issue of Clinical and Experimental Optometry is on this subject and is well worth your attention. It critically reviews the various kinds of visual acuity tests for children in the light of published evidence. Among other matters the paper shows the extent to which paedriatic visual acuity tests conform to logMAR principles. The paper provides the opportunity for clinicians to review the suitability of the children’s tests they use. REFERENCES 1. De Valdes BD. The use of eye glasses (Uso de los Antoiios). Originally published in Spanish in 1623.
© 2013 The Author Clinical and Experimental Optometry © 2013 Optometrists Association Australia
A N D
E X P E R I M E N T A L
OPTOMETRY EDITORIAL
Measuring visual acuity is not as simple as it seems Clin Exp Optom 2014; 97: 1–2
Barry L Cole AO PhD MAppSc BSc LOSc FAAO Chairman Editorial Board Clinical and Experimental Optometry E-mail: [email protected]
Measuring visual acuity is the most ubiquitous and useful test in the optometric armamentarium and one of the easiest tests to do. It is a good predictor of the presence or absence of refractive error and of a good number of sight-threatening ocular diseases. Visual acuity is vitally important for monitoring the progression of ocular disease. Reduced visual acuity after refraction signals the need to find an explanation. Visual acuity is a good measure of visual capability in everyday life and best of all, it is measured on an easy to understand scale. It is surprising that it took so long for a test of visual acuity to be devised. Glasses were invented nearly 800 years ago and for the next 700 years, spectacle makers and opticians worked industriously to improve the quality of spectacles and of course, continue to do so. The science of measuring refractive error did not flower until the 19th century, so it was not until 1862 that Hermann Snellen developed his visual acuity chart. The Spanish friar Benito Daze de Valdes, who is credited with writing the first optometric textbook, The Use of Glasses, in 1623 saw the need for a measure of visual acuity.1 He used a visual acuity test that involved counting mustard seeds but the idea did not catch on. Eduard von Jaeger developed his standard reading cards and his J scale a few years before Snellen but it was Snellen’s chart and notation that brought about a standard universal language for expressing visual acuity. A problem soon emerged: the design of the Snellen letter chart was not standardised. Aspects of Snellen’s letter chart were respected over the ensuring years, such as a 5 × 5 or 5 × 4 matrix for letters, limb width one-fifth of the letter height and the Snellen notation but there was no standardi-
DOI:10.1111/cxo.12123
sation of letter size sequences, chart layout and optotype design.2 Snellen charts varied wildly for the next 100 years. They were useful to individual practitioners but by no means an international metric. Numerous attempts were made to rectify this problem. Several international ophthalmological congresses from 1872 to 1922 attempted to do so.3 The American ophthalmologist, John Green in 1867 advocated use of non-serif letters, 11 letters per line and logarithmic progression of size, but the world was not yet ready for this. Louise Sloan in 1959 found that not all letters were of equal legibility and proposed a set of 10 equi-legible letters. The British Standards Institute issued a standard for visual acuity charts in 1968 but still there was great variability in acuity chart design. It is a matter of national pride that it was two Australian optometrists, Ian Bailey and Jan Lovie, who had the right idea at the right time. Working in the National Vision Research Institute of Australia, they developed the Bailey–Lovie logMAR visual acuity chart in 1976.4 It took the ophthalmic world by storm. Their principles for the design of a visual acuity chart covered all the bases so that letter size is the only significant variable in measuring visual acuity. The principles included non-serif letters on a 5 × 4 matrix, a limited family of equi-legible letters, the same number of letters in each line, a logarithmic progression of letter size in 0.1 log steps (constant ratio of size), proportional spacing of lines and letters to hold contour interaction constant and letter-by-letter scoring. Their ideas had something of a setback because the US National Academy of Science National Research Council Working Group 39 produced its report on the measurement of visual acuity a few years later, in 1980, without reference to the Bailey and Lovie paper. The Working Group got some things right but by no means all; however, good ideas always triumph. About 1978, the
© 2013 The Author Clinical and Experimental Optometry © 2013 Optometrists Association Australia
Early Treatment of Diabetic Retinopathy Study (ETDRS) Group was planning a multicentre research study and saw the need for a standard visual acuity chart in each participating centre. It adopted all of the Bailey– Lovie principles in devising the ETDRS logMAR chart, although there are some differences from the Bailey–Lovie chart (for example, four metre test distance, 5 × 5 letter matrix instead of 5 × 4, Sloan letters instead of BSI letters). The ETDRS chart is probably the best known logMAR chart and has firmly ensconced the Bailey–Lovie principles. The principles were also adopted by the International Council of Ophthalmology in 1984. The dutiful reader may have got this far, perhaps with some interest in being reminded of the history of measuring visual acuity but might well be asking if there is any practical import to this editorial. Researchers whose experiments have a measurement of visual acuity as a variable have adopted the logMAR principles universally. Many optometrists have logMAR charts in their consulting rooms. Is it all done and dusted? The point of this editorial is to ask practising optometrists to do a self-audit of their measurement of visual acuity. Have you moved with the times, from the 19th to the 21st century or do you still use one of those wildly diverse Snellen charts? If you use a logMAR chart do you use it to its full advantage?
SELF-AUDIT Here is a series of questions for a self-audit: • Do you use a logMAR chart for routine clinical work? If not perhaps you should. • Do you have a standard protocol for testing? Do you point to letters? Do you allow patients to correct their response? Research studies do not allow these. Do you encourage guessing? It is usually encouraged. When do you stop testing? One recommendation for best sensitivity Clinical and Experimental Optometry 97.1 January 2014
1
Editorial Cole
is to stop when four errors are made in a line. • Do you know the luminance of the chart you use? Perhaps it is not a critical factor: in the range of 40 to 600 cd/m2 a doubling of luminance improves visual acuity by one letter in a five-letter line.5 British Standard BS 4274-1 2003 recommends a minimum luminance of 120 cd/m2 but other countries differ (85 cd/m2 to 300 cd/m2); however, make sure that the luminance is bright rather than dull and that it is uniform. • Do you record the number of correct responses in the smallest line read either as number of letters read or by adding 0.02 log units for each letter read? More precise recording of visual acuity gives better sensitivity to change allowing more precise monitoring of visual acuity for patients with progressive ocular disease. If visual acuity is recorded only to the nearest line, the clinician can only be sure (95% confidence) that there is a real difference, if visual acuity has changed by two lines (0.20 log units). Letter-by-letter scoring gives confidence (95%) that a five letter change of visual acuity is a real difference.6 • What do you do when the 1.0 logMAR letters cannot be read? Do you reduce the distance? Just recording ‘< 6/60’ is no longer acceptable practice. • When no letters can be read do you hold up fingers, wave hands and flash lights for eyes? This is very qualitative and imprecise. Have you thought of using the Berkeley Rudimentary Vision test (Precision Vision, USA)? It is a very clever alternative to finger counting and hand waving.7 It is a pretty new test: why not be among the first to adopt it? • Do you use a logMAR reading chart? • Do you use the logMAR system for predicting magnification needed for patients with low vision? A recent paper by Bailey and Lovie8 is well worth reading and helpful if you choose to engage in a self-audit of your methods of measuring visual acuity. It might also be the time to reflect again on the terminology we use to differentiate the various conditions, under which visual ability is measured, as the Editor of this journal enjoined us to do five years ago.9 He deprecates use of the term ‘best corrected visual acuity’ and its terrible acronym BCVA, since visual acuity is by definition the best spatial resolution of the eye and the words Clinical and Experimental Optometry 97.1 January 2014
2
‘best corrected’ are redundant. He advocates use of ‘presenting vision’ and ‘unaided vision’ for the other two common circumstances, in which vision is measured using a visual acuity chart.
2. 3.
Computerised visual acuity charts If you use a computer display for measuring visual acuity, there are some words of caution in the Bailey and Lovie-Kitchin paper8 that should be considered carefully. It is presently impractical to display a full logMAR chart on a computer screen: it would need to be a very large and very high resolution monitor. A solution to this is to display a line at a time or single letters, perhaps surrounded by contour interaction bars. The risk with this is that visual acuity measurement will again descend into such a morass of meaningless diversity as software geeks develop yet another ‘app’ for visual acuity. The result will be that measurement of visual acuity on one chart will not be reproducible with another; however, Bailey and Lovie-Kitchin8 report that the E-ETDRS chart, a one-at-a-time letter computer chart with flanking bars correlates well with the standard ETDRS chart.
4.
5.
6.
7.
8. 9. 10.
English translation of the 1923 translation into modern Spanish by Runge PE is published as part of the series Hischberg J ed. History of Ophthalmology. Belgium: G Schmidt, 2004. Bennett AG. Ophthalmic test types. Br J Physiol Opt 1965; 22: 238–271. The College of Optometrists. Test charts. URL: http://www.college-optometrists.org/en/ knowledgecentre/museyeum/online _exhibitions/optical_instruments/charts.cfm. Bailey IL, Lovie JE. New design principles for visual acuity charts. Am J Optom Physiol Opt 1976; 53: 740– 745. Sheedy JE, Bailey IL, Raasch TW. Visual acuity and chart luminance. Am J Optom Physiol Opt 1985; 61: 595–600. Bailey IL, Bullimore MA, Raasch TW, Taylor HR. Clinical grading and the effect of scaling. Invest Ophthalmol Vis Sci 1991; 32: 422–432. Bailey IL, Jackson AJ, Minto H, Greer RB, Chu MA. The Berkeley Rudimentary Vision Test. Optom Vis Sci 2012; 89: 1257–1264. Bailey IL, Lovie-Kitchin JE, Visual acuity testing: from laboratory to the clinic. Vis Res 2013; 90: 2–9. Collin HB. Is BCVA an invention of ophthalmology? Clin Exp Optom 2008; 91: 425–426. Anstice NS, Thompson B. The measurement of visual acuity in children: an evidence-based update. Clin Exp Optom 2013; doi: 10.1111/cxo.12086.
Visual acuity of young children Thought should also be given to the measurement of visual acuity of young children. Testing visual acuity of children presents its own challenges with issues such as holding the child’s attention, accounting for cognitive immaturity and the lack of comprehensive age norms. The classic tumbling E and Landolt’s ring were devised in the late 19th century so the visual acuity of patients who are illiterate by reason of their very young age, lack of education or intellectual disability can be measured. Since then, quite a number of tests have been especially devised for testing the visual acuity of very young children. The lead paper10 in this issue of Clinical and Experimental Optometry is on this subject and is well worth your attention. It critically reviews the various kinds of visual acuity tests for children in the light of published evidence. Among other matters the paper shows the extent to which paedriatic visual acuity tests conform to logMAR principles. The paper provides the opportunity for clinicians to review the suitability of the children’s tests they use. REFERENCES 1. De Valdes BD. The use of eye glasses (Uso de los Antoiios). Originally published in Spanish in 1623.
© 2013 The Author Clinical and Experimental Optometry © 2013 Optometrists Association Australia
