Behavioural Brahz Research, 49 (1992) 141-147 9 1992 Elsevier Science Publishers B.V. All rights reserved. 0166-4328/92/$05.00
141
BBR01324
The Rotterdam C-chart: norm values for visual acuity and interocular differences in 5-year-old children J.W.R. Pott and J. van Hof-van Duin Department of Physiology L Erasmus University Rotterdam, Rotterdam (The Netherlands) (Received 1 August 1991) (Revised version received 16 September 1991) (Accepted 3 October 1991)
Key words: Visual development; Child; Visual acuity; Acuity chart; Landolt C; Rotterdam C-chart; Interocular acuity difference
Common C-charts are often not suitable for developmental studies and assessment of visual acuity in young and visual impaired children, because of unequal steps between lines, uncomparable crowding, inadequate number of large optotypes on the first line and ceiling effects. Therefore a new C-chart (the Rotterdam C-chart) was developed on which optotypes on successive lines decreased in 1/3-octave steps (0.llog steps) and on which crowding was comparable over the entire chart. Norm values for binocular and monocular visual acuity at test distances of 6 m and 40 cm and norm values for interocular acuity differences in 5-year-old children were determined by testing 201 5-year-olds with this chart. Results were compared to those obtained in 24 young adults. Although mean binocular and monocular acuity of the 5-yearolds were significantly lower than in adults, the distribution of acuity assessments were similar in the children and adults. Therefore, the Rotterdam C-chart seems suitable for developmental studies and for the assessment of visual acuity in young and visually impaired children.
INTRODUCTION
It is well known that visual acuity improves throughout the first years of life, although opinions differ about the age that adult values are reached. In children, tested with Landolt C rings, reduced acuities have been reported up to around puberty 8"t5. Actual acuity values depend on whether Landolt C rings are presented as single optotypes, in lines or are surrounded by contours. If optotypes are presented in lines and/or are surrounded by contours, lower acuity values are obtained both in adults and in children by the so-called crowding or contour interaction effect. In adults, crowding has been reported to depend on the separation between contours and on the minimum angle of resolution (MAR) of"a subject. Maximal crowding occurs when the separation between adjacent contours and optotypes is approximately halfthe height of the optotype ~3. There is some discrepancy about the magnitude of crowding in children. Atkinson et al. 2 found the crowd-
Correspondence: J. van Hof-van Duin, Department of Physiology I, Erasmus University Rotterdam, P.O. Box 1738, 3000 DR Rotterdam, The Netherlands.
ing effect larger for 3~/2-year-old children than for 5 6-year-olds, whereas other authors found similar crowding in children from 2 to 7 years of age t t,2o. Although the presence of crowding is known to have a substantial influence on acuity assessment, most commercial available C-charts do not have comparative contour interaction over the entire chart. Usually interletter and inter-line distances are relatively large on the rows with smaller optotypes. As a result smaller optotypes are relatively easier to recognize. This gives normal adults an advantage over visually impaired subjects and over both normal and visually impaired children, since acuity assessed with letter- or C-charts is defined by the line with the smallest optotypes on which a certain number of correct responses are given. On low vision charts, suitable for developmental studies, visual requirements should be comparable all over the chart. Consequently, such a chart should meet a number of criteria. Since children's and visually impaired subjects' acuity estimates are liable to the same inaccuracies of test circumstances and test charts as adults, the same standards four assessment as recommended for adults by the Committee on Vision 7 should be applied. These include the use of Landolt-C rings (or equivalent letters or digits) presented in lines, the size of which alter in equal steps of 0.1 on a logarithmic
142 scale ( = 1/3-octave step). The change in size of optotype from one line to the next needs to progress equally and regularly throughout the chart, which is obtained when a logarithmic scale is used. On a logarithmic scale as much weight is given to acuity changes near adult values as near (very) low vision values 1~ This is not the case with linear scales. Linear scales have a coarser step size in the range of lines with large optotypes, resulting in less accurate assessment of acuity and interocular acuity differences in children and subjects with low vision. Besides, when a logarithmic scale is used, testing at non-standard distances is simplified ~2. Furthermore, crowding should be comparable over the entire chart. This is achieved by keeping the inter-letter and inter-line distances proportional to the optotype height. A second requirement for a low vision chart is that for reliable assessment of acuity in young or visually impaired children an adequate number of optotypes should be available on the lines with the larger optotypes. Most available charts only have one to three optotypes on the line with the largest optotypes. A third requirement concerns the availability of small optotypes. On many commerciable available letter and C-charts a ceiling effect is present. The highest measurable acuity valuable is often 6/5* (50 s of arc), whereas as early as 1874, Snellen and Landolt reported adults' mean acuity, assessed with letters presented in a linear array, to be 6/5 (ref. 22). As a consequence, one expects individual acuity estimates of normal adults often to be better than 6/5 (less than 50 or 40 s of arc), especially when C-rings instead of letters are used. According to the criteria discussed above, we developed a new C-chart, 'the Rotterdam C-chart', incorporating the recommendations of the Committee on Vision 7. The purpose ofour study was to obtain binocular and monocular acuity norms in 5-year-old children, not only a t 6 m but also at 0.4 m distance. Results were compared to those obtained in adults. The second aim was to examine the suitability of the Rotterdan C-chart for developmental studies. Whereas mean visual acuity of 5-year-olds can be expected to be significantly lower than of adults, comparable distributions of acuity esti-
* Visual acuity Can be expressed as the Snellen fraction, i.e. the reciprocal value of the minimum angle of resolution (MAR) in min.arc, converted for the used test distance, which is indicated in the nominator. Snelle n acuity is usually assessed at a distance of 6 m. An acuity of 6/6 indicates an acuity threshold of 1 rain.arc at 6 m. An acuity of 1.6 min.arc, if assessed at 5 m, will result in a Snellen fraction of 518. Results of the present study will be presented as M A R in min.arc.
mates in both groups is required, if the Rotterdam Cchart would be suitable for developmental studies.
SUBJECTS
Visual acuity was assessed in 201 healthy children with a mean age of 5.0 years (S.D. + 0.2 year). They were all fullterm born children whose birthweight had ranged between 2,500 and 4,500 g, and who had experienced an uncomplicated perinatal period (fullterm means gestational age ranging from 37 to 42 weeks). Children were screened for eye misalignment. Refractive state was examined by means of isotropic photorefraction without cycloplegia ~7"~8. This method has been proven to be reliable both with and without the use ofcycloplegia, when compared to results ofretinoscopy under cycloplegia ~'5. Results obtained in 15 children were excluded from calculations because photorefraction values exceeded one of the following criteria: spherical equivalent > 2.5 D; cilindrical error > 1~5 D; interocular difference of spherical equivalent > 1 D or ofcilindrical error > 1 D. Children wore their own correction if prescribed. Informed consent was obtained from the parents, who were approached for their cooperation through physicians of the Department of Youth Health Care of the Municipal Health Office of Rotterdam and Papendrecht (The Netherlands). In addition visual acuity was assessed in 24 young adults. If necessary, adults were corrected to a visual acuity of 6/6 in each eye on a standard Snellen letter chart (n = 15). All adult subjects had normal eye alignment. Informed consent was obtained from each subject.
METHODS
The Rotterdam C-chart The Rotterdam C-chart consisted of black Landolt C's on a white background. The height and width of each optotype was both five times the thickness, of the line and five times the width of the opening in the circle. The orientation of the C's was either horizontal or vertical, with a random distribution of the 4 directions over the chart. As can be seen in Fig. 1, the Landolt C's were arranged in 16 horizontal rows. The number of optotypes on each line varied from 5 on the upper row to 11 optotypes in the lower 6 rows. Successive rows decreased in equal steps of 1/3 octave**, this corre-
** An octave is a doubling or halving of the optotypeheight.
143
O00CO C O 0 0 0 0 O00CO00 OCO000C COOCCO0 oocoooo O O C O 0 0 0 C CoCOOOCOO oococooooo oooo~ooonoo
...,....~..
Fig. 1. Real size Rotterdam C-chart ~ r acuity measurements at 40cm.
sponds to steps of 0.1 on a logarithmic scale (i.e. a decrease in optotype height by a factor 2 on every third row). Comparable crowding was maintained over the chart by keeping inter-letter and inter-line distances relatively constant. The inter-letter distance on each row was 0.25 octave smaller than the horizontal diameter of each optotype. Inter-line distances were 0.26 octave smaller than the optotype height on the preceding line. The size of the chart for acuity measurements at 6 m distance was 95 cm wide by 110 cm. For acuity measurements at 0.4 m a reduced version of the large chart was used, with a size of 6.3 cm wide by 7.3 cm (reduction 1:15). An additional chart was made for near acuity measurements in visually impaired children, with a 1:3.2 reduction of the original 6 m chart. Charts were printed on photographical paper. The contrast of the optotypes was 88%. Acuity assessment Binocular and monocular acuity was assessed at 6 m and 40 cm. For 6 m assessment, the luminance of the chart was 100 cd/m 2. Subjects were seated at 6 m distance, with a helper seated beside the child, to encourage the child during testing. One of the parents was seated nearby. The examiner was positioned beside the chart and pointed at the optotypes from below, using a black pointer. The children were asked to indicate the position of the gap in the Landolt C by pointing with a finger or with the hand in the relevant direction. If the children preferred, they could use a black cardboard 'C' to mimic the indicated optotype. This was practised with large optotypes until the examiner felt confident that the child understood the procedure. Starting with lines with large optotypes, the examiner pointed out five optotypes on each row in a random order. When a child made a mistake, the examiner would go back to the previous line with larger optotypes, to keep the child's attention. After two or three trials on the preyious row
the examiner would resume testing the line with the smaller optotypes. Acuity threshold was determined by the line with the smallest optotypes on which the child could still make four correct responses out of five. For testing near acuity the luminance of the chart was 450 cd/m 2 (variations in illumination above 1.0 log cd/ m z have little effect on visual acuityg). The child was seated at a distance of 40 cm with the head supported by a headrest. Only one examiner was neaded. The procedure was similar as for distance acuity assessment. Monocular testing was achieved by means of occluding glasses. Adult assessment was identical as for the children. The same criteria were used, but adults could verbally indicate the direction of the gap in the Landolt C's. For calculation of mean acuities, variances and comparison of group means, acuity estimates were converted into the logarithm of the minimal angle of resolution (log MAR) 16"24.
RESULTS
Binocular, acuity could be assessed in 198 out of the 201 children. Failure in 3 children was due to lack of cooperation. Results of 18 children were excluded because of strabismus in 3 cases (1.5%), binocular refractive error,s in 6 children (3%), and monocular refractive errors in 9children (4.5%). Consequently, binocular and monocular acuity norms were calculated from values obtained in 180 children. Monocular assessment was possible in all children in whom binocular acuity assessment had been possible. As indicated in Table I, mean binocular acuity in 5-year-old children (1.50 min.arc, S.D. + 0.31 octave at 6 m, and 1.55 min.arc, S.D. +0.31 octave at 40 cm) appeared at both test distances significantly lower than in adults with acuities of 0.82 min.arc (S.D. + 0.27 octave) at 6 m and 0.69 min.arc (S.D. +0.31 octave) at 40 cm (t-test, P
141
BBR01324
The Rotterdam C-chart: norm values for visual acuity and interocular differences in 5-year-old children J.W.R. Pott and J. van Hof-van Duin Department of Physiology L Erasmus University Rotterdam, Rotterdam (The Netherlands) (Received 1 August 1991) (Revised version received 16 September 1991) (Accepted 3 October 1991)
Key words: Visual development; Child; Visual acuity; Acuity chart; Landolt C; Rotterdam C-chart; Interocular acuity difference
Common C-charts are often not suitable for developmental studies and assessment of visual acuity in young and visual impaired children, because of unequal steps between lines, uncomparable crowding, inadequate number of large optotypes on the first line and ceiling effects. Therefore a new C-chart (the Rotterdam C-chart) was developed on which optotypes on successive lines decreased in 1/3-octave steps (0.llog steps) and on which crowding was comparable over the entire chart. Norm values for binocular and monocular visual acuity at test distances of 6 m and 40 cm and norm values for interocular acuity differences in 5-year-old children were determined by testing 201 5-year-olds with this chart. Results were compared to those obtained in 24 young adults. Although mean binocular and monocular acuity of the 5-yearolds were significantly lower than in adults, the distribution of acuity assessments were similar in the children and adults. Therefore, the Rotterdam C-chart seems suitable for developmental studies and for the assessment of visual acuity in young and visually impaired children.
INTRODUCTION
It is well known that visual acuity improves throughout the first years of life, although opinions differ about the age that adult values are reached. In children, tested with Landolt C rings, reduced acuities have been reported up to around puberty 8"t5. Actual acuity values depend on whether Landolt C rings are presented as single optotypes, in lines or are surrounded by contours. If optotypes are presented in lines and/or are surrounded by contours, lower acuity values are obtained both in adults and in children by the so-called crowding or contour interaction effect. In adults, crowding has been reported to depend on the separation between contours and on the minimum angle of resolution (MAR) of"a subject. Maximal crowding occurs when the separation between adjacent contours and optotypes is approximately halfthe height of the optotype ~3. There is some discrepancy about the magnitude of crowding in children. Atkinson et al. 2 found the crowd-
Correspondence: J. van Hof-van Duin, Department of Physiology I, Erasmus University Rotterdam, P.O. Box 1738, 3000 DR Rotterdam, The Netherlands.
ing effect larger for 3~/2-year-old children than for 5 6-year-olds, whereas other authors found similar crowding in children from 2 to 7 years of age t t,2o. Although the presence of crowding is known to have a substantial influence on acuity assessment, most commercial available C-charts do not have comparative contour interaction over the entire chart. Usually interletter and inter-line distances are relatively large on the rows with smaller optotypes. As a result smaller optotypes are relatively easier to recognize. This gives normal adults an advantage over visually impaired subjects and over both normal and visually impaired children, since acuity assessed with letter- or C-charts is defined by the line with the smallest optotypes on which a certain number of correct responses are given. On low vision charts, suitable for developmental studies, visual requirements should be comparable all over the chart. Consequently, such a chart should meet a number of criteria. Since children's and visually impaired subjects' acuity estimates are liable to the same inaccuracies of test circumstances and test charts as adults, the same standards four assessment as recommended for adults by the Committee on Vision 7 should be applied. These include the use of Landolt-C rings (or equivalent letters or digits) presented in lines, the size of which alter in equal steps of 0.1 on a logarithmic
142 scale ( = 1/3-octave step). The change in size of optotype from one line to the next needs to progress equally and regularly throughout the chart, which is obtained when a logarithmic scale is used. On a logarithmic scale as much weight is given to acuity changes near adult values as near (very) low vision values 1~ This is not the case with linear scales. Linear scales have a coarser step size in the range of lines with large optotypes, resulting in less accurate assessment of acuity and interocular acuity differences in children and subjects with low vision. Besides, when a logarithmic scale is used, testing at non-standard distances is simplified ~2. Furthermore, crowding should be comparable over the entire chart. This is achieved by keeping the inter-letter and inter-line distances proportional to the optotype height. A second requirement for a low vision chart is that for reliable assessment of acuity in young or visually impaired children an adequate number of optotypes should be available on the lines with the larger optotypes. Most available charts only have one to three optotypes on the line with the largest optotypes. A third requirement concerns the availability of small optotypes. On many commerciable available letter and C-charts a ceiling effect is present. The highest measurable acuity valuable is often 6/5* (50 s of arc), whereas as early as 1874, Snellen and Landolt reported adults' mean acuity, assessed with letters presented in a linear array, to be 6/5 (ref. 22). As a consequence, one expects individual acuity estimates of normal adults often to be better than 6/5 (less than 50 or 40 s of arc), especially when C-rings instead of letters are used. According to the criteria discussed above, we developed a new C-chart, 'the Rotterdam C-chart', incorporating the recommendations of the Committee on Vision 7. The purpose ofour study was to obtain binocular and monocular acuity norms in 5-year-old children, not only a t 6 m but also at 0.4 m distance. Results were compared to those obtained in adults. The second aim was to examine the suitability of the Rotterdan C-chart for developmental studies. Whereas mean visual acuity of 5-year-olds can be expected to be significantly lower than of adults, comparable distributions of acuity esti-
* Visual acuity Can be expressed as the Snellen fraction, i.e. the reciprocal value of the minimum angle of resolution (MAR) in min.arc, converted for the used test distance, which is indicated in the nominator. Snelle n acuity is usually assessed at a distance of 6 m. An acuity of 6/6 indicates an acuity threshold of 1 rain.arc at 6 m. An acuity of 1.6 min.arc, if assessed at 5 m, will result in a Snellen fraction of 518. Results of the present study will be presented as M A R in min.arc.
mates in both groups is required, if the Rotterdam Cchart would be suitable for developmental studies.
SUBJECTS
Visual acuity was assessed in 201 healthy children with a mean age of 5.0 years (S.D. + 0.2 year). They were all fullterm born children whose birthweight had ranged between 2,500 and 4,500 g, and who had experienced an uncomplicated perinatal period (fullterm means gestational age ranging from 37 to 42 weeks). Children were screened for eye misalignment. Refractive state was examined by means of isotropic photorefraction without cycloplegia ~7"~8. This method has been proven to be reliable both with and without the use ofcycloplegia, when compared to results ofretinoscopy under cycloplegia ~'5. Results obtained in 15 children were excluded from calculations because photorefraction values exceeded one of the following criteria: spherical equivalent > 2.5 D; cilindrical error > 1~5 D; interocular difference of spherical equivalent > 1 D or ofcilindrical error > 1 D. Children wore their own correction if prescribed. Informed consent was obtained from the parents, who were approached for their cooperation through physicians of the Department of Youth Health Care of the Municipal Health Office of Rotterdam and Papendrecht (The Netherlands). In addition visual acuity was assessed in 24 young adults. If necessary, adults were corrected to a visual acuity of 6/6 in each eye on a standard Snellen letter chart (n = 15). All adult subjects had normal eye alignment. Informed consent was obtained from each subject.
METHODS
The Rotterdam C-chart The Rotterdam C-chart consisted of black Landolt C's on a white background. The height and width of each optotype was both five times the thickness, of the line and five times the width of the opening in the circle. The orientation of the C's was either horizontal or vertical, with a random distribution of the 4 directions over the chart. As can be seen in Fig. 1, the Landolt C's were arranged in 16 horizontal rows. The number of optotypes on each line varied from 5 on the upper row to 11 optotypes in the lower 6 rows. Successive rows decreased in equal steps of 1/3 octave**, this corre-
** An octave is a doubling or halving of the optotypeheight.
143
O00CO C O 0 0 0 0 O00CO00 OCO000C COOCCO0 oocoooo O O C O 0 0 0 C CoCOOOCOO oococooooo oooo~ooonoo
...,....~..
Fig. 1. Real size Rotterdam C-chart ~ r acuity measurements at 40cm.
sponds to steps of 0.1 on a logarithmic scale (i.e. a decrease in optotype height by a factor 2 on every third row). Comparable crowding was maintained over the chart by keeping inter-letter and inter-line distances relatively constant. The inter-letter distance on each row was 0.25 octave smaller than the horizontal diameter of each optotype. Inter-line distances were 0.26 octave smaller than the optotype height on the preceding line. The size of the chart for acuity measurements at 6 m distance was 95 cm wide by 110 cm. For acuity measurements at 0.4 m a reduced version of the large chart was used, with a size of 6.3 cm wide by 7.3 cm (reduction 1:15). An additional chart was made for near acuity measurements in visually impaired children, with a 1:3.2 reduction of the original 6 m chart. Charts were printed on photographical paper. The contrast of the optotypes was 88%. Acuity assessment Binocular and monocular acuity was assessed at 6 m and 40 cm. For 6 m assessment, the luminance of the chart was 100 cd/m 2. Subjects were seated at 6 m distance, with a helper seated beside the child, to encourage the child during testing. One of the parents was seated nearby. The examiner was positioned beside the chart and pointed at the optotypes from below, using a black pointer. The children were asked to indicate the position of the gap in the Landolt C by pointing with a finger or with the hand in the relevant direction. If the children preferred, they could use a black cardboard 'C' to mimic the indicated optotype. This was practised with large optotypes until the examiner felt confident that the child understood the procedure. Starting with lines with large optotypes, the examiner pointed out five optotypes on each row in a random order. When a child made a mistake, the examiner would go back to the previous line with larger optotypes, to keep the child's attention. After two or three trials on the preyious row
the examiner would resume testing the line with the smaller optotypes. Acuity threshold was determined by the line with the smallest optotypes on which the child could still make four correct responses out of five. For testing near acuity the luminance of the chart was 450 cd/m 2 (variations in illumination above 1.0 log cd/ m z have little effect on visual acuityg). The child was seated at a distance of 40 cm with the head supported by a headrest. Only one examiner was neaded. The procedure was similar as for distance acuity assessment. Monocular testing was achieved by means of occluding glasses. Adult assessment was identical as for the children. The same criteria were used, but adults could verbally indicate the direction of the gap in the Landolt C's. For calculation of mean acuities, variances and comparison of group means, acuity estimates were converted into the logarithm of the minimal angle of resolution (log MAR) 16"24.
RESULTS
Binocular, acuity could be assessed in 198 out of the 201 children. Failure in 3 children was due to lack of cooperation. Results of 18 children were excluded because of strabismus in 3 cases (1.5%), binocular refractive error,s in 6 children (3%), and monocular refractive errors in 9children (4.5%). Consequently, binocular and monocular acuity norms were calculated from values obtained in 180 children. Monocular assessment was possible in all children in whom binocular acuity assessment had been possible. As indicated in Table I, mean binocular acuity in 5-year-old children (1.50 min.arc, S.D. + 0.31 octave at 6 m, and 1.55 min.arc, S.D. +0.31 octave at 40 cm) appeared at both test distances significantly lower than in adults with acuities of 0.82 min.arc (S.D. + 0.27 octave) at 6 m and 0.69 min.arc (S.D. +0.31 octave) at 40 cm (t-test, P
