Normal visual acuity in year olds

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1 Normal visual acuity in year olds Josefin Ohlsson 1 and Gerardo Villarreal 2 1 Department of Ophthalmology, Institute of Clinical Neuroscience, Go teborg University, Sweden 2 Department of Ophthalmology, Nuevo Leon University, Monterrey, Mexico ABSTRACT. Purpose: The aim of this work was to establish visual acuity norms in year-olds. Methods: In a previous, population-based study carried out in 1998, a total of year-old children were examined with a full eye examination. In 23, 25% (n = 262) of these children were randomly selected and invited to a re-examination; 147 subjects agreed to participate and 116 attended. The examined group did not significantly differ from the original sample in terms of the prevalence of ocular and visual disorders. Best corrected monocular visual acuity (VA) was assessed with the revised 2 ETDRS logmar chart. Results: Mean best corrected VA was.1 logmar across the examined group. There was no significant difference between right and left eyes. By excluding nine subjects who had significant ametropia and/or ocular or visual pathology, mean VA increased to.12 logmar (SD.7). The mean interocular difference in VA among normal subjects was.4 logmar. Conclusions: Visual acuity in teenagers is significantly better than. logmar and the interocular difference is low in healthy eyes. Key words: adolescent human reference values visual acuity Acta Ophthalmol. Scand. 25: 83: Copyright # Acta Ophthalmol Scand 25. doi: /j x Introduction In order to separate the abnormal from the normal, the criteria for defining normality must be established. This may seem obvious, but, surprisingly often, the normal distribution of physiological parameters is undefined. Unfortunately, this also applies to the range of normal visual acuity (VA), which is insufficiently described. The custom of using 1. (. logmar or 2/2) as a criterion for normal VA in adults originates from Snellen & Landolt (1874) and many visual charts do not contain optotypes above this value. This is despite the fact that Snellen himself cautioned against interpreting his norm value as an accurate limit for normal VA. Some authors even argue that the use of 1. as a norm value is based on a miscalculation. According to Velasco e Cruz (199), Snellen used calculations of a full grating cycle when determining the minimum visual angle in his E-chart. Snellen s norm value should thus be 2. instead of 1.! Because VA develops from birth until the mid-2s, norms for normal VA need to be age-specific. Reviews on the development of VA suggest that 1. is reached at the age of 5 6 years (Simons 1983; Fern & Manny 1986), but some studies have reported high levels of VA early in life. Sheridan (1974) reported that most (83%) 5 7-year-old children have VA of at least 1.3. Studying VA in young children involves a methodological problem as the co-operation and testability of young children is very often limited. Assessment of VA most often relies on testing charts with optotypes, and it would consequently be beneficial to agree on standards for VA charts. One such attempt involves the increasing use of charts with equally difficult and equidistant optotypes in logmar steps, such as the Early Treatment Diabetic Retinopathy Study (ETDRS, 1985) chart (Ferris et al. 1982). There are, moreover, methodological problems in defining normal VA. To establish the distribution of normal values in a study, we must first decide on inclusion and exclusion criteria. What inclusion criteria should we use? How do we decide on exclusion without including VA? If we find an individual without signs of ocular or visual pathology, but with VA of.5 or even.3, should this individual be included in the normal group? To our knowledge, very few studies on what is to be considered normal VA in adolescents and adults have been published. Reporting on letter acuity 487

2 ACTAOPHTHALMOLOGICA SCANDINAVICA 25 in normal subjects aged 1 19 years, Frise n & Frise n (1981) found a mean VA of.9 logmar (1.23 decimal). Elliott et al. (1995) found similar results with a mean VA of.13 logmar in year-olds. The aim of this study was to establish VA in a population-based sample of year-olds without significant refractive errors or ocular or visual pathology. Methods Subjects In a previous, population-based study performed in 1998, a total of year-old children were examined with a full eye examination (Ohlsson et al. 21). All the children were born in Sweden in In 23, 25% (n ¼ 262) of these subjects (at that time aged years), were randomly selected and invited to a re-examination. The study was carried out at the Department of Ophthalmology, Institute of Clinical Neuroscience, Gothenburg University, Sweden and approved by the local committee of medical ethics. A total of 147 of the invited subjects agreed to participate and 116 attended (44% of 262). Out of the attending subjects, 75 (65%) were female. Out of the 146 subjects who did not attend, 31 (12%) accepted the invitation but did not show up on the examination day, 51 (19%) did not want to participate and 64 (24%) subjects did not respond to the invitation. The re-examined group (n ¼ 116) did not significantly differ from the original total sample in terms of the prevalence of ocular and visual disorders (Table 1), although it had a significantly higher proportion of females than the original sample (p ¼.2). Nine subjects in the re-examined group (n ¼ 116) had ametropia of 5 dioptres sphere and/or 2 D cylinder and/or ocular or visual pathology (results taken from the examination in 1998), and were excluded from the analysis of results in normal subjects. In total, 17 subjects were classified as normal. Details of the nine excluded subjects can be seen in Table 2. The examination in 1998 included best corrected VA, stereopsis, cover testing, red reflex, refractive retinoscopy and examination of the posterior pole. Subjects with subnormal VA or other eye pathology were referred to the Paediatric Eye Clinic for a full clinical evaluation. In selected cases visual evoked potentials (VEPs) was also performed. The re-examination in 23 was performed at the subjects schools. Monocular best corrected VA was assessed with the revised 2 ETDRS chart (Ferris et al. 1993) (Precision Vision, La Salle, Illinois, USA). Chart 1 was used for the right eye and chart 2 for the left eye. Subjects were tested with optimal refractive correction and not only habitual correction. Visual acuity scoring was performed with the letter by letter method according to ETDRS standards (Early Treatment Diabetic Retinopathy Study Group 1985). The examiner was not aware of the results of previous testing at the time of the current examination. Statistics Mean VA was calculated using logmar scores, according to recommendations Table 1. Characteristics of 116 re-examined subjects versus those of the total group. (All results are taken from the examination in 1998). Results are based on the prevalence of each condition (i.e. a subject with strabismic amblyopia is represented both under amblyopia and strabismus). Total 116 p-value population re-examined n ¼ 146 Female 49% 65% <.1 Amblyopia <1. (decimal) 4.8% 4.3%.79 Amblyopia.5 (decimal) 2.5% 1.7%.52 Myopia.5 D right eye 43.7% 45.7%.65 Myopia 5 D right eye 1.6% 1.7%.93 Hyperopia þ 3 D right eye 1.9%.9%.22 Strabismus 2.3% 2.6%.83 Anisometropia 1.5 D 3.2% 2.6%.69 (Moseley & Jones 1993). Student s t- test for paired observations was used for testing interocular difference in VA. The homogeneity of the original sample and the re-tested group was tested using a statistic test based on a normal approximation of the binomial distribution. The test was performed separately for the different diagnoses and no adjustment for multiplicity was performed. The possible correlation between anisometropia and difference in interocular VA was tested using Spearman rank correlation analysis. Results The mean best corrected VA was.1 logmar across the examined group. There was no significant difference between right and left eyes (p ¼.1). By excluding nine subjects who had ametropia of 5 D sphere and/or 2 D cylinder and/or ocular or visual pathology (results taken from the examination in 1998), mean best corrected VA increased to.12 logmar (SD.7). Mean interocular difference of VA among normal subjects was.4 logmar (SD.5). Mean anisometropia among normals was.25 D; there was no correlation between the degree of ansiometropia and the difference in interocular VA (p ¼.77). In the group without significant ametropia or known visual or ocular pathology (n ¼ 17), no eye had VA worse than.12 logmar. One eye with VA of.12 logmar was found, as were one eye with.8 logmar, four eyes with.6 logmar and six eyes with VA of.4 logmar (Fig. 1). Four subjects had bilateral VA <. logmar. It is unfortunately not possible to compare the current VA results to those found in 1998, as the results for VA in 1998 were truncated above 1. (logmar.). Discussion Visual acuity in healthy teenagers is better than. logmar. Our results fit well with those of previous studies on normal mean VA. By plotting our results together with the results from Elliott et al. (1995), Frise n & Frise n 488

3 Table 2. Details of the nine subjects excluded from results on normal subjects. Results regarding pathology are taken from the full clinical examination in Case Sex Current refraction Current Pathology/ OD/OS visual acuity OD/OS excluded due to (sphere cyl axis) (logmar) 8 F þ1.5.5 *9 þ.1 Amblyopia plano.25 * M þ.25. Ocular albinism þ.5 1 *17.1 amblyopia 27 M þ.5 1 *16 þ.4 Abnormal optical þ.5 2 *16.18 nerve OD 124 M *18 þ.16 Abnormal optical 9 2 *18 þ.16 nerve bilaterally 153 F þ.25.4 Amblyopia plano M þ.5 1 *16.8 Astigmatism OS.5 2 *16 þ.4 24 F 4.5 *18.6 Amblyopia * F þ.5.12 ocular albinism þ2.5 þ.12 amblyopia 234 M *1 þ.6 Astigmatism *16.4 (1981), Myers et al. (1999) and Lam et al. (1996), we get a description of mean VA in normal, healthy eyes from the age of 5 years to the age of 75 years (Fig. 2). The development from 5 25 years of age is surprisingly linear. The studies by Frisén & Frisén (1981) and Elliott et al. (1995) suggest that average VA peaks in the mid to late 2s at.17 logmar and.16 logmar, respectively, whereafter it gradually declines. However, even at the age of 75 years mean VA is better than. logmar; Frise n & Frisén (1981) found an average VA of.4 logmar and Elliott et al. (1995) of.2 logmar. The distribution of VA (logmar) in our sample resembles normal (Gaussian) distribution, even if the peak is bifid and somewhat skewed to the left (Fig. 1). Normal distribution requires a shape completely determined by the mean and the standard deviation, and a continuous frequency distribution of infinite range with identical arithmetic mean, mode and median. In our sample, these criteria are not altogether fulfilled, but the sample is probably too small to reliably determine normal distribution. We have found very few previous reports or discussions on VA and normal distribution. Lam et al. (1996) found a Gaussian-like distribution of logmar VA in normal year-old children. It is interesting to note that the mean interocular difference was very low (.4 logmar). This is in agreement with previous publications: Brown & Yap (1995) reported a difference of.33 logmar between the eyes in normal subjects OD OS visual acuity (logmar) Fig. 1. Distribution of best corrected visual acuity (logmar) in subjects without significant refractive errors or known ocular or visual pathology (n ¼ 17, mean.12 logmar [(SD.7], median.14 logmar, mode.16 logmar). 489

4 ACTAOPHTHALMOLOGICA SCANDINAVICA 25 Visual acuity (logmar).1.5 Elliott Ohlsson Myers Frisen Lam Age (years) Fig. 2. Mean normal visual acuity at 5 75 years of age. Comparison of current study with Elliott et al. (1995), Frisén & Frisén (1981), Myers et al. (1999) and Lam et al. (1996). The low participation rate (44%) in this study and the fact that a greater proportion of responders was female represent drawbacks and possible sources of bias. The original sample was population-based and the re-examined group (n ¼ 116) did not significantly differ from the original total sample in terms of the prevalence of ocular and visual disorders (Table 1). Normality in the re-examined group was based on the examination in 1998, and visual and ocular pathology that arose after the age of years may have been missed. Intraocular pressure was not assessed in 1998, but glaucoma is very rare in this age group. The study by Lam et al. (1996) dealt with randomly selected subjects from a geographic cohort. The three other previous studies on VA dealt with populations that were possibly biased. In the study by Frise n & Frisén (1981) the subjects consisted of patients attending an eye clinic with normal findings on examination. In the study by Elliott et al. (1995) the majority of subjects consisted of experienced psychophysical observers. Myers et al. (1999) recruited their 1- year-old subjects from non-ophthalmologic paediatric clinics and nearby schools. Does it then matter that 1. (. logmar) seems to be clearly below normal VA in adolescents and adults? Snellen s norm value has been used for more than 1 years and is deeply rooted in clinical practice. Firstly, as argued above, it is necessary to define what is normal before we can learn anything about abnormal conditions. Secondly, physiologically speaking, there is a major difference between a VA of 2. and a VA of 1.. A decrease in VA from 2. to 1. means the loss of 75% of foveal cell density and a decrease from 1.5 to 1. means the loss of 55% of foveal cell density (Frisén&Frisén 1979). Acknowledgements We are grateful to all subjects who participated in the study and to all schools who granted use of their premises for examinations. Part of this work was presented as a poster at the Association for Research in Vision and Ophthalmology (ARVO) meeting in Fort Lauderdale, Florida, USA, in April 24. We wish to thank Sofia Andersson PhD for excellent help with the statistical analysis. This study was supported by the Gothenburg Medical Society (Ahrnberg Foundation), the KMA Foundation (Kronprinsessan Margaretas Arbetsna mnd fo r Synskadade), the Swedish Society of Medicine and the Royal Society of Arts and Sciences in Gothenburg. References Brown B & Yap MK (1995): Differences in visual acuity between the eyes: determination of normal limits in a clinical population. Ophthalmic Physiol Opt 15: Early Treatment Diabetic Retinopathy Study Group (1985): Photocoagulation for diabetic macular oedema. Early Treatment Diabetic Retinopathy Study Report No. 1. Arch Ophthalmol 13: Elliott DB, Yang KC & Whitaker D (1995): Visual acuity changes throughout adulthood in normal, healthy eyes: seeing beyond 6/6. Optom Vis Sci 72: Fern KD & Manny RE (1986): Visual acuity of the preschool child: a review. Am J Optom Physiol Opt 63: Ferris FL III, Freidlin V, Kassoff A, Green SB & Milton RC (1993): Relative letter and position difficulty on visual acuity charts 49

5 from the Early Treatment Diabetic Retinopathy Study. Am J Ophthalmol 116: Ferris FL III, Kassoff A, Bresnick GH & Bailey I (1982): New visual acuity charts for clinical research. Am J Ophthalmol 94: Frisén L & Frisén M (1979): Micropsia and visual acuity in macular oedema. A study of the neuro-retinal basis of visual acuity. Graefes Arch Clin Exp Ophthalmol 21: Frisén L & Frisén M (1981): How good is normal visual acuity? A study of letter acuity thresholds as a function of age. Graefes Arch Clin Exp Ophthalmol 215: Lam SR, LaRoche GR, De Becker I & Macpherson H (1996): The range and variability of ophthalmological parameters in normal children aged years. J Pediatr Ophthalmol Strabismus 33: Moseley MJ & Jones HS (1993): Visual acuity: calculating appropriate averages. Acta Ophthalmol (Copenh) 71: Myers VS, Gidlewski N, Quinn GE, Miller D & Dobson V (1999): Distance and near visual acuity, contrast sensitivity and visual fields of 1-year-old children. Arch Ophthalmol 117: Ohlsson J, Villarreal G, Sjo stro m A, Abrahamsson M & Sjo strand J (21): Visual acuity, residual amblyopia and ocular pathology in a screened population of year-old children in Sweden. Acta Ophthalmol Scand 79: Sheridan MD (1974): What is normal distance vision at 5 7 years? Dev Med Child Neurol 16: Simons K (1983): Visual acuity norms in young children. Surv Ophthalmol 28: Snellen H & Landolt E (1874): Ophthalmometrologie. Die Funktionspru fung des Auges. I. Eidoptometrie (Bestimung der Sehschärfe). In: Graefe A & Saemisch T (eds). Handbuch der Gesamten Augenheilkunde. Vol. III. Leipzig: Engelman Velasco e Cruz AA (199): Historical roots of 2/2 as a (wrong) standard value of normal visual acuity. Optom Vis Sci 67: 661. Received on June 3rd, 24. Accepted on May 3rd, 25. Correspondence: Josefin Ohlsson Department of Ophthalmology Sahlgrenska University Hospital SE Mo lndal Sweden Tel: þ Fax: þ josefin.ohlsson@neuro.gu.se 491