Astigmatic axis and amblyopia in childhood

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1 Astigmatic axis and amblyopia in childhood Maths Abrahamsson and Johan Sjo strand ABSTRACT. Purpose: This study is part of a larger project whose aim is to evaluate the relationship between refractive errors and amblyopia. In an earlier study,we showed that there is a substantial increase in the prevalence of amblyopia among children with oblique astigmatism. To further evaluate this relationship,we examined children with astigmatisms of dioptre or more and varying directions of the astigmatic axes. Methods: Two groups of astigmatic children,with oblique and orthogonal astigmatism,respectively,were selected for the study at year of age via a general health screening programme. The most emmetropic axis was identified in each child and used in the study. Visual acuity (VA) was tested when the children were between and.5 years of age. The presence of amblyopia,defined as difference in VA between the s of. log unit or more,and any increase in acuity following occlusion therapy were noted. Results: The angle of the astigmatic axis strongly relates to the risk of developing amblyopia. Axes 5 degrees from the main axes did not affect the risk of amblyopia but oblique astigmatism significantly increased the risk of developing amblyopia (p ¼.). Conclusions: The results accord with earlier findings that oblique astigmatism increases the risk of developing amblyopia. Key words: oblique astigmatism amblyopia Acta Ophthalmol. Scand. : 8: 7 Copyright # Acta Ophthalmol Scand. ISSN Introduction It is common knowledge that infants have a high incidence of clinically significant astigmatism. Rates of occurrence of astigmatism vary between % and 7% in different studies (Atkinson et al. 98; Gwiazda et al. 98; Saunders 995; Ehrlich et al. 997). To some degree, this range of incidence can be explained by variations in definitions of astigmatism and study methodology. However, both photorefraction (Atkinson et al. 98; Howland et al. 978) and cycloplegic retinoscopy (Ehrlich et al. 997; Mohindra et al. 978; Dobson et al. 98) have shown that astigmatism is frequent among infants. In very young children the most common type of astigmatism is against-the-rule astigmatism (i.e. plus cylinder axis in 8 degrees) (Gwiazda et al. 98). School children, on the other hand, show a significant predominance of with-the-rule astigmatism (plus cylinder axis in 9 degrees) (Hirsch 96; Abrahamsson et al. 99; Zadnik et al. 99), although they have a lower overall incidence of astigmatism than infants. The typical incidence of astigmatism among school children is less than %. This discrepancy in incidence and change of axis between infants and school children can be interpreted as indicative of either the elimination of cylinder refractive errors between and 6 years of age, or of new cases generated from the previously non-astigmatic population. Several studies of refractive development during childhood, especially of astigmatism, have focused on this change of axis (Dobson et al. 98; Gwiazda et al. 985; Zadnik et al. 99). The progression of change of axis is interesting for several reasons, especially as the data can be interpreted into different models of refractive development. While most studies focus on the orthogonal axis, this study concentrates on patients with oblique astigmatism. This is comparatively rare, with an incidence of only %. In an earlier study, we found there to be a considerable increase in the relative risk of developing amblyopia in this particular group (Abrahamsson et al. 99; Sjo strand & Abrahamsson 99). Although the relative risk was very high, the total numbers of individuals involved were low and, subsequently, a considerable confidence interval had to be taken into account. There is one further issue that must be discussed in relation to astigmatism, namely, that of meridional amblyopia. As its name implies, this refers to a visual loss that is more pronounced for structures in a given direction than for structures perpendicular to that direction. Several authors have underlined the close relationship between meridional amblyopia and astigmatism (Gwiazda et al. 985, 986; Charman & Voisin 99; Ehrlich et al. 997). By studying line-spread function (LSF) and modulation transfer function (MTF) in astigmatic optical systems, Charman & Voisin (99) showed

2 there to be a considerable oblique effect. In other words, the MTF and LSF were significantly more depressed in directions that were oblique (5 degrees) to the cardinal axis of the astigmatism. In patients with oblique astigmatism, MTF and LSF are most depressed at 9 and 8 degrees, which coincides with the essential direction of letter charts and text. This gives rise to the interesting theoretical speculation that oblique astigmatism is especially amblyogenic. The aim of this study was to evaluate oblique astigmatism as a risk factor for amblyopia. Comparison of the prevalence of amblyopia at the age of years in children who had either oblique or orthogonal astigmatism as infants allows us to estimate the relative risk of developing amblyopia for the two groups. Based on earlier studies, we know that, in general, astigmatism at the age of year does not imply a significant increase in relative risk for developing amblyopia. An additional aim was to evaluate changes in magnitude and axes of astigmatism with respect to age in the two groups. Methods All children in the city of Va stera s, Sweden, were offered a voluntary ophthalmological examination at the age of year administered through children s health care centres. The actual examinations took place some time during their second year of life. They consisted of assessment of motility and alignment as well as of cycloplegic retinoscopy. The children were refracted by retinoscopy min after instillation of cyclopentolate (%). All children with systemic diseases or craniofacial disorders were excluded from the study. A total of 55 -year-old children underwent refractive screening. Among these, 8 ( boys; girls) were found to fulfil the inclusion criteria for oblique astigmatism. Inclusion criteria required the astigmatism to be bilateral and greater than. D in the least emmetropic, with the axis falling into one of the intervals 67.5 degrees or 57.5 degrees. Differences in the magnitude of the astigmatism between the s had to be less than D and the axes of the astigmatism had to be symmetrical or, in some cases, parallel. Differences of more than D in any of the emmetropic axes were not allowed. A plus convention was used. The axis and magnitude of astigmatism were measured again and were required to fall within the inclusion criteria at a second examination within months of the first. An equal number of control subjects ( boys and girls) with orthogonal astigmatism (astigmatism within 5 degrees of the cardinal axes) were selected from next consecutive cases. Control subjects had to fulfil the same inclusion criteria as the oblique cases, with the only difference being the axis angle. The control group was not required to undergo a second examination within months. In Sweden, 97% of all children take part in a general health screening programme for -year-olds. The health screening includes a test of visual acuity (VA) (HOTV chart). If a child fails the test (by exhibiting binocular vision of.65 or less or a difference in acuity between the s of. log unit or more), he or she is re-tested. A second failure results in the child s referral to an ophthalmological clinic. Approximately 5% of referred children manage to pass the VA re-test at the clinic. The remaining referred children are further examined by an ophthalmologist. Fixation, motality and binocularity, as well as refractive errors, are tested and evaluated and a diagnosis of visual loss is established. According to the procedure, the children in our study were prescribed glasses that fully corrected the astigmatism at years of age if the astigmatism was greater than.75 D. For astigmatism below.75 D, the amount of correction varied with the refractive error of the least emmetropic axis. The use of spectacles was only determined by means of parental reporting. Reported spectacle use did not differ significantly between the two groups of patients. Refraction data were obtained from the records of the doctors who carried out the voluntary examinations at year of age and also determined the refractive error at years of age. For a large proportion of the subjects, refraction data had also been collected during the interval between and years. In the case of children who passed the initial screening test, acuity data was obtained from the screening test. In the case of those children who failed the initial screening test, acuity data was obtained from the ophthalmological examination. Thus, there is a difference in quality between acuity values obtained at the screening test, which are truncated, and those obtained at the ophthalmological examination. In the screening situation, no acuities above. were tested, while for the children tested at the ophthalmological clinic, acuity levels higher than. were tested. Amblyopia was defined by two criteria. There should be an acuity difference (approximately. log unit or more) between the s measured at the ophthalmological clinic with best correction in at least two separate, consecutive test sessions. The acuity of the amblyopic should have improved by at least one line following occlusion treatment. A cross-tabulation (Table ) was used to determine the strength of oblique astigmatism as a risk indicator for amblyopia compared to orthogonal astigmatism. The confidence interval was determined with Fischer s exact test. Results Table. Cross-tabulation for calculating the risk of amblyopia. In this study, we examined refractive error during the second year of life and correlated it to amblyopia diagnosed at years of age. Six control subjects were lost to follow-up due to our inability to identify their files from the general health screening programme at years of age. The Oblique Orthogonal Total Amblyopia 8 Non-amblyopia 7 7 Total Relative risk OR.7 (. :.); p-value., two-sided Fisher s exact test.

3 prevalence of oblique astigmatism in our sample was approximately % (8/ 55) among -year-old children in the Va stera s area. Distribution of the magnitude of the spherical and cylindrical components of the refractive error was similar in both groups (Fig. ). The residual astigmatism at years of age differed significantly between the oblique and orthogonal groups. In the orthogonal group, the astigmatism decreased in approximately 89% (n ¼ 7) of cases, with 7% (n ¼ 9) showing no astigmatism at years of age. In the four patients with amblyopia, the astigmatism increased or remained unchanged during the period between and years of age. Among the patients with oblique astigmatism, there was a mean decrease in the magnitude of the cylindrical component of the refractive error. However, the mean decrease was of much smaller amplitude and the variation in the individual changes more pronounced. In five of 8 cases (6%), astigmatism had totally vanished by the age of years. In cases (9%) the astigmatism decreased. The cases with amblyopia at years of age showed a general lack of emmetropization. Figure presents the distribution of the cylindrical component of the refractive error of the two groups. Comparison of the results in Figs. and shows the difference in emmetropization between the two groups. The spherical component of the refractive error decreased in both groups. There was a somewhat larger decrease in the orthogonal group (mean spherical component difference at years ¼.7 D). The difference in the decrease between the two groups was non-significant. The relationship between axis and magnitude of astigmatism at year of age is presented in polar form in Fig., where subjects with amblyopia at years of age are indicated. Because of the particularities of the inclusion criteria, our sample included no cases of anisometropia of D or more at year of age. At years of age, 8% (n ¼ ) of the children had developed a difference in refractive error in the most emmetropic axis of D or more. Among the children with amblyopia, significant anisometropia was present in two children with orthogonal astigmatism and in five children with oblique astigmatism. Two of the children with both oblique astigmatism and amblyopia had strabismus. The calculated relative risk was.7 (. :.) (p ¼.). Patients with oblique astigmatism at year of age were at significant risk of later developing amblyopia. Visual acuity A comparison of best-corrected VA at years of age in the two groups indicated that the oblique group had slightly lower VA in general than the orthogonal group. The difference in mean acuity was.8 log units. Table presents VA data for the patients with amblyopia. Because of the small number of study participants, it is impossible to determine whether there is a difference in the magnitude of amblyopia between the two groups. Discussion In order to understand the relationship between astigmatism and amblyopia, we must be able to describe the natural course of the astigmatism. Several studies have focused on the age dependent changes in both magnitude and axis of astigmatism. Most studies agree that astigmatism is common during the first years of life and that its occurrence is greatly reduced or eliminated during the following years. The crucial age for axis transition is 5 years of age. Before that age, against-the-rule astigmatism is most common, while after that age with-the-rule astigmatism becomes the most frequent type seen (Dobson et al. 98; Howland & Sayles 98). Longitudinal data indicate the same basic results. Astigmatism that is present during the first years of life decreases with age and the distribution of the axes changes (Atkinson et al. 98; Dobson et al. 98; Howland & Sayles 98; Abrahamsson et al. 988). A Distribution of the magnitude of the spherical component of the refractive error B Distribution of the magnitude of the cylindrical component of the refractive error Diopters Oblique axis Orthogonal axis Oblique axis Orthogonal axis Fig.. (A, B) Box plots describing the spherical and cylindrical components of the refractive errors in orthogonal and oblique astigmatism at year of age. The squares indicate mean values for each group. The central bars indicate median values. 5

4 Diopters Oblique axis The children in this study were selected with the inclusion criterion of astigmatism of. D or more at two consecutive sessions during the second year of life. Several studies indicate that the incidence of astigmatism of. D or more is approximately % at this age and that incidence of oblique astigmatism is much lower, at approximately %, and subject to substantial geographical variations (Abrahamsson et al. 988; Ingram et al. 979). The box plots in Fig. indicate reasonable similarity in magnitude of both spherical and cylindrical components of the Orthogonal axis Fig.. Box plot comparing cylindrical power of the orthogonal and oblique astigmatism groups at years of age. refractive error in the oblique group and in the control group. The extent of emmetropization years later was very different in the two groups. Refractive errors disappear or decrease during childhood in the group with orthogonal astigmatism, but not in the group with oblique astigmatism. Some support for this finding can be found in a paper by Erlich et al. (997), which shows that oblique astigmatism disappears much more slowly than orthogonal astigmatism during the period between nine and months of age. Although there is a significant difference in the distribution of cylindrical power at years of age, the median cylindrical power differs only by half a dioptre. Our interest in oblique astigmatism stems from an earlier study (Abrahamsson et al. 988) in which we explored the relationship between astigmatism and amblyopia. Oblique astigmatism was found to be a significant risk factor for developing amblyopia. The problem was that the number of infants with oblique astigmatism in that study was extremely small. In the present study we increased the number. The odds ratio estimated here (OR ¼.7) was much smaller than that in the original study, but the relative risk seems much more reasonable due to the greater number of patients. The study indicates that oblique astigmatism is related to amblyopia. It further implies that this relationship is present at all age levels from one to years of age. The mechanisms behind the relationship are obscure, but several can be proposed. In most of the amblyopic cases, the astigmatism axis of the two s is separated by approximately 9 degrees, forming either a V or an A. The dissimilarity of the two images should make proper cortical processing difficult. In our previous work we studied the relationship between emmetropization and amblyopia. In most cases of risk factors related to refractive errors, an Fig.. Polar diagrams showing the distribution of magnitude and axis of patients with oblique astigmatism (A) and orthogonal astigmatism (B) at year of age. Black circles indicate patients with amblyopia. The small numbers indicate several data points at the same position. 6

5 Table. Visual acuity of both s in all children with amblyopia. Patients Orthogonal astigmatism Oblique astigmatism Amblyopic added lack of emmetropization or increasing refractive errors significantly elevate the relative risk. This was also true in the present study, where most amblyopic cases showed no emmetropization or even increases of refractive errors. If we take into account the lack of emmetropization in the two groups, the relative risk is increased. In this study, the change in refractive error with age in the group with oblique astigmatism differed significantly from that expected in the general population. It is possible that this type of astigmatism represents a more fundamental defect in the refractive parameters that falls outside the handling capacity of the regulatory system. This study implies that oblique astigmatism emmetropizes differently from orthogonal astigmatism. In the oblique group, neither the cylindrical nor the spherical components of the refractive error showed as pronounced an emmetropization as they did in the group with orthogonal astigmatism. Acknowledgements This study was supported by the Swedish Medical Research Council (grant no. 6), the Kronprinsessan Margaretas Better Arbetsna mnd, the Handlanden H. Svensson Foundation, the Margit Thyselius Foundation, the De Blindas Vänner Association, the Sigvard and Marianne Bernadotte Foundation, the Wilhelm and Martina Lundgren Science Foundation and the Ahrnberg Foundation. We would like to express our special gratitude to Professor Anders Heijl for his valuable comments. References Amblyopic Better *.9.65*. *Patients with strabismus. Abrahamsson M, Fabian G, Andersson A-K & Sjo strand J (99): A longitudinal study of a population based sample of astigmatic children. I. Refraction and amblyopia. Acta Ophthalmol (Copenh) 68: 8. Abrahamsson M, Fabian G & Sjo strand J (988): Changes in astigmatism between the ages of and years: a longitudinal study. Br J Ophthalmol 9: 5 9. Atkinson J, Braddick O & French J (98): Infant astigmatism: its disappearance with age. Vision Res : Charman W & Voisin L (99): Astigmatism, accommodation, the oblique effect and meridional amblyopia. Ophthalmic Physiol Opt : 7 8. Dobson V, Fulton A & Sebris L (98): Cycloplegic refractions of infants and young children: the axis of astigmatism. Invest Ophthalmol Vis Sci 5: Ehrlich D et al. (997): Infant emmetropization: Longitudinal changes in refraction components from nine to months of age. Am Optom Vis Sci 7: 8 8. Gwiazda J, Bauer J, Thorn F & Held R (986): Meridional amblyopia does result from astigmatism in early childhood. Clin Vision Sci : 5 5. Gwiazda J, Mohindra I, Brill S & Held R (985): Infant astigmatism and meridional amblyopia. Vision Res 5: Gwiazda J, Scheiman M, Mohindra I & Held R (98): Astigmatism in children: changes in axis and amount from birth to 6 years. Invest Ophthalmol Vis Sci 5: Hirsch M (96): Changes in astigmatism during the first 8 years of school: an interim report from the Ojai longitudinal study. Am J Optom : 7. HowlandH,AtkinsonJ,BraddickO&FrenchJ (978): Infant astigmatism measured by photorefraction. Science :. Howland H & Sayles N (98): Photorefractive measurements of astigmatism in infants and young children. Invest Ophthalmol Vis Sci 5: 9. Ingram R, Traynar M, Walker C & Wilson J (979): Screening for refractive errors at age year: a pilot study. Br J Ophthalmol 6: 5. Mohindra I, Held R, Gwiazda J & Brill S (978): Astigmatism in infants. Science : 9. Saunders K (995): Early refractive development in humans. Surv Ophthalmol : 7 6. Sjo strand J & Abrahamsson M (99): Risk factors in amblyopia. Eye : Zadnik K, Mutti D & Adams A (99): Astigmatism: what s the rule? OSA Techn Digest : Received on December 7th,. Accepted on September th,. Correspondence: Maths Abrahamsson Department of Clinical Neuroscience Gothenburg University SE- 5 Gothenburg Sweden Tel: þ 6 77 Fax: þ maths@oft.gu.se 7