Transactions 29th European Strabismological Association Meeting – de Faber (ed) © 2005 European Strabismological Association, ISBN 04 1537 211 9

The effects of experimentally induced spherical myopic anisometropia on stereoacuity

Ali Akbar Saber Moghaddam, Abbas Kargozar & Mohammad Etezad Razavi

Assistant Prof. of ophthalmology, Mashhad University of Medical Sciences, Mashhad, Iran

ABSTRACT:

Purpose: To determine the effects of experimentally induced anisometropia on stereopsis in healthy adults to assess the potentially detrimental effects of uncorrected anisometropia on the development of stereoacuity during childhood.

Materials & Methods: The study performed on 100 healthy adult volunteers, 58 male and 42 female (ranging in age from 20–30 years). 3 type of myopic anisometropia were induced in all 100 patients in their left eyes (by plus lenses 0.75 to 2.25 diopter in 0.75 diopter increment). Then stereoacuity was measured by Titmus stereotest. Each patient’s highest level of stereoacuity was recorded (in each step of anisometropia induction). The statistical analysis performed by paired t-test.

Conclusion: Producing of anisometropia to 1.25 dipter does not significantly affects stereopsis, but anisometropia more than 1.75 diopter can significantly reduce stereoacuity. Also reduction of visual acuity to 20/40 doesn’t create statistically significant decrease in stereopsis but reduction of visual acuity to 2/100 or less, significantly effects stereopsis.

1INTRODUCTION

Anisometropia is considered a causal factor in the pathogenesis of amblyopia and strabismus in the developing human eye. It is estimated that as many as 6%–38% of all cases of amblyopia are caused

by anisometropia without strabismus, whereas approximately 12%–18% of children with strabismus also have anisometropia (1,2). However, data on the prevalence of anisometropia and its com-

plications in children are rare and conflicting. Although is generally agreed that anisometropic refractive errors should be corrected in patients with established amblyopia or strabismus to ensure optimal visual development and maturation, the exact levels of anisometropia and age at which corrections should be undertaken in otherwise healthy children remain to be determined.

Amblyopia may be defined as a unilateral or bilateral decrease in visual acuity caused by deprivation of form vision, abnormal binocular interaction, or both, for which no organic cause can be detected(3). This definition clearly implicates amblyopia as the functional consequence during early developmental plasticity; it does not encompass any abnormalities in binocular vision that also may exist. Normal neural development of binocular cortical cells requires clear and equal retinal images during the critical period of visual development. Research shows that unilateral image blur during the early period of visual development results in loss of binocular function such as fusion and stereopsis (4). Thus, it is important to consider the associated effects on binocular vision when establishing guidelines for the empiric management of potentially amblyogenic problems.

Stereopsis is the unique quality or binocular vision that enables depth perception in visual space. It arises from the horizontal retinal image disparity between the two foveas or other corresponding retinal points; differing amounts of such disparity give rise to differing sensations of depth (5).

It has been suggested (5,6) that empiric correction should be considered for the following anisometropic refractive errors: astigmatism 1.5 diopters (D), hyperopia of 1.5 D, and myopia

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of 3D. Due to a lack of clinical evidence in support of these recommendations, the potential effects of uncorrected anisometropia on stereopsis were investigated.

In this prospective study, the stereoacuity levels of experimentally induced anisometropia in binocularly healthy adults were measured. We speculated that the sensory consequences induced by monocular blur in this group might mimic those experienced by the anisometropic young child with an equivalent amount of monocular blur. Although these two populations clearly are not identical, this approach may yield insights into the relationship between anisometropia and stereopsis, as well as identify levels of stereopsis possible in individuals affected by various degrees of anisometropia.

2MATERIALS & METHODS

100 health adult volunteers (58 male and 42 female) ranging in age from 20–30 years and free of ocular disease participated in the study. Eligibility criteria for participation was 20/20 snellen visual acuity without correction in each eye in both distance and near, no significant refractive error, normal ocular alignment, and normal stereoacuity (40 second of arc or better measured by Titmus fly test).

3 different type of myopic anisometropia were induced in all 100 patients in their left eyes by 0.75, 1.50 and 2.25 diopter lenses (in 0.75 diopter increment). So there were 3 types of unilateral myopia ( 0.75, 1.50 and 2.25). Testing was carried out under normal room lighting condition.

Stereoacuity was measured using the Titmus stereotest with patients placing the cross-polarizing stereoacuity glass over their trial frame.

Each patient’s highest level of stereoacuity was recorded. The statistical analysis was performed using paired t-test to evaluate the differences in stereopsis test results between different levels of anisometropia, P equal or less than 0.05 was considered statistically significant.

3RESULTS

Levels of stereoacuity were measured in each step of anisometropia. When 0.75 diopter anisometropia induced, mean visual acuity decreased to 20/30 and mean stereoacuity decreased to 60 second of arc (stage 1). This decrease was not statistically significant (p 0.5). With 1.50 diopter anisometropia induction, visual acuity decreased to 20/40 and stereoacuity decreased to 200 40 second of arc (stage 2). This difference also was not statistically significant (P 0.1). In third step, anisometropia produced by 2.25 diopter lens. Mean visual acuity decreased to 20/100 and stereopsis decreased to 900 100 second of arc (P 0.0001) which is statistically significant

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4DISCUSSION

The visual acuity and stereoscopic acuity depends on many factors and is influenced greatly by the method used in determining it, (in refined laboratory examination and with highly trained subjects, stereoscopic acuities as low as 2 to 7 seconds of arc have been found (7)). It is clear that visual acuity has some relation to stereoscopic acuity. Stereoscopic acuity decrease (as doe’s visual acuity), from the center to the periphery of the retina(8). However, despite this relationship, stereopsis is a function not linearly correlated with visual acuity, so that reduction of visual acuity with neutral density filter over one eye does not rise the stereoscopic threshold, even if the acuity was lowered to as low as 20/70. Further decreases in vision to 20/100 greatly increase the threshold and with a decrease in acuity of the covered eye to 20/200, stereopsis will be absent(9).

Poor visual acuity was generally accompanied by reduced stereoscopic acuity but that there were no correlation between these two functions and of special clinical interest is the fact that

stereoacuity in patients with amblyopia may be better than what one would expect from their visual acuity (10, 11).

Anisometropia and also anisokonia have been seen to affect stereoscopic acuity as well. For this, we study the effects of induced anisometropia by glass lenses on stereopsis, as well as visual acuity. As shown by our study the induced anisometropia also affects stereoscopic acuity in nonlinear method.

Anisometropia equal or less than 1.50 diopter have not significant effect on stereoscopic view, but increase in anisometropia (to 2.25 diopter or more) will affect stereopsis significantly (P 0001). Oguzh(12) had been indicated such relationship but in lesser number of patients. Weakly(13) studied the effect of innate (natural) anisometropia on stereoscopic vision and found that spherical myopic anisometropia greater than 2 diopter and spherical hyperopic anisometropia greater than 1 diopter results in statistically significant decrease in binocular function. White(14) studied the effect of post eximer laser photorefractive keratectomy (PRK) anisometropia on binocular function and concluded that anisometropia greater than 2.5 diopter can affect the stereoacuity. Brooke(15) studied binocular function in similar to our study method in 19 adults and founds that stereoacuity was significantly decreased with as little as one diopter of spherical anisometropia.

The Titmus stereotest was used due to its ease of presentation, with the understanding that it may not be sensitive enough to make quantitative measurements of stereoacuity or sufficient to detect small differences in stereoacuity. The test can not completely exclude monocular clues or the use of memory to identify correct targets, which can result in falsely high measured stereoacuity levels. As described above, we attempted to minimize this problem by carefully instructing patients to identify axially displaced circle only. Data from other investigators show similar reductions in stereopsis using random dot stereograms (16); the reduction in stereoacuity in our study was highly significant for some increase in anisometropia, suggesting the use of Titmus stereotest did not bias the results of our study. Although, the precise mechanisms by which anisometropia leads a decrease in stereoacuity are not clear, it has been suggested foveal suppression in the defocused eye is the cause of decreased stereopsis.

5CONCLUSION

Anisometropia can have a significant adverse effect on high-grade binocular function. The mechanisms underlying the loss of stereopsis seem to involve foveal suppression, the extent of which is directly related to the degree of anisometropia. Also we concluded that relation between decrease in visual acuity and stereopsis is not linear. Our results the effects of anisometropia on stereopsis should be considered in the empiric correction of anisometropic refractive errors in children. This study may help the development of guidelines in the future.

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