Correlation between stereoacuity and induced astigmatism: a prospective study

Background and Aim: Stereopsis represents the highest grade of Binocular Single Vision wherein two dissimilar images of the same object are formed, simultaneously, at the maculae of the two eyes, and these images are then processed in the visual cortex to produce a single fused image, whose location in space can be perceived. Material and Methods: Present Randomized, Prospective, Comparative, study was performed at Department of Ophthalmology Bharati Hospital & Research Centre, Pune. Two thousand subjects were divided into two groups, they were divided into 2 groups using randomized table. Group 1 was subjected to binocular induced astigmatism. Group 2 was subjected to monocular induced astigmatism. Binocular Induced Astigmatism was induced in two groups, (500 participants in each group) either Myopic Astigmatism or Hypermetropic Astigmatism, similarly Monocular was further subdivided in to two groups (500 participants in each group): Myopic Astigmatism and Hypermetropic Astigmatism. Results: With both +1 Dcyl & +2 Dcyl binocular induced myopic astigmatism, stereoacuity was affected maximum at 45 and least affected at 180. With both -1 Dcyl & -2 Dcyl binocular induced hypermetropic astigmatism, stereoacuity was affected maximum at 45 and least affected at 180. Stereoacuity in binocular induced hypermetropic astigmatism was more affected than binocular induced myopic astigmatism. Stereoauity was grossly affected in both monocular myopic and hypermetropic astigmatism. With the rule astigmatism had remarkable difference in stereoacuity in both myopic and hypermetropic induced astigmatism of 1 and 2 diopters cylinders. Stereoacuity was more affected in hypermetropic group. Conclusion: Stereoacuity is most affected at oblique axis (450). There is gross reduction in stereoacuity in induced monocular astigmatism than binocular astigmatism. Both myopic and hypermetropic against the rule astigmatism has comparable deterioration of stereoacuity.


Introduction
Humans live in a three-dimensional world and the ability to perceive depth is, therefore, invaluable in everyday life. Precise judgments of depth are essential for daily activities. Indeed, a high grade of depth perception is a necessary requirement in many professions. This depth perception is termed as Stereopsis, a word derived from the Greek language wherein στερεο-stereo-means "solid", and ὄψιςopsis, means "sight".
In ophthalmology, stereopsis is defined as the relative ordering of visual objects in depth, and represents the highest grade of binocular single vision, wherein two Stereopsis represents the highest grade of Binocular Single Vision wherein two dissimilar images of the same object are formed, simultaneously, at the maculae of the two eyes, and these images are then processed in the visual cortex to produce a single fused image, whose location in space can be perceived. Wheatstone proved that horizontal disparities of the images, rather than vertical disparities, were of greater relevance for the sensation of depth [1]. The minimal disparity detectable by an individual is termed as stereoacuity. Stereoacuity may be quantitatively assessed by means of a stereoscope. The first Stereoscope was invented by Wheatstone in 1838.
Stereoacuity is affected by various factors, such as misalignment of visual axes, refractive errors, anisometropia, and amblyopia. Astigmatism is an important factor, as its prevalence is fairly high, being between 6.33% [2] to 36.9% [3] in children. However, previous references in the literature contain little information regarding the relationship between stereoacuity and refractive errors, especially astigmatism [4,5]. This study was therefore conducted to determine the correlation between stereoacuity and induced astigmatism in emmetropes.

Inclusion criteria
Individuals between 8-35 years. of age having Unaided Visual Acuity 6/6 in both eyes and with binocular single vision.

Exclusion criteria
Individuals having squint. Individuals with already diagnosed amblyopia. Individuals having any other ocular disease Written informed consent for the study was obtained from all the participants prior to enrolment in the study. Demographic data and detailed history were captured on a predetermined proforma. Complete ocular examination in all participants was performed.
All individuals were tested on a contour-based test i.e. Titmus fly stereotest (range 20-400 seconds of arc) using Polaroid glasses. Final Sample Size calculated was 2000. They were divided into 2 groups using randomized table.
 Group 1 was subjected to binocular induced astigmatism.  Group 2 was subjected to monocular induced astigmatism. Descriptive statistics included computation of percentages, means and standard deviations. For all tests, confidence level and level of significance were set at 95% and 5% respectively.

Results
The present study was conducted during the period of August 2012 to August 2014 at Ophthalmic OPD of a teaching hospital and various school and college screening programmes. There were total of 2000 participants from ages 8-35 years, mean age being 20.8 years. There were 1012 males and 988 females (Table 1). Mean baseline stereoacuity was 28.8 ± 4.9 second of arc (range 20-40 second of arc). With both +1 Dcyl & +2 Dcyl binocular induced myopic astigmatism, stereoacuity was affected maximum at 45 0 and least affected at 180 0 ( Table 2). Values are Mean ± SD of mean.

Original Research Article
Tropical Journal of Ophthalmology and Otolaryngology Available online at: www.medresearch.in 369|P a g e With both +1 Dcyl & +2 Dcyl monocular induced myopic astigmatism, there was gross reduction in stereoacuity specially with +2 D cylinder (Table 4). Values are Mean ± SD of mean.
With both -1 Dcyl & -2 Dcyl monocular induced hypermetropic astigmatism, there was gross reduction in stereoacuity specially with -2 D cylinder (Table 5). Stereoacuity in binocular induced hypermetropic astigmatism was more affected than binocular induced myopic astigmatism (Table 6). Values are Mean ± SD of mean. Values are Mean ± SD of mean.
Stereoauity was grossly affected in both monocular myopic and hypermetropic astigmatism (Table 7).

Original Research Article
Tropical Journal of Ophthalmology and Otolaryngology Available online at: www.medresearch.in 370|P a g e With the rule astigmatism had remarkable difference in stereoacuity in both myopic and hypermetropic induced astigmatism of 1 and 2 diopters cylinders. Stereoacuity was more affected in hypermetropic group (Table 8). Against the rule astigmatism had comparable deterioration of stereoacuity in both myopic and hypermetropic induced astigmatism of 1 and 2 diopters cylinders (Table 9).  1A and 1B, the steroacuity was significantly affected. The deterioration was proportional to increase in diopters of astigmatism irrespective of myopic or hypermetropic astigmatism.

Discussion
Maximum deterioration was noted at 45 0 axes with both myopic and hypermetropic astigmatism and although significant, stereoacuity was least affected at 180 0 in both the groups. Amongst the two groups stereoacuity was significantly affected in hypermetropic astigmatism as compared with respective myopic group. Astigmatism induces image blur which causes deterioration of stereoacuity. The image blur is proportional to the dioptric power of astigmatism and thus the stereoacuity deteriorates with increase in diopteric power of astigmatism irrespective of myopia or hypermetropia.
The findings in this group are consistent with the results seen by Chen, et al [8] reported the reduction in depth discrimination was dependent on the axis of the induced astigmatism (p < 0.01). The maximum effect occurred with orthogonal-oblique orientations (x45 left; x135 right), followed by against-the-rule (ATR) astigmatism; with-the-rule (WTR) astigmatism had the least effect (p < 0.001). Shinichiro Nakano et al [7] also reported similar results.
Horizontal disparity is the basis of steropsis. The astigmatism at vertical axis (90 0 ) increases the horizontal disparity causing greater reduction in stereopsis [9,10]. The present study showed that there was minimal deterioration of stereoacuity at horizontal axis (180 0 ) regardless of whether the astigmatism was myopic or hypermetropic, which indicated that vertical disparity affected depth discrimination to a lesser extent than horizontal disparity. Oblique astigmatism, affects both horizontal and vertical disparity, thus may account for the greater effect on stereoacuity. In both groups 2A and 2 B the steroacuity was significantly affected. The deterioration was proportionate to increase in diopters of astigmatism irrespective of myopic or hypermetropic astigmatism.
Maximum deterioration was noted at 45 0 axes with both myopic and hypermetropic astigmatism and although significant, stereoacuity was least affected at 180 0 in both the groups. Amongst the two groups stereoacuity was significantly affected in hypermetropic astigmatism as compared with respective myopic group. In monocular astigmatism in addition to image blur caused by astigmatism, binocular disparity of retinal images also plays role in reduction of stereoacuity.
The findings in this group are consistent with the results seen by Ouguz et al [10] reported that stereoacuity levels were reduced in proportion to the degree of anisometropia. Three diopters of anisometropia, regardless of type, produced a marked reduction of stereoacuity in all patients. Similar results were found by Heravian J et al [4], they reported that there was significant reduction in stereopsis level with respect to the degree and types of meridional anisometropia (p< 0.001).
Maximum reduction of stereopsis occurred with 4 D myopic and hypropic astigmatic anisometropia in oblique axis. It was 773.33±101/4 and 693.33±/179.9 second of arc for Titmusstereo test.
Comparison between mean stereopsis in myopic or hypropic astigmatic anisometropia was statistically significant in all three axes; horizontal, vertical and oblique (p< 0.001).
The comparison between monocular vs binocular induced astigmatism showed significant greater deterioration in monocular astigmatism than in binocular group. This again highlights the role of binocular retinal image disparity in stereopsis.
The results in this group are in consistent with the results seen by Shinichiro Nakano et al [7] who reported that stereopsis degraded in both binocular and monocular astigmatic induction as astigmatic power increased but there were no apparent differences in stereopsis between binocular and monocular astigmatic induction.
In the present study against the rule astigmatism had comparable deterioration of astigmatism in both myopic cylinder at 90 0 and hypermetropic cylinder at 180 0 of 1 and 2 diopters in both monocular and binocular groups. There was remarkable difference in with the rule astigmatism in myopic and hypermetropic group.
Stereoacuity is more affected by hypermetropic with the rule astigmatism compared to myopic with the rule astigmatism. Chen et al [8] reported that the effect of WTR astigmatism was lesser compared with ATR astigmatic blur on depth discrimination.
The study was conducted in emmetropic individuals with normal binocular function by experimentally inducing astigmatism. They were not adapted to the image blur and disparity. The effect on stereoacuity might be different in individuals who have astigmatism during critical period of binocular vision development Limitation of the study were, it was conducted in emmetropic individuals with normal binocular function by experimentally inducing astigmatism.
They were not adapted to the image blur and disparity. The effect on stereoacuity might be different in individuals who have astigmatism during critical period of binocular vision development.