Characteristic and Pattern of Bilateral Astigmatism in North India

Saptarshi Mukherjee *¹, Shangchingla S Chang ², Brajesh Kumar ³

1. Centre for Sight Eye Hospital.

2. Assam Downtown University

3. Dr Shroff’s Charity Eye Hospital.

Corresponding Author: Saptarshi Mukherjee, Centre for Sight Eye Hospital.

Copy Right: © 2023 Saptarshi Mukherjee, This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Received Date: April 29, 2023

Published Date: May 10, 2023

Refractive errors, is one of the most common vision disorders, are known as the second cause of visual impairment and low vision.[1,2,3,4] Astigmatism is a clinically important condition and accounts for about 13% of the refractive errors of human eye.[4] Its prevalence (≤0.50 D) has been reported to vary with ethnicity,5 age,6 and sex.[5]  Regional and ethnic variation is also found in adult populations.[7] Till now  prevalence rates up to 75% have been reported.[9] The most common type of astigmatism orientation is with-the-rule (the axis of the minus cylinder is placed between 30° and 150°), and the most rare type is oblique astigmatism (axes between 30° and 60°).[12, 10] The prevalence of with-the-rule astigmatism (axes between 60° and 120°) usually decreases with age, and against-the-rule increases.[11] The characteristic of astigmatism in fellow eyes has been assessed in some studies. They have revealed that the amounts of astigmatism in fellow eyes are usually similar, and significant aniso astigmatism is rare.[13, 14]

Most astigmatic refractive conditions have an identical level of astigmatism in each eye. However, there's discussion over whether the astigmatic axes in fellow eyes typically show direct or symmetry. This study was conducted to find the connection between the astigmatic axes of right and left eye pairs, with particular thought given to determining the degree to which either directs or mirror symmetry (enantiomorphism) of the astigmatic axes exists.

Since the relationship and symmetry of fellow eye axes are important in a clinical setting and also in etiologic and genetic studies of astigmatism, it is important to conduct comprehensive studies with complete bilateral analysis of the astigmatism status. In this study, we researched the bilateral similarity in the astigmatism axis orientation (rule similarity), direct and mirror symmetry of the axes, and the prevalence of each type of symmetry pattern. The main objective of this study was to provide a comprehensive report on the profile of bilateral astigmatism. It should be noted that the present data focuses on symmetry of astigmatism in an Indian population, as results may vary by ethnicity or race.

Methodology

This is a cross-sectional study carried out in a tertiary eye care hospital in New Delhi. The subject population comprised of patients from age group 10years and above that satisfied the inclusion criteria.

The inclusion criteria included patients with bilateral astigmatism not less than ± 0.50D in each eye. Physically and mentally well-being and who were able to cooperate adequately. Exclusion criteria in this study integrated the presence of any corneal disease, including corneal ectasia (keratoconus and pellucid marginal degeneration), corneal dystrophy, any active corneal inflammation or infection, corneal scarring, history of any eye surgery affecting the cornea including cataract, refractive, and pterygium surgery, a history of ocular trauma, presence of any opacity in the ocular media such as corneal opacity and cataract which can cause erroneous refraction results, and the presence of other anterior segment diseases such as pterygium and phlyctenulosis which may involve the cornea.

Total of 1218 patients were enrolled in the study. A complete eye examination was carried out that included vision taking (Both aided and unaided tested with Snellen’s Chart, Number Chart or Broken C Chart), Retinoscopy(Heine Beta 200 retinoscope (HEINE Optotechnik, Germany) and Subjective refraction. The final subjective refraction was recorded in minus cylindrical form.

Definitions

Amount of astigmatism- We categorized it in three groups of mild (less than 1.00D), moderate (1.00–2.00D) and high (2.00D or more).

Type of Astigmatism - Astigmatism with axes from 60 to 120° (90±30) as against-the-rule, from 150 to 30° (180±30) as with-the-rule, and the rest as oblique.

Rule Similiarity- Bilateral astigmatism was classified into two general caterogy as Isorule and Anisorule based on the orientation status of fellow eyes. Isorule astigmatism was defined as fellow eyes having the same orientation type and had three subgroups: WW (WTR astigmatism in both eyes), AA (ATR astigmatism in both eyes), and OO (oblique astigmatism in both eyes). Anisorule astigmatism was present when fellow eyes had different orientation types and was categorised as WA (WTR in one eye and ATR in the other), WO (WTR in one eye and oblique in the other), and AO (ATR in one eye and oblique in the other).

Symmetry Pattern- The Symmetry pattern was assessed under 3 models(Direct Symmetry, Mirror Symmetry and Asymmetry). In Direct Symmetry, the axes of the two eyes were numerically equal or exception of ± 5°, which means that when AxisR−AxisL is evaluated the resultant is 0 or 0±5°(For example, if the axis of the right eye is 95° and left eye is 90°, the difference between them is 5° which will still be included as direct symmetry). For Mirror Symmetry if the axes of the fellow eyes were superposed, they form a cross (For example, right eye axis is 100° and left eye axis is 80°). If we calculate AxisR−(180−AxisL) and resultant is 0 or 0±5° then it will be considered as mirror symmetry.We calculated the deviation of the inter-ocular axis difference (AD) from exact symmetry (for direct symmetry AxisR−AxisL is equal to 0 and for mirror symmetry AxisR−(180−AxisL) is equal to 0)  in these two models. The first model was the direct symmetry model defined as:

In each of these models, three calculations are done for each pair of eyes, and the smallest value is considered as the absolute deviation from mirror or direct symmetry.

The rest that does not fulfill both the direct and mirror symmetry categories were considered as Asymmetry.

Result

Total of 1218 cases were analyzed. The mean age of the study population was 51 ± 18 years (10 -96years). Female was 49% and male was 51% (Fig1).

Rule Similiarity and Symmetry Pattern with Age

In the total sample, the percentage of Isorule in male and female were 82.85% and 85.67% respectively and that of Anisorule were 17.15% and 14.33% respectively. The age of the study sample were categorized into 9 groups (interval of 10years each). As illustrated in Fig 2, the prevalence of Isorule in age group 10-59years (total 695 patients) was 81.15% and in age group 60-99 years (total 523 patients) was 88.34%. On the other hand, the prevalence of Anisorule in age group 10-59years was 18.85% and in age group 60-99years was 11.66%. The p-value is 0.001.

As illustrated in Fig3, The Asymmetry pattern had a non-linear relationship with respect to age. The Direct Symmetry had a linear increasing pattern with age till 60-69yrs (56.27%).The Mirror Symmetry Pattern seem more to be inversely proportional to direct symmetry , although there were fluctuations in between.

Rule Similiarity and Symmetry Pattern with Refractive Error

Refractive error was divided into three categories- Plano (Only astigmatic component), Myopia (Spherical component in minus form) and hypermetropia (Spherical component in plus form). As shown in Fig-4, Isorule was seen to be more than Anisorule. Isorule was more prevalent among the hyperopes (87.70%) then Plano(83.20%) followed by myopia(80.45%).In case of Anisorule, the prevalence was seen more among the myopes (19.55%) than plano(16.80%) and  hyperopes(12.30%).

In Fig-5, The Asymmetry pattern was more among the Myopes(40.51%) followed by the Plano with 39.02%  and the hyperopes(32.26%). The prevalence of direct symmetry seem to be more among the hyperopes(51.81%) then plano(42.28%) and lastly myopes(41.64%). Mirror symmetry was seen more among Plano(18.70%) then myopes(17.85%) and hyperopes(15.93%).

Isorule in Relation to Age

As illustrated in Fig6, WTR(With the rule astigmatism) in both eyes, decreases with age whereas ATR(Against the rule) increases with age. OBL(Oblique Astigmatism)has no significant relationship with age.

Anisorule in Relation to Age

As illustrated in Fig7, all the three sub-group of anisorule showed no significant relationship with age.

Rule Similiarity with Amount of Astigmatism

The amount of astigmatism in both eyes were considered together and grouped as ML-ML(Mild in both eyes), ML-MO (Mild in one & moderate in other), ML-H(Mild in one &high in other), MO-MO(Moderate in both), MO-H(Moderate in one &high in other) and H-H(High in both). In Fig-8 Isorule showed more prevalence then Anisorule. Isorule was seen highest in MO-MO group (89.3%) and lowest in ML-H group (65.7%). Anisorule on the other hand was highest in ML-H group (34.3%) and lowest in MO-MO group (10.7%).

Symmetry Pattern with Amount of Astigmatism

In Fig9, Overall the Asymmetry pattern was more prevalent. The Asymmetry pattern was highest in group ML-H(62.9%) and lowest in group ML-ML(31.1%). Direct Symmetry was highest in group ML-ML(54.5%) and lowest in group H-H(20.5%). Mirror Symmetry was highest in H-H group (43.8%) and lowest in ML-MO group (13.3%).

Discussion

It is the comprehensive study based on tertiary clinical set up where we demonstrate the relationship and symmetry pattern of Bilateral Astigmatism. Our study showed that bilateral astigmatism is mostly isorule. Anisorule astigmatism was seen in less than one-sixth of the sample. The higher prevalence of isorule astigmatism was similar in male and female. This finding is compatible with our clinical knowledge. The high prevalence of isorule astigmatism indicates the relative similarity of the orientation of the main meridians in fellow eyes. According to the findings of this study, the majority of cases of bilateral astigmatism are isorule astigmatism, and anisorule astigmatism is rare in the general population. Therefore, when we encounter patients with anisorule astigmatism with a large inter-ocular difference (for example, axes of 85° and 150° in the right and left eyes), we should consider the possibility of a number of acquired corneal disorders that can affect astigmatism. For example, the presence of unilateral corneal scarring,[15,16] pterygium,[17,18]  post cataract surgery and even keratoconus[19,20] may interfere with the isorule pattern in astigmatism. Therefore, based on the results of this study and clinical experience, we can say that cases of anisorule astigmatism need to be examined to ensure corneal health and rule out possible disease conditions.

As can be deduced from the results, gender is not a determinant factor of the prevalence of rule similarity, and the distribution of rule similarity is comparable in both genders in this study population. The age-related changes in different types of isorule astigmatism, including with the rule, Against the rule, and oblique, were similar to the findings of previous studies, such that with-the-rule astigmatism had a decreasing trend, against-the-rule astigmatism had an increasing trend, and oblique astigmatism had a relatively steady trend. [21-25]

It is also observed that the prevalence of isorule astigmatism in higher level according to magnitude of astigmatism and type of astigmatism i.e Simple form, Compound form or mixed form. In case of bilateral astigmatism isorule rule is seen higher prevalence with magnitude similarity of both eyes i.e mild-mild(ML-ML), Moderate-moderate (MO-MO), High-High(H-H). But Anisorule is showing increasing prevalence in higher magnitude difference i.e Mild-High (ML-H), Moderate-High (Mo-H).

Our study showed that the axes in fellow eyes had a higher tendency toward direct symmetry. Although the prevalence of exact symmetry was not very high, the prevalence of exact direct symmetry was higher than exact mirror symmetry. In addition, the prevalence of exact mirror symmetry is increasing in higher magnitude of astigmatism (table 5).And we also observed that asymmetry is more prevalent in case of different magnitude of astigmatism in fellow eye i.e mild-high, moderate-high.  These findings indicate that symmetry is the common pattern for the axes in fellow eyes. These findings differ from the results of the study by McKendrick and Brennan.[26] who were the first to evaluate bilateral axis symmetry in 192 participants and found that neither direct nor mirror symmetry were predominant. The results of the study by McKendrick and Brennan are not reliable due to two main limitations: the small sample size and ignoring the clock arithmetic nature of astigmatism axes. Disregarding modulus 180 could have biased their calculations.

Symmetry and being isorule is important in the clinical management and prescribing glasses to cases of astigmatism as well. Given their meridional magnification effect, cylindrical glasses can cause spatial distortions and consequently asthenopia and headaches for patients.[27] Such spatial distortions become more disturbing when the axes in fellow eyes deviate from the 180 and 90 symmetric position and in fact become anisorule and asymmetric.[28] These problems worsen under binocular vision, and involve the binocular spatial perception.[28] To reduce or prevent such potential problems, previous studies have suggested that the axes on the prescription be as close to 180 and 90 symmetry as possible, albeit without compromising visual acuity.[29,30] Asharlous et al. evaluated the association of astigmatism axes in fellow eyes in 2016.[31]This study presented a comprehensive analysis of the association of astigmatism axes in the fellow eyes of individuals visiting ophthalmology clinics. Although the involvement of the axes was well analyzed, the results may not be generalisable because the sample did not characterize the general population. Therefore, the pattern of the association of bilateral astigmatism axes in the general population and the changes of these patterns with age are not clear.[31]

The findings of this study indicate that most cases of astigmatism tend toward symmetry, and fortunately, most bilateral cases are isorule with-the-rule and against-the-rule, and spatial problems and binocular stereoscopic perceptions may be a problem only for a small percentage of patients.

Conclusion

Similar to the results of a previous clinical-based study, we found that the majority of cases of bilateral astigmatism in the population are isorule. Gender is not a determinant in the prevalence of isorule astigmatism, while age is associated with significant changes in its prevalence. Overall, the prevalence of isorule and anisorule astigmatism increases with age in the population, with the slope being much steeper in isorule astigmatism. The prevalence of isorule astigmatism increases with spherical ametropia. Axis symmetry is a common feature in bilateral astigmatism in the population, and the prevalence of direct symmetry is higher than mirror symmetry. Axis symmetry usually reduces at older ages, and the least symmetry is seen after the age of 50 years. Most cases of isorule astigmatism are with-the-rule (WW) and the majority of symmetric axes are also in this group, while the isorule oblique type is uncommon in the population and the cases show less symmetry. The rarest form of isorule astigmatism is oblique astigmatism with direct symmetry.

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