Astigmatism in Duane Retraction Syndrome

Abstract

Purpose

To compare the prevalence, magnitude, and type of astigmatism among patients with different Duane Retraction Syndrome (DRS) types.

Method

This retrospective cross-sectional study reviewed the records of 312 DRS patients. Patients were categorized into DRS Types 1, 2, 3, and bilateral cases. Refractive errors and visual acuity were analyzed, emphasizing the prevalence of astigmatism types, specifically with-the-rule (WTR), against-the-rule (ATR), and oblique, as well as the power vectors for each DRS subtype.

Results

Our study included 312 patients with DRS, comprising 44.6% males and a median age of 18.5 years (interquartile range [IQR]: 7–27). Of these, 280 had unilateral DRS and 32 had bilateral DRS. The median age for unilateral DRS patients was 19 years (IQR: 7–27; 43.2% males), whereas for bilateral DRS patients, it was 16 years (IQR: 6-29.2; 56.3% males). In all DRS patients, 180 (57.7%) were diagnosed with Type 1 DRS, 87 (27.9%) with Type 2 DRS, and 45 (14.4%) with Type 3 DRS. In unilateral cases, WTR astigmatism was the most prevalent (56.8%), followed by oblique (23.6%) and ATR astigmatism (19.6%). Bilateral DRS patients exhibited a similar distribution in both eyes, with WTR astigmatism also being predominant. The comparative analysis of power vectors indicated that Type 1 DRS exhibited a greater prevalence of WTR and oblique astigmatism, whereas ATR astigmatism was the predominant pattern in Types 2 and 3. A comparison of cylindrical powers and power vectors among fellow eyes of different DRS subtypes revealed that Type 2 DRS had significantly higher cylindrical power (p = 0.017) and a greater tendency toward ATR astigmatism (p = 0.038) than fellow eyes in other subtypes, suggesting that astigmatic changes may also occur in fellow eyes.

Conclusion

Our study demonstrates a high prevalence of astigmatism, particularly WTR astigmatism, in DRS patients. Furthermore, we found significant associations between specific astigmatism patterns and DRS subtypes, suggesting a potential link between extraocular muscle innervation, co-contraction, and corneal shape. These findings highlight the importance of comprehensive astigmatism assessment in DRS patients for optimal refractive management.

Introduction

Duane Retraction Syndrome (DRS) is classified as a congenital cranial dysinnervation disorder. It is characterized by the lack of typical nerve innervation, which can be observed through advanced imaging techniques that reveal either the absence or hypoplasia of the abducens nerve. Additionally, DRS involves misinnervation of the lateral rectus muscle, which receives signals from an aberrant branch of the oculomotor nerve [1,2,3].

Huber classified DRS into three distinct types based on electrophysiological studies: Type 1, characterized by restricted abduction; Type 2, marked by limited adduction; and Type 3, which presents with restrictions in both abduction and adduction. Furthermore, the dysfunction of the abducens nucleus, along with the misinnervation hypothesis, contributes to the co-contraction of the medial and lateral rectus muscles, resulting in the narrowing of the palpebral fissure during attempted adduction [4, 5].

The tension applied to the globe by extraocular muscles plays a significant role in influencing refractive errors. In patients with unilateral Duane syndrome, the chronic co-contraction of the medial and lateral rectus muscles can deform the globe and alter corneal curvature, resulting in increased astigmatism in the affected eye compared to the fellow eye [6]. Globe retraction, often accompanied by a narrowing of the palpebral fissure and a shift in the eyeball’s position within the orbit, can intensify the vector forces acting on the cornea, further distorting its shape [7].

In this study, we aim to explore the astigmatic profiles in eyes affected by DRS compared to their fellow eyes, to elucidate the dynamic effects of eyelid position, muscle contraction, and globe position on corneal curvature. Additionally, we seek to investigate the patterns and variations of astigmatic changes among different subsets of DRS. By doing so, we aim to contribute to a more standardized approach to the assessment and management of astigmatism in patients with DRS, ultimately enhancing patient care outcomes.

Method

In this retrospective cross-sectional study, we reviewed the hospital records of patients diagnosed with various types of DRS who were candidates for surgical intervention at Farabi Eye Hospital between 2015 and 2022. The study protocol was meticulously reviewed and approved by the Institutional Review Board of Tehran University of Medical Sciences, ensuring compliance with ethical standards and guidelines. The research was conducted using the principles outlined in the Declaration of Helsinki, which safeguard the rights and well-being of study participants.

Patients were eligible for inclusion in the study if their medical records confirmed a diagnosis of DRS types 1, 2 and 3. Additionally, eligibility required the absence of systemic comorbidities, such as motor or mental disabilities, and no evident anterior or posterior segment pathologies. To minimize potential confounding factors, patients with a history of prior ocular surgeries, including strabismus correction or refractive procedures performed at other eye care facilities, were excluded from the study. A total of 312 DRS patients met the inclusion criteria for the study, comprising 180 cases of DRS type 1, 87 cases of DRS type 2, and 45 cases of DRS type 3.

Our clinic’s standard ophthalmic examination protocol encompasses a comprehensive range of tests and assessments. Initially, patients undergo a detailed evaluation, including dry and cycloplegic refraction, corrected distance visual acuity (CDVA) measurement, and a fundus examination. Refractive error is assessed using the Topcon KR-800 Auto Kerato-Refractometer (Topcon, Japan) and verified through retinoscopy with the Heine Beta 200 retinoscope (Heine Optotechnik, Herrsching, Germany). CDVA is measured using an E Snellen chart at a distance of 6 m, with results converted to logarithm of the minimum angle of resolution (logMAR) units using the formula: logMAR = -log (VA E-Snellen). The angle of deviation is quantified using the alternate prism cover test, where exotropia is defined as a misalignment of one prism diopter or more between the visual axes of the eyes, as determined by the unilateral prism cover test [8]. Following the initial ophthalmic examination, eye movements were carefully assessed using a motility test, which enabled the detection of any overshoot or undershoot of the extraocular muscles. All evaluations required participants to wear their prescribed refractive corrections to ensure optimal visual performance. The procedures were conducted while patients focused on 20/30 optotypes displayed on a near Snellen chart positioned 33 cm away, providing a standardized visual stimulus for accurate assessment of ocular motility [9].

Unilateral amblyopia was defined as a difference of ≥ 2 Snellen (decimal) lines in CDVA between the eyes, with the amblyopic eye having a CDVA of ≥ 0.1 logMAR (8/10 decimal) and the presence of at least one of the following amblyogenic factors: (1) anisometropia (a refractive difference of ≥ 3.00 D for myopia, ≥ 1.00 D for hyperopia, or ≥ 1.50 D for astigmatism) (2), strabismus, or (3) a combination of anisometropia and strabismus [10].

WTR astigmatism was defined as occurring between 160° and 20°, oblique astigmatism was classified as occurring between 21° and 69° and between 111° and 159°, while ATR astigmatism was identified when the axes were positioned between 70° and 110° [11].

The classification of DRS includes several types. Type 1 is characterized by restricted abduction, which may present as esotropia, exotropia, or orthophoria in the primary gaze position. This type also exhibits globe retraction during adduction and a widening of the palpebral fissure during abduction. Type 2 is noted for limitations in adduction, while Type 3 presents with restrictions in both abduction and adduction. Type 4, a recently identified variant known as the synergistic divergence type, is marked by significant restriction in adduction, coupled with simultaneous abduction of both eyes when attempting to gaze toward the opposite side. A pivotal clinical feature across all types is the retraction of the globe, particularly evident during adduction [12, 13].

In the final phase of the study, we analyzed the distribution patterns of astigmatism power and axis in patients. To facilitate this analysis, we employed the power vector method as outlined by Thibos and Horner, comparing J0 and J45 values using the following formulas:

$$\:\text{J}0=\:(-\text{C}/2)\:\text{c}\text{o}\text{s}\:\left(2\right),\:\:\text{J}45=\:(-\text{C}/2)\:\text{s}\text{i}\text{n}\:\left(2\right)$$

C represents the positive cylindrical power (C > 0) in these equations. α denotes the meridian of the minus cylinder, which corresponds to the meridian of flat keratometry. The J0 value reflects the cylinder power at the orthogonal 90° and 180° meridians, with a positive J0 indicating WTR astigmatism and a negative J0 indicating ATR astigmatism. Meanwhile, J45 assesses the cross-cylinder set at the 45° and 135° meridians, which pertains to oblique astigmatism [14] .

Statistical analysis

Descriptive statistics, including means and standard deviations, were calculated to determine the refractive error values for patients within each DRS category. Frequency distributions were generated to identify the prevalence of different astigmatism axis types among patients with DRS types I, II, and III, and percentages were calculated to quantify the overall prevalence of astigmatism in these groups. A paired t-test was employed to assess refractive errors and visual acuity between DRS-affected eyes and their fellow eyes. ANOVA test was used to evaluate refractive errors across the different types of DRS, followed by LSD post hoc analyses to compare these parameters pairwise between the DRS types. A p-value of less than 0.05 was considered statistically significant.

Result

The study included a total of 312 cases, of which 173 (55.4%) were female, and 139 (44.6%) were male. The median age of the total patient was 18.5 years (interquartile range [IQR]: 7 to 27 years). Among the cases, 180 (57.7%) were diagnosed with Type 1 DRS, 87 (27.9%) had Type 2DRS, and 45 (14.4%) had Type 3 DRS. A total of 280 cases (89.7%) were identified with unilateral DRS, while 32 cases (10.3%) exhibited bilateral DRS. Among the unilateral cases, the gender distribution closely mirrored that of the total population, comprising 159 females (56.8%) and 121 males (43.2%). The median age of unilateral patients was 19 years (IQR:7 to 27 years). Within this subgroup, the distribution of DRS types was as follows: Type 1 accounted for 159 cases (56.8%), Type 2 for 81 cases (28.9%), and Type 3 for 40 cases (14.3%).

The most common type of deviation was esotropia, observed in 179 (57.4%) cases, followed by exotropia in 88 (28.2%) cases, and orthotropia in 45 (14.4%) cases. A similar pattern was observed in the unilateral DRS subgroup, with esotropia being the most prevalent (158 cases, 56.4%), followed by exotropia (82 cases, 29.3%), and orthotropia (40 cases, 14.3%).

Regarding the affected eye(s), the left eye was involved in the majority of cases (206, 66.0%), followed by the right eye (74, 23.7%), and bilateral involvement (32, 10.3%). In the unilateral DRS subgroup, the left eye was affected in 206 cases (73.6%).

Table 1 The descriptive statistics for refractive error components and visual acuity in the different types of unilateral DRS cases

In the unilateral DRS cases, the most common type of astigmatism in the DRS eye was WTR astigmatism, observed in 159 cases (56.8%), followed by oblique astigmatism in 66 cases (23.6%), and ATR astigmatism in 55 cases (19.6%). A similar pattern was observed in the fellow eyes of unilateral cases, with WTR astigmatism being the most common (168 cases, 60.0%), followed by oblique astigmatism (62 cases, 22.1%), and ATR astigmatism (50 cases, 17.9%).

Unilateral DRS group

Table 1 compares refractive error components and visual acuity parameters across all unilateral cases of DRS and the different types of unilateral DRS cases. CDVA was recorded in 246 cooperative patients. In unilateral cases of DRS, the fellow eyes demonstrated statistically significantly better CDVA compared to the DRS-affected eyes (0.0463 ± 0.11642 vs. 0.0942 ± 0.15898 LogMAR, P < 0.001). This tendency for worse CDVA in the DRS eyes remained statistically significant in Type 1 DRS (0.09 ± 0.13 vs. 0.04 ± 0.10 LogMAR; P = 0.001); however, it lost its significance in Types 2 and 3 DRS. DRS eyes tended to be more hyperopic than fellow eyes (SE: 0.1603 ± 2.09340 vs. 0.1085 ± 1.73551 D); however, this difference was not statistically significant in either the total DRS or among the subtypes. In unilateral cases, the mean cylindrical power in the DRS-affected eye was significantly higher than that in the fellow eye, with a mean difference of -0.16071 (p = 0.038). This finding reflects associated astigmatic changes in the DRS-affected eye. The astigmatic tendency observed in DRS eyes remained statistically significant in Type 1 DRS (mean difference of cylindrical power =-0.254, p = 0.005); however, this significance was not maintained in Type 2 and 3 DRS.

In unilateral cases, there were no statistically significant differences concerning power vectors between DRS and fellow eyes. However, in Type 1 DRS, the J45 vector in DRS-affected eyes exhibited a significantly more negative value compared to fellow eyes, signifying a propensity for oblique astigmatism in this subtype (J45 vector values: -0.18 ± 0.53 DRS eyes, -0.05 ± 0.46 fellow eyes; p = 0.014). Furthermore, within the Type 3 DRS subset, the J0 vector in DRS-affected eyes was significantly more negative than in fellow eyes, indicating a tendency toward ATR astigmatism in Type 3 DRS (J0 values: -0.14 ± 0.57 DRS eye, 0.11 ± 0.41 fellow eye; p = 0.03).

Table 2 LSD post hoc analysis between different types of unilateral DRS patients

The results of one-way ANOVA revealed that CDVA did not differ significantly across the various types of DRS in either the affected or fellow eyes. The investigation of mean spherical power and SE demonstrated a trend toward myopia as the classification progressed from Type 1 DRS to Type 2 and subsequently to Type 3 DRS, observed in both the affected and fellow eyes; However, it did not reach statistical significance. The mean cylindrical power did not differ significantly across the three types of DRS in either the affected or fellow eyes. The investigation of J0 vector values indicated that they become increasingly negative as the classification progresses from Type 1 DRS (0.03 ± 0.42 D) to Type 2 (-0.11 ± 0.53 D) and then to Type 3 DRS (-0.14 ± 0.57 D) in the affected eyes (p = 0.035). This observation represents a significant trend, shifting from a predominance of WTR astigmatism in Type 1 DRS-affected eyes toward a more pronounced ATR astigmatism in Type 3 DRS-affected eyes.

Table 2 presents a post hoc analysis comparing the different types of unilateral DRS patients for various refractive and visual acuity parameters. The results of the post-hoc analysis revealed that the mean spherical power and SE in Type 1 DRS-affected eyes were significantly more hyperopic compared to those in Type 3 DRS-affected eyes (sphere power: mean difference = 0.73 D, p-value = 0.048; SE: mean difference = 0.76 D, p-value = 0.040). Furthermore, fellow eyes in Type 1 demonstrated a pronounced hyperopic pattern compared to fellow eyes in Type 3, with this difference reaching statistical significance (mean difference = 0.67 D, p-value = 0.028). While the mean cylinder power in the DRS eye did not differ significantly among the types, Type 2 cases had a significantly higher mean cylindrical power than Type 1 cases in the fellow eye, representing more astigmatic changes (mean difference = 0.28657 D, p-value = 0.017). In Type I DRS cases, the J0 values in the affected eyes exhibited a significantly higher mean compared to Type 2 cases (mean difference = 0.14153 D, p-value = 0.030), indicating a greater prevalence of WTR astigmatism in Type 1 DRS. Moreover, in Type 2 DRS cases, the J0 values in the fellow eyes demonstrated a significantly lower mean compared to those in Type 3 cases (mean difference = 0.14 D, p-value = 0.038), suggesting a greater prevalence of ATR astigmatism in Type 2 DRS compared to Type 3 in the fellow eye.

Amblyopia was observed in 19.3% of the total sample, with a slightly higher prevalence in Type III patients (22.5%) compared to Type I (18.9%) and Type II (18.5%). Also, abnormal head posture was far more prevalent, affecting 81.4% of the total sample. The highest occurrence was in Type II patients (87.7%), followed by Type I (79.9%) and Type III (75.0%).

Table 3 The comparison for refractive error components and visual acuity between right and left eyes of bilateral Duane retraction syndrome

Bilateral DRS group

Of the total 32 bilateral DRS cases, 14 cases (43.8%) were females, and 18 (56.3%) were males. The median age of bilateral patients was 16 years (IQR:6 to 29.2 years). Type 1 was the most common, accounting for 21 cases (65.6%), followed by Type 2 with 6 cases (18.8%), and Type 3 with 5 cases (15.6%). Concerning the deviation patterns, 21 cases (65.6%) demonstrated esotropia, 6 cases (18.8%) exhibited exotropia, and 5 cases (15.6%) were classified as orthotropic. In the right eye, WTR astigmatism was the most prevalent, occurring in 19 cases (59.4%), followed by oblique astigmatism in 10 cases (31.3%) and ATR astigmatism in 3 cases (9.4%). Similarly, in the left eye, WTR astigmatism was the most common, observed in 17 cases (53.1%), followed by oblique astigmatism in 13 cases (40.6%) and ATR astigmatism in 2 cases (6.3%).

Table 3 compares refractive error components and visual acuity parameters between the right and left eyes in cases of bilateral DRS. However, no significant differences were observed in refractive error components or visual acuity between the right and left eyes in these bilateral DRS cases.

Discussion

In this retrospective cross-sectional study, the most prevalent type of astigmatism in DRS-affected eyes, whether in unilateral or bilateral patients, was found to be WTR, followed by oblique and ATR astigmatism. In unilateral cases, DRS-affected eyes exhibited significantly worse CDVA and higher cylindrical power compared to the fellow eyes. This trend remained significant in Type I DRS cases as well. Regarding power vectors, eyes with Type 1 DRS demonstrated a significantly higher tendency toward WTR and oblique astigmatism when compared to both the fellow eyes and other DRS subtypes. Conversely, eyes with Type 3 DRS exhibited a significantly greater trend toward ATR astigmatism.

Our study revealed that eyes affected by unilateral DRS exhibit worse CDVA compared to their fellow eyes. This disparity can be attributed to the fixation preference for the fellow eye and the suppression of the DRS eye, resulting in diminished visual stimulation for the affected eye and exacerbating the acuity disparity. Notably, the prevalence of amblyopia in unilateral and bilateral DRS cases has been reported to be 18.5% and 36.5%, respectively [15].

We found that the pattern and axis of astigmatism vary across the different subtypes of DRS, reflecting the distinct underlying pathophysiology associated with each subtype. Consistent with the findings of our study, Young et al. reported higher cylindrical power and greater astigmatic changes in DRS-affected eyes compared to their fellow eyes [7]. Given that multiple synergistic factors influence changes in corneal topography and astigmatism in patients with DRS over an extended period, Yeniad et al. evaluated the dynamic changes in corneal topography in both DRS-affected eyes and their fellow eyes across two gaze positions: primary and preferred. They observed a significant decrease in the mean horizontal sim K value when transitioning from preferred to primary gaze. This flattening of the cornea in the horizontal direction is attributed to the co-contraction of misinnervated extraocular muscles, coupled with the narrowing of the palpebral aperture and alterations in eyelid position, which collectively lead to permanent astigmatic changes over the long term. Their findings also indicate that conventional refractive assessments conducted in primary gaze may not adequately reflect the visual experiences of patients with DRS, as these individuals often adopt specific head postures to enhance their vision. It is recommended that a thorough evaluation of DRS patients includes an assessment of corneal topography in both their preferred gaze positions and primary gaze [6]. Despite previous studies confirming astigmatic changes in DRS eyes compared to their fellow eyes, Yuzbasıoğlu et al. reported no significant differences. This discrepancy may be attributed to the smaller sample size in their study population [16].

The eyelids exert a significant molding effect on the cornea, which is why patients with congenital ptosis frequently exhibit higher levels of astigmatism. This phenomenon is attributed to the impact of the ptotic eyelids on the corneal curvature, leading to alterations in visual acuity [17]. The mechanical pressure exerted by ptosis of the upper eyelid results in the steepening of the corneal curvature in the vertical meridian, contributing to a WTR drift [18]. Notably, surgical intervention for congenital ptosis leads to a significant reduction in cylindrical power and an improvement in CDVA at three months post-operatively. Additionally, corneal topography indicates a decrease in astigmatism, although average keratometry values remain unchanged [19,20,21].

Chronic co-contraction of the medial and lateral rectus muscles in DRS patients would significantly affect corneal curvature, resulting in tightening along the 180° axis and the development of ATR astigmatism [6, 7]. However, variations in astigmatism across different subtypes of DRS can be elucidated by evaluating the pathophysiology of each subtype from an alternative perspective. The number of nerve fibers originating from the oculomotor nerve that innervate the lateral rectus muscle may give rise to different subsets of DRS patients. In cases where there is a limited number of nerve fibers, the medial rectus muscle is stronger than the lateral rectus, resulting in minimal impairment of adduction while causing marked impairment of abduction. This characterizes Type 1 DRS, where the co-contraction is highly asymmetric. Conversely, when the number of nerve fibers is greater, innervation to the medial rectus decreases while that to the lateral rectus increases, leading to impaired adduction. This scenario is representative of Type 2 DRS, where co-contraction becomes less asymmetric. In cases where there is a substantial number of nerve fibers, the medial rectus loses its functional strength, along with limitation in normal abduction due to the absence of the abducens nucleus, this condition corresponds to Type 3 DRS. Here, the forces become more balanced, resulting in symmetric co-contraction. The symmetric co-contraction observed in Type 3 DRS exerts the most significant force on corneal curvature, leading to horizontal steepening. In contrast, the highly asymmetric co-contraction in Type 1 DRS does not significantly impact horizontal steepening [22, 23]. Furthermore, when fewer nerve fibers reach the lateral rectus muscle during embryonic development, the muscle becomes increasingly fibrotic. This fibrotic lateral rectus may then apply abnormal deformational forces on the ocular globe, potentially contributing to the development of oblique astigmatism [24]. Therefore, we conclude that there is a notable trend toward ATR astigmatism in Type 3 DRS, primarily attributed to the highest level of co-contraction observed in this subtype. In contrast, WTR and oblique astigmatism are more prevalent in Type 1 DRS, primarily attributed to the scarce innervation of the lateral rectus muscle and the subsequent fibrotic changes, as demonstrated in our study. Furthermore, the significantly greater astigmatic changes observed in Type 1 DRS compared to other subtypes may highlight the predominant role of fibrotic lateral rectus muscles over co-contraction in the development of astigmatism.

The predominant WTR astigmatism observed in the overall patient in our study and previous research can be attributed to the higher prevalence of Type 1 DRS within these populations [15]. Consequently, given that astigmatism variations differ among the various DRS subtypes, it is reasonable to note that Young et al. reported ATR and oblique astigmatism in their study of DRS patients, reflecting the varying distribution of subtypes across different studies [7].

The tension exerted on the ocular globe by extraocular muscles and its impact on refractive errors can be effectively illustrated by examining the outcomes of rectus muscle recession on refraction. Studies consistently show that horizontal muscle recession is linked to an increase in astigmatism postoperatively. Additionally, a notable disparity in astigmatism has been observed between operated and unoperated eyes. The reduction in tension from recessed muscles may lead to flattening the cornea along the 180-degree meridian while causing steepening along the 90-degree meridian, resulting in WTR astigmatism. However, the changes in astigmatism are frequently minor and transient, typically lasting up to six months [25,26,27,28]. Reports regarding muscle recession in patients with DRS appear to be contrasting. It has been established that muscle recession has resulted in a significant reduction in astigmatism, which remained stable over an 18-month follow-up period. These notable long-term reductions underscore the significant role of muscle co-contraction in the development of astigmatic changes in DRS patients. Therefore, it is important to emphasize that strabismus surgery not only corrects strabismus but also positively impacts refractive errors, facilitating amblyopia treatment through improved ocular alignment [29].

Additionally, observing that all refractive and visual parameters were significantly worse in bilateral cases of DRS compared to unilateral cases adds a new dimension to our understanding of the condition. This finding suggests that the impact of DRS on visual function may be cumulative when both eyes are affected, highlighting the necessity for comprehensive care in these instances.

The present study on astigmatism in various types of DRS demonstrates several notable strengths. It includes a robust sample size of 312 cases, enhancing the credibility and statistical power of the findings. Furthermore, the comprehensive analysis of astigmatism’s prevalence, magnitude, and types across DRS Types 1, 2, and 3 facilitates a detailed comparative assessment of this clinically significant aspect of the syndrome. However, this study has several limitations. First, its retrospective design inherently limits the ability to establish causality or explore changes over time. Additionally, the lack of a control group restricts the ability to compare the results with the general population. Another notable limitation is the absence of keratometry and corneal topography data, which prevented us from definitively attributing astigmatic changes to corneal distortion, as these changes may have originated from a lenticular source. Furthermore, this study was conducted exclusively predominantly on Iranian patients, which may limit the generalizability of the findings to other populations due to potential ethnic and genetic differences.

Future research should adopt a prospective design to better elucidate the detailed pathophysiological mechanisms and genetic factors contributing to astigmatic changes in DRS patients. Incorporating a control group would allow for robust comparisons to assess the age-related shifts in astigmatism patterns. Additionally, advanced diagnostic tools such as posterior corneal assessment and confocal biomicroscopy are recommended to provide deeper insights into corneal morphology and biomechanics. Keratometry and corneal topography should also be included in future studies to distinguish between corneal and lenticular contributions to astigmatism.

This study provides valuable insights into the prevalence, magnitude, and type of astigmatism across different DRS subtypes. Our findings indicate that astigmatism is common in this patient population, with WTR astigmatism being the most prevalent type. The significant associations observed between specific astigmatism patterns and DRS subtypes suggest a potential interplay between extraocular muscle function and corneal shape. Specifically, the association of Type 1 DRS with WTR/oblique astigmatism and Type 3 DRS with ATR astigmatism warrants further investigation to elucidate the underlying biomechanical mechanisms. Additionally, the observation of astigmatic differences in fellow eyes, particularly in Type 2 DRS, raises intriguing questions about the potential impact of DRS on the seemingly unaffected eye.