Iridocorneal angle (ICA) or anterior chamber angle is the anatomical structure responsible for aqueous humor outflow and regulating intraocular pressure inside the eye [1]. ICA anatomy plays a vital role in the pathogenesis of different types of glaucoma (especially angle closure). In the Far East, ICA closure is considered a crucial feature for primary angle-closure glaucoma, identifying COL18A1 as an ICA candidate gene for angle closure in humans. Therefore, the assessment of ICA is an essential part of the ophthalmic examination [2, 3]. A gonioscopy examination is a gold standard for assessing the angle. However, it is uncomfortable for some patients due to the huge measurement discrepancy between the examiners [4]. With the advancement of new technology, anterior segment optical coherence tomography (AS-OCT) is increasingly used in the ophthalmic field to obtain precise anterior chamber angle measurements [5]. It is a private and secure imaging modality that offers many useful trabecular-iris space area (TISA) and ICA parameters, such as trabecular-iris angle (TIA), iris thickness (IT), iris volume (IV), iris curve (IC), and angle opening distance (AOD) [6].

Studying the normal angle and providing baseline normative data for the general population will help understand and diagnose ICA-related diseases and identify their risk factors. Many existing studies evaluated the ICA by AS-OCT; they showed a discrepancy in their measurements, which is explained by the difference in ethnicity [7-17]. Considering the Saudi population, some ICA parameters, namely (IT) and (IV), were measured in healthy eyes [18, 19]. However, other parameters of ICA (specifically AOD and TISA) need further evaluation in the Saudi population. Thus, this study aims to provide population-based reference data of iridocorneal angle parameters measured in healthy Saudi eyes with spectral-domain AS-OCT MS-39® (Costruzione Strumenti Oftalmici, Florence, Italy).

Eighty-seven optical reports from 87 participants were assessed in this study. Demographics and characteristics reported from the clinical trial of study participants are demonstrated in Table 1. There was an insignificant difference in the percentages of males (55.2%) and females (44.8%) included in this study (p=0.335). The mean age of the participants was 41.1 ± 14.6 (18 to 78) years. Patients showed excellent visual acuity (20/20 to 20/32), low cylinder (≤1.00D), and normal intraocular pressure (9.0 to 20.0mmHg). The mean axial length was 24.3 ± 0.33mm. Table 2 compares nasal and temporal angle parameters with their mean values. Our participants had almost insignificant differences between nasal and temporal values except for AOD_500, which was higher in the temporal angle (p=0.005). The correlations between patient factors and the angle parameters are demonstrated in Table 3. There were moderate to strong positive correlations (r = 0.6 to 0.9) between the grade of the angle and the nasal and temporal angle parameters indicative of higher angle grade (wider angle) association with more extensive angle parameters. Fair negative correlations (r = -0.4) existed between age and angle parameters (Fig. 2); older age is associated with smaller angle parameters. Also, there were fair negative correlations (r = -0.3 to -0.4) between IOP and angle parameters; higher IOP was associated with smaller angle parameters (Fig. 3).

Fig. (2). Scatterplot diagram demonstrating the correlation between Age and Angle Opening Distance (AOD).

Fig. (3). Scatterplot diagram demonstrating the correlation between Intraocular Pressure (IOP) and Angle Opening Distance (AOD).

Fig. (4). Diagram showing the poor correlation between gender and angle parameters.

The results also showed no significant differences between genders in angle parameters (Fig. 4), representing poor correlations (r = 0 to 0.2). No correlations were found among laterality, BCVA, spherical equivalent, cylinder, and angle parameters. Multiple linear regression with backward elimination of non-significant factors found age (p=0.002) and angle grade (p<0.001) as predicting factors for nasal AOD₅₀₀ (Adjusted R²=0.772, p<0.001). Multiple linear regression with backward elimination of non-significant factors found angle grade (p<0.001) and temporal iris thickness (p=0.004) as predicting factors for temporal AOD₅₀₀ (Adjusted R2=0.511, p<0.001). Multiple linear regression with backward elimination of non-significant factors found IOP (p =0.026) and angle grade (p<0.001) as predicting factors for nasal TISA₅₀₀ (Adjusted R2=0.639, p<0.001). Multiple linear regression with backward elimination of non-significant factors found angle grade (p<0.001) and temporal iris thickness as predicting factors for temporal TISA₅₀₀ (Adjusted R2=0.314, p<0.001).

Measurement of Iridocorneal angle (ICA) appeared to help evaluate and establish the diagnosis of any glaucoma [20]. Recent studies showed that ethnicity is an important discriminating factor to be considered when assessing ICA (Caucasians were found to have wider angles than others) [9-16]. Due to our population's lack of reference normative data for ICA parameters, such a study was necessary. This study analyzed the ICA parameters using AS-OCT on the Saudi population, which is a unique contribution to the literature. AS-OCT usage in angle-closure subjects to examine angle requires comprehensive angle biometry assessment through accurate credentials of the scleral spur as the sclera point of the inward protrusion [21]. In our study, we analyzed the AOD500, AOD750, TISA500, and TISA750 from temporal and nasal angles (from 0 to 180 degrees), which were found to be adequate for assessing the angle by many studies [10, 12, 17]. However, other studies included ICA measurement from superior and inferior quadrants, which might be susceptible to measurement errors due to the need for eyelid manipulation [9, 12]. In our study, the mean angle values were higher compared to what was reported by Leung et al. [17]. However, they were found to be substandard compared to those reported by other studies found in the literature. This could be explained by anatomical variation between different ethnic groups [10-12].

Our study found insignificant differences between temporal and nasal values except for AOD500, which was higher in the temporal angle (p=0.005). This result agreed with Romkens et al. [16], which reported higher values in the temporal angle. However, this contrasts with several other studies reporting lower values in the temporal area [12, 22-24]. Regarding the correlation between age and angle values, our study suggested a strong negative correlation between age and angle values; that is, older age is associated with smaller angle parameters. The lens thickness increases with aging might explain this negative correlation, and several studies strongly agree with this correlation [11, 12, 23]. The study also indicated a strong and positive correlation between the grade of the angle (based on Shaffer classification) and angle values, and a higher angle grade (wider angle) was associated with larger angle values. Friedman et al. [24] reported a strong correlation between gender and angle values; women have narrower angles than men; however, Ruiz-Belda et al. [25] reported a highly significant difference between genders regarding nasal IA measurements at three different meridians. However, our study found a poor correlation between gender and angle parameters, with no significant difference between males and females. A Chinese-American study examined AS-OCT images and found no statistically significant difference in biometric measurements pre and post-gender and age adjustment in detecting primary angle-closure glaucoma [26]. Another study highlighted the significance of angle assessment in the dark to light in attenuating beneficial angle-widening results in PACD eyes to closely evaluate the angle-closure mechanism [27]. Friedman et al. [28] stressed the dangerously increasing rate of Primary angle-closure glaucoma (PACG), a vision loss problem, in older adults across Asia. The study reviews the PACG research and elaborates on improving the imaging of the anterior segment to identify the angle closure and the treatments that are approached to cure PACG [28].

Our study found no correlation between spherical equivalent, cylindrical power, and angle parameters. This contrasts with other studies that established an impact of refractive status on angle measurements (hyperopes found to have shallower angles) [11, 24]. We attribute our finding to the fact that all participants enrolled in our study were chosen to have minimal refractive error, which was not the case in other studies. Lastly, our study had several limitations. The first limitation is that our measurements of angle parameters were limited to nasal and temporal angles, with no measures taken from superior and inferior quadrants. Due to the poor quality images attained for the superior and inferior quadrants and the requirement for eyelid manipulation, all quadrants needed to be evaluated in cases of glaucoma. The second limitation is that iris volume (IV) and curve (IC) were not included in our study, which could dynamically influence the angle. Also, our study used subjective utilization of the angle imaging and manual localization of the scleral spur, which could induce measurement errors. However, this was the limitation of the other studies in the relative literature [9-11, 15, 17].

In conclusion, AS-OCT was a valuable, easy, non-invasive method to evaluate the iridocorneal angle. We provided population-based reference data of iridocorneal angle parameters, which can be used in future studies. Our study suggests that age, angle grade, CDR, and IOP influence iridocorneal angle measurements.