Comparative analysis of iridocorneal angle in cats and dogs using ultrasound biomicroscopy: implications for glaucoma prevalence

Abstract

Background

This study aims to investigate the anatomical differences in the anterior segment of the eyes between dogs and cats using ultrasound biomicroscopy (UBM) to understand the higher prevalence of primary angle-closure glaucoma (PACG) in dogs compared to cats. Retrospective analysis was performed on clinical data from 16 eyes of 16 dogs and 14 eyes of 14 cats with normal eye conditions. UBM was utilized to measure nine specific parameters, including Schwalbe’s Line Distance (SLD), Iridocorneal Angle (ICA), Angle-Opening Distance (AOD), and three ciliary cleft parameters: width (CCW), length (CCL), and area (CCA). To account for differences in body size, ciliary cleft parameters were adjusted accordingly.

Results

Significant anatomical differences in the anterior segment were found between the two species. Dogs had smaller values for SLD, ICA, AOD, and ciliary cleft parameters (CCW, CCL, CCA) compared to cats. Even after body-size adjustment, the rectified ciliary cleft parameters remained smaller in dogs.

Conclusion

The anatomical differences, particularly the smaller ciliary cleft and narrower drainage angles in dogs, may contribute to the higher prevalence of PACG in this species. Conversely, the larger ciliary cleft in cats may explain the lower occurrence of primary glaucoma in cats.

Background

Glaucoma is a critical concern in veterinary ophthalmology due to its potential to cause significant vision loss and severe pain in affected animals [1, 2]. In dogs, primary glaucoma and secondary glaucoma occur at rates of 0.89% and 0.80%, respectively, in North America, with primary angle-closure glaucoma (PACG) accounting for approximately 87% of cases within primary glaucoma [1, 3,4,5]. This contrasts with cats, where glaucoma typically arises as a secondary condition, accounting for approximately 95–98% of feline glaucoma cases, often due to pre-existing ocular or systemic disease such as uveitis, lens luxation, or trauma [6, 7].

The anterior segment of the eye is presumed to exhibit notable differences between dogs and cats; however, these differences have not been clearly elucidated yet. Some studies have identified that cats have a deeper anterior chamber depth, measuring approximately 4.52 mm compared to 3.76 mm in dogs [8, 9] However, research that comprehensively explains differences in the anterior chamber anatomy, including the iridocorneal angle(ICA) and uveal tract, which are crucial in regulating the production and drainage of aqueous humor, remains lacking [10, 11].

Ultrasound biomicroscopy (UBM) is an excellent tool for observing the anterior segment of the eye [12, 13]. This high-resolution imaging technique allows for detailed visualization of the structures involved in the production and drainage of aqueous humor [14, 15]. Particularly in veterinary medicine, UBM has been used to study the ICA, including the ciliary cleft (CC), which is critical for regulating aqueous humor outflow and, consequently, intraocular pressure (IOP) [16,17,18]. However, to the best of the author’s knowledge, there have been no studies to date that compare the ICA across species to determine the differences in the causes of glaucoma.

In this study, we aim to conduct a comparative analysis of the ICA anatomy in cats and dogs using UBM to elucidate the structural differences that may underlie the higher incidence of primary glaucoma in dogs. By establishing a clear correlation between the ICA anatomy and glaucoma prevalence, we hope to contribute to the development of targeted interventions and preventive strategies for glaucoma in veterinary practice.

Results

Demographic of study subjects

The study included 14 dogs and 16 cats. Among the dogs, Shih tzus (n = 3) were the most common breed, followed by Maltese (n = 2) and Spitz (n = 2), with the remaining breeds represented by a single dog each, including Poodle, Cavalier king charles spaniel, Beagle, Chihuahua, Pomeranian, and Mixed breeds. The average age of the dogs was 7.29 years, with a range from 2 to 15 years. The average weight was 5.16 kg, ranging from 2.85 to 8.30 kg. In terms of sex, 5 dogs were spayed females and 9 were castrated males. Regarding eye laterality, 9 eyes were OD (right eye) and 5 were OS (left eye).

For the cats, the study included 16 cats, predominantly Domestic Shorthairs (DSH, n = 15), with one Turkish Van represented. The average age of the cats was 4.28 years, ranging from 1 to 9 years. The average weight was 5.18 kg, with a range from 2.55 to 7.20 kg. In terms of sex, there were 8 spayed females and 8 castrated males. Regarding eye laterality, 9 eyes were OD and 7 were OS.

Statistical analysis showed that there was a significant difference in age between the dogs and cats (p = 0.023), with dogs being older on average. However, there was no significant difference in weight between the two groups (p = 0.948). In addition, no statistically significant differences were observed in eye laterality (OD vs. OS) in either dogs (9 OD vs. 5 OS) or cats (9 OD vs. 7 OS), indicating that the distribution of left and right eyes was balanced within each species. Furthermore, the sex distribution did not differ significantly.

Post-hoc power analysis

To evaluate the adequacy of our sample size, a post-hoc power analysis was performed. The analysis was based on the observed effect sizes and standard deviations for key anatomical parameters—including Schwalbe’s Line Distance (SLD), Angle-Opening Distance (AOD), ICA, and the CC parameters (CCW, CCL, and CCA). The results indicated that, for these primary outcome measures, the study achieved a statistical power exceeding 80% at an alpha level of 0.05. These findings confirm that the sample sizes of 14 eyes from 14 dogs and 16 eyes from 16 cats were statistically sufficient to detect clinically meaningful differences between the two groups.

Analysis of IOP

The IOP was assessed to evaluate differences between canine and feline subjects. In dogs, the mean IOP was 17.13 ± 3.096 mmHg, with values ranging from 11.00 to 22.00 mmHg. For cats, the mean IOP was 18.29 ± 2.016 mmHg, ranging from 13.00 to 21.00 mmHg. Statistical analysis revealed no significant difference in IOP between dogs and cats (p = 0.2414), indicating that the variance in IOP between the two species was not statistically significant.

Analysis of ICA parameters

The SLD was measured to compare the anterior segment parameters between canines and felines. The mean SLD in dogs was 2.306 ± 0.1539 mm, ranging from 1.980 to 2.510 mm. In cats, the mean SLD was 3.089 ± 0.2143 mm, with a range from 2.630 to 3.410 mm. The difference in SLD between dogs and cats was significant (p < 0.0001), indicating a marked anatomical difference in this parameter (Fig. 1A).

Fig. 1

Analysis of Iridocorneal Angle Parameters. A. Schwalbe’s Line Distance (SLD) comparison between dogs and cats shows that the mean SLD in dogs was 2.306 ± 0.1539 mm, ranging from 1.980 to 2.510 mm, whereas in cats, the mean SLD was 3.089 ± 0.2143 mm, with a range from 2.630 to 3.410 mm. B. The Angle-Opening Distance (AOD) comparison reveals that in dogs, the mean AOD was 0.9908 ± 0.3474 mm, with values ranging from 0.6200 to 1.810 mm, while in cats, the mean AOD was 1.833 ± 0.3874 mm, ranging from 1.340 to 2.580 mm. C. The Iridocorneal Angle (ICA) comparison shows that the mean ICA in dogs was 15.56 ± 3.241 degrees, ranging from 10.40 to 22.00 degrees, whereas in cats, the mean ICA was significantly larger, at 30.27 ± 3.954 degrees, with a range from 23.73 to 37.00 degrees. For ease of interpretation of our results, statistical significance is indicated as follows: not significant (ns), p < 0.05 (*), p < 0.01 (**), p < 0.001 (***), and p < 0.0001 (****)

The AOD was assessed to evaluate the openness of the ICA. In dogs, the mean AOD was 0.9908 ± 0.3474 mm, with values ranging from 0.6200 to 1.810 mm. For cats, the mean AOD was 1.833 ± 0.3874 mm, ranging from 1.340 to 2.580 mm. Statistical analysis revealed a significant difference in AOD between dogs and cats (p < 0.0001), highlighting the anatomical variance in the drainage angle between these species (Fig. 1B).

The ICA was measured to understand the configuration of the angle where the sclera, cornea, and iris base converge. In dogs, the mean ICA was 15.56 ± 3.241 degrees, ranging from 10.40 to 22.00 degrees. In cats, the mean ICA was significantly larger, at 30.27 ± 3.954 degrees, with a range from 23.73 to 37.00 degrees. This significant difference (p < 0.0001) between the two species underscores a fundamental anatomical distinction (Fig. 1C).

The SLD indicates the peripheral anterior chamber depth(p-ACD), with cats showing a greater depth compared to dogs. Additionally, both ICA and AOD are traditional parameters used to evaluate the exit pathway of aqueous humor, and these values were found to be significantly larger in cats.

Analysis of CC parameters

The CCW was measured to compare the CC width between canines and felines. The mean CCW in dogs was 0.6357 ± 0.2544 mm, ranging from 0.3800 to 1.450 mm. In cats, the mean CCW was 1.227 ± 0.2369 mm, with a range from 0.8400 to 1.870 mm. The difference in CCW between dogs and cats was significant (p < 0.0001), indicating a marked anatomical difference in this parameter (Fig. 2A).

Fig. 2

Analysis of Ciliary Cleft Parameters. A. Ciliary Cleft Width (CCW) comparison between dogs and cats shows that the mean CCW in dogs was 0.6357 ± 0.2544 mm, ranging from 0.3800 to 1.450 mm, whereas in cats, the mean CCW was 1.227 ± 0.2369 mm, with a range from 0.8400 to 1.870 mm. B. The Ciliary Cleft Length (CCL) comparison reveals that in dogs, the mean CCL was 1.259 ± 0.2662 mm, with values ranging from 0.9400 to 2.050 mm, while in cats, the mean CCL was 2.039 ± 0.2518 mm, ranging from 1.660 to 2.480 mm. C. The Ciliary Cleft Area (CCA) comparison shows that the mean CCA in dogs was 0.3343 ± 0.09171 mm², ranging from 0.2200 to 0.5300 mm², whereas in cats, the mean CCA was significantly larger, at 1.149 ± 0.2388 mm², with a range from 0.7300 to 1.540 mm². For ease of interpretation of our results, statistical significance is indicated as follows: not significant (ns), p < 0.05 (*), p < 0.01 (**), p < 0.001 (***), and p < 0.0001 (****)

The CCL was assessed to evaluate the length of the CC. In dogs, the mean CCL was 1.259 ± 0.2662 mm, with values ranging from 0.9400 to 2.050 mm. For cats, the mean CCL was 2.039 ± 0.2518 mm, ranging from 1.660 to 2.480 mm. Statistical analysis revealed a significant difference in CCL between dogs and cats (p < 0.0001), highlighting the anatomical variance in the CCL between these species (Fig. 2B).

The CCA was measured to understand the area of the CC. In dogs, the mean CCA was 0.3343 ± 0.09171 mm², ranging from 0.2200 to 0.5300 mm². In cats, the mean CCA was significantly larger, at 1.149 ± 0.2388 mm², with a range from 0.7300 to 1.540 mm². This significant difference (p < 0.0001) between the two species underscores a fundamental anatomical distinction (Fig. 2C).

These results demonstrate the significant anatomical differences in the CC structures between dogs and cats, which could contribute to the varying prevalence of glaucoma in these species. The CCW, CCL, and CCA were approximately 1.9 times, 1.6 times, and 3.5 times larger in cats than in dogs, respectively. This indicates that the CC, which regulates the flow of aqueous humor, is larger in cats.

Analysis of rectified CC parameters

The r-CCW was measured to compare the adjusted CC width between canines and felines. The mean r-CCW in dogs was 0.6364 ± 0.2410 mm, ranging from 0.3500 to 1.370 mm. In cats, the mean r-CCW was 1.227 ± 0.2181 mm, with a range from 0.9200 to 1.750 mm. The difference in r-CCW between dogs and cats was significant (p < 0.0001), indicating a substantial anatomical difference in this parameter (Fig. 3A).

Fig. 3

Analysis of Rectified Ciliary Cleft Parameters. A. Rectified Ciliary Cleft Width (r-CCW) comparison between dogs and cats shows that the mean r-CCW in dogs was 0.6364 ± 0.2410 mm, ranging from 0.3500 to 1.370 mm, whereas in cats, the mean r-CCW was 1.227 ± 0.2181 mm, with a range from 0.9200 to 1.750 mm. B. The Rectified Ciliary Cleft Length (r-CCL) comparison reveals that in dogs, the mean r-CCL was 1.264 ± 0.2937 mm, with values ranging from 0.9600 to 2.180 mm, while in cats, the mean r-CCL was 2.043 ± 0.2513 mm, ranging from 1.720 to 2.610 mm. C. The Rectified Ciliary Cleft Area (r-CCA) comparison shows that the mean r-CCA in dogs was 0.3386 ± 0.1088 mm², ranging from 0.2300 to 0.6000 mm², whereas in cats, the mean r-CCA was significantly larger, at 1.145 ± 0.1917 mm², with a range from 0.8800 to 1.590 mm². For ease of interpretation of our results, statistical significance is indicated as follows: not significant (ns), p < 0.05 (*), p < 0.01 (**), p < 0.001 (***), and p < 0.0001 (****)

The r-CCL was assessed to evaluate the adjusted length of the CC. In dogs, the mean r-CCL was 1.264 ± 0.2937 mm, with values ranging from 0.9600 to 2.180 mm. For cats, the mean r-CCL was 2.043 ± 0.2513 mm, ranging from 1.720 to 2.610 mm. Statistical analysis revealed a significant difference in r-CCL between dogs and cats (p < 0.0001), highlighting the anatomical variance in the adjusted CC length between these species (Fig. 3B).

The r-CCA was measured to understand the adjusted area of the CC. In dogs, the mean r-CCA was 0.3386 ± 0.1088 mm², ranging from 0.2300 to 0.6000 mm². In cats, the mean r-CCA was significantly larger, at 1.145 ± 0.1917 mm², with a range from 0.8800 to 1.590 mm². This significant difference (p < 0.0001) between the two species underscores a fundamental anatomical distinction (Fig. 3C).

These results demonstrate that even after rectification for different body sizes, the CC parameters (r-CCW, r-CCL, and r-CCA) are significantly larger in cats than in dogs.

Discussion

This study aimed to investigate the underlying causes of PACG in dogs by comparing the anatomical features of the anterior segment between dogs and cats. Through this research, we confirmed that dogs have significantly smaller anatomical structures involved in the regulation of aqueous humor compared to cats. These findings suggest that the reduced size of these structures in dogs may contribute to the higher prevalence of PACG in this species. By highlighting these anatomical differences, our study provides valuable insights into the potential mechanisms driving the development of PACG in dogs, supporting the hypothesis that structural variations in the anterior segment play a crucial role in this disease’s etiology.

In the present study, the canine cohort included a diverse representation of breeds with varying predispositions to glaucoma. In dogs, breeds known to be predisposed to PACG—specifically Shih Tzu and Beagle—were included. Notably, Beagle is also recognized as being predisposed to primary open-angle glaucoma (POAG). In addition, breeds generally considered to be at a lower risk for glaucoma, including Maltese, Spitz, Poodle, Chihuahua, and Pomeranian, were part of our sample [19]. This balanced representation among dogs with different glaucoma predisposition profiles suggests that the observed differences in anterior segment anatomy are broadly applicable across the canine population. Although breed-specific analyses were limited by the small number of cases per breed, the inclusion of both glaucoma-prone and non-glaucoma-prone breeds in dogs strengthens the generalizability of our findings.

In our study, only one eye per animal was included to ensure the statistical independence of our observations. Although both eyes of many animals exhibited normal findings, including both could introduce intra-individual correlations that might bias the results. Therefore, when both eyes met the inclusion criteria, one eye was randomly selected to represent each subject. Moreover, as reported in previous studies, there is no anatomical difference between the left and right eyes [12]. This methodological decision aligns with standard practices in ophthalmologic research, ultimately strengthening the validity of our statistical analyses by ensuring that the findings are not confounded by correlated data from bilateral observations [20].

Under the controlled conditions of our standardized protocol, potential anatomical differences among breeds did not affect the positioning of the ultrasound probe. Our primary aim was to evaluate the inherent anatomical variations between species using UBM under consistent examination conditions. To achieve this, we standardized our measurement technique by consistently positioning the probe at the corneoscleral limbus, a well-defined and reproducible anatomical landmark. This approach minimized variability in probe placement across all subjects, regardless of breed-specific anatomical differences, and ensured that parameters such as SLD, ICA, AOD, and IOP were measured uniformly. Consequently, the observed differences in these parameters can be attributed to true interspecies anatomical variations rather than inconsistencies in measurement technique.

In this study, dogs exhibited a smaller SLD compared to cats. The SLD measures the distance from the endpoint of Descemet’s membrane to the anterior lens capsule, representing the p-ACD. Previous research has indicated that cats have a deeper anterior chamber depth (ACD) than dogs, which is consistent with our findings [8]. In human studies, a smaller ACD has been identified as a triggering mechanism for PACG [21]. A reduced ACD causes the iris to move closer to the cornea, increasing the probability of peripheral anterior synechiae formation [22, 23]. Based on these findings, the smaller p-ACD observed in dogs compared to cats suggests a higher likelihood of obstructed aqueous humor drainage in dogs. This obstruction can precipitate a rapid and significant increase IOP, characteristic of PACG.

Furthermore, dogs were found to have smaller ICA and AOD compared to cats. It is well-established that smaller ICA and AOD can contribute to the development of glaucoma in human medicine [24,25,26]. A reduced ICA and AOD indicate an anterior and external displacement of the iris root, potentially leading to compromised access of aqueous humor to the trabecular meshwork [27, 28]. In veterinary medicine, the exposed CC in dogs and cats contrasts with the trabecular meshwork embedded in the sclera in humans. This difference means that ICA and AOD are traditionally considered less significant in evaluating aqueous humor flow in animals [16, 18]. However, the differences observed in ICA and AOD between dogs and cats in this study, indicative of an anterior and external displacement of the iris root, might provide an explanation for the higher incidence of PACG in dogs.

In this study, dogs exhibited smaller CC parameters compared to cats. It is known that the CC has a significant effect on aqueous humor outflow in canine eyes, and a closed CC increases the likelihood of PACG by 20 times [17, 18, 29]. Therefore, we concluded that the smaller CC in dogs compared to cats might explain the relatively higher incidence of PACG in dogs. However, the average axial globe length for cats is around 22.3 mm, while for dogs it is about 20.8 mm [9]. To account for the relative CC differences due to the axial length of the eye, we rectified the CC parameters based on the study by Kawata et al. [30]. The results showed that even after rectification, the r-CCW in cats was still 1.9 times larger, the r-CCL was 1.6 times larger, and the r-CCA was 3.4 times larger compared to dogs. These findings support the hypothesis that smaller CCs in dogs may contribute to the higher prevalence of PACG in this species compared to cats.

This study has several limitations. Firstly, as a retrospective study, the measurement conditions between the two species were not consistently maintained. Specifically, in cats, due to the structural difficulty of exposing the limbus, measurements were challenging and were therefore taken in the operating room after sedation and anesthesia in a sternal position. In contrast, in dogs, the measurements were taken in the examination room without sedation in a sitting position. The anesthesia-induced changes in blood pressure could have potentially impacted the IOP and parameters such as the ICA, AOD, and CC [31,32,33,34]. However, in this study, both dog and cat IOP values remained within the normal range, and the difference between the two groups was not statistically significant [2, 6]. Additionally, efforts were made to stabilize anesthesia and maintain consistent blood pressure during IOP measurement and UBM imaging, including the use of fluids during the procedure. Furthermore, in our preliminary study, we found no significant correlation between IOP and parameters like SLD, AOD, ICA, or CC (Figure S1-2). Based on these findings, it can be inferred that the effect of anesthesia on feline IOP was minimal, reducing the likelihood that any anatomical differences in the anterior chamber were induced by anesthesia.

Secondly, there was a significant age difference between the dogs (7.29 years) and cats (4.28 years) in this study. Research in human medicine indicates that as age increases, lens thickness also increases, leading to a decrease in ACD [35, 36]. This age difference could potentially have influenced the p-ACD measurements. However, a study by Boillot et al. found no correlation between age and CC parameters in dogs [13]. Based on this, it is unlikely that the age difference between the two species caused the observed differences in CC parameters.

Thirdly, the method used for rectification might be less applicable to cats, as it was originally designed for dogs with varying body weights. Additionally, there was no established method for rectifying CCL in previous studies, so we adapted the approach used for CCW. Despite these limitations, considering the similarities in the anterior chamber between dogs and cats, applying this method seems appropriate. Moreover, in this study, there was no significant difference in body weight between the two species, suggesting that the eye size difference might not be substantial. Taking these factors into account, the differences in CC parameters between dogs and cats are likely to be significant.

Conclusion

In conclusion, considering the differences in the types of glaucoma observed in dogs and cats, along with the findings of this study, it is suggested that the higher prevalence of PACG in dogs is likely due to their smaller CC (Fig. 4). Conversely, the lower incidence of primary glaucoma in cats, at approximately 2%, may be attributed to their larger CC [6]. These anatomical distinctions in the anterior segment structures between dogs and cats provide significant insights into the mechanisms underlying the development of glaucoma in these species and highlight the importance of targeted diagnostic and therapeutic approaches based on species-specific anatomical features.

Fig. 4

Comparison of Ciliary Clefts in Dogs and Cats. A. Iridocorneal angle in dogs. B. Iridocorneal angle in cats. The comparison of the two images shows that cats have a wider ciliary cleft than dogs

Methods

Clinical information

This retrospective study collected clinical data from animals that visited the Chungbuk National University Veterinary Medical Teaching Hospital between July 2022 and October 2023. Animals were included if they met al.l of the following criteria: [1] normal findings on a comprehensive ophthalmic examination—including slit-lamp biomicroscopy, Schirmer tear test, menace response, pupillary light reflex, dazzle reflex, and intraocular pressure (IOP) measurement with IOP values within the normal range; [2] no history of ocular disease or ocular surgery; and [3] complete clinical records documenting all required ophthalmic evaluations. Based on these criteria, a total of 14 eyes from 14 dogs and 16 eyes from 16 cats were included in the study.

Ophthalmological evaluations

Ophthalmological evaluations involved a variety of diagnostic tests to ensure a comprehensive examination of ocular health. These included slit-lamp biomicroscopy (MW50D, SHIGIYA, Hiroshima, Japan) and the Schirmer Tear Test (Schirmer Tear Flow Strips, Gulden Ophthalmics, PA) to assess tear production. Assessments of the menace response, pupillary light reflex, and dazzle reflex were also conducted. IOP as measured using rebound tonometry with the TonoVet Plus^® (icare, Vantaa, Finland). Further procedures included gonioscopy (Ocular Koeppe Diagnostic Lenses, Ocular Instruments Inc., Bellevue, WA). Notably, fundus examinations were not performed in this study, as the protocol required that the procedures be conducted without pupil dilation. Only eyes that were confirmed to be normal based on these tests were included in the study.

UBM examination

UBM was performed with the VuPAD^® (Sonomed Escalon, Lake Success, NY) using a transparent thin film probe cover filled with distilled water (ClearScan^®, ESL, Inc.), providing detailed visualization of the anterior segment structures.

For the UBM examination in dogs, measurements were consistently taken under uniform lighting conditions. To ensure the dogs’ comfort and cooperation, topical anesthesia was administered using a 0.5% solution of proparacaine hydrochloride (Alcaine^®; Alcon). During the UBM procedure, all dogs were kept in a sitting position. The eyelid was gently held by hand without applying pressure to the globe, ensuring clear visualization of the eye’s dorsal quadrant. The UBM probe was then placed perpendicularly to the corneoscleral limbus in this quadrant to obtain precise imaging.

For the UBM examination in cats, the patients were given 100 mg of gabapentin an hour before the procedure to reduce anxiety. Unlike the dogs, UBM measurements in cats were performed under anesthesia. Anesthesia was induced with 6 mg/kg propofol (Freepol-MCT inj.) intravenously and maintained with isoflurane (Terrell^®, Piramal Critical Care Inc.). During the UBM procedure, all cats were kept in a sternal position. The lighting conditions in the operating room were dim and consistent for all animals throughout the study. Topical proparacaine (Paracaine Eye Drops 0.5%) was applied to the ocular surface. The upper eyelid was slightly raised to expose the sclera, and images were obtained perpendicular to the limbus in the superotemporal quadrant of both eyes.

Measurement parameters

In this study, a total of nine parameters were utilized to analyze the anatomical features relevant to glaucoma. These parameters are divided into three main categories: ICA parameters, CC parameters, and rectified CC parameters.

The first category includes the distance of the Schwalbe’s line to the anterior lens capsule (SLD), the ICA, and the angle-opening distance (AOD). The SLD was measured from the borderline of the cornea and sclera to the anterior lens capsule [37]. The geometric ICA was identified at the peripheral circumferential section where the sclera, cornea, and iris base converge. To measure the ICA, lines were drawn along the inner layer of the sclera and the outer layer of the iris root, with the angle between these two lines being recorded. The AOD was assessed by drawing a perpendicular line from the endpoint of Descemet’s membrane to the anterior surface of the iris, providing a quantifiable measure of the ICA’s openness [38] (Fig. 5A).

The second category focuses on the CC parameters, including CC Width (CCW), CC Length (CCL), and CC Area (CCA). The CCW was measured as the distance from the point where the outer layer of the pectinate ligament meets the inner boundary of the sclera to the point where it reaches the upper surface of the iris root. The CCL was determined by measuring the distance from the angle recess to the midpoint of the CCW (Fig. 5B). The CCA was calculated by mapping the area formed by tracing the inner scleral surface from the angle recess to its inner boundary and the superior surface of the iris root up to the CCW (Fig. 5C). This measurement provides detailed insights into the anatomical structure of the CC [18, 39].

The third category involves the rectification of CC parameters to account for different body sizes and weights. This was achieved by using the previously measured SLD and CC parameters. The optional fixed SLD (OFS), such as the mean of the SLDs of the examined dogs and cats, was used. The formulas for rectified CCW (r-CCW), rectified CCL (r-CCL), and rectified CCA (r-CCA) were as follows:

r − CCW = CCW×(OFS/SLD), r − CCL = CCL×(OFS/SLD), and r − CCA = CCA×(OFS/SLD)^{2 (30)}.

Fig. 5

UBM Examination Parameters. A. Iridocorneal angle parameters: Schwalbe’s Line Distance (SLD) was measured from the borderline of the cornea and sclera to the anterior lens capsule. The Iridocorneal Angle (ICA) was identified at the junction of the sclera, cornea, and iris base, with measurements taken between the inner scleral layer and the outer iris root. The Angle-Opening Distance (AOD) was measured perpendicularly from Descemet’s membrane to the anterior iris surface. B. Ciliary cleft parameters: Ciliary Cleft Width (CCW) was measured from the pectinate ligament’s outer layer to the iris root’s upper surface. Ciliary Cleft Length (CCL) was measured from the angle recess to the midpoint of the CCW. C. Ciliary Cleft Area (CCA) was calculated by tracing the inner scleral surface from the angle recess to its boundary and the superior iris root up to the CCW, providing insights into the CC’s anatomical structure

Statistical analysis

Statistical analyses were conducted using SPSS software (version 17.0; SPSS Inc., Chicago) to thoroughly investigate the data. To analyze factors affecting the comparison between dogs and cats, several statistical methods were employed. First, the Shapiro-Wilk test was used to assess the normality of the age and weight data. Once normality was confirmed, mean values were compared using the t-test. Additionally, the t-test was applied to compare the nine parameters measured by UBM. For the evaluation of eye laterality, a binomial test was used to determine whether the distribution of right (OD) and left (OS) eyes deviated from the expected 50:50 ratio within each species, and a chi-square test (or Fisher’s exact test when appropriate) was performed to compare laterality between dogs and cats. Furthermore, a post-hoc power analysis was conducted using G*Power (version 3.1) based on the observed effect sizes and standard deviations of key UBM parameters. For clarity in the presentation of results, asterisks were used in the figures to indicate significance, while detailed p-values were provided in the text.