Imaging analysis of the biological parameters of the lens in patients with cortical age-related cataracts using ultrasound biomicroscopy

Cortical cataract is the most common type of age-related cataract [14]. Especially during the intumescent stage, when the opacity of the lens increases with age, the cortex absorbs water and the lens volume increases, causing the anterior chamber to become shallower [15, 16] and possibly triggering an acute attack of glaucoma in patients with narrow anterior chamber. In the process of anterior chamber angle narrowing, the changes and roles of lens-related parameters have attracted more and more attention from scholars, with LV, LP, and RLP being more frequently studied [17‐19]. In our previous study, we found that the ILA and ILCD were characteristically present in eyes with acute lens-induced angle closure [20]. However, there are no studies on whether the ILA and ILCD are different in each quadrant of patients with CARC. On the other hand, whether this change in lens-related parameters that occur with age affects the improvement of visual quality after cataract surgery remains debatable.

Most studies regarding the lens have been performed using anterior segment optical coherence tomography (AS-OCT), and relatively few using UBM. AS-OCT is widely used in clinical practice due to its non-contact, non-invasive, and maneuverable. It has the advantage of being safe, unaffected by the degree of corneal clouding and clearly showing some anterior chamber angle structures, such as the scleral spur [21]. However, the posterior iris structures are poorly displayed with AS-OCT due to the influence of the iris pigment epithelium and refractive media. UBM is a high-resolution ultrasound technique that provides a detailed assessment of the anterior segment of the eye [22], which is independent of light and refractive media, and has the advantages of reproducibility, objectivity, and reliability [23]. However, it is relatively complex to perform and requires a water bath. In the previous study on the consistency of these two measurement methods for ILA and ILCD, we found that the consistency of the two measurement methods in the superior, inferior, nasal, and temporal directions is good for ILCD, but the consistency for ILA measurement is poor, which may be related to poor imaging of the posterior iris tissue by AS-OCT [24]. This means that although AS-OCT is more convenient, the use of UBM is more dominant for measuring the lens-related parameter, such as ILA. Cataract surgery is generally performed in the supine position, which is the same as the examination position for UBM, the results of UBM are closer to the intraoperative state of the anterior segment. Therefore, we chose UBM to study and analyse the characteristics of the lens-related parameters in different genders, different eyes, and different quadrants of CARC.

LV, a new risk factor [25], is closely associated with anterior chamber angle closure [4]. Furthermore, a greater forward protrusion of the lens may result in a narrower anterior chamber angle which is more likely to induce progressive anterior chamber angle closure [26]. The larger the LV, the more the iris pushes anteriorly, increasing the contact between the iris and the lens, subsequently increasing the ILCD, thereby causing pupillary block [27]. A study by Tan et al. [28] showed that the LV increased significantly with age, its value in female patients was greater than that in male patients, and it was associated with narrow anterior chamber angle. Our study showed that the mean TIA in males was greater than that in females, while the mean LV in males was less than that in females. Moreover, the mean ILA in males was greater than that in females, while the mean ILCD in males was less than that in females. However, none of the differences were significant. The LV represents the extent to which the lens protrudes into the anterior chamber [3]. Further, as shown in our study, the larger the LV in females, the more the lens protrudes forward and pushes the iris, thus corresponding to a larger ILCD and a smaller ILA.

Both LP and RLP reflect the influence of the anatomical position of the lens on the anterior chamber structure. Lim et al. [29] assessed the eyes with the acute attacks and the fellow eyes in patients with PACG and compared the ACD, LP, RLP, and other related variables. They found that the LP was more anterior in the eyes with acute attacks than in the fellow eye in those patients, while there was no significantly different for RLP between the two groups. However, Nongpiur et al. [3] used AS-OCT to compare PACG patients with normal subjects, and found that LP and RLP had no correlation with the occurrence of PACG. Our study showed that the mean LP was larger in males with CARC than in females, with a statistically significant difference. Contrastingly, the difference in RLP between the male and female patients was not significant. A study by Pakuliene et al. [30] concluded that the LP could be used to predict narrow angles, and it was greater in males with cataracts than in females. Moreover, the TIA was also greater in males than in females, which is consistent with our results. The LP and RLP are affected by the ACD and LT, the correlation between these two parameters and anterior chamber angle closure needs further investigation, but women have a more anteriorly positioned lens, crowded anterior structures in the eye, and narrower anterior chamber angles, which are more likely to induce acute glaucoma attacks.

The preoperative position and inclination of the lens will affect its postoperative visual acuity and visual quality. Therefore, many scholars have conducted studies on this. Chen et al. [31] studied the characteristics of lens inclination and eccentricity in 1097 patients proposed for cataract surgery using CASIA2. They found that the average inclination of the natural lens to the infratemporal direction is 5.16°, and the average eccentricity to the temporal side is 0.22 mm. It was concluded that a certain degree of inclination and eccentricity existed in the preoperative lens of patients with cataracts. Li et al. [32] studied 230 patients aged 7–90 years using CASIA2 and found that the lens tilted to the infratemporal, with a mean inclination of 4.3 ± 1.5° and an eccentricity of 0.17 ± 0.12 mm to the super-temporal. In contrast, Wang et al. [33] retrospectively studied the lens and IOL of 333 patients (mean age, 70 ± 8 years) who underwent cataract surgery using the IOL Master 700. They found that both the lens and IOL tilted to the nasal side with a mean amplitude of 3.7 ± 1.1° and 4.9 ± 1.8°, respectively. Kimura et al. [34] assessed 100 eyes (the lens of 41 eyes and the IOL of 59 eyes) in 49 patients using CASIA2. They found that in the non-dilated state, the lens tilted at an average angle of 5.15° to the infratemporal, and the IOL tilted at an average angle of 4.31° to the infratemporal. On the other hand, in the dilated state, the lens tilted at an average angle of 5.25° to the infratemporal, and the IOL tilted at an average angle of 4.65° to the infratemporal. Our measurements showed that the distribution of the ILA tended to be nasal < inferior < temporal < superior. The H-test showed that there were significant differences in anterior chamber angles of the four quadrants, while the mean distribution trend of ILCD in the four quadrants is just the opposite: superior < temporal < inferior < nasal, with significant differences between the four quadrants. Combining the results of ILA and ILCD, the lens shifted upward and temporally. Differences in these measurement results may be related to differences in the study population, measurement positions, measurement methods, and instruments used. Cui et al. [35] used UBM to observe the preoperative lens zonulas in 50 eyes of 50 patients with cataracts combined with PACG. They found that the zonulas was abnormal in 29 eyes (58%), of which nine eyes (18%) were disconnected, and 20 (40%) were loose. The zonulas of the lens may become loose with age and degeneration. Therefore, we hypothesise that during a seating examination, such as an AS-OCT, the lens is more likely to be deflected downwards due to gravity. During the operation position (supine position) examination, the lens of some patients will have a slight upward deviation when the head is slightly tilted back.