Abstract
Introduction: Cataract surgery has been reported to have a reducing effect on intraocular pressure (IOP) in glaucomatous and non-glaucomatous eyes. This effect seems to be more noticeable in eyes with narrow angles (NAs) than in eyes with open angles (OAs). Decrease in IOP may be a result of the increase in anterior chamber angle and Schlemm canal (SC) after cataract surgery. The purpose of this paper was to evaluate the relationship between Schlemm canal-cross-sectional area (SC-CSA) changes and trabecular meshwork, ciliary muscle changes after cataract surgery and the difference in non-glaucomatous eyes with NAs and OAs. Methods: IOP, SC-CSA, Schlemm canal diameter (SCD), trabecular meshwork width (TMW) and thickness (TMT), trabecular-iris angle at 500 µm from the scleral spur (TIA500), and the distance between the inner apex of the ciliary muscle and scleral spur (IA-SS) were measured by swept-source-optical coherence tomography preoperatively and 1-week post-surgery. Patients were divided into NA and OA groups according to the degree of TIA500, and SC-CSA-related parameters were compared. Results: Seventy-five patients (89 eyes) were included. IOP significantly decreased, SC-CSA, SCD, TMW, TMT, and TIA500 significantly increased post-surgery (p < 0.001). Changes in nasal SC-CSA were associated with TMW (p = 0.003) and TIA500 (p < 0.001) changes; changes in temporal SC-CSA were associated with TMW (p = 0.001) and TMT (p < 0.001) changes. SC-CSA expansion was correlated with changes in TMW (β3.726 ± 1.085, p = 0.001 nasally; β3.405 ± 0.945, p = 0.001 temporally), TMT (β5.224 ± 2.033, p = 0.012 nasally; β11.853 ± 3.059, p < 0.001 temporally), and TIA500 (β40.330 ± 15.100, p = 0.009 nasally; β35.453 ± 17.527, p = 0.047 temporally). There was no association between SC-CSA expansion and IA-SS changes. SC-CSA expansion was greater in the NAs than in the OAs group (p < 0.001). Conclusion: Cataract surgery results in IOP reduction and SC-CSA expansion. Increased SC-CSA correlates with increases in TMW, TMT, and TIA500. SC-CSA increased more in NAs than in OAs group post-surgery, which may explain the greater decrease in IOP in eyes with NAs.
Introduction
Cataract surgery has been reported to have a reducing effect on intraocular pressure (IOP) in glaucomatous and non-glaucomatous eyes [1‒3]. This effect seems to be more noticeable in eyes with narrow angles (NAs) than in eyes with open angles (OAs) [4]. However, the exact mechanism of this effect is still not fully understood. Upon cataract removal, the decrease in IOP may result from increasing the anterior chamber depth (ACD) [4‒7] and anterior chamber angle (ACA), including the trabecular-iris angle (TIA) [8], the angle opening distance [2, 4, 9], and trabecular-iris surface area [6, 9]. In addition, Schlemm canal (SC) expansion after cataract surgery has been positively correlated with decreased IOP [3]. It is assumed that SC size would be improved due to widening of the drainage angle. However, few studies have examined the relationship between changes in SC size and ACA-related parameters following cataract surgery.
Anatomically, the SC, trabecular meshwork (TM), and ciliary muscle (CM) are tightly connected to constitute a complete tension structure [10‒14]. The relaxation and contraction of the TM and SC tissues, which are regulated by the CM, adjust the outflow rate of aqueous humor [10, 12, 15]. As a mean resistance point of the aqueous humor outflow pathway [16], smaller SC size was negatively correlated with higher IOP in non-glaucomatous and glaucomatous eyes [17, 18]. Qi et al. [19] indicated that a smaller vertical diameter of SC and a thinner TM were associated with early IOP elevation after cataract surgery. Zhao et al. [3] found a significant increase in SC diameter (SCD), SC-cross-sectional area (SC-CSA) of SC, and TM width (TMW) after cataract surgery. However, the specific correlation between morphological changes in the SC, TM, and CM after cataract surgery has not been investigated.
Therefore, this study aimed to further investigate the effect of cataract surgery on the aqueous outflow pathway and the underlying mechanism wherein eyes with NAs show a greater decrease in IOP post-cataract surgery than that with OAs. Swept-source-optical coherence tomography (SS-OCT) was used to evaluate the relationship between SC-CSA and TM, CM, and TIA changes after cataract surgery and to compare the difference in SC-CSA changes in non-glaucomatous eyes with NAs and OAs.
Methods
This prospective study was conducted at the Eye Hospital of Wenzhou Medical University, Hangzhou Branch, Southeast China, between April 2022 and September 2022. This study and the required data were prospectively approved by the Eye Hospital of Wenzhou Medical University’s Institutional Review Board (No. 2022-014-K-11) and complied with the 1975 Helsinki Declaration, as revised in 1983. Written informed consent was obtained from each participant after the nature of the procedures and possible risks were explained. This trial was registered at NIH (clinicaltrial.gov) on April 24, 2022 (NCT05352854). Patients requiring cataract surgery and having normal IOP and NAs/OAs were included.
The exclusion criteria were as follows: (1) major intraoperative and postoperative complications (e.g., posterior capsule rupture or endophthalmitis); (2) previous intraocular surgery and penetrating or laser surgery, including peripheral iridotomy; (3) glaucoma (glaucomatous vision loss or optic neuropathy changes), uveitis, severe retinal diseases, pseudoexfoliation syndrome, and other serious eye diseases; (4) corneal and conjunctival abnormalities, including scarring, malnutrition, and corneal opacity, affecting SS-OCT imaging; (5) glaucoma medication; (6) SS-OCT images in which the SC could not be identified; and (7) failure to complete follow-up.
Demographic information was recorded. Each patient underwent a comprehensive ocular examination, including visual acuity testing, slit-lamp biomicroscope, fundus examination, IOP measurement (non-contact tonometer; TX-F; Cannon), axial length (AL), and ACD measurement (IOL-Master 700). Slit-lamp biomicroscope and fundus examinations were performed by a practical ophthalmologist (L.Z.L.).
Swept-Source-Optical Coherence Tomography
SS-OCT (CASIA 2) was performed for all patients under the same indoor lighting conditions (the same illumination of approximately 500 lx of the indoor fluorescent lamp and room temperature between 20 and 26 °C). Scans were centered on the ACA structure in the temporal and nasal quadrants (at the 3 o’clock or 9 o’clock positions) using the angle HD (high definition) 2D line scanning mode. To ensure that the ACA was in the instrument’s field of view, the patients were instructed to focus on the left or right built-in fixation lights during the test. Images with good corneal vertex reflections were captured. The same position was determined based on the texture of the vessels on the scleral and iris surfaces. At least three consecutive images were taken by the same experienced operator (J.X.P.) during each measurement, and the images with the best visibility of SC morphology were selected for analysis.
The TIA at 500 µm from the scleral spur (TIA500) was then automatically processed using built-in software and measurement tools provided by the manufacturer (Fig. 1a). The angle was graded using the Shaffer’s classification system [20]. According to the preoperative TIA500, the patients were divided into two groups: NAs (TIA500 < 25°) and OAs (TIA500 ≥ 25°).
a SS, scleral spur; TIA500, TIA at 500 μm from the scleral spur. b IA-SS, distance from the scleral spur to the inner apex of the ciliary muscle (green line); SCD, Schlemm’s canal diameter (red line); SC-CSA, cross-sectional area of Schlemm’s canal (yellow line); TMT, trabecular meshwork thickness (blue line); TMW, trabecular meshwork width (pink line).
a SS, scleral spur; TIA500, TIA at 500 μm from the scleral spur. b IA-SS, distance from the scleral spur to the inner apex of the ciliary muscle (green line); SCD, Schlemm’s canal diameter (red line); SC-CSA, cross-sectional area of Schlemm’s canal (yellow line); TMT, trabecular meshwork thickness (blue line); TMW, trabecular meshwork width (pink line).
The SC-, TM-, and CM-related parameters were manually measured using ImageJ software (http://imagej.nih.gov/ij/). Each image was magnified by 200%, 150%, and 75% to measure SC, TM, and CM, respectively. All information and ocular characteristics of the patients were blinded during the measurements to avoid biases by the observer (J.X.P.). To assess the reproducibility of the image analysis using ImageJ, measurements were repeated twice preoperatively with an interval of 2 weeks in 15 randomly chosen eyes, and the coefficient of variation (CV) and intraclass correlation coefficient were calculated.
Preoperative and Postoperative Parameters Evaluation
The measured SS-OCT parameters and their definitions are as follows: SC-CSA was drawn as a long circular region inside the SC outline; SCD was the distance between the anterior and posterior ends of the SC-CSA; TMW was the distance between the scleral spur and the Schwalbe’s line, which was defined as the boundary between the high-reflective inner corneal lining and the low-reflective TM; trabecular meshwork thickness (TMT) was the vertical distance between the posterior endpoint of the SC and the inner surface of the cornea; and the distance between the inner apex of the CM and scleral spur was defined as IA-SS. Figure 1b illustrates these parameters.
A single surgeon (Z.Y.E.) performed the surgeries, and all patients received topical anesthesia. No complications occurred during any of the surgeries. IOP and SS-OCT measurements were repeated according to the protocol when patients returned for postoperative care, 1-week post-surgery. The same ophthalmologist (J.X.P.) examined all the enrolled patients postoperatively. The main outcome measurements were changes in IOP, SC-CSA, SCD, TMW, TMT, TIA500, and IA-SS after phacoemulsification.
Statistical Analysis
All statistical analyses were performed using SPSS (v.21.0). The Kolmogorov-Smirnov test was used to assess normal distribution. Variables are expressed as mean ± standard deviation or median (interquartile range) based on normality. An independent sample t test or the Mann-Whitney U test was used to compare preoperative and postoperative variables and OA and NA groups, depending on the normality of data. Univariate analysis was used to assess the correlation between the changes in SC-CSA and related variables. Multiple linear regression was performed to determine the variables that were associated with SC-CSA expansion, including those with p < 0.10 in the univariate analysis. Statistical significance was set at p < 0.05.
Results
The present study comprised 115 eyes of 92 patients who underwent cataract surgery, of which 14 eyes were excluded due to loss to follow-up, and 12 eyes were excluded because the SC cannot be defined due to poor quality of SS-OCT images. Therefore, 75 patients (89 eyes), with an average age of 69.0 ± 9.3 years (range 36–86), were selected for the final analysis. Table 1 shows the demographics and ocular characteristics of participants. The average ACD and AL before surgery was 2.89 ± 0.48 mm and 23.59 ± 1.81 mm, respectively.
Demographics and ocular characteristics of the participants
Parameters . | ||
---|---|---|
n | 75 | |
Age, years | 69.0±9.3 | 36–86 |
ACD, mm | 2.89±0.48 | 1.84–4.08 |
AL, mm | 23.59±1.81 | 20.99–32.70 |
IOP, mm Hg | 15.4±3.3 | 9.5–23.9 |
Parameters . | ||
---|---|---|
n | 75 | |
Age, years | 69.0±9.3 | 36–86 |
ACD, mm | 2.89±0.48 | 1.84–4.08 |
AL, mm | 23.59±1.81 | 20.99–32.70 |
IOP, mm Hg | 15.4±3.3 | 9.5–23.9 |
Values are means ± SD and ranges, unless indicated otherwise.
ACD, anterior chamber depth; AL, axial length; IOP, intraocular pressure.
The measurements of SC-CSA, SCD, TMW, and TMT were reproducible with an intraclass correlation coefficient ≥0.8 (all p < 0.01). The coefficient of variations of the SC-CSA, SCD, TMW, and TMT were 34.4%/35.0%, 28.0%/28.2%, 17.2%/19.7%, and 28.4%/23.8%, in the nasal and temporal sections, respectively.
Table 2 summarizes the preoperative and postoperative ocular parameters measured by SS-OCT. Preoperatively, the average IOP was 15.4 ± 3.3 mm Hg, which dropped to 13.3 ± 3.3 mm Hg at 1 week postoperatively (p < 0.001). The average SC-CSA, SCD, TMW, TMT, and TIA500 increased significantly after surgery (all p < 0.001) in both the nasal and temporal sections. Conversely, the IA-SS score decreased after surgery, although the difference was not significant (p = 0.094 in the nasal section and p = 0.063 in the temporal section).
Preoperative and postoperative ocular parameters
Parameters . | Preoperative . | Postoperative . | t value . | p value . |
---|---|---|---|---|
IOP, mm Hg | 15.4±3.3 | 13.3±3.3 | 5.374 | <0.001* |
Nasal | ||||
SC-CSA, μm2 | 3,596.1±1332.9 | 6,293.7±1653.9 | −17.99 | <0.001* |
SCD, μm | 143.9±40.9 | 187.6±41.9 | −14.48 | <0.001* |
TMW, μm | 694.0±118.6 | 821.2±132.7 | −8.498 | <0.001* |
TMT, μm | 256.1±78.3 | 293.7±81.4 | −4.627 | <0.001* |
TIA500, ° | 26.7±12.7 | 43.5±8.8 | −15.663 | <0.001* |
IA-SS, μm | 841.5±127.8 | 823.5±125.2 | 1.695 | 0.094 |
Temporal | ||||
SC-CSA, μm2 | 3,966.3±1409.7 | 6,583.1±1911.7 | −8.610 | <0.001* |
SCD, μm | 160.9±45.7 | 201.5±45.5 | −7.551 | <0.001* |
TMW, μm | 716.2±145.0 | 867.0±156.4 | −8.044 | <0.001* |
TMT, μm | 260.3±63.7 | 297.3±74.4 | −6.349 | <0.001* |
TIA500, ° | 31.0±12.4 | 46.4±10.8 | −15.133 | <0.001* |
IA-SS, μm | 872.9±143.7 | 850.5±130.6 | 1.889 | 0.063 |
Parameters . | Preoperative . | Postoperative . | t value . | p value . |
---|---|---|---|---|
IOP, mm Hg | 15.4±3.3 | 13.3±3.3 | 5.374 | <0.001* |
Nasal | ||||
SC-CSA, μm2 | 3,596.1±1332.9 | 6,293.7±1653.9 | −17.99 | <0.001* |
SCD, μm | 143.9±40.9 | 187.6±41.9 | −14.48 | <0.001* |
TMW, μm | 694.0±118.6 | 821.2±132.7 | −8.498 | <0.001* |
TMT, μm | 256.1±78.3 | 293.7±81.4 | −4.627 | <0.001* |
TIA500, ° | 26.7±12.7 | 43.5±8.8 | −15.663 | <0.001* |
IA-SS, μm | 841.5±127.8 | 823.5±125.2 | 1.695 | 0.094 |
Temporal | ||||
SC-CSA, μm2 | 3,966.3±1409.7 | 6,583.1±1911.7 | −8.610 | <0.001* |
SCD, μm | 160.9±45.7 | 201.5±45.5 | −7.551 | <0.001* |
TMW, μm | 716.2±145.0 | 867.0±156.4 | −8.044 | <0.001* |
TMT, μm | 260.3±63.7 | 297.3±74.4 | −6.349 | <0.001* |
TIA500, ° | 31.0±12.4 | 46.4±10.8 | −15.133 | <0.001* |
IA-SS, μm | 872.9±143.7 | 850.5±130.6 | 1.889 | 0.063 |
Values are means ± SD.
IA-SS, distance from the scleral spur to the inner apex of the ciliary muscle; IOP, intraocular pressure; SC-CSA, Schlemm canal cross-sectional area; SCD, Schlemm canal diameter; TMW, trabecular meshwork width; TMT, trabecular meshwork thickness; TIA500, the trabecular-iris angle at 500 μm from the scleral spur.
*Indicates statistical significance.
The results of the univariate linear regression analysis of the association between SC expansion and changes in related parameters, including preoperative SC-CSA, are shown in Tables 3 and 4. Changes in SC-CSA were associated with changes in TMW (β = 3.393 ± 1.103, p = 0.003 nasally; β = 3.560 ± 1.048, p = 0.001 temporally), temporal TMT (β = 14.253 ± 3.233, p < 0.001), and nasal TIA500 (β = 61.758 ± 14.647, p < 0.001). There was no association between preoperative SC-CSA, changes in IA-SS and changes in the SC-CSA in our cases.
Association between SC expansion and changes in related parameters
Parameters . | Univariate analysis . | Multivariate analysis . | ||||
---|---|---|---|---|---|---|
B±SD . | 95% CI . | p value . | B±SD . | 95% CI . | p value . | |
Preoperative SC-CSA (μm2) | −0.205±0.112 | −0.427 to 0.017 | 0.07 | −0.081±0.100 | −0.280 to 0.119 | 0.423 |
Changes in TMW (μm) | 3.393±1.103 | 1.194–5.592 | 0.003* | 3.726±1.085 | 1.562–5.889 | 0.001* |
Changes in TMT (μm) | 4.141±2.107 | −0.057 to 8.340 | 0.053 | 5.224±2.033 | 1.169–9.279 | 0.012* |
Changes in TIA500 (°) | 61.758±14.647 | 32.567–90.949 | <0.001* | 40.330±15.100 | 10.213–70.447 | 0.009* |
Changes in IA-SS (μm) | 0.900±1.656 | −2.400 to 4.199 | 0.588 | - | - | - |
Parameters . | Univariate analysis . | Multivariate analysis . | ||||
---|---|---|---|---|---|---|
B±SD . | 95% CI . | p value . | B±SD . | 95% CI . | p value . | |
Preoperative SC-CSA (μm2) | −0.205±0.112 | −0.427 to 0.017 | 0.07 | −0.081±0.100 | −0.280 to 0.119 | 0.423 |
Changes in TMW (μm) | 3.393±1.103 | 1.194–5.592 | 0.003* | 3.726±1.085 | 1.562–5.889 | 0.001* |
Changes in TMT (μm) | 4.141±2.107 | −0.057 to 8.340 | 0.053 | 5.224±2.033 | 1.169–9.279 | 0.012* |
Changes in TIA500 (°) | 61.758±14.647 | 32.567–90.949 | <0.001* | 40.330±15.100 | 10.213–70.447 | 0.009* |
Changes in IA-SS (μm) | 0.900±1.656 | −2.400 to 4.199 | 0.588 | - | - | - |
IA-SS, distance from the scleral spur to the inner apex of the ciliary muscle; SC-CSA, Schlemm canal cross-sectional area; TMW, trabecular meshwork width; TMT, trabecular meshwork thickness; TIA500, the trabecular-iris angle at 500 μm from the scleral spur.
*Indicates statistical significance.
Association between SC expansion and changes in related parameters
Parameters . | Univariate analysis . | Multivariate analysis . | ||||
---|---|---|---|---|---|---|
B±SD . | 95% CI . | p value . | B±SD . | 95% CI . | p value . | |
Preoperative SC-CSA (μm2) | −0.197±0.128 | −0.452±0.058 | 0.128 | - | - | - |
Change in TMW (μm) | 3.560±1.048 | 1.471–5.649 | 0.001* | 3.405±0.945 | 1.520–5.289 | 0.001* |
Change in TMT (μm) | 14.253±3.233 | 7.810–20.696 | <0.001* | 11.853±3.059 | 5.753–17.953 | <0.001* |
Change in TIA500 (°) | 37.720±20.285 | −2.708 to 78.148 | 0.067 | 35.453±17.527 | 0.504–70.401 | 0.047* |
Change in IA-SS (μm) | 1.827±1.774 | −1.709 to 5.363 | 0.307 | - | - | - |
Parameters . | Univariate analysis . | Multivariate analysis . | ||||
---|---|---|---|---|---|---|
B±SD . | 95% CI . | p value . | B±SD . | 95% CI . | p value . | |
Preoperative SC-CSA (μm2) | −0.197±0.128 | −0.452±0.058 | 0.128 | - | - | - |
Change in TMW (μm) | 3.560±1.048 | 1.471–5.649 | 0.001* | 3.405±0.945 | 1.520–5.289 | 0.001* |
Change in TMT (μm) | 14.253±3.233 | 7.810–20.696 | <0.001* | 11.853±3.059 | 5.753–17.953 | <0.001* |
Change in TIA500 (°) | 37.720±20.285 | −2.708 to 78.148 | 0.067 | 35.453±17.527 | 0.504–70.401 | 0.047* |
Change in IA-SS (μm) | 1.827±1.774 | −1.709 to 5.363 | 0.307 | - | - | - |
IA-SS, distance from the scleral spur to the inner apex of the ciliary muscle; SC-CSA, Schlemm’s cross-sectional area; TMW, trabecular meshwork width; TMT, trabecular meshwork thickness; TIA500, the trabecular-iris angle at 500 μm from the scleral spur.
*Indicates statistical significance.
In multivariate linear regression analysis, after adjusting for preoperative SC-CSA and changes in IA-SS, changes in TMW (β = 3.726 ± 1.085, p = 0.001 nasally; β = 3.405 ± 0.945, p = 0.001 temporally), TMT (β = 5.224 ± 2.033, p = 0.012 nasally; β = 11.853 ± 3.059, p < 0.001 temporally), and TIA500 (β = 40.330 ± 15.100, p = 0.009 nasally; β = 35.453 ± 17.527, p = 0.047 temporally) were significantly associated with changes in SC-CSA after cataract surgery (Tables 3, 4).
Table 5 shows the SC-CSA-related variables in the OAs and NAs groups. The preoperative SC-CSA was greater in OAs eyes than in NAs eyes, although the difference was not significant in the temporal section (p = 0.17). After cataract surgery, the changes in SC-CSA were greater in NAs eyes compared with OAs eyes (3,363.2 ± 1,156.3 vs. 2,049.5 ± 1,095.0 in the nasal section, p < 0.001; 3,556.5 ± 1,703.8 vs. 2,088.1 ± 1,207.7 in the temporal section, p < 0.001).
Comparison of parameters between OA and NA eyes
. | OA . | NA . | p value . |
---|---|---|---|
Nasal, n | 38 | 37 | |
Preoperative SC-CSA, μm2 | 4,167.5±1,327.2 | 3,009.4±1,069.3 | <0.001* |
Change in SC-CSA, μm2 | 2,049.5±1,095.0 | 3,363.2±1,156.3 | <0.001* |
Postoperative SC-CSA, μm2 | 6,442.6 (2,462.8) | 6,427.5 (1,671.0) | 0.564 |
Temporal, n | 48 | 27 | |
Preoperative SC-CSA, μm2 | 4,254.3±1,369.1 | 3,454.4±1,357.3 | 0.17 |
Change in SC-CSA, μm2 | 2,088.1±1,207.7 | 3,556.5±1,703.8 | <0.001* |
Postoperative SC-CSA, μm2 | 6,342.4±1,777.1 | 7,010.9±2,096.5 | 0.147 |
. | OA . | NA . | p value . |
---|---|---|---|
Nasal, n | 38 | 37 | |
Preoperative SC-CSA, μm2 | 4,167.5±1,327.2 | 3,009.4±1,069.3 | <0.001* |
Change in SC-CSA, μm2 | 2,049.5±1,095.0 | 3,363.2±1,156.3 | <0.001* |
Postoperative SC-CSA, μm2 | 6,442.6 (2,462.8) | 6,427.5 (1,671.0) | 0.564 |
Temporal, n | 48 | 27 | |
Preoperative SC-CSA, μm2 | 4,254.3±1,369.1 | 3,454.4±1,357.3 | 0.17 |
Change in SC-CSA, μm2 | 2,088.1±1,207.7 | 3,556.5±1,703.8 | <0.001* |
Postoperative SC-CSA, μm2 | 6,342.4±1,777.1 | 7,010.9±2,096.5 | 0.147 |
Values are mean ± SD or median (IQR).
SC-CSA, Schlemm canal cross-sectional area.
*Indicates statistical significance.
Discussion
This study confirms previous reports of IOP reduction after cataract surgery [1, 3, 21, 22]. In our series of non-glaucomatous patients with either OAs or NAs, the average IOP dropped significantly 1 week after cataract surgery. Since decreased ACA and SC are fundamental mechanisms of glaucoma, it follows that the decrease in IOP may be a result of the increase in ACD [4‒7], ACA [2, 4, 6, 8, 9], and SC after cataract surgery [3, 19]. However, previous studies focused only on the relationship between changes in IOP and the related parameters outlined above. Studies on SC-TM-CM changes and their relationship with ACA are rare.
To our best knowledge, this is the first observation, using OCT, which shows the specific correlation between the changes in SC-CSA and changes in TMW, TMT, and TIA500 1 week after cataract surgery. This bridges the gap between studies that have shown an increase in SC-CSA following cataract surgery and OCT imaging of aqueous outflow structures. In the univariate analysis, we found that, in the nasal section, for a 1 µm increase in TMW and a 1° increase in TIA500, SC-CSA increased by approximately 3.39 µm2 and 61.76 µm2, respectively. In the temporal section, for a 1 µm increase in both TMW and TMT, SC-CSA increased by approximately 3.56 µm2 and 14.25 µm2, respectively.
In terms of the structure of the SC-TM-CM complex [14], the CM diverges into external and internal branches. The external branches are inserted into the juxtacanalicular elastic net; the internal branches connect to the trabecular lamella, which is fixed in the scleral spur (SS) at the posterior TM. The cell processes of SC endothelial cells are attached to the cell processes of juxtacanalicular cells, whose cell processes are sequentially attached to the trabecular lamella, thus forming a hemidesmosome structure [10, 11]. In several clinical studies [23, 24], CM contraction induced by pilocarpine stretched the TM and increased the SC-CSA, which indicated a possible relationship between SC-CSA, TM, and CM.
A previous study [25] demonstrated that increasing age and development of cataracts were associated with thickening of the lens, a steeper anterior curvature of the lens, and therefore an anteriorly located and smaller CM (smaller cross-sectional area of the CM and IA-SS), as well as a narrower SC. It follows that the alterations seen in the present study, including the increase in TIA500, could be a result of the posterior displacement of these anterior structures (including the CM, SS, and iris root), caused by lens exchange with phacoemulsification and intraocular lens implantation. In addition, Zhao et al. [3] found that the anterior vault, which was defined as the distance between the posterior corneal surface and the horizontal line connecting the two SS, increased after cataract surgery, indicating posterior displacement of the SS. This movement would increase posterior traction on the SS and adjust the zonular tension vectors transmitted to the SS, CM, and TM [26], thus facilitating aqueous outflow by pulling the TM toward the center of the eye and enlarging the SC lumen, which were represented as increases in TMW, TMT, SCD, and SC-CSA. Conversely, the IA-SS decreased after cataract surgery, although the difference was not significant. It appeared to be associated with posterior displacement of the SS after cataract surgery.
Therefore, cataract surgery seems to have a “chain reaction” on TIA500 and the aqueous humor outflow pathway. Since the SC acts as the primary resistance point in the outflow facility [16], we investigated the relationship between changes in SC-CSA and other related variables. However, no correlation between changes in SC-CSA and IA-SS was found in this study. This outcome may be owing to the lack of significant difference between the preoperative and postoperative IA-SS. Upon further investigation, the changes in SC-CSA were significantly associated with changes in TMW, TMT, and TIA500, as shown in the multivariate linear regression analysis.
Cataract surgery has been shown to have a greater effect on lowering IOP in eyes with NAs than in OAs [4, 27, 28]. Huang et al. [4] found that increases in ACD and the angle opening distance were both greater in NAs eyes than in OAs eyes between any two points of follow-up time after cataract surgery. Therefore, these more significant increases in ACD and ACA, induced by cataract removal, may be the primary contributing factors for greater IOP change in eyes with NAs [4, 29]. In the present study, we found that the increase in SC-CSA was greater in NA eyes than in OA eyes after cataract surgery, which adds further evidence for the greater decrease in IOP seen in NAs eyes. It also agreed with the correlation between the change in SC-CSA and TIA500 found in this study.
Our study had certain limitations. First, due to the poor quality of SS-OCT images and the inability to define the SC or TM, we have had to exclude 10.5% eyes. Although scans were centered in the temporal and nasal quadrants and located by the texture of the vessels on the scleral and iris surfaces, the SC and TM may not be in exactly the same position of the eye in the pre- and postoperative period. Second, we only examined one postsurgical timepoint. However, the parameters involved in this study have the potential to change over a longer period after surgery. Therefore, further investigations, with longer follow-up periods, are warranted.
Conclusion
After cataract surgery, SC-CSA increased significantly, and this increase was accompanied by an increase in the TMW, TMT, and TIA500. Compared to OAs eyes, the increase in SC-CSA was greater in NA eyes, which may explain the greater decrease in IOP seen in NA eyes after cataract surgery.
Acknowledgments
The authors express their gratitude to the department of cataracts of Eye Hospital of Wenzhou Medical University Hangzhou Branch for their cooperation and assistance.
Statement of Ethics
This study and the required data were prospectively approved by the Eye Hospital of Wenzhou Medical University’s Institutional Review Board (No. 2022-014-K-11) and complied with the 1975 Helsinki Declaration, as revised in 1983. Written informed consent was obtained from each participant. This trial was registered at NIH (clinicaltrial.gov) on April 24, 2022 (NCT05352854).
Conflict of Interest Statement
The authors have no conflicts of interest to declare.
Funding Sources
This work was supported by research grants from the Basic scientific research projects in Wenzhou [Y20220145]; the Zhejiang Medical Health Science and Technology Project [No. 2023KY913]; the Science and Technology Department of the State Administration of Traditional Chinese Medicine – Zhejiang Province Joint Construction Project [GZY-ZJ-KJ-24089]; the “Pioneer” and “Leading Goose” R&D Program of Zhejiang [2022C03070].
Author Contributions
All authors contributed to the study conception and design. Zhangliang Li, MD, provided assistance with experimental design, data analysis, and manuscript revision; Xueer Wu, MD, provided assistance with data analysis and manuscript writing; Xinpei Ji, MD, provided assistance with data collection and analysis; Zehui Zhu, MD, and Nan Zhe, MD, provided assistance with data collection; Yun-e Zhao, MD, provided assistance with experimental design and manuscript revision.
Additional Information
Zhangliang Li and Xueer Wu contributed equally to this work.
Data Availability Statement
The data that support the findings of this study are not publicly available due to their containing information that could compromise the privacy of research participants but are available from the corresponding author (Y.E.Z.) upon reasonable request.