FormalPara Key Summary Points

Why carry out this study?

During phacoemulsification, the infusion pressure can cause the liquefied vitreous fluid to escape through the ciliary fiber interspace in highly myopic eyes, leading to reduced vitreous cavity pressure similar to vitrectomized eyes.

This study uses the quantitative index of intraocular pressure (IOP) to provide a more intuitive understanding of IOP fluctuations during cataract surgery in high myopia eyes.

What was learned from the study?

Highly myopic eyes are more likely to develop low IOP during cataract surgery, and the longer the axial length (AL), the higher the total probability of low IOP.

In patients with IOP < 9.5 mmHg intraoperatively, fluid supplementation via a side-port incision can effectively raise IOP to about 16 mmHg before intraocular lens (IOL) implantation and about 14 mmHg after incision sealing, facilitating smoother IOL implantation and reducing the risk of postoperative low IOP.

Digital Features

This article is published with digital features, including a video to facilitate understanding of the article. To view digital features for this article go to https://doi.org/10.6084/m9.figshare.24081567.

Introduction

Highly axial myopia, defined as an axial length (AL) ≥ 26 mm, is characterized by long AL [1], deep anterior chamber (AC) [1, 2], floppy and large lens capsule [2], weakness of the zonular fibers [2,3,4], and loss of support from the liquefied vitreous body [4]. These factors contribute to the increased difficulty of performing cataract surgery and elevate the risk of intraoperative complications [4]. The weakness of the ciliary zonules amplifies the interspace between zonular fibers, enabling an abnormal flow pattern of intraocular fluid via the zonular fibers. This leads to fluid misdirection syndrome (FMS) during phacoemulsification [5,6,7]. Conversely, reverse pupil block can also occur during the surgery, leading to posterior bowing of the iris-lens diaphragm [5, 8], compressing the vitreous body, and causing the flow of vitreous cavity fluid to the AC [8, 9] through the ruptured anterior hyaloid membrane (AHM) [10, 11] and weakened ciliary zonules [6, 11]. This phenomenon results in a decrease in intraocular pressure (IOP), deepened AC, and even eyeball collapse. The severity of vitreous liquefaction increases with longer AL, making this situation more likely to occur.

Despite sealing the incision and achieving a well-formed AC, low IOP increases the risk of choroidal detachment and expulsive choroidal hemorrhage (SEH) [12, 13]. This poses potential safety concerns during and after surgery. In some special cases, despite injecting ophthalmic viscosurgical device (OVD) until a well-formed AC is achieved, there was still low IOP or even eyeball collapse. In the previous literature, two studies [14, 15] proposed a surgical technique to improve this phenomenon, that is, using a cannula to supplement balanced salt solution (BSS) to the vitreous cavity through the interspace of weakened zonules under the iris via a side-port incision. This technique can increase the pressure in the vitreous cavity intraoperatively, improve IOP, and prevent complications caused by low IOP.

However, it is not known how much IOP should be reached for BSS supplementation. Excessive supplementation of BSS can result in high IOP during surgery, which may affect the fundus blood supply; moreover, the above studies only focus on the eyes after vitrectomy and lack experience with highly axial myopic eyes, which are prevalent among Asian people. Therefore, we conducted a study in which patients with high myopia were selected and divided into groups based on their AL, with the normal AL eyes as the control, to assess the probability of low IOP during cataract surgery under different ALs, and to restore the IOP through the key step of supplementing BSS into the vitreous cavity through the interspace of zonules, so as to facilitate intraoperative operation and postoperative stability of IOP. At the same time, the relationship between high myopia and intraoperative IOP in different AL groups and the appropriate IOP value should be achieved by supplementing BSS were analyzed.

Methods

Patients

This prospective clinical trial included cataract patients with high myopia (AL longer than 26 mm) and those with normal AL (22–24.5 mm) who underwent cataract surgery performed by the same surgeon (Z.Y.E) at the Eye Hospital of Wenzhou Medical University, Hangzhou, between June 2021 and September 2022. The surgical eyes were included in patients undergoing unilateral cataract surgery, whereas one eye was randomly selected by authors using a random table in patients undergoing bilateral cataract surgery. Highly axial myopic eyes were divided into three groups (26 mm ≤ AL < 28 mm, 28 mm ≤ AL < 30 mm, and AL ≥ 30 mm, respectively), with normal AL eyes (22 mm ≤ AL < 24.5 mm) as the control group. The preoperative IOP of all selected eyes was within the normal range. The exclusion criteria were eyes with corneal scars, glaucoma, a history of trauma, a history of ophthalmic surgery, implantation of devices other than intraocular lens (IOL) during cataract surgery, or intraoperative complications. Preoperative assessments included tonometry (iCare pro tonometer, iCare Finland Oy, Helsinki, Finland), slit-lamp examination with the Lens Opacities Classification System III (LOCS III), dilated fundus examination, and biometry measurement (IOL Master 700, Carl Zeiss Meditec AG, Jena, Germany). Data on the age, sex, anterior chamber depth (ACD), and lens thickness (LT) were recorded for each eye. The study was performed in accordance with the Helsinki Declaration of 1964 and its later amendments. It was approved by the Ethics Committee of Wenzhou Medical University (2021-133-K-112-01) and was registered with ClinicalTrials.gov (NCT05201677). All patients were aware of the collection of their data for this study and signed a consent form at the time of enrollment.

Preparation and Tonometric Measurement

The intraoperative IOP measurement was performed using the iCare pro tonometer under aseptic conditions. The iCare pro tonometer was covered with an aseptic surgery membrane (Fig. 1A). The iCare pro tonometer was placed perpendicular to the central area of the cornea (Fig. 1B). The average value was calculated automatically by the tonometer and considered as the result of IOP data at this time, when the background color of the tonometer screen turned green (standard deviation is in the normal range) (Fig. 1C). IOP was measured by the same measurer.

Fig. 1
figure 1

Use of the iCare pro tonometer. A The sticky part of the aseptic surgery membrane completely covers the upper part of the iCare pro tonometer. B The surgical assistant placed the iCare pro tonometer perpendicular to the cornea of the patient at an appropriate distance to measure the intraocular pressure (IOP). C The average value was calculated automatically by the tonometer and considered as the result of IOP data when the background color of the tonometer screen turned green

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Surgical Technique and Time Points of IOP Measurements

All surgeries were performed by the same experienced surgeon. Phacoemulsification was performed using the Centurion Vision System with gravity infusion mode (Alcon Laboratories, Inc., Fort Worth, TX, USA). OVD (Medical Sodium Hyaluronate Gel, Bausch & Lomb Freda, Shandong, China) was transferred into a 2.5-ml injector to record the dose conveniently while ensuring there was no air at the top of the injector.

After administering topical anesthesia, a 2.2-mm clear corneal incision was made at the 11 o’clock position, and a 1.0-mm side-port incision was made at the 3 o’clock position. Central continuous curvilinear capsulorhexis (CCC) was performed with a diameter of 5–5.5 mm. Hydrodissection, hydrodelineation, and phacoemulsification were performed, and the cortex was removed with automated irrigation/aspiration (I/A). A single-piece acrylic foldable IOL was implanted, and the incisions were sealed with hydration. OVD was injected twice, before CCC and IOL implantation, and the total dose was recorded. The surgical time and cumulative dissipated energy (CDE) were also recorded.

Under the 2.2-mm corneal incision, if CT ASPHINA 509M IOL (Carl Zeiss Meditec AG, Jena, Germany) is implanted, even if the IOP is very low, the tip of the injector can be easily inserted into the AC to implant the IOL. However, when AcrySof IOL (Alcon Laboratories, Inc., Fort Worth, TX, USA) is implanted through the Monarch III injector (Alcon Laboratories, Inc., Fort Worth, TX, USA) or Proming A1-UV IOL (Eyebright, Beijing, China) is implanted through the ILIS injector (Eyebright, Beijing, China), because of the small incision, it is generally necessary to put the tip of the injector outside the incision, so as to push the IOL into the incision and then into the capsular bag. When the IOP is low, this operation cannot be completed unless the IOP is increased. Otherwise, the incision needs be enlarged so that the tip of the injector will be inserted into the AC, resulting in additional damage to the eyeball.

In this study, fluid supplementation was performed before IOL implantation and/or after incision sealing if low IOP appeared. Lower than 10 mmHg indicates low IOP. The cannula of the injector was introduced via the side-port incision and extended peripherally between the space of the iris and zonules, and then BSS was injected into the vitreous space via the interspace of the ciliary fibers to maintain normal IOP (Fig. 2, See video 1 in the online/HTML version of the manuscript or follow the digital features link under the abstract). The IOP data were recorded at six time points during surgery (Fig. 3).

Fig. 2
figure 2

Degree of eyeball deformation before and after fluid supplementation. A Eyeball deformation during intraocular lens (IOL) implantation at low intraocular pressure (IOP). B If the IOP is low and the procedure of IOL is difficult, the cannula of the injector was introduced via the side-port incision and balanced salt solution (BSS) was injected into the vitreous space via the interspace of the ciliary fibers. C The IOP returned to normal and IOL was successfully implanted

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Fig. 3
figure 3

Intraocular pressure (IOP) data recorded at six time points. BSS balanced salt solution, IOL intraocular lens

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Fluid supplementation through weakened zonules via side-port incision to maintain normal IOP before IOL implantation. (MP4 75,176 KB)

Statistical Analysis

The data were presented using Microsoft Office Excel (Microsoft, Redmond, WA, USA), and statistical analysis was performed using SPSS (version 26.0, IBM, Armonk, NY, USA) and GraphPad Prism (version 9.0, GraphPad Software Incorporated, San Diego, CA, USA). The Shapiro–Wilk test was used to verify the normality of data distribution. The normally distributed data were expressed as mean ± standard deviation, and the non-normally distributed data were expressed as median (P25, P75). The chi-squared test was used to compare the probability of low IOP between the groups. Based on the results of the normality test, paired t test or Kruskal–Wallis test was used to compare the value of IOP before and after fluid supplementation in all groups. Paired t test was used to compare the effect of the eye speculum on IOP. A P value < 0.05 was considered statistically significant. All reported P values were two-sided.

The sample size was calculated using PASS (version 15.0.5, NCSS LLC, Kaysville, UT, USA), based on the probability of low IOP requiring BSS supplementation between the groups, which was the most important parameter in this study. A two-sided 5% significance level and a power of 80% were considered, with an anticipated dropout rate of 0%. The calculation indicated that each group should consist of 23 participants and a total of 92.

Results

Baseline

A total of 95 eyes were included in this study. The average age of the patients was 65.28 ± 10.68 years (range, 34–88 years). No significant difference was observed in terms of gender ratio, eye ratio, the ratio of LOCS III scores ≥ 2, age, IOP, LT, surgical time, and CDE among the groups (Table 1). The average ACD of the normal AL group was significantly shallower compared to the high myopia groups (P < 0.05). The median intraoperative OVD dose of the normal AL group was significantly lower than those of the high myopia groups (P < 0.05). No intra- or postoperative complications were observed.

Table 1 Baseline demographic and clinical characteristics
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Comparison of IOP Before and After Placing the Eye Speculum

To account for the potential impact of the eye speculum on IOP, we measured IOP before and after placing the eye speculum. It was found that IOP increased significantly after placing the eye speculum (P < 0.05) (Table 2).

Table 2 Comparison of IOP before and after placing the eye speculum
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The Trends of IOP Change During Surgery

There was no difference in IOP among four groups after placing the eye speculum (Table 3). Low IOP and eyeball collapse (Fig. 2A) were more likely to occur before IOL implantation and after incision sealing, so fluid supplementation was needed to increase IOP. Fluctuations in IOP were found to be greater in the three high myopia groups compared to the normal AL group, and the fluctuation amplitude of IOP in group 4 was the most obvious (Fig. 4). Moreover, the highest increase in IOP was observed in group 4 after the first BSS supplementation. The total probability of low IOP requiring at least one BSS supplementation was 16.67% for group 1, compared to 56.52, 62.50, and 70.83% for groups 2, 3, and 4, respectively, indicating a statistically significant difference (Table 4).

Table 3 The IOP data recorded at five time points
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Fig. 4
figure 4

Changes of mean intraocular pressure (IOP) at different time points. Number 1 means the time point after placing the eye speculum. Number 2 means the time point before intraocular lens (IOL) implantation. Number 3 means the time point after the first balanced salt solution (BSS) supplementation. Number 4 means the time point after incisions sealing. Number 5 means the time point after the second BSS supplementation. AL axial length

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Table 4 Probability of low IOP requiring BSS supplementation at different time points in the surgical procedure
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First BSS Supplementation

Before IOL implantation, the probability of low IOP requiring BSS supplementation was significantly higher in groups 2, 3, and 4 (43.48, 41.67, and 62.50%) compared to group 1 (4.17%, P < 0.05) (Fig. 4, Table 4). Due to low IOP, the eyeball was obviously deformed at the incision when IOL was going to be inserted (Fig. 2C). In all patients, IOP ranged from 6 to 24.9 mmHg before IOL implantation. Patients with IOP < 9.5 mmHg required the first BSS supplementation. In patients with IOP between 9.5 and 13.6 mmHg (Fig. 5A), the first BSS supplementation depended on the type of IOL and the tip size of the IOL injector, sub-2 mm tip size can easily pass through the incision without BSS supplementation. After BSS supplementation, the IOP in the three high myopia groups increased significantly compared to the pre-supplementation levels (P < 0.05, Table 3, Fig. 6A). There was no significant difference before and after the first BSS supplementation in group 1 (P > 0.05). After the first BSS supplementation, the IOP in the high myopia groups rose from 12.10 (10.10, 14.60) mmHg (range, 6.0–24.9 mmHg) to 16.60 (13.60, 20.60) mmHg (range, 10.2–34.4 mmHg). All IOLs were successfully implanted into capsular bags.

Fig. 5
figure 5

Intraocular pressure (IOP) before and after the balanced salt solution (BSS) supplementation and IOP requiring BSS supplementation before intraocular lens (IOL) implantation/after incision sealing. A IOP requiring BSS supplementation before IOL implantation. B IOP requiring BSS supplementation after incision sealing

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Fig. 6
figure 6

Intraocular pressure (IOP) before and after the balanced salt solution (BSS) supplementation. A IOP before and after the first BSS supplementation. B IOP before and after the second BSS supplementation. AL axial length

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Second BSS Supplementation

At the end of the surgery, the incision was sealed and the AC was well formed. The probability of low IOP requiring BSS supplementation in group 3 was higher compared to the other three groups, although the difference was not statistically significant (13.04, 19.05, 45.00, and 12.50%, respectively, P > 0.05) (Table 4). IOP ranged from 6.4 to 25.3 mmHg in all patients after incision sealing. Patients with IOP lower than 9.8–10.2 mmHg required a second BSS supplementation (Fig. 5B). After the second BSS supplementation in all highly myopic groups, the IOP improved from 12.60 (10.00, 17.25) mmHg (range, 7.0–25.3 mmHg) to 14.60 (12.05, 17.35) mmHg (range, 9.8–25.3 mmHg), which was higher than that before in each groups, while the difference was statistically significant in group 3 (Table 3, Fig. 6B, P < 0.05).

Discussion

This study examined the relationship between AL and intraoperative IOP in highly axial myopia eyes. No significant difference was observed in terms of gender ratio, eye ratio, the ratio of LOCS III scores ≥ 2, age, IOP, LT, surgical time, and CDE among the groups. We preliminary demonstrated that longer AL was associated with a higher probability of low IOP requiring BSS supplementation, with group 2 showing a probability of 56.52%, group 3 at 62.50%, and group 4 at 70.83%. After the first and second BSS supplementation, the IOP in high myopia groups increased to 16.60 (13.60, 20.60) mmHg (range, 10.2–34.4 mmHg) and 14.60 (12.05, 17.35) mmHg (range, 9.8–25.3 mmHg), respectively.

Previous studies have focused on the abnormal fluctuation of AC in cataract surgery after vitrectomy [5, 8, 16]. Some scholars pointed out that there was a difference between the pressure of AC and posterior chamber (PC), leading to a series of surgical complications [14, 15]. In fact, high myopic eyes share similarities in anatomical structure with vitrectomized eyes, characterized by weakened zonules and lack of support from the liquefied vitreous body. Therefore, high myopic eyes are also likely to occur with the above-mentioned surgical risks during phacoemulsification, which surgeons should pay close attention to.

Currently, surgeons have explored several techniques to address AC fluctuation during cataract surgery in vitrectomized eyes, such as injecting OVD [5], minimizing wound leakage [17, 18], reducing the height of the infusion bottle and fluidic parameters [19,20,21,22], adding a second infusion line [8, 23], and using a blunt-ended second instrument (other than a phaco tip or an I/A tip, usually the chopper) to lift the edge of the iris or depress the anterior capsule [8, 16, 19, 24, 25], and so on. However, as described by Lyu [14], these methods are insufficient in achieving proper fluid flow balance and restoring support to the vitreous cavity, which in turn fails to adequately balance the pressure between the AC and PC. Some scholars have proposed stabilizing vitreous cavity pressure by injecting BSS into the vitreous body through the pars plana [9, 22]. However, this technique is not recommended due to its significant surgical trauma and prolonged cataract surgery duration. Nowadays, a simple, non-invasive and low-cost surgical technique proposed by Lyu [14] and Li [15], involving the injection of BSS through the interspace of the zonular fibers under the iris to supplement the vitreous fluid, is utilized to maintain AC stability, minimize intraoperative complications, and enhance surgical safety [14, 15]. We also employed this technique in our high myopia eye study.

Furthermore, the above two articles relied on the finger measurement of IOP to determine the cessation of BSS injection, and the sample sizes were small, with one study comprising only a few cases and the other involving only a dozen cases. Consequently, we gathered a total of 71 high myopia eyes, which are prevalent in Asia and share similarities with vitrectomized eyes, and compared them with 24 eyes exhibiting normal AL. Intraoperative IOP measurements were obtained using the iCare pro tonometer to provide a more intuitive understanding of IOP fluctuations during cataract surgery in high myopia eyes.

In this study, IOP was measured at six selected time points. High myopia eyes exhibited a higher likelihood of experiencing low IOP before IOL implantation and after incision sealing, which was significantly different from that of patients with normal AL. Although there was no significant difference in the probability of low IOP requiring BSS supplementation between the high myopia groups, the total probability of low IOP at least once occurred in eyes with AL > 30 mm was the highest (56.52, 62.50, 70.83%). We hypothesized that greater increases in AL corresponded to more severe vitreous liquefaction, leading to increased loss of liquefied vitreous humor intraoperatively. A similar trend was observed before IOL implantation, where the probability of low IOP requiring BSS supplementation in eyes with an AL greater than 30 mm (62.5%) exceeded that of the two other high myopia groups (43.48 and 41.68%, respectively). After incision sealing, the probability of low IOP requiring BSS supplementation progressively increased in all groups (13.04, 19.05, and 45.00%, respectively) as AL increased, with the exception of group 4 (12.50%). This may be attributed to the relatively low probability of low IOP in group 3 before IOL implantation, but subsequent vitreous loss during the later stages of the procedure resulted in an increased probability of low IOP. In contrast, group 4 exhibited a probability of low IOP before IOL implantation, prompting the sufficient supplementation of vitreous fluid through BSS injection at that stage and consequently reducing the probability of low IOP after incision sealing.

Our findings indicated that the IOP of the three high myopia groups increased after BSS supplementation, providing confirmation that such supplementation through the interspace of the ciliary fibers under the iris effectively replenishes vitreous fluid, resulting in increased IOP. The basic parameters of each group were similar, ensuring that they had no effect on the analysis of the main parameters of IOP. After BSS supplementation, the degree of eyeball deformation was decreased, the implantation of IOL was smoother, and this technique can effectively maintain normal IOP until the end of surgery, thereby preventing potential postoperative complications related to low IOP [26,27,28,29].

The present study revealed that some eyes with normal AL also required BSS supplementation, possibly due to age-related severe vitreous liquefaction and the outflow of liquefied vitreous through the original interspace of the zonular fibers, but the occurrence of low IOP requiring BSS supplementation in these eyes was not as severe as observed in high myopia eyes. Injecting BSS into the vitreous cavity through the original interspace of the zonular fibers can also improve low IOP to some extent.

Additionally, IOP measurements were obtained before and after placing the eye speculum to examine its impact on IOP. The results showed that IOP increased significantly after placing the eye speculum, which was consistent with the results of previous studies conducted on children [30, 31]. Placing the eye speculum resulted in an increase in IOP ranging from 2 to 4.5 mmHg. We speculated that the actual IOP during surgery might be lower than the measured IOP, which justifying the need for BSS supplementation in patients with IOP > 10 mmHg before IOL implantation.

Our study has some limitations. Firstly, the actual dose of BSS injected could not be quantified due to the unrestricted outflow of intraocular fluid through the incision. Secondly, we did not unify the type of IOL. Thirdly, real-time monitoring of IOP could not be conducted. In addition, this surgical technique should be done by highly skilled ophthalmologist. When the IOP is too high, a part of BSS can be released by gently pressing the lateral corneal incision. Nonetheless, this study offers a practical strategy for managing low IOP in high myopic eyes during cataract surgery, which is of great clinical significance.

Conclusions

High myopic eyes seems more likely to experience low IOP during cataract surgery, with a higher total probability of low IOP requiring BSS supplementation associated with increasing AL. Therefore, we should pay more attention to the phenomenon of low IOP in cataract surgery in high myopic eyes. Among patients with intraoperative IOP below 9.5 mmHg, fluid supplementation through a side-port incision can effectively elevate IOP to about 16 mmHg before IOL implantation and about 14 mmHg after incision sealing, which may promote smoother IOL implantation and reducing the risks of postoperative low IOP.