FormalPara Key Summary Points

Why carry out this study?

High myopia with shallow anterior chamber depth (ACD) is common among East Asian patients. Several studies have highlighted that primary angle closure can also occur in eyes with long axial length (AL). To date, few studies have investigated anterior segment characteristics of highly myopic eyes with shallow ACD, especially in patients younger than 50 years old.

This study aimed to investigate anterior segment parameters of eyes with ACD of < 2.8 mm and AL of ≥ 25.0 mm in patients aged between 18 and 50 years.

What was learned from the study?

A smaller anterior segment, as well as a thicker and more anteriorly positioned lens, may contribute to shallow ACD in eyes with long axial length.

Ciliary body and iris mechanisms may not correlate with a shallow ACD in this group of patients.

Further longitudinal studies would be needed to clarify the anterior segment changes over time.

Introduction

High myopia with shallow anterior chamber depth (ACD) is common among East Asian patients. It has been reported that the percentage of eyes with ACD of < 2.8 mm was 4.46% in a group of Chinese patients with myopia (mean age 25.14 ± 5.41 years) and 3.59% in patients with high or extremely high myopia [spherical equivalent (SE) < −6.0 diopters (D)] [1]. Myopia is usually regarded as exerting a protective effect against primary angle closure (PAC). However, several studies have highlighted that PAC can also occur in eyes with long axial length (AL) [2,3,4]. According to a recent report on the biometric characteristics of myopia in primary angle closure disease (PACD) in rural China, the prevalence of myopia in eyes with PACD was 41.7% at age 30–39 years, 12.3% at age 40–49 years, and 8.7% at age 50–59 years [5].

Further, the surge in refractive surgery in recent years has led to the discovery of many patients with shallow ACDs at a younger age. Likewise, the implantation of an implantable Collamer lens (ICL, STAAR Surgical, Nidau, Switzerland) can lead to a shallower anterior chamber; consequently, the suggested ACD for ICL surgery is ≥ 2.80 mm in Europe, or ≥ 3.00 mm in the USA. Nevertheless, several studies have shown that patients with high myopia and shallow ACD (i.e., < 2.8 mm) achieved satisfactory and stable visual outcomes following ICL implantation. Meanwhile, studies regarding the long-term safety of this procedure are limited [6,7,8]. Moreover, only a few studies have investigated anterior segment characteristics of highly myopic eyes with shallow ACD, especially in patients aged < 50 years, to date.

Therefore, this study aimed to investigate anterior segment parameters of eyes with ACD of < 2.8 mm and AL of ≥ 25.0 mm. We believe that the results of our present study will contribute to a better understanding of eyes with low ACD and long AL.

Methods

The subjects were recruited from those attending the optometry clinic of Eye and ENT Hospital of Fudan University, for the correction of myopia or myopic astigmatism from September 2021 to February 2022. The research followed the tenets of the Declaration of Helsinki, and all procedures were approved by the human subjects review committee of the Eye and ENT Hospital of Fudan University, Shanghai, China. All patients provided written informed consent.

The inclusion criteria were as follows: (1) patient aged between 18 and 50 years, (2) spectacle spherical power ranging from −0.50 to −18.00 D and cylindrical power of up to −6.00 D, (3) AL ≥ 25.0 mm, (4) corneal endothelial cell density > 2000 cells/mm2, (5) no contact between the peripheral iris and the posterior trabecular meshwork on gonioscopy, and (6) intraocular pressure (IOP) < 21 mmHg. If both eyes met the criteria, one eye was randomly selected as the study eye. Patients were separated into the study group (ACD < 2.8 mm) and the control group (ACD ≥ 2.8 mm). The exclusion criteria were (1) any history of ocular diseases (kerataconus, lens opacity, retinal disease, etc.) or (2) trauma, chronic systemic diseases, or previous ocular surgeries.

All participants underwent a comprehensive ophthalmic examination under the same darkened ambient light conditions by an experienced operator, including a detailed slit-lamp examination of the anterior segment, fundus examination, IOP measurement by noncontact tonometry, manifest and cycloplegic refraction, gonioscopy, Scheimpflug camera imaging (Pentacam, Oculus GmbH) for measurements of the horizontal corneal diameter (CD), central corneal thickness (CCT), ACD (measured from the corneal endothelium to the anterior lens) and anterior chamber volume (ACV), and IOL-Master 700 (Carl Zeiss Meditec, Jena, Germany) for the measurements of AL.

Ultrasound biomicroscopy (UBM, Quantel Medical, France) measurements were performed by an experienced operator (Niu L.L.) in a light room (illumination of approximately 120 lx). The examiner was blinded to the patient grouping. The linear scanning frequency was 50 MHz, the scanning depth and width were 9 × 16 mm, and the axial and vertical resolutions were 35 and 60 μm, respectively. Examinations were performed using an eye-cup, filled with methylcellulose solution after topical oxybuprocaine hydrochloride was instilled. To control the influence of accommodation, patients were asked to fixate on a ceiling target using the contralateral eye, in the supine position. Radial scans at the 12, 3, 6, and 9 o’clock positions centered over the limbus, and the horizontal (3 o’clock to 9 o’clock) and vertical (6 o’clock to 12 o’clock) perpendicular full-view scans centered over the pupil, were obtained. Each measurement was repeated three times, and the mean values were calculated. The reliability of the procedure mentioned above has been reported in a previous study [9].

Then, all UBM images were measured manually by another well-trained examiner (Luo X.M.) blinded to the clinical data. The following parameters were measured (Fig. 1): (1) horizontal and vertical anterior chamber width (ACW, the distance between the two scleral spurs) and anterior vault (AV, the maximum perpendicular distance from the posterior corneal surface to the horizontal line between the scleral spurs) [2], (2) horizontal and vertical ciliary sulcus-to-sulcus diameter (STS) and the distance between cornea and sulcus (CSD, the maximum perpendicular distance from the corneal endothelium to the STS line), (3) lens thickness (LT) and lens vault (LV, perpendicular distance from the anterior pole of the lens to the STS line) [2], (4) angle-opening distance at 500 μm (AOD500, the distance between the posterior corneal surface and the anterior iris surface on a line perpendicular to the trabecular meshwork of 500 μm from the scleral spur), and trabecular–anterior iris surface angle (TIA, the angle between the posterior corneal surface and the anterior iris surface) [2], (5) iris–ciliary angle (ICA, the angle between the posterior iris surface and the anterior surface of the ciliary body) [10], (6) iris thickness at 500 μm (IT500, iris thickness at 500 μm from the scleral spur) and iris curvature (IC, the perpendicular distance from a line between the most central to the most peripheral points of the iris pigment epithelium to the posterior iris surface at the point of greatest convexity) [2], (7) trabecular–ciliary process distance (TCPD, a line extending from the corneal endothelium 500 μm anterior to the scleral spur toward the ciliary processes) [2], and (8) maximum ciliary body thickness (CBTmax, the distance from the innermost point of the ciliary body to the inner wall of the sclera or its extended line) and ciliary process length (CP length, the distance from the innermost point of the ciliary process to the intersection point of the iris and ciliary body) [10]. For ACW, AV, STS, CSD, and LV, the average values from the horizontal and vertical directions were obtained for the analysis. For AOD500, TIA, ICA, IT500, IC, TCPD, CBTmax, and CP length, the average values from the four quadrants were obtained for the analysis. Afterward, three relative parameters were calculated as follows: relative AV (RAV) = AV/AL, relative CSD (RCSD) = CSD/AL, and relative lens position (RLP) = (ACD + 1/2 lens thickness)/AL.

Fig. 1
figure 1

A Determination of the parameters on an ultrasound biomicroscopy image of the horizontal perpendicular full-view scans centered over the pupil. ACD anterior chamber depth, SS scleral spur, ACW anterior chamber width, AV anterior vault, LV lens vault, STS sulcus-to-sulcus diameter, CSD distance between cornea and sulcus, LT lens thickness. B Determination of the parameters on an ultrasound biomicroscopy diagram of the radial scans centered over the limbus. A circle with a radius of 500 µm centered on the scleral spur (SS) is drawn. TIA trabecular–anterior iris surface angle, AOD500 angle-opening distance at 500 µm, IT500 iris thickness at 500 µm, IC iris curvature, ICA iris–ciliary angle, TCPD trabecular–ciliary process distance, CBTmax maximum ciliary body thickness, CPlength ciliary process length

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Statistical Analysis

Statistical analyses were performed using SPSS software (version 23.0; SPSS Inc.). The normality assumption of the data was assessed using the Kolmogorov–Smirnov test. Continuous variables with normal distribution between groups were compared using the independent t-test, and data with a non-normal distribution were analyzed using the Mann–Whitney U test. Linear regression analyses were performed for indices with respect to age. A general linear model was used to compare main outcomes while adjusting for age and SE. Statistical significance was set at p < 0.05.

Results

A total of 56 eyes of 56 patients (17 males and 39 females) with ACDs of < 2.8 mm and ALs of ≥ 25.0 mm were included in the study group. A total of 124 eyes of 124 patients with ACDs of ≥ 2.8 mm and ALs of ≥ 25.0 mm served as the controls (44 males and 80 females). The mean ACD was 2.72 ± 0.06 mm (range 2.60–2.79 mm) for the low ACD group, and 3.16 ± 0.22 mm (range 2.80–3.75 mm) for the normal ACD group (Mann–Whitney U test, p < 0.001). No significant difference was found between ALs (low ACD group 26.85 ± 1.48 mm, normal ACD group 26.97 ± 1.45 mm, Mann–Whitney U test, p = 0.889). The mean age was greater for the low ACD group (35.27 ± 6.5 years) than for the normal ACD group (31.92 ± 6.25 years, Mann–Whitney U test, p < 0.001). The mean SE was −9.10 ± 3.26 D for the low ACD group and −8.19 ± 2.66 D for the normal ACD group (Mann–Whitney U test, p = 0.017).

The correlation analysis between the individual parameters and age is presented in Table 1. ACD was negatively correlated with age for both ACD groups, while IC, LT, and LV were positively correlated with age for both ACD groups (Fig. 2).

Table 1 Correlation analysis between individual parameters and age
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Fig. 2
figure 2

Distributions of ACD (A), IC (B), LT (C), and LV (D) with respect to age. ACD anterior chamber depth, IC iris curvature, LT lens thickness, LV lens vault

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The results of the general linear model comparing the main outcomes while adjusting for age and gender are presented in Table 2. Notably, IOP was higher in the low ACD group (F = 10.844, p = 0.001). Compared with the normal ACD group, the low ACD group had lower values for the following parameters: CD, PD, TIA, AOD500, ACV, ACW, AV, IT500, STS, CSD, TCPD, CBTmax, CP length, RAV, RCSD, and RLP (p < 0.05). Conversely, the values of CCT, IC, LT, LV, and ICA were greater in the low ACD group (p < 0.05, Table 2).

Table 2 Comparisons between the low and normal ACD groups after adjusting for age and SE using a general linear model
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Parameters regarding the dimensions of the anterior segment (CD, ACW, and STS) are summarized in Fig. 3A. Figure 3B shows the lens thickness and vault. The relative dimensions of the anterior segment (RAV, RCSD) and the relative position of the lens (RLP) are summarized in Fig. 3C. Figure 3D and E shows the parameters concerning the ciliary body and iris. Figure 3F illustrates the ICA.

Fig. 3
figure 3

Comparisons of individual parameters between the low and normal ACD groups. ACD anterior chamber depth, CD corneal diameter, ACW anterior chamber width, STS sulcus-to-sulcus diameter, LT lens thickness, LV lens vault, TCPD trabecular–ciliary process distance, CBTmax maximum ciliary body thickness, CPlength ciliary process length, IT500 iris thickness at 500 µm, IC iris curvature, ICA iris–ciliary angle; relative anterior vault (RAV) = AV/AL; relative distance between cornea and sulcus (RCSD) = CSD/AL; relative lens position (RLP) = (ACD + 1/2 lens thickness)/AL

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Discussion

In the current study, we investigated the four recognized biometric parameters related to PAC (i.e., anterior chamber dimensions, lens thickness and vault, iris thickness and curve, and structural features of ciliary body) in a group of eyes with ACDs of < 2.8 mm and ALs of >  25.0 mm, and found that a smaller anterior segment (comprising lower CD, ACW, and STS, and more anteriorly located scleral spurs and ciliary sulcus), as well as the lens element (thicker and more anteriorly positioned lens), might be correlated with the occurrence of shallow ACD for eyes with long AL.

The smaller CD, ACW, and STS found in this study confirmed that a shallow anterior segment may be the most important and consistent biometric feature predisposing patients to a shallow ACD [11,12,13]. Meanwhile, the lower values for RAV and RCSD suggested that the anterior and posterior portions of the eyeball elongate disproportionately in these eyes. Previous studies on the development of the eyeball showed that from ages 4–17 years, the growth of the AL is mainly caused by the increasing AL of the vitreous cavity, whereas the small increase in ACD is mainly caused by the decrease in lens thickness [14]. Khokhar et al. found that, for myopic eyes with AL of > 24.5 mm, there was a disproportionate elongation of the eyeball, with very weak or no correlation between anterior biometry and AL [15]. Therefore, a long AL may not be a protective factor against shallow ACD and PAC.

Lens-induced mechanisms (i.e., LT, LV, and RLP) have been acknowledged as one of the most important predispositions to PAC [16, 17]. In the current study, the age-related LT was consistent with other studies, as measured with the same device (UBM) [10, 18], and LT was significantly greater in eyes with shallow ACD. LV is defined as a parameter that reflects the degree of protrusion of the lens into the anterior chamber, and it correlates to the thickness, shape, and position of the lens. Importantly, LV has been reported to be one of the strongest predictors for angle closure [11, 16]. The definition of RLP was first proposed [3] to reflect the relative position of the lens. An anteriorly positioned lens may result in a decreased iris–lens canal and a higher posterior–anterior chamber pressure gradient [19, 20].

Greater IC, iris area, and IT were found to be independently associated with narrow angles [12]. In the current study, we found that the IC was greater in the shallow ACD group (0.10 mm versus 0.02 mm), whereas the IT500 was thinner in the shallow ACD group. Therefore, the greater IC in this study may be ascribed to the greater LV, rather than the thicker iris. Notably, the IC values obtained in the current study were much lower than those in previous studies [11, 12], which may be attributed to the younger age of our participants. Furthermore, no plateau irises were observed in the current study. Consequently, the iris mechanism does not contribute to the shallow ACD in this group of patients.

The effect of structural features of the ciliary body on PAC remains unclear. The anterior position of the ciliary processes has been shown to be a predisposing factor for angle closure. However, the effect of ciliary body thickness on angle closure remains controversial [21]. In this study, we used TCPD and ICA to measure the position of the ciliary body and CP length, and CBTmax to measure the shape of the ciliary body [22, 23]. The smaller TCPD, CP length, and CBTmax, as well as the greater ICA for the low ACD group, seemed to indicate that the ciliary body mechanism does not contribute to the shallow ACD in this group of patients.

PAC with myopia has received increasing attention in recent years [3, 23]. Li et al. [2] compared biometric parameters between PAC with longer AL (≥ 23.5 mm) and those with shorter AL, and reported that PAC with longer AL had a flattened cornea, a more anterior position of the lens, and less anteriorly rotated ciliary body, compared with PAC with shorter AL. Loh et al. compared ACD and AL among myopia, emmetropia, and hyperopia in subjects with primary angle-closure suspect and found that AL and vitreous cavity length in myopic eyes were significantly longer than in emmetropic and hyperopic eyes, while ACD was not significantly different. These results suggest that the anterior segment may be crowded in myopic eyes, similar to that in hyperopic and emmetropic eyes, despite the longer AL [24]. A 7-years cross-sectional study on the relationship between ocular biometry and severity of PAC glaucoma showed that eyes with longer AL have a higher mean deviation of the visual fields than those with shorter AL, although this difference was significant only in the female subgroup. The underlying mechanism could be that female subjects with PAC glaucoma, who have a more pronounced AL elongation, have weaker scleral support at the optic nerve, and thus a greater susceptibility to glaucomatous damage [25]. Another study found that an increase in AL was related to slower visual field progression in the inferior hemifield, and faster visual field progression for the papillomacular bundle area [26]. The mechanism of the development of glaucoma in eyes with a longer AL is still unclear, and further study is required.

One of the limitations of this study is that patients with extremely shallow ACD, apparent narrow angle, and iris bombe could have been excluded from the optometry clinic; thus, no testing was performed on them. Second, we performed UBM examinations only in an ambient lighting condition and not in a dark condition; therefore, we could not compare the dynamic changes in iris parameters in these patients.

Conclusion

We investigated the biometric parameters of eyes with ACD of < 2.8 mm and AL of ≥ 25.0 mm in a group of patients between the ages of 18 and 50 years and found that a smaller anterior segment (comprising lower CD, ACW, and STS and more anteriorly located scleral spurs and ciliary sulcus), as well as the lens element (thicker and more anteriorly positioned lens) might be correlated with the occurrence of shallow ACD in patients with high myopia.