The procedures in this study conformed to the tenets of the Declaration of Helsinki, that was approved by the Ethics Committee of the He Eye Specialist Hospital, Shenyang, China and by the Institutional Review Board of the He Eye Specialist Hospital (No. AF/07.08/02.1). Written informed consent was waived due to the retrospective design of the study.

A database search of imaging reports at He Eye Specialist Hospital was performed using “posterior staphyloma” as the keyword. The charts of all patients with posterior staphyloma seen between January 2020 and December 2020 were gathered. Posterior staphyloma was defined as “an outpouching of the wall in the eye with less radius of curvature compared to the surrounding curvature” [7]. The pigmentary abnormalities on the Optos images were used to ascertain the border of a staphyloma [3,5]. We only collected macular staphyloma (wide and narrow macular type) for the correlation analysis between staphyloma area and central foveal thickness (CFT), subfoveal choroidal thickness (SFCT), subfoveal scleral thickness (SFST). Exclusion criterion included: eyes with a history of vitrectomy, buckling, or corneal surgery; eyes with macular hole, choroidal neovascularization, foveal retinal split, or macular edema; blurry scanning laser ophthalmoscope or OCT images; and posterior staphyloma without involvement of the macula.

All patients underwent a detailed ophthalmological examination and Optos fundus imaging, including best-corrected visual acuity (BCVA) measurement using the Snellen chart, slit-lamp biomicroscopy, and indirect ophthalmoscopy through dilated pupil. Pseudocolor fundus images were recorded by an Optos scanning laser ophthalmoscope (Optos Daytona P200T, Optos PLC). The fundus autofluorescence images and near-infrared images were also obtained in some patients. As a routine imaging protocol, 512 × 128 mode and 21 lines enhanced depth imaging mode through the foveal were obtained on spectral domain OCT (SD-OCT; Cirrus HD-OCT5000, Carl Zeiss Meditec, Dublin, USA).

The data was collected from every patient’s record including age, sex, ethnicity, BCVA, and AL. The corneal curvature K1 (minimum corneal curvature) and K2 (maximum corneal curvature) were measured by the IOLMaster 700 (Carl Zeiss AG, Jena Germany). Image data was measured by two experienced retinal specialists (XZ and AZ), taking note of CFT, SFCT, and SFST on enhanced depth imaging OCT. According to the International Photographic Classification and Grading System of Myopia and Macular Disease, all eyes were classified into five categories of myopic maculopathy atrophy-based classification including “no myopic retinal degenerative lesion (category 0),” “tessellated fundus (category 1),” “diffuse chorioretinal atrophy (category 2),” “patchy chorioretinal atrophy (category 3),” and “macular atrophy (category 4)” [8].

Macular staphyloma area (MSA) was measured and calculated on UWF image by its own software. The staphyloma was outlined according to the obvious change of pigmentary abnormalities [3,5] combined with the curvature of retinal and choroidal vessels meaning the slope of retina (Fig. 1), the edge of staphyloma would be checked on SD-OCT when it was not clear on UWF images, as an irregularly shaped Bruch’s membrane curvature at the ectasia margin on OCT images would be observed [9].

The pixel area would be transferred to mm². Because the resolution of the Optos image is 14 microns, the actual MSA is the pixel area multiplied by the square of 14, then divided by 10⁶ to get the actual area in mm².

A total of 135 eyes from 92 patients with macular staphyloma were included in this study, with a female predominance (65.9%). All patients were Asians (Chinese origin). The mean age was 64.73 ± 10.84 years (range, 28 to 85 years). The mean AL was 29.92 ± 2.46 mm (range, 24.50 to 35.66 mm). AL shorter than 26.5mm was found in 10 eyes (7.4%). The mean MSA on UWF images was 279.67 ± 71.70 mm². The mean CFT, SFCT, and SFST were 188.51 ± 51.39, 65.88 ± 47.21, and 241.04 ± 48.39 μm, respectively (Table 1). The ICC of MSA measured by two experienced physicians (XZ and AZ) was 0.965 (95% confidence interval [CI], 0.946 to 0.977; p < 0.001) (Fig. 2).

Regarding the MSA in the subgroups (categories 1 to 4), significant difference was found when category 4 was compared to other categories (category 4 vs. 1, p < 0.001; category 4 vs. 2, p < 0.001; category 4 vs. 3, p = 0.006), as well as category 1 compared to category 3 ( p = 0.008) (Table 1). However, no significant difference was observed when category 2 paired comparison with other categories (category 1 vs. 2, p = 0.155; category 2 vs. 3, p = 0.096) was performed (Fig. 3).

In the multiple linear regression analysis, after adjusting for SFCT, BCVA, CFT, and SFST, the factors independently related to MSA were AL (β = 8.352; 95% CI, 3.306 to 13.398; p = 0.001), sex (β = −26.673; 95% CI, −51.759 to −1.586; p = 0.037), age (β = 1.184; 95% CI, 0.020 to 2.348; p = 0.046) (Table 1). MSA was positively correlated with AL (Fig. 4) and age (Fig. 5).

Posterior staphyloma is histologically characterized by relatively abrupt scleral thinning beginning at the margin of the staphyloma, the presence of structural defects in the arrangement of scleral collagen fibers, and marked choroidal thinning. Choroidal thinning was most pronounced at the posterior staphyloma margin and secondary to axial elongation of the eye. The important cause of myopic maculopathy is posterior staphyloma, which can also be found in the eyes without significant choroidal thinning such as retinitis pigmentosa or Bruch’s membrane limited defects [10].

There are limited population cohort studies on the epidemiologic data of posterior staphyloma, and its prevalence varies from 0.6% to 2.05% [11-13]. The prevalence of posterior staphyloma varies from 19% to 90% in highly myopic eyes [9]. Posterior staphylomata can be a challenge for conventional fundus images because the 50° field of the camera does not always involve their borders. Optos UWF imaging is an advantageous device for detecting the whole area of the staphyloma because of its wide view field and its multimodal features could help maximize the sensitivity of detection. The diagnosis of posterior staphyloma on this convenient device has already been testified [3,5,14]. Ohno-Matsui [5] found a sensitivity of 85% and a specificity of 85.7% for detecting staphyloma borders by using a combination of pseudocolor, autofluorescence, and near-infrared images.

In this study, we outlined macular staphyloma on UWF image based on the pigmentary abnormalities along the staphyloma edge [3,5]. Some people would argue the accuracy of this measurement, because sometimes it is not obvious that the inferior edge is there. Ohno-Matsui et al. [3,5] reported that the pigmentary abnormalities along the staphyloma edge on the Optos 200Tx retinal images were highly correlated with the outpouching edge of the eye in the 3D MRI images. We also used SD-OCT to check the edge of staphyloma in the ambiguous part [9]. Two experienced retina specialists measured the area in this study, and ICC was high.

Some research reported that the grade of posterior staphyloma had a linear relationship with chorioretinal atrophy [15]. Miyake et al. [16] found that eyes with chorioretinal atrophy exhibited a curved, steep shape with an undulated surface. Similarly, we found that the degree of chorioretinal atrophy was positively correlated with MSA in this study.

Although AL was reported to be not correlated with presence of posterior staphyloma in several studies [2,3,17], different types of staphyloma were involved in those studies. In contrast, Jonas et al. [18] found the prevalence of scleral staphyloma increased significantly with the increase of the eye axis. Wakazono et al. [19] also found AL significantly affected the speed of the posterior pole shape change. The measurement of AL is usually through the macula, and the macular outpouching would elongate the ocular axis. The length of the ocular axis is significantly positively correlated with the MSA in this study. Excessive axial elongation may initiate stress at the posterior pole, which may lead to localized degeneration of the sclera, retina, and/or choroid. Either of these degenerative changes could cause pathological changes in other tissues [6]. However, staphyloma also occurs in nonhigh myopic eyes [20,21]. We also found AL shorter than 26.5 mm was in 7.4% of the eyes with macular staphyloma, which suggests other factors associated with staphyloma.

In this study, age was found as another positive factor correlated with MSA. Various studies have reported that the incidence or prevalence of myopic posterior staphyloma is positively correlated with age [3,9,21-27]. Liu et al. [13] found that myopic retinopathy (i.e., staphyloma, myopic choroidal atrophy, lacquer cracks, Fuchs spot) was associated with increasing age. Degenerative changes were common in staphyloma eyes of people over 40 years old, followed by increasing age [2]. Interestingly, we also found the mean age of patients with AL shorter than 26.5 mm was older compared to other patients (73.10 ± 7.05 years vs. 64.06 ± 10.83 years) in this study, the increasing age might be the reason of the formation of staphyloma in these normal AL eyes. Previous studies had found increasing age and AL were relevant risk factors for developing lesions in high myopia patients and in eyes with progressive staphyloma [28-31]. In the patients aged 60 to 86 years, the prevalence of posterior staphyloma rose to 53.5% [2].

In addition, we also found that sex was also an independent factor affecting MSA. Female patients tended to have a larger MSA. According to previous studies, myopic maculopathy progression was associated with female sex in a late myopic stage, and the staphyloma shape was steeper and less smooth in female and elderly patients [9,19,20].

So far, there is no hypothesis that explains the mechanism of total posterior scleral staphyloma formation. In the eyes with high myopia, posterior staphyloma appears to be associated with scleral structural defects, including diffuse localized thinning, thinning of collagen bundles, greater interfibrillar separation, reduced collagen striations, and a lamellar structure resembling the corneal stroma that appears to slide the scleral lamellar [32]. Jonas et al. [18] suggest that significant scleral thinning results from decreased scleral local resistance to backward push of Bruch’s membrane, leading to localized outpouching of the sclera. Some studies have shown that the AL and age are independent related factors that affect the thickness of the foveal sclera [33]. According to clinical experience and previous research, the retinal thickness could reflect the degree of posterior scleral staphyloma to a certain extent [34,35]. The depth of the staphyloma was related to the extent of scleral thinning. Posterior staphyloma is frequently associated with chorioretinal atrophy [2,36]. However, choroidal thickness was not significantly different between staphylomatous eyes and highly myopic control eyes in corresponding regions after adjusting for age and AL [21]. Compared to the data of normal eyes in previous studies [37-40], CFT, SFCT, and SFST were also thinner in the eyes with macular staphyloma in this study, but the regression analysis did not find that CFT, SFCT, and SFST could be considered as independent factors affecting the size of the staphyloma.

This study has several limitations, including a relatively small study population and no 3D MRI images. Because of the bowl shape of staphyloma, we could not measure the true area of staphyloma. The UWF image just reflected the 2D image of staphyloma. This problem also occurs in other retinal measurements due to ocular curvature, such as geography atrophy measurement lots of studies have done before. Because the position of staphyloma would influence the size measurement on UWF images, we only selected the macular staphyloma to maximumly reduce this bias. Fortunately, the ICC in this study is excellent; it is reliable in reflecting MSA. However, it should be mentioned that the value corrected through the projection was not obtained because the area might be affected by the warpage depending on its size and position. The correlation between MSA and posterior staphyloma height, foveal curvature, and some plus signs, such as choroidal neovascularization and lacquer crack, should also be further studied in the future. To the best of our knowledge, this is the first study to measure the staphyloma area using UWF imaging. This study is also void of any bias from pharmaceutical sponsorship.

In conclusion, our data suggests that it is feasible to measure MSA on UWF images, that could be helpful to the study of staphyloma in the future. A larger MSA is associated with female sex, longer AL, and older age.