This case–control study covered patients with high myopia who visited the Tianjin Medical University Eye Hospital between December 2020 and November 2022. All patients signed an informed consent form and underwent eye examinations. All study procedures conformed to the tenets of the Declaration of Helsinki and were approved by the Ethics Committee of Tianjin Medical University Eye Hospital. The study was registered in the Chinese clinical trial registry (http://www.chictr.org.cn/, Registration number: ChiCTR2100046590). The inclusion criteria were age > 18 years, spherical equivalent refraction (SER) <  − 6.00 D, AL ≥ 26.5 mm, and the type of PS being narrow macular staphyloma. Patients with intraocular pressure (IOP) > 21 mmHg, previous vitreoretinal surgery, primary or secondary glaucoma, systemic disease, refractive interstitial opacity affecting fundus imaging, and poor OCT and OCTA imaging quality were excluded.

Standardized ophthalmic examination given to all patients, including IOP measurement (CT-1; Topcon, Japan), color fundus examination (CR-2, Canon, Japan), slit-lamp biomicroscopy, AL (Lenstar LS-900; Haag-Streit AG, Switzerland) and Autorefraction (KR-800, Topcon, Japan). SS-OCT/OCTA (VG200S; SVision Imaging, Henan, China).

We measured the horizontal, vertical, minimal, and maximal diameter of the optic disc, distance between the optic disc centre and the fovea (“disc–fovea distance,” DFD), optic disc area, and PPA area by using the ImageJ system (National Institutes of Health, Bethesda, MD, USA) in fundus photographs. We calculated optic disc tilt ratio and optic disc rotation degree. The optic disc tilt ratio was measured as the ratio of the maximum and minimum diameters of the optic disc, and the optic discs were considered to be tilted when the tilt ratio exceeded 1.30 [15]. The optic disc rotation was defined as the angle between the long axis of the optic disc and the vertical meridian, and angle greater than 15° was considered to be a rotated optic disc [10]. The PPA is the area of choroidal retinal atrophy where the scleral and choroidal vessels are visible (Fig. 1). Using the Littmann–Bennett method, we corrected for bias in the measurements because of image magnification. All measurements were taken three times by two independent measurers and averaged.

Fundus photography and SS-OCT images of the MTM and NMTM groups. A–C Left eye images of a 43-year-old male with MTM, whose AL was 30.27 mm. D–F Left eye images of a 34-year-old female with MTM, whose AL was 28.96 mm. B, E Fundus photograph of a highly myopic eye with the horizontal (red line), vertical (green line), minimal (white line), maximal diameter (blue line), and the PPA (area surrounded by grey lines)

SS-OCT/OCT can analyse an area of 16*16 mm at the centre of the optic disc and a depth of 6 mm. Cube scans measuring 6 mm × 6 mm were centred on the optic disc. Simultaneously, the scan size was adjusted according to the different ALs to avoid differences in magnification. PCT was defined as the vertical distance from the outer edge of the hyperreflective line of the retinal pigment epithelium to the reflective line of the inner sclera at the centre of the optic disc. Scanning the optic disc region automatically obtains 10 clock orientations, superior(S), inferior (I), nasosuperior (NS), nasoinferior (NI), inferonasal (IN), inferotemoporal (IT), temporoinferior (TI), temporosuperior (TS), superotemporal (ST), and superonasal (SN).

The system automatically identifies and records the PST. PST was measured manually using SS-OCT. The retina was scanned horizontally at 0° in a single-line scan pattern of 16 mm (centred on the optic disc), and the scleral thickness was measured manually at a distance of 1000 μm from the temporal scleral canal on the acquired OCT image. The scleral thickness was measured three times by two experienced ophthalmologists to obtain the average. We analysed vascular density in the optic disc from the internal limiting membrane to the retinal nerve fibre layer, the optic disc region within 2 mm in diameter, and the peri-optic disc region between 2 and 4 mm in diameter. The peri-optic disc region (2–4-mm range) was unevenly divided into eight zones according to the Garway-Heath division of the optic disc: nasosuperior (NS), nasoinferior (NI), inferonasal (IN), inferotemoporal (IT), temporoinferior (TI), temporosuperior (TS), superotemporal (ST), and superonasal (SN) [16], measured vascular density within the optic disc and peri-optic disc region (Fig. 2D).

Diagram of microcirculation parameters. A, B The peripapillary choroidal thickness is automatically identified by the system and defined as the vertical distance between the outer edge of the retinal pigment epithelium (RPE) and the outer edge of the choroid. C, D The vascular density of Optic disc and peri-optic disc region are automatically identified and calculated by the system. The range of measurement is defined as from the internal limiting membrane to Bruch’s membrane. E Schematic diagram of PST measurement

The presence and location of PS were confirmed using SS-OCT. The diagnosis of PS was the outpouching of the eye wall with a radius of curvature less than that of the surrounding eye wall [17]. The location of the PS was determined according to the Ohno–Matsui classification [18]. Using SS-OCT, all patients were divided into two groups, namely the MTM group and the NMTM group. In the MTM group, based on the MSS staging system, MTM was divided into stage 1, inner/outer maculoschisis, and stage 2, predominantly outer maculoschisis [19]. PS and MTM were observed by two independent measurers. When results were inconsistent, the retinal specialist made the judgement.

Statistical analyses were performed with SPSS software version 26.0 (IBM-SPSS, Inc, Chicago, IL). Continuous variables are expressed as mean ± standard deviation. We compared the groups of eyes with or without MTM by the measured parameters, using the chi-square test, independent t-test, and Mann–Whitney U test. Due to the fact that the generalized estimating equation (GEE) can eliminate the correlation between left and right eye, we used it to explore the factors influencing the presence of MTM. In the GEE, we used MTM as the independent variable and structure and microcirculation of the optic disc region as the dependent variables. Further, we used linear correlation analysis to analyse the correlation between the PST as the dependent variable and the ocular parameters as the independent variables in highly myopic eyes. P < 0.05 was considered a statistically significant difference.