Central foveal distribution based on posterior scleral morphologies in high myopia

Wei-Feng Liu, Jiang-Bo Fan2, Yue-Yuan Xu, Xuan Zhu, Ya-Lan Ming, Yi-Qi Liu, Xue-Li Lan, Guo-Fu Huang

Abstract

•KEYWORDS:high myopia; posterior scleral morphology; central fovea; vision

INTRODUCTION

With the increasing prevalence of myopia[1-3], the occurrence of high myopia (HM) is also increasing[4-6]. As HM progresses, the axial length (AL) increases and the sclera expands, resulting in different posterior scleral morphologies. Some of the most convex parts of the posterior sclera are located at the posterior pole or below, some are located at the optic nerve area, and others are mixed. Whether this change in the morphology of the posterior sclera affects the central foveal position has yet to be reported. This study used Topcon DRI-OCT to measure the horizontal distance between the fovea and the vertical line pass of the optic nerve head (ONH) center (HDFCO) and the vertical distance between the fovea and the horizontal line pass of the ONH center (VDFCO). The distribution of the fovea in the posterior zone was additionally evaluated to explore the influence and relationship of different posterior scleral morphologies.

SUBJECTS AND METHODS

Patients with refractive errors were consecutively recruited between May 2016 and February 2018 at the Third Affiliated Hospital of Nanchang University. The exclusion criteria were patients who underwent fundus surgery and patients with other ocular diseases that affected the evaluation of the foveal position. For patients with an eye AL larger than 26 mm, the 12 mm×9 mm scan mode of Topcon DRI-OCT Atlantis was used to acquire OCT images of the eye. This study included 126 cases (207 eyes): 74 females (128 eyes), 52 males (79 eyes), 110 cases of the right eye, and 97 cases of the left eye. The age range was from 7-81 (average 38.55±18.36) years, spherical equivalent (SE) from -6.00 to -31.50 (mean: -13.13±5.89) Diopters, and AL from 26-36.10 (average: 28.98±2.04) mm. The study complied with the tenets of the Declaration of Helsinki, and approval was obtained from the Institutional Review Board of the Third Affiliated Hospital of Nanchang University.

GroupClassificationClassification of the posterior scleral morphology was determined using the OCT-3D model. The imaging model visualized 3D images of the posterior sclera. According to Curtin’s posterior scleral staphyloma classifications[7], the most common types were analyzed (Types Ⅰ, Ⅱ, Ⅲ, Ⅶ, Ⅸ, and Ⅹ). Types Ⅰ and Ⅱ were classified as group A, type Ⅲ as group B, and types Ⅶ, Ⅸ, and Ⅹ as group C. The posterior scleral morphology was evaluated and confirmed by two retina specialists. If there were any inconsistencies in the evaluations, the sample was assessed by a third specialist.

ZoneDefinitionAccording to three horizontal line passes of the ONH, the posterior polar 30°eye fundus was divided into three zones. Line 1 was the horizontal line pass of the superior rim of the ONH, Line 2 was the pass of the center ONH, and Line 3 was the pass of the inferior rim of the ONH. Zone A was between Line 1 and Line 2, Zone 2 was between Line 2 and Line 3, and Zone 3 was below Line 3. (Figure 1).

LocationDefinitionThe distribution of the central fovea position was defined according to several measurements. As shown in Figure 1, the macular fovea (red intersection) and the center of the ONH (red intersection) determined the HDFCO (solid long blue line) and VDFCO (solid short blue line). Two sets of data, HDFCO and VDFCO, were used to determine the location of the fovea.

Figure 1 Distance between the central fovea and the ONH: HDFCO and VDFCO.

RefractiveErrorRefractive error was determined using computerized refractor and subjective refraction examinations. A Nidek autorefractometer ARK-1 was initially used to determine the refractive degree and other notable features. Patients (<18 years of age) were then examined retinoscopically following an instillation of three drops of tropicamide 0.5%, which were instilled at 5-min intervals. After the instillation of the third drop (>30min), subjective refraction was performed. Patients >18 years of age were examined directly using subjective refraction. SEs were calculated following the examination. Subjective refraction was performed using a Nidek RT3100 refractometer (NIDEK CO. LTD.). Best corrected visual acuity (BCVA) were tested and recorded. The visual acuity of the myopic maculopathy C0 and C1 HM were analyzed according to META-PM classification.

AxialLengthMeasurementIOL Master (Carl Zeiss Meditec) was used to measure the AL 5 times. The average was taken, and if there was a deviation within the 5 measurements, the datum was not used in the study.

StatisticalAnalysisDescriptive analyses of the data were undertaken using SPSS 16.0 software (SPSS, Inc, Chicago, IL, USA). A one-way analysis of variance (ANOVA) was used. A probability value of less than 5% was considered statistically significant.

RESULTS

GeneralPosteriorScleralMorphologyGroupThere was no statistically significant difference in age, SE, or AL between groups A and B. However, the difference among groups A, B, and C was statistically significant. The width of the ONH ranged between 941 and 2424 μm, and the average was 1451±374 μm. Among them, the horizontal length of group A was shortest than that of groups B and C, and the difference was statistically significant. The vertical length of the ONH ranged between 1341 and 3176 μm, with an average of 2083±351 μm. The vertical length of group C was longer than that of groups A and B, and the difference was statistically significant (Table 1).

Table 1 Comparison of different posterior scleral morphology groups under general conditions

ParametersoftheCentralFovealPositionThe HDFCO ranged between 3629 and 7290 μm, with an average of 5048±608 μm. The HDFCO was 5042±555 μm, 4662±530 μm, and 5466±795 μm in groups A, B, and C, respectively. There was a statistically significant difference among the three groups (F=10.27,P<0.01). The HDFCO of group B (Type Ⅲ) was the shortest, followed by group A (Types Ⅰ and Ⅱ). The longest HDFCO is in Group C.

The VDFCO ranged between 71 and 3317 μm, with an average of 831±510 μm. The VDFCO values were 811±497 μm, 828±397 μm, and 979±674 μm in groups A, B, and C, respectively. There was no significant difference among the three groups (F=1.05,P=0.35).

TheCentralFovealPositioninDifferentZonesOf the 207 eyes, there were 3 eyes in zone 1, accounting for 1.5% (3/207) of the samples-all of which were from group A (Type Ⅰ and Type Ⅱ). There were 145 eyes in zone 2, accounting for 70% (145/207) of the samples. A total of 117 eyes were from group A (56.5%), 15 eyes were from group B (7.2%), and 13 eyes were from group C (6.3%). There were 59 eyes in zone 3, which accounted for 28.5% (59/207) of the samples; 43 eyes were from group A (20.7%), 7 eyes were from group B (3.4%), and 9 eyes were from group C (4.4%).

In zone 2, the HDFCO was 5037±507 μm, 4771±509 μm, and 5585±773 μm in groups A, B, and C, respectively. There was a statistically significant difference among the three groups (F=8.38,P<0.01). The VDFCO values were 615±297 μm, 643±322 μm, and 504±363 μm in groups A, B, and C, respectively. There was no significant difference among the three groups (F=0.87,P=0.41).

In zone 3, the HDFCO was 5048±683 μm, 4444±540 μm, and 5293±840 μm in groups A, B, and C, respectively. There was no statistically significant difference among the three groups (F=3.13,P=0.05). The VDFCO values were 1385±484 μm, 1225±201 μm, and 1664±318 μm. There was no significant difference among the three groups (F=2.18,P=0.12).

VisualAcuityThe visiual acutiy of HM were LogMAR 4.87±0.22 and 4.77±0.27 in zone 2 and zone 3; these values were not significantly different (t= -1.55,P=0.12). The vision of HM in zone 1 was LogMAR 4.2 in just one eye.

DISCUSSION

The visual function decrease gradually in HM. Most ophthalmologies think it corrected with much factors in clinical practice, include refractive error, optical aberration in early stage and retinal structural damage, choroidal vascular reduction in late stage of HM. But all changes attribute to long AL and posterior staphyloma. Long AL and posterior staphyloma will change the macular fovea and eye position. It cause visual axis light can’t focus on the macular fovea. This is a question worth discussing.

In previous qualitative and quantitative reports on the eye and retina, the morphology of the eye was rarely analyzed and taken into account. In 1977, Curtin used the first binocular indirect ophthalmoscope to classify posterior staphyloma into 10 types, providing a clinical reference for the evaluation of posterior scleral morphologies in HM. In recent years, with the development of imaging technology in ophthalmology, Ohno-Matsui reclassified posterior staphyloma into 5 categories using 3D-MRI images. This resulted in the removal of the latter 5 categories of Curtin’s classifications, namely, the wide, narrow, peripapillary, nasal, and inferior types of posterior staphyloma[8]. In addition, Moriyamaetal[9]used 3D-MRI imaging to classify the posterior segment into barrel, cylindric, nasally distorted, and temporally distorted types, whereas Fledeliusetal[10]quantified the appearance of posterior scleral grape cysts by reconstructing curvature plots with OCT. In this study, a 3D composite image of the posterior sclera was formed using DRI-OCT Atlantis to evaluate the posterior scleral pattern. Types Ⅰ and Ⅱ were classed together into group A because it is difficult to accurately assess. Type Ⅲ was grouped into group B, and Types Ⅶ, Ⅸ, and Ⅹ were classed as group C. Types Ⅳ and Ⅴ were not observed in this study.

To analyze the distribution of the central foveal position, a reference point must be formed. Researchers have used the temporal disc, myopic arc[11-12], and optic disc[13-14]as previous reference points. The myopic arc often changes with age and AL[15]. These results showed that the widest width of the ONH was Type Ⅲ of group B, followed by group C and group A with the narrowest width. From a vertical length of the ONH, groups A, B, and C all gradually increased. This indicates that different posterior morphologies can lead to significant changes in the ONH. Therefore, in this study, the center of the ONH was established as the reference point, which was used to evaluate the position of the central fovea using HDFCO and VDFCO measurements. There were significant differences in groups A, B, and C by comparing HDFCOs. The HDFCO of Type Ⅲ of group B was the shortest, followed by Types Ⅰ and Ⅱ of group A. The longest HDFCO was found in group C. However, there were no significant differences in VDFCO among groups A, B, and C. This shows that the central fovea changes mainly in the horizontal direction in different posterior scleral morphologies, with the fovea closest to the ONH in group B.

It is evident that the central fovea of the 70% sample are normally distributed in zone 2. Approximately 30% of the samples have significant displacement in zone 1 (1.5%) and zone 3 (28.5%). A total of 1.5% of the eyes moved up to zone 1, which were mostly from the group with an AL longer than 29 mm. In zone 2, HDFCO was longest in group C than in other groups, and there was no difference between groups A and B. No difference in VDFCO among three groups, which indicates that the central fovea location mostly changes in the horizontal line and in mixed mode posterior scleral morphologies.

In zone 3,the longest HDFCO was in group C, and the shortest HDFCO was in group B. No difference in VDFCO among three groups, which indicates that the central fovea location mostly changes in the horizontal line and in mixed mode posterior scleral morphologies. The visual outcomes were determined, including refractive status, AL, and aging in HM[16]. With the progression of myopia, myopic maculopathy influences visual outcomes, including fusion of patchy atrophy, CNV, macular atrophy and lacquer cracks[17-18]. From this study, C2, C3, and C4 myopic maculopathy were excluded. The central fovea position was related to the vision of HM of C1 myopic maculopathy. Therefore, this suggests that the visual outcomes are influenced by the above factors, and the position of the fovea is not affected.

In this study, groups B and C had relatively small samples. However,aside from the limitations of this study, the age, AL, and SE of groups A and B were complementary, producing more accurate analysis results. For group C, the same factors were significantly higher than those of groups A and B. Interestingly, this in itself is a feature of the different types of posterior scleral morphologies. In summary, the position of the fovea changes horizontally with different posterior scleral morphologies in different zones in HM, but this change has no effect on vision.