Background
In eastern Asia, myopia has reached epidemic proportions, and the prevalence of myopia has continued to increase[
1‐
3]. High myopia is one of the commonest types of human ocular optical disorders, which can result in complications such as cataract, posterior scleral staphyloma, retinal detachment, choroidal neovascularization, and macular holes[
4‐
8]. The retina can degenerate and become thinner in myopic eyes, especially at the posterior pole[
9], so characterizing the associations between macular thickness of myopia and macular diseases has important clinical implications.
Optical coherence tomography (OCT) is a noninvasive, cross-sectional imaging technique that can measure macular thickness, and is highly reproducible[
10‐
12]. Apart from its diagnostic value in uveitis, OCT has enabled objective assessment of treatment response, provided predictive value for visual recovery and prognosis of uveitic entities, and detected toxic effects of the drug[
13,
14]. The purpose of this study was to evaluate the variations in macular retinal thickness and volume in young Chinese myopic patients by using time-domain optical coherence tomography (Stratus TD-OCT) and spectral-domain optical coherence tomography (Cirrus HD-OCT), to identify the morphological features of the macula in macular diseases. These techniques could evaluate risk factors associated with myopia, which may assist in early diagnosis and monitoring of macular changes.
Material and methods
Subjects
Ninety-two eyes of 92 consecutive refractive surgery candidates with spherical equivalents ≥ -0.5 diopters (D) were recruited for the study. Liu et al. have reported[
15] corneal astigmatism effects on the OCT measurements among high myopes, therefore corneal astigmatism of each subject was less than -1.0 D, especially in the high myopia group. The study has been approved by the Institutional Ethnic Committee (Institutional Review Board) of Shanghai Jiaotong University Affiliated Sixth People’s Hospital, and informed consent was obtained for each patient.
Each subject underwent a full ophthalmic examination, which included measurements of visual acuity, refraction, and intraocular pressure (IOP) by a noncontact tonometer. Axial length measurements were obtained in each eye with the IOL Master (Carl Zeiss Meditec, Inc, Dublin, CA, USA). Optic nerve head evaluation was performed with a 90-D lens. Macular retinal thickness and volume were measured by a Stratus TD-OCT and Cirrus HD-OCT (Carl Zeiss Meditec, Dublin, CA, USA).
Inclusion criteria were as follows: best corrected visual acuity (BCVA) of 20/20 or better, an IOP lower than 21 mm Hg in either eye, a healthy optic nerve head (ONH) without glaucomatous damage (i.e., no disc hemorrhage, notching, or thinning of the neural rim).
Those with a history of severe ocular trauma, intraocular or refractive surgery, or any ocular or neurological disease that could have affected the ONH or retinal nerve fiber layer (RNFL) were excluded from the study. Subjects with evidence of macular disease or peripapillary atrophy extending more than 1.73 mm from the center of the optic disc or with glaucoma or an IOP higher than 21 mmHg in either eye were also excluded. In addition, participants with a history of systemic diseases including hypertension and diabetes were excluded.
OCT imaging
After pupillary dilation to a minimum diameter of 5 mm, eyes of the subjects that satisfied the study criteria were scanned using the Cirrus HD-OCT system with software version 5.0 and Stratus TD-OCT system with software version 4.0. The mean retinal thickness and volume maps were determined for nine sectors in three concentric circles of diameters 1, 3, and 6 mm. The inner and outer rings were divided into four quadrants as follows: outer of temporal (To), outer of superior (So), outer of inferior (Io), outer of nasal (No), inner of temporal (Ti), inner of superior (Si), inner of inferior (Ii), and inner of nasal (Ni). The foveal thickness (F) was defined as the average thickness in the central 1 mm diameter. All the scans had signal strength of at least six and all measurements were taken by a single, trained examiner.
Statistical analysis
Statistical analyses were performed with commercially available software (SPSS ver. 12.0; SPSS Inc, Chicago, IL, USA). The total average and mean macular retinal thickness and macular volume were compared between the two groups with an independent t-test (except for the gender, which was compared by the chi-square test). Agreements between the two OCT systems were measured by Bland–Altman method and the Pearson correlation. A P-value less than 0.05 was considered statistically significant.
Discussion
Myopia is one of the most common causes of visual impairment and blindness, so early detection and management of degenerative eye diseases is of utmost importance. The results of the current study demonstrated that average macular retinal thickness of the fovea did not vary with myopia, while total volume and the retinal thickness of the parafovea were thinner with increased myopia. There was good agreement between the two OCTs in myopic eyes in all macular subfields, and the Cirrus HD-OCT system provided thicker macular retinal thickness measurements than the Stratus TD-OCT system.
Previous studies reported that Cirrus HD-OCT measurements resulted in a significantly thicker retinal thickness than the Stratus TD-OCT[
16‐
18]. We also found that Cirrus HD-OCT measurements resulted in thicker retinal thickness measurements than the Stratus TD-OCT in myopic eyes. Different technologies and segmentation algorithms of the two OCT systems may explain the thicker retinal thickness measurements observed in Cirrus HD-OCT. The most important difference is that the Cirrus segmentation identified the thickness of the retina from the retinal pigment epithelium (RPE) to the internal limiting membrane (ILM), while the Stratus segmentation identified retinal thickness from the inner segment/outer segment junction (IS/OS) to the ILM, thus Cirrus HD-OCT measurements resulted in an inherently thicker distance between the IS/OS and the RPE[
19].
Our study showed that the two OCTs showed macular retinal thickness of the parafovea, especially in outer retinal subfields, that was significantly lower in the high myopia group compared with the low to moderate myopia group, except at the fovea (all P-values less than 0.001, except at the fovea). Other studies demonstrated that the average macular retinal thickness of the parafovea and total macular volume decreased with axial length (all P-values less than 0.001, except at the fovea). So with an increase of axis length, macular thickness becomes thinner, especially in the outer retinal subfields. Marcus et al. and Lim et al. also reported that the parafovea was thinner in myopic patients’ myopia[
20]. This observation may be the result of the anatomical features of the retinal macular foveolar region. The foveolar is the thinnest area of the retina, which consists of the five innermost cell layers of the retina, including the outer plexiform layer, the outer nuclear layer, the external limiting membrane, the photoreceptor layer, and the retinal pigment epithelium. Previous histopathologic studies reported that the thickness of neurosensory retina (RNL), especially the inner nuclear layer, were thinner in several animal models of myopia[
21]. The foveolar lacks the innermost retinal layers, and there is no inner nuclear, so the retinal thickness variation is not obvious between the high myopia group and moderate myopia groups. Therefore, the macular retinal thickness in the parafovea was thinner in the high myopia group than in the moderate myopia group, but no difference was found in the foveolar. Moreover, other pathological processes, such as choroid retinal degeneration, and choroid and RPE atrophy, could likewise play a role in changes of the retina.
The results of the current study characterized macular retinal thickness in young myopic patients. These characteristics can be used as an auxiliary diagnosis for making a distinction with other macular retinal related diseases. If the macular thickness in the foveolar is abnormally thin, pathological disease should be considered. However, if the changes occurred in the parafovea, other factors can first be eliminated, such as refractive factors, which could influence the measurements of macular thickness by the OCTs, especially among high myopia patients. In conclusion, when the macular thickness is abnormal, clinicians should consider the effect of the refractive factor combined with clinical manifestations to comprehensively evaluate its clinical significance.
Open Access
This article is published under license to BioMed Central Ltd. This is an Open Access article is distributed under the terms of the Creative Commons Attribution License (
https://creativecommons.org/licenses/by/2.0
), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (
https://creativecommons.org/publicdomain/zero/1.0/
) applies to the data made available in this article, unless otherwise stated.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
LL participated in the study design, reviewed the literature, collected the clinical data, and drafted the manuscript. JZ provided the conception and design of the study and reviewing the manuscript. L-lJ collected the clinical data and selected the material. J-gY took part in the study design and performed the statistical analysis. S-rC participated in collected the clinical data. All authors have read and approved the manuscript.