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ORIGINAL ARTICLE
Year : 2018  |  Volume : 5  |  Issue : 2  |  Page : 35-40

Factors affecting choroidal thickness in normal myopic eyes in Egyptians using swept-source optical coherence tomography


1 Department of Ophthalmology, Assiut Ophthalmology Hospital, Assiut, Egypt
2 Department of Ophthalmology, Faculty of Medicine, Assiut University, Assiut, Egypt

Date of Web Publication19-Feb-2019

Correspondence Address:
Dr. Khaled Abdelazeem
Department of Ophthalmology, Faculty of Medicine, Assiut University, 71515 Assiut
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/erj.erj_16_18

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  Abstract 


Purpose: To study the influence of age, sex, refractive error, and axial length (AL) on choroidal thickness (CT) in normal myopic eyes, among Egyptian population, using swept-source optical coherence tomography (SS-OCT). Patients and Methods: CT was measured by an SS-OCT in 97 eyes of 49 normal myopic volunteers. The subjects were classified according to age, degree of myopia, and AL. Correlation between CT and age, AL, and myopia was done for different groups. AL was measured using IOL Master. OCT measurements were performed using Topcon DRI-1 SS-OCT. CT was automatically calculated and shown as a colored topographic map with nine subfields defined by the Early Treatment Diabetic Retinopathy Study style grid. Results: CT tends to decrease with advance of age. A negative correlation found between the central subfoveal CT (SFCT) and the age (r = −0.329, P = 0.001, R2 = 0.108). Although there was no significant correlation between CT and degree of myopia (r = 0.159, P = 0.120, R2 = 0.025), CT decreases with increase of myopia except with >−8 D group. CT tends to decrease as the AL increases. A negative correlation found between the central SFCT and AL (r = −0.340, P = 0.001, R2 = 0.115). Conclusions: AL and age have a negative correlation with CT, while the sex and spherical equivalent of refractive error in myopes do not affect CT.

Keywords: Choroidal thickness, myopic eyes, optical coherence tomography, swept source


How to cite this article:
Mokhtar ER, Abdelazeem K, Abdalla A, Fahmy HL. Factors affecting choroidal thickness in normal myopic eyes in Egyptians using swept-source optical coherence tomography. Egypt Retina J 2018;5:35-40

How to cite this URL:
Mokhtar ER, Abdelazeem K, Abdalla A, Fahmy HL. Factors affecting choroidal thickness in normal myopic eyes in Egyptians using swept-source optical coherence tomography. Egypt Retina J [serial online] 2018 [cited 2019 Sep 18];5:35-40. Available from: http://www.egyptretinaj.com/text.asp?2018/5/2/35/252540




  Introduction Top


The choroid, being a major vascular layer of the eye, plays an important role in ocular health. Its abnormalities such as vascular hyperpermeability, vascular changes, loss, and thinning are critical to the onset and progression of many ocular diseases. It is involved in the pathogenesis of many intraocular diseases such as age-related macular degeneration (AMD), myopic chorioretinopathy, central serous chorioretinopathy (CSCR), and polypoidal choroidal vasculopathy (PCV).[1],[2],[3],[4],[5] Accurate measurement of choroidal thickness (CT) in vivo is an essential step in monitoring disease onset and progression that lead to choroidal thinning. Based on histologic study, CT ranges from 170 to 220 μm.[6] Diagnostic techniques such as ultrasonography,[7] magnetic resonance imaging (MRI),[8] and Doppler laser have been used to study the choroid, but they were of limited use due to insufficient resolution. On the other hand, indocyanine green angiography gives clinical information but does not provide cross-sectional images of the choroid for in vivo study.[9],[10]

Optical coherence tomography (OCT) is a noninvasive and noncontact imaging modality that enables two-dimensional cross-sectional and three-dimensional volumetric imaging of tissue architecture.[11] It has evolved over the past decade as one of the most important ancillary tests in ophthalmic practice. It provides high-resolution cross-sectional images of the retina, retinal nerve fiber layer, and optic nerve head. With axial resolution in 5–7 μm range, it provides close to an in vivo “optical biopsy” of the retina.[12] Time-domain OCT could not be used for choroidal imaging as it has poor penetration below the retinal pigment epithelium (RPE) and relatively low resolution.[11],[13] In 2006, spectral-domain OCT (SD-OCT) became commercially available. Spaide et al.[14] introduced a technique to allow choroidal imaging using SD-OCT devices: enhanced depth image OCT, which provides consistent choroidal visualization It allows quantitative thickness measurements of the choroid.[15],[16],[17]

Recently, a new type of OCT instrument, swept-source OCT (SS-OCT), was introduced. The SS-OCT uses a tunable laser (swept-source) as a light source with a longer wavelength (1050–1310 nm) that allows the light to penetrate deeper into tissues than the conventional SD-OCT instruments. This, then, enabled the imaging of the choroid.[18] Copete et al.[19] and Ruiz-Moreno et al.[18] confirmed that reliable and reproducible measurement of CT was possible using an SS-OCT device. The current study was designed to measure CT and to study the influence of age, sex, refractive error, and axial length (AL) on CT in normal myopic eyes, among Egyptian population, using SS-OCT.


  Patients and Methods Top


This prospective, cross-sectional study included 97 eyes of 49 normal Egyptian volunteers between April 2017 and November 2017. The subjects were recruited from the Lasik Assessment Unit of Al Forsan Centre, Assiut, Egypt. It was reviewed and approved by Assiut University Institutional Review Board. All study conduct adhered to the tenets of the Declaration of Helsinki. All subjects provided written informed consent to participate in the study following a discussion about the nature of the study and the risks/benefits of participation.

Cases were divided into groups according to age, AL, spherical equivalent (SE) of refractive error. Subjects with the following criteria were excluded from the study: (1) corneal abnormalities such as ectasia, (2) ocular pathology or previous surgery, (3) glaucoma, (4) choroidal abnormalities, (5) myopic fundus changes, (6) patients with diabetes mellitus, and (7) patients with optical media opacity.

A complete ophthalmic examination for all subjects included measurement of uncorrected visual acuity, corrected distance visual acuity, intraocular pressure using a Goldmann applanation tonometer, anterior-segment examination with a slit-lamp, dilated fundus examination, manifest and cycloplegic refraction using an Autorefractor KR-8900 (Topcon, Tokyo, Japan) after application of cyclopentolate hydrochloride 1%.

AL was measured using IOL Master (Carl Zeiss Meditec, Dublin, CA, USA). CT measurements were performed by a single expert operator using Topcon DRI-1 SS-OCT (Topcon, Tokyo, Japan). After pupillary dilation with 1% tropicamide, a 12, 9-mm radial line scan protocol was used. Each radial line was automatically scanned repeatedly, 32 times in the same position, and then, 12 high-resolution averaging B-scan images were produced. Each scan was reviewed to ensure its centration on the fovea. Only good-quality scans were included. All measurements were performed at the same time of day.

CT was calculated as the perpendicular distance between the outer border of the RPE and the junction between choroid and sclera. It was automatically calculated using the built-in mapping software and shown as a colored topographic map with nine subfields defined by the Early Treatment Diabetic Retinopathy Study (ETDRS) style grid. It is formed of three concentric rings centered at the center of the fovea. The inner ring is 1 mm in diameter, the middle ring is 3 mm in diameter, and the outer ring is 6 mm in diameter. The middle and outer rings were subdivided into four quadrants: superior, inferior, nasal, and temporal quadrants.

A topographic map of CT was then automatically generated. The nine ETDRS subfields are subfoveal CT (SFCT) at the inner ring, nasal inner macula, superior inner macula, temporal inner macula, inferior inner macula, nasal outer macula, superior outer macula, temporal outer macula, and inferior outer macula. The automatically plotted reference lines were inspected for any misalignment and corrected manually, if necessary.

Line measurement of the SFCT was performed manually from the outer border of the RPE and the junction between choroid and sclera [Figure 1]. Three-dimensional macular protocol was performed, at the same session, to measure central macular thickness to exclude any retinal abnormality.
Figure 1: Manual measurement of choroidal thickness at a central subfoveal line

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Data analysis

Statistical Package for the Social Sciences (SPSS 20.0; SPSS, Chicago, IL, USA) was used for data analysis. Analysis of Variance was applied using a general one-way model for comparison between means to evaluate the changes of CT with the parameters tested. Pearson correlation coefficient (r) was used to assess correlation between the SFCT with age, AL, and SE. Statistical significance was defined as P < 0.05.


  Results Top


This study included 97 eyes of 49 normal myopic Egyptian subjects. Males were 33 (34.02%) and females were 64 (65.98%). The mean age was 27.6 ± 6.2 (range, 18–46 years). The mean AL was 24.92 ± 1.11 mm (range, 23–28 mm) and the mean SE was − 3.59 ± 2.12 D (range, 0–10 D). The mean CT in the different subfields is listed in [Table 1]. Mean SFCT was 275.18 ± 74.61 μm for the ETDRS map measurements and 284.24 ± 78.70 μm for the manual SFCT measurements. The CT was greater in the superior and temporal subfields compared with the inferior and nasal subfields in both 3 mm ring and 6 mm ring of ETDRS map explained by [Figure 2].
Table 1: Mean choroidal thickness evaluated by the Early Treatment Diabetic Retinopathy Study map

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Figure 2: Mean choroidal thickness (μm) in nine areas of Early Treatment Diabetic Retinopathy Study map

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Regarding age, a significant difference was found in CT at different measured subfields except at the nasal outer region and inferior outer region as shown in [Table 2]. CT tends to decrease with advance of age as shown in [Figure 3]. A negative correlation found between the central SFCT and the age (r = −0.329, P = 0.001, R2 = 0.108). CT was significantly different, in relation to AL, in different measured areas. CT tends to decrease as the AL increases except at central subfoveal line and superior outer ring [Table 3]. A negative correlation was found between the central SFCT and AL (r = −0.340, P = 0.001, R2 = 0.115) [Figure 4].
Table 2: Choroidal thickness (μm) and age

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Figure 3: Scatter plot of the age (years) and central subfoveal choroidal thickness (μm)

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Table 3: Choroidal thickness (μm) and axial length

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Figure 4: Scatter plot of the axial length (mm) and subfoveal choroidal thickness (μm)

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A significant difference was found in CT, regarding SE of refractive error, at different ETDRS subfields except at the temporal outer ring [Table 4]. The mean CT was decreasing as the SE increases, except for >−8 D group, which showed an increase in CT. Pearson correlation showed nonsignificant correlation between CT and SE (r = 0.159, P = 0.120, R2 = 0.025) [Figure 5]. According to gender, there was no significance difference (P > 0.05) between males and females in the current study [Table 5].
Table 4: Choroidal thickness (μm) and spherical equivalent of refractive error

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Figure 5: Scatter plot of the spherical equivalent of refractive error (D) and subfoveal choroidal thickness (μm)

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Table 5: Choroidal thickness (μm) and gender

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  Discussion Top


The SS-OCT uses a longer wavelength source (1050 nm), which facilitates accurate visualization of the chorioscleral interface. Thus, CT can be measured accurately. The role of the choroid in a number of diseases, including CSCR, high myopia, AMD, choroidal melanoma, and PCV, confirm the importance of understanding the choroidal structure.[20] Until recently, information regarding CT in normal eyes was based primarily on histologic results, which do not necessarily reflect the true measurements of this dynamic tissue.[21] Based on histologic study, CT ranges from 170 to 220 μm.[22]

The mean SFCT was 275.18 ± 74.61 μm for ring measurements while it was greater for line measurements (284.24 ± 78.70 μm). Moussa et al.'s[23] study revealed the same notice and explained that by the difference in the nature of the measurements. The line protocol measures the thickest central point while the ring measures the entire 1-mm ring. Comparison of SFCT in our study and other studies [Table 6] revealed thickness >C275 μm in five studies[23],[24],[25],[26],[27] and <275 μm in three studies.[28],[29],[30] The SFCT in these studies ranged from 261.93 ± 88.42 μm[30] and 345.67 ± 81.8 μm.[25] Regarding normal Egyptian eyes, our results was less than Moussa et al.'s[23] and Gomma's[27] studies. This might be due to difference of refractive error of study subjects; SE in our study was −3.59 ± 2.12 D and −0.9 D in Moussa et al.'s[23] study, while Gomma's[27] study included only emmetropes with range of refractive error between −1 D and +1 D. On the other hand, the AL in the current study was more than Moussa et al.'s[23] study. The mean SFCT in this study was (275.18 ± 74.61 μm) for ring measurements (ETDRS protocol) while it was greater for line measurements (284.24 ± 78.70 μm) and that was comparable with another Egyptian study.[23] The difference between the ring measurement and line measurement could be explained by the difference in the nature of the measurements. The line protocol measures the thickest central point while the ring measures the entire 1-mm ring. Other studies indicated that the choroid was thicker at the fovea than at temporal and nasal locations probably because of high metabolic demand.[28],[32],[33]
Table 6: Comparison between current study and different studies regarding the mean subfoveal choroidal thickness

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Comparing CT in three groups of different AL, there was a significant difference between AL and CT in all ETDRS subfields except at superior outer ring. The central subfoveal line showed no significant difference between AL and CT. Negative correlation between AL and SFCT was found using regression analysis (r = −0.340, P = 0.001, R2 = 0.115). Many studies agree with these results.[19],[23],[26],[28] According to SE, there was a significant difference between SE and CT of different measured subfields except for outer temporal ring. However, unlike many studies,[23],[29] which reported a negative correlation between SE and CT, in our study, there was no significant correlation between SE and SFCT in agreement with Michalewski et al.[34] This can be explained by the lack of hyperopic and small number of high myopic (>−8 D) populations in our study. It was done on people with normal eyes who were seeking for Lasik vision correction; the majority of those subjects are low-to-moderate myopes.


  Conclusion Top


AL and age have negative correlation with CT, while sex and SE do not affect CT. This should be considered during construction of reference database for CT in the future.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]



 

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