The corneal epithelium plays a very important role in protecting eyes as it is the outermost layer and in maintaining high optical quality [
Since the corneal epithelium contributes a lot in corneal refraction and it helps in the design of the above surgeries, it is very important to get a better knowledge of the characteristics of corneal epithelial thickness distribution. Previously, a few instruments have been invented and applied to corneal epithelium thickness measurement in vivo, including very high-frequency (VHF) digital ultrasound and confocal microscopy. A few studies on corneal epithelial thickness mapping have been done using very high-frequency (VHF) digital ultrasound and confocal microscopy [
Furthermore, little knowledge in distinctions of epithelial thickness among different myopia degrees is known. Therefore, with the support of a large sample size, this study aims to investigate the distinction of corneal epithelial thickness in different myopic degrees. The description of corneal epithelial thickness distribution in more detailed parts and correlation between corneal epithelial thickness and various corneal parameters, such as age, corneal thickness, IOP, astigmatism, and corneal front curvature were also analyzed.
Two hundred and fifteen eyes from 215 healthy subjects (102 women, 113 men) with a mean age of 21.26 ± 4.35 years(18 to 40 years) and mean manifest refraction spherical equivalent (MRSE) of −5.34 ± 2.19 D (ranging from −1.125 D to −12.00 D) participated in this study. Subjects reached a complete ophthalmologic evaluation, including the intraocular pressure (IOP) measurement, best-corrected distance visual acuity (BCVA), slit lamp and ophthalmoscope examination, corneal topography (Pentacam HR, OCULUS GmbH, Wetzlar, Germany), Schirmer I test, and tear break-up time test. Every subject had best-corrected distance visual acuity of 20/25 or better. All measurements were taken without the application of artificial tears or mydriatic eye drops. And the exclusion criteria included suspicious and frank keratoconus, a history of contact lens wear, current or prior ocular pathology, and dry eye disorder. All subjects were informed of the aim of the study, and their consent was obtained at the time of their first clinical visit. This prospective study was performed at the Refractive Surgery Center at the Tianjin Ophthalmology Hospital, Nankai University, and received the approval of the Ethics Committee of our Institution, in accord with the Declaration of Helsinki.
An ultrahigh resolution SD-OCT (RTVue-100, Optovue Inc., Fremont, CA) was used in this study. The system worked at 830 nm wavelength and had a scan speed of 26,000 axial scans per second. The setting’s axial resolution was 5
The RTVue-100 corneal epithelial thickness mapping and pachymetry software (software version 6.11.0.12) automatically processed the OCT scan to provide the corneal epithelial thickness and pachymetry (corneal thickness) maps, corresponding to a 6 mm diameter area. A well-trained investigator conducted all the measurements, and three repeated measurements were collected and averaged in each case.
The analyzing area was two 6 mm diameter disks of corneal thickness and corneal epithelial thickness maps. Each map was divided into 3 zones by diameter: central 2 mm, inner ring from 2 to 5 mm, and outer ring from 5 to 6 mm, according to the set of the analyzing system (Figure
Details of the mapping of corneal thickness and corneal epithelial thickness over the 6 mm diameter cornea from the analyzing report in the set. The analyzing area is divided into three main parts (center, Ring1, and Ring2) and 17 sectors. In Ring1, the sectors were named, respectively, anticlockwise for OD as R1a, R1b, R1c, R1d, R1e, R1f, R1g, and R1h. Similarly, the sectors from Ring2 for OD were named from R2a to R2h. The naming all started from superior to temporal, then inferior to nasal. The left eye map was mirrored.
Distinction of corneal epithelial thickness between the right and left eyes.
Mean difference ( |
SEM | Sig. | ||
---|---|---|---|---|
Right − left (R-L) | Center | −0.34237 | 0.20142 | 0.175 |
Ring1 | −0.35241 | 0.21179 | 0.068 | |
Ring2 | −0.34456 | 0.21583 | 0.093 | |
Avg. | −0.35361 | 0.21945 | 0.113 |
A set of groups were formed considering the average MRSE of the study population. Group Myopia-L consisted of a low-myopia population, defined as MRSE less than or equal to −3.00 D (
Anterior segment was imaged with Pentacam (OCULUS GmbH, Wetzlar, Germany). In each acquisition, the rotating Scheimpflug camera captured 50 images automatically and measures 25,000 true elevation points. Due to the good repeatability of this device [
Statistical Products and Services Solution (SPSS version 20.0, Chicago, Illinois, USA) were used for the statistical analysis. Normal distribution of data was assessed using the Kolmogorov-Smirnov test.
Analysis of variance (ANOVA) was used to compare epithelial thickness in each sector of the 6 mm diameter of cornea and the differences of corneal epithelial thickness in different MRSE groups. The Student’s independent-samples
Two hundred and fifteen eyes from 215 subjects were assigned to calculate myopic corneal epithelial thickness and corneal thickness of 17 sectors (Table
The corneal epithelial thickness and corneal thickness in different locations.
a | b | c | d | e | f | g | h | Avg. | |||
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Epithelial thickness ( |
Center | Avg. | 53.26 | ||||||||
SD | 2.66 | ||||||||||
Ring1 | Avg. | 52.21 | 52.74 | 53.43 | 53.59 | 54.08 | 54.12 | 53.51 | 52.73 | 53.30 | |
SD | 2.62 | 2.66 | 2.58 | 2.56 | 2.57 | 2.56 | 2.60 | 2.64 | 2.48 | ||
Ring2 | Avg. | 51.08 | 52.29 | 53.38 | 53.51 | 54.00 | 54.16 | 53.53 | 52.40 | 53.04 | |
SD | 2.68 | 2.70 | 2.52 | 2.54 | 2.64 | 2.54 | 2.53 | 2.68 | 2.38 | ||
|
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Total thickness ( |
Center | Avg. | 534.24 | ||||||||
SD | 29.89 | ||||||||||
Ring1 | Avg. | 566.99 | 556.14 | 542.89 | 539.83 | 544.30 | 550.44 | 557.72 | 566.80 | 553.14 | |
SD | 31.88 | 31.55 | 30.94 | 30.79 | 30.39 | 30.10 | 30.64 | 31.33 | 30.56 | ||
Ring2 | Avg. | 602.28 | 585.08 | 564.26 | 562.04 | 568.37 | 573.62 | 584.28 | 597.17 | 579.64 | |
SD | 33.82 | 33.06 | 32.13 | 31.69 | 31.39 | 31.12 | 31.88 | 32.34 | 31.31 |
Box plots to show the thickness differences of three locations (center, Ring1, and Ring2) in the corneal epithelial thickness map (a) and corneal thickness map (b). Corneal thickness increased from the center to the periphery (b) while corneal epithelial thickness remained constant (a).
Significant differences in each sector of corneal epithelial thickness value and corneal thickness value were found (Figures
The detailed corneal epithelial thickness (a) and corneal thickness (b) of different sectors in Ring1 and Ring2.
Figure
The distribution of corneal epithelial thickness in each sector using color gradations with average thickness on it.
Table
Correlations between corneal epithelial thickness and some parameters.
Location | Age | CT | Km | Axis-C | Axis-T | IOP | ||||||
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|
|
|
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|
|
|
|
|
Center | −0.11 | 0.045 | 0.157 | 0.021 | 0.065 | 0.340 | −0.004 | 0.953 | 0.033 | 0.628 | 0.023 | 0.741 |
Ring1 | −0.14 | 0.038 | 0.148 | 0.030 | 0.091 | 0.185 | −0.061 | 0.373 | 0.051 | 0.454 | −0.005 | 0.941 |
Ring2 | −0.11 | 0.058 | 0.140 | 0.040 | 0.099 | 0.148 | −0.087 | 0.201 | 0.041 | 0.553 | −0.037 | 0.585 |
Avg. | −0.13 | 0.042 | 0.148 | 0.031 | 0.088 | 0.201 | −0.051 | 0.456 | 0.043 | 0.527 | −0.006 | 0.934 |
CT: corneal thickness; Km: mean front corneal surface curvature; Axis-C: cornea front astigmatism axis (flat); Axis-T: astigmatic axis; IOP: intraocular pressure.
As shown in Figure
For three locations (center, Ring1, and Ring2), differences of corneal epithelial thickness among different MRSE groups which were divided according to manifest refraction (group myopia-L for less than or equal to 3.00 D, group myopia-M for 3.00 D to 6.00 D, group myopia-H for more than 6.00 D).
The differences of corneal epithelial thickness between the right and left eyes were calculated and described in Table
As shown in Figure
Difference of corneal epithelial thickness between male and female in three locations.
In Table
A good knowledge of the corneal epithelium distribution may help a lot in many aspects of clinical work, such as screening for keratoconus before corneal refractive surgery [
The distribution of both corneal thickness and corneal epithelial thickness follow a nonuniform pattern (Table
The thinnest part of corneal thickness is R1d and R2d, namely, temporal-inferior part. The thickest part is R1a and R1h for Ring1 and R2a and R2h for Ring2, namely, nasal-superior part. The result is in agreement with previously reported values in the use of other evaluation tools [
However, the distribution of corneal epithelial thickness is quite different from that of corneal thickness. On the map of corneal epithelial thickness, the thinnest part is R1a for Ring1 and R2a for Ring2. The thickest part is R1e and R1f for Ring1 and R2e and R2f for Ring2. In another word, the thinnest part is the superior and the thickest part is the nasal-inferior. Reinstein et al. [
Concerning the nasal-inferior part to be the thickest part of corneal epithelium over the entire corneal area, one possible explanation of the asymmetry is the eye abrasion caused by the eyelid. Doane [
It is a limitation here that the tear film was included in the measurement due to the restriction of the machine. Previous study [
The corneal thickness increases gradually from the center to the periphery. However, there is no significant difference among the center, Ring1, and Ring2 in corneal epithelial thickness map in this study. It means that the corneal epithelial thickness remains constant on average from the center to the periphery over the 6 mm diameter area. Tao et al. [
The low to moderate myopia groups (group Myopia-L and Myopia-M) were statistically thicker than group Myopia-H. According to this, we could deduce that people with high myopia tend to have thinner corneal epithelium than others do. In a clinical study done by Gowrisankaran et al. [
Male subjects have thicker corneal epithelial thickness than female subjects do in all three locations (center, Ring1, and Ring2, M-F = 1.39
The correlation between corneal epithelial thickness and age is also negative in this study. Kanellopoulos and Asimellis [
Since many young patients suffered from myopia, the information provided by this study may to some degree help researchers or others who are interested in corneal epithelial mapping to get more information and develop further research.
Due to the measuring limitation of the SD-OCT, the axial resolution of the system is 5 microns. Because the subjects were healthy except for myopia, their corneal epithelial thicknesses were in the normal range (45–60 microns, 53.26 on average). Therefore, there would not be too much difference numerously among them. Some of the differences observed were lower than 5 microns. Some previous studies [
To sum up, the profile of the corneal epithelial thickness in myopic eyes was described in this study and confirmed to be nonuniform over the entire cornea. People with high myopia tend to have thinner corneal epithelium than low–moderate myopic patients do. Many factors can be related to the corneal epithelial thickness, such as age, gender, and corneal thickness. Further investigation of the correlation with corneal epithelial thickness might also be needed to expose a specific role for corneal epithelium, such as corneal biomechanics and corneal wounding healing after corneal surgery.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This study was supported by research grants from the National and Science Program Grant (no. 81670884), China.