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(Investigative Ophthalmology and Visual Science. 2006;47:341-347.)
© 2006 by The Association for Research in Vision and Ophthalmology, Inc.
DOI:  10.1167/iovs.05-0480
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Quantitative In Vivo Retinal Thickness Measurement in Chinese Healthy Subjects with Retinal Thickness Analyzer

Haidong Zou, Xi Zhang, Xun Xu, and Suqin Yu

From the Shanghai First People’s Hospital, Shanghai Jiaotong University, Shanghai, Peoples Republic of China.


   Abstract
Top
 Abstract
Subjects and Methods
 Results
 Discussion
 References
 
PURPOSE. To measure the retinal thickness of the posterior pole in healthy Chinese subjects using a retinal thickness analyzer and to compare the retinal thicknesses between different age, gender, and refractive error groups.

METHODS. Between February 2002 and September 2004, healthy Chinese subjects (age range, 21–50 years), underwent measurements of visual acuity, refractive error, intraocular pressure, and posterior pole retinal thickness with the RTA, as well as ophthalmoscopy and slit lamp examinations. Eleven retinal thickness parameters were directly derived from the reports of the RTA measurement. The average value of each parameter was compared between the male and female groups, the emmetropia and low-myopia groups, and the three age groups (21–30, 31–40, and 41–50 years). Linear regression analysis was also applied to determine the effect of age on 11 parameters. Some participants were also enrolled in the reproducibility assessment procedure.

RESULTS. The 272 eyes of 272 subjects (mean age, 34.4 years) were included. The intraclass correlation coefficients of intravisit and intervisit reproducibility were 0.95 and 0.88, respectively. The average retinal thicknesses at the foveola and fovea are 147.6 and 160.0 µm, respectively. No significant difference was found in any of the parameters between the different gender groups, emmetropia and low-myopia groups, or age groups. Regression analysis revealed no significant negative linear correlation between any of the parameter values and age.

CONCLUSIONS. The retinal thickness of macular region in the Chinese is a little lower than in Westerners, but not statistically significant. The Chinese retinal thickness was not gender or refractive error related.

Assessment of retinal thickness is important for treatment and follow-up of a variety of ocular diseases. Early diagnosis and precise monitoring of these thickness alterations may be possible only if an accurate measurement of the thickness is available. Currently, the clinical methods that are commonly used for estimation of retinal thickness are slit-lamp biomicroscopy, stereophotography, optical coherence tomography (OCT), and retinal thickness analysis. The first two methods are subjective, and neither of them can provide a quantitative measurement of retinal thickness. OCT gives quantitative measurement of the retinal thickness of a randomly shaped path. Few data regarding retinal thickness measurements with OCT in healthy subjects have been published.1

The retinal thickness analyzer (RTA; Talia Technology Ltd., Neve-Ilan, Israel) is another noninvasive method for in vivo quantitative measuring of retinal thickness. The RTA uses a green HeNe laser slit (540 nm) to image retinal cross-sections at an angle in a manner similar to slit lamp biomicroscopy. The reflected image of the intersection of the two retinal boundaries is recorded digitally. The separation between the reflections from the vitreoretinal interface and the chorioretinal interface is the measure of the retinal thickness. The depth (axial) resolution and the lateral (transverse) resolution of the RTA are 52 and 11.7 µm, respectively. The detail principles of RTA have been described elsewhere.2 3 4

The RTA was first introduced to evaluate retinal thickness of the posterior pole in 1989. From then on, some studies have involved measurement of retinal thickness at the posterior pole in living healthy human eyes using this new method.2 3 5 6 7 8 9 10 11 Among them, only four studies have reported measurement of retinal thickness in healthy Asian people. In Japan, to measure the change of the posterior pole retinal thickness in glaucoma, Tanito et al.5 reported the macular and posterior retinal thickness of 31 eyes of 31 healthy subjects. To compare foveal thickness measurements using OCT and RTA, Konno et al.2 reported the retinal thickness measurements in 24 eyes of 12 healthy Japanese subjects. In China, Yang and Du6 11 measured and compared the posterior pole retinal thicknesses in 116 eyes of 77 healthy subjects. Obviously, the number of healthy eyes involved in the two Japanese studies was low, and the investigators did not report the retinal thickness in different age or gender groups. The Chinese studies described that thickness maps of the retina of 6 x 6-mm size at the posterior pole around the macula rendered a U-shaped pattern, extending from the disc to the superior and inferior fovea. They compared the retinal thickness difference in different age or gender groups, but they gave only the mean retinal thickness of the 6 x 6-mm posterior area, which could not be further used to establish an Asian normal retinal thickness data or be compared with other studies, perhaps because of the analysis software used, or because they only meant to introduce the new method to the Chinese ophthalmologists.

In the present study, we report our in vivo measurement of 11 retinal thickness parameters of the posterior pole in 272 eyes of 272 healthy Chinese subjects using RTA and compare the retinal thickness values between different age, gender, and refractive error groups.


   Subjects and Methods
Top
 Abstract
 Subjects and Methods
Results
 Discussion
 References
 
Between February 2002 and September 2004, healthy subjects were enrolled in the study at Shanghai First People’s Hospital. They were recruited from the staff or friends and relatives of the staff, outpatients requiring ametropia correction, and local residents included in an epidemiologic survey for eye fundus diseases. Inclusion criteria were set as: emmetropic refractive error (spherical equivalent between +0.50 and –0.50 D, astigmatism in minus cylinder up to 0.50 D) or low myopia (spherical equivalent up to –5.00 D, astigmatism in minus cylinder up to 1.50 D), age range between 21 and 50 years, clear ocular media or with mild cataract, no abnormal findings in slit lamp and funduscopic examinations, best spectacle corrected visual acuity 6/9 (20/25 or 0.8) or better, no systemic diseases (hypertension, diabetes mellitus), no previous ocular surgery, no ocular morbidity (uveitis, glaucoma, retinitis), and no anisometropia (both eyes are the same). All participants were Chinese. This study was conducted at a site that has no internal review board or ethics committee. However, it did not involve any invasive or irreversible interventions, and informed consent was obtained from all patients in accordance with the tenets of the Declaration of Helsinki.

The size (n) of the group for comparing could be calculated as follows: Based on former work,3 the known mean thickness in the white population at the foveola was 178 µm (µ0), SD was 44 µm ({sigma}), we set {alpha} at 0.05 and ß at 0.1, then µ1 = 178 · 0.90 = 160.2,

Posterior Pole Retinal Thickness Measurements
Because the eyes correlated highly, we did not use both eyes of any participant. For the subject’s convenience, we randomly selected one eye for measurement.

After case collection, all participants underwent measurement of visual acuity, refractive error with an autorefractometer (model 600; Nidek, Gamagori, Japan), intraocular pressure (IOP) by a noncontact tonometer (XPERT; Reichert Jung, Vienna, Austria), and evaluation of the eye diseases by ophthalmoscopy and slit lamp examination.

Before the RTA measurement, we dilated pupils with 0.5% tropicamide and 2.5% phenylephrine hydrochloride eye drops (Santen, Osaka, Japan). Twenty to 30 minutes later, participants underwent retinal thickness measurements at the posterior pole using the RTA (Talia Technologies, Ltd.) performed by one of us (HZ), who had been personally trained in the use of RTA by a professional trainer from the manufacturer.

Just before the RTA measurements, the spherical equivalent refractive error were input by the examiner, and the posterior pole of the fundus was then scanned with the posterior pole thickness mode of the RTA. One scan covers 3 x 3 mm, which consists of 16 optical sagittal cross sections scanned with the green laser (543 nm). Each cross section is 187 µm apart and 3 mm long. In the posterior pole thickness mode, five such scans at the center, superotemporal and inferotemporal, and superonasal and inferonasal areas of the posterior pole, which cover an area of 6 x 6 mm, were performed for each measurement. After the measurements, the images were analyzed using the RTA software, version 4.075. Using this software, the parameters of posterior pole retinal thickness were displayed with a color-coded map. The images with poor quality were excluded from the study, and only high-quality images were used for subsequent analyses. The following images were regarded as poor quality: those with incomplete scanning throughout the 6 x 6-mm area and fuzzy images (i.e., slit lamp images with fuzzy edges).

The 11 parameters used for calculation and comparison were the following: foveola average thickness (VAV), foveola minimum thickness (VMI), foveola maximum thickness (VMA), foveal average thickness (FAV), foveal minimum thickness (FMI), perifoveal average thickness (PFAV), perifoveal minimum thickness (PFMI), perifoveal maximum thickness (PFMA), posterior pole average thickness (PPAV), posterior pole minimum thickness (PPMI), and posterior pole maximum thickness (PPMA). The definitions of these parameters are presented in Table 1 . All values for these parameters were directly derived from the reports of the measurements.


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  		TABLE 1. Parameters Definition

 
Reproducibility Assessments
Between March 2002 and August 2002, participants included in the present study were also enrolled in the reproducibility assessment procedure. Two different reproducibilities were assessed. First, the intravisit reproducibility, evaluating two scans in a single session in each of the subjects. The instrument was realigned after each scan. The intraclass correlation coefficient was calculated using the VAVs of the two scans (two-way random effects model). Second, the intervisit reproducibility, evaluating two scans obtained from two different sessions (2 days apart) in each of the subjects. The intraclass correlation coefficient was calculated using the VAV values of the two scans (two-way random-effects model).

Statistical Analysis
The average value of each parameter was compared between the male and female groups and between the emmetropia and low-myopia groups by the two-tailed Student’s t-test. The average of each parameter was compared among the three age groups (21–30, 31–40, and 41–50 years) using one-way analysis of variance followed by Bonferroni post hoc test. Linear regression analysis was also applied to determine the effect of age on 11 parameters. The intraclass correlation coefficient was considered to be adequate if greater than or equal to 0.70. All the other tests were considered to be statistically significant at P < 0.05.

Data were recorded on a spreadsheet (Excel software; Microsoft, Redmond, WA), and all statistical analyses were performed on computer (SPSS II software; SPSS ver. 10; SPSS Inc., Chicago, IL).


   Results
Top
 Abstract
 Subjects and Methods
 Results
Discussion
 References
 
Subjects
Retinal thickness measurements were performed in 289 eyes of 289 healthy human subjects. After the RTA measurement, 17 (5.9%) of 289 eyes were excluded because of poor image quality, as follows: incomplete scanning in 6 (35.3%) of 17 eyes, and fuzzy images in 11 (64.7%) of 17 eyes. Eventually, 272 (94.1%) of 289 eyes were included in the analyses of posterior pole retinal thickness.

The average age of the 272 participants was 34.4 years (range, 21–50). The male group consisted of 126 participants, with an average age of 33.6 ± 9.1 years (SD), and the female group consisted of 146 participants, with an average age of 35.1 ± 8.3 years. No statistically significant difference in age was found between the two gender groups (t = 1.36, P = 0.18).

We examined 128 right eyes and 144 left eyes. The 133 eyes of the 133 participants were classified in the emmetropia group, with an average age of 34.8 ± 8.4 years, and 139 eyes of the 139 participants were classified in the low-myopia group, with an average age of 34.0 ± 8.9 years, no statistically significant difference in age was found between the two refractive error groups (t = 0.63, P = 0.53).

The subjects in the gender and refractive error groups totaled more than 122, indicating that the size of this study was reliable for comparisons of those groups.

Reproducibility
Twenty-four participants volunteered to take part in the intravisit reproducibility testing. The intraclass correlation coefficient was 0.95. Nineteen participants volunteered to take part in the intervisit reproducibility testing. The intraclass correlation coefficient was 0.88.

Retinal Thickness
The results of the retinal thickness measurements are summarized in Tables 2 3 and 4 . No significant difference was found between men and women on any of the parameters (Table 2) . When we divided the subjects into emmetropia or low-myopia groups, no significant difference in any of the parameters was found between the two groups (Table 3) . No significant difference was found among the three age groups (Table 4) , and regression analysis revealed no significant negative linear correlation between any of the parameters and age (P > 0.05).


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  		TABLE 2. Results of Retinal Thickness Measurements in Different Gender Groups

 

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  		TABLE 3. Results of Retinal Thickness Measurements in Different Refractive Error Groups

 

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  		TABLE 4. Results of Retinal Thickness Measurements in Different Age Groups

 

   Discussion
Top
 Abstract
 Subjects and Methods
 Results
 Discussion
References
 
Both the RTA and the OCT have been shown to be sensitive tools for detecting macular thickening. As described by Neubauer et al.,7 both the OCT and RTA can detect small increases of 20 to 40 µm, and differentiate the normal eyes and eyes with macular edema. Several studies have described the scanning methods of OCT and RTA,3 4 12 13 14 and have given comparison of them elsewhere.4 The RTA software we used gave the minimum or maximum retinal thickness in the foveola, foveal, perifoveal or posterior pole regions, which are necessary for determining the overall change of the central retina, or diagnosing glaucomatous change, and these values could not be obtained by the OCT scanner.

There are a few published study aiming for measuring retinal thickness in healthy subjects using RTA.3 9 10 The largest sample size in healthy volunteers was found in a study by Landau et al.,3 who reported the average retinal thickness at the foveola to be 178 µm in 50 healthy white subjects. In other studies designed for assessment of diabetic macular edema or comparing the retinal thickness determined by RTA and OCT, which were conducted in the United States, Germany, and Canada, the mean foveal thickness reported was from 165.5 to 181 µm.7 8 15

However, the eyes of Asian people may differ from the eyes of humans of other races.16 17 18 19 20 21 The retinal thickness in Asian people should be assessed only a study sample made up only of Asians. To our knowledge, the present study is the first to report retinal thickness data determined by RTA in healthy Asian people. Moreover, the number of healthy subjects involved in the present study is more than any earlier studies in Asian people, which facilitated for subgroup comparison.

Because the intraclass correlation coefficients of the intravisit and intervisit reproducibility assessment were both higher than 0.70, the RTA measurement in the present study was considered to be reproducible.

A major problem with the RTA measurement seems to arise from fuzzy images, as described before.7 8 Those scans cause errors in the calculation of retinal thickness and yield too high values. To give the measurement precisely, we excluded the eyes with fuzzy images.

In the present study, the average thicknesses at the foveola and foveal in Chinese subjects were 147.6 ± 26.3 µm (95% confidence interval [CI], 144.5–150.7 µm) and 160.0 ± 23.0 µm (95% CI, 157.2–162.7 µm), both lower than the values in the studies of the Westerners.3 7 8 Since Landau et al.3 used a prototype of the RTA and Neubauer et al.7 used RTA software version 3 in their study, the different versions of software used for the analysis and calibration cannot be omitted as the reason for the different results. In the study of Guan et al.,8 who used the same RTA software version 4.075 as was used in the present study, the average foveal retinal thickness in the normal Canadians was 165.5 ± 10.6 µm. When equal variances are assumed, the t value calculated using the average value and SD in this study and the study of Guan et al. is 0.98 (independent-samples t-test; P > 0.05), suggesting that the foveal retina in Asian subjects is slightly thinner than in Westerners, but the difference is not statistically significant. However, as the normal subjects included in the study of Guan et al. was only 17, further study including more normal Western subjects (maybe equal to or more than the size in the present study) would help to support this conclusion.

Tanito et al.5 measured the retinal thickness in 31 healthy Japanese subjects using the same RTA software version as was used in the present study, and the VAV, VMI, PFAV, PFMI, PPAV and PPMI results reported by them were 7 to 21 µm higher than the corresponding results in the present study.5 The significant disparity in study size may be the cause of the difference. Because of the different versions of software used, the values in the present study cannot be compared with two other Asian studies (Table 5) .


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  		TABLE 5. Results of Retinal Thickness Measurements in Different Studies of Healthy Asian Populations Using RTA

 
In myopic eyes, the axial length of the globe exceeds normal dimensions, and the sclera becomes thinner, especially at the posterior pole. Therefore, the retina at the posterior pole may be stretched in myopic eyes. One hypothesis of the present study was that the myopic eyes would have thinner retinas than would the emmetropic eyes. Because the optical power of the eye being examined may cause a slight difference in the calculated magnification power in the equation for retinal thickness, we chose the myopia subjects as spherical equivalent up to –5.00 D, astigmatism in minus cylinder up to 1.50 D. To our surprise, no parameter of the retinal thickness assessed in the low-myopia group was found to be significantly lower than the emmetropia group. Wakitani et al.22 evaluated the retinal thickness of 203 healthy Japanese subjects with OCT, they classified the subjects into emmetropia, low myopia, medium myopia, and high-myopia groups according to the axial length of the eye and found no statistically significant difference between these groups on the average retinal thickness in three circular areas surrounding the central fovea (350, 1850, and 2850 µm in diameter). They speculated that the retinal thickness in healthy myopic eyes is thinner in the periphery than that in emmetropic eyes, but not in the central portion. Decreased thickness in the peripheral retina may compensate for the stretching force seen over the whole retina and may preserve the thickness in the central retina. Our results helped to prove the speculation of Wakitani et al., but two factors may influence the reliability of this proof. First, eyes with spherical equivalent more than –5.00 D were excluded in the present study. Second, the classification of the myopia group was different in the two studies. Further studies including high myopia, and classifying the myopia group combined with axial length and refractive error of the eye may help to prove the hypothesis.

No significant correlation was found between gender and retinal thickness in the present study, which is the same as some other studies involving the use of RTA.6 Of note, this conclusion differs from other studies of OCT, which found that male gender is associated with a significantly thicker central retina.22 23 One possible explanation is that the retinal regions measured in the studies using RTA or OCT are different.

Landau et al.3 reported that the foveolar thickness increases with age. Yang and Du6 did not found any relationship between age and retinal thickness. With OCT examination, the controversy also exists: Almounti and Funk24 found a significant decrease in retinal thickness with increasing age (0.53 µm per year), and they suggested that retinal nerve fiber bundles loss with age leads to the decrease. Kanai et al.25 also demonstrated that macular thickness decreases with aging. But some studies did not find any correlation between retinal thickness and age.22 26 In the present study, no significant correlation was founding linking any of the parameters to age. However, some limitation of this assessment should be mentioned: First, the subject age groups may not be of sufficient size, as the size of each group was lower than 122; second, we doubt that the age difference, which causes differences in tear film quality and media clarity between the age groups, may affect the quality of the scans, especially the foveal images, resulting in this paradox. More subjects in different age groups and further work focused on the influence of tear film quality and media clarity to the scan may help to validate the relationship between retinal thickness and age.

In conclusion, we measured the retinal thickness in healthy Chinese subjects with RTA, and our results show that the retinal thickness of macular region in the Chinese was slightly lower than in the Westerners, but not to a statistically significant degree. No discrepancy in the thickness between different genders or emmetropia and low myopia was found. The relationship between retinal thickness and age is still in doubt. The retinal thickness values given in the present study may significantly contribute to early, accurate diagnosis and better monitoring of treatment of clinical macular diseases in Chinese or Asian people. This study also helps to determine differences in retinal thickness between the different races and between the sexes within each racial group.


   Footnotes
 
Supported by Shanghai First People’s Hospital Grant 02B15.

Submitted for publication April 19, 2005; revised August 18 and September 29, 2005; accepted November 22, 2005.

Disclosure: H. Zou, None; X. Zhang, None; X. Xu, None; S. Yu, None

The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Corresponding author: Haidong Zou, Department of Ophthalmology, Shanghai First People’s Hospital, No. 85 Wujin Road, Shanghai, P.R. China 200080; zouhaidong8{at}yahoo.com.cn.


   References
Top
 Abstract
 Subjects and Methods
 Results
 Discussion
 References
 

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