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(Investigative Ophthalmology and Visual Science. 2006;47:3276-3285.)
© 2006 by The Association for Research in Vision and Ophthalmology, Inc.
DOI:  10.1167/iovs.06-0072
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Distribution of Optic Disc Parameters Measured by OCT: Findings from a Population-Based Study of 6-Year-Old Australian Children

Son C. Huynh,1 Xiu Ying Wang,1,2 Elena Rochtchina,1 Jonathan G. Crowston,3 and Paul Mitchell1,2

1From the Centre for Vision Research, Department of Ophthalmology and the Westmead Millennium Institute, University of Sydney, Sydney, Australia; the 2Vision Co-operative Research Centre, School of Optometry, University of New South Wales, Sydney, Australia; and the 3Hamilton Glaucoma Center, University of California-San Diego, La Jolla, California.


   Abstract
Top
 Abstract
Methods
 Results
 Discussion
 References
 
PURPOSE. To study the distribution of optic disc, cup, and neural rim size by ocular and demographic variables in a population-based sample of 6-year-old children.

METHODS. The Sydney Childhood Eye Study examined 1765 of 2238 eligible 6-year-old children (78.9%) from 34 randomly selected Sydney schools during 2003 to 2004. Comprehensive standardized eye examination included cycloplegic autorefraction, optical biometry and "fast optic disc" scans performed using optical coherence tomography.

RESULTS. Scans of adequate quality were available for 1309 children (75% of participants), with 70% aged 6 years; 50.9% were boys. Mean (± SD) horizontal and vertical disc diameter and disc area was 1.53 ± 0.21 mm, 1.79 ± 0.28 mm, and 2.20 ± 0.39 mm2, respectively. Corresponding cup dimensions were 0.70 ± 0.28 mm, 0.73 ± 0.27 mm, and 0.48 ± 0.32 mm2. A definable optic cup was absent in 7.4%, 87% of whom were European white. Cup-to-disc diameter ratios were 0.46 ± 0.16 horizontally and 0.42 ± 0.15 vertically, whereas cup-to-disc area ratio was 0.22 ± 0.13. Mean ± SD neural rim area was 1.76 ± 0.44 mm2 and increased with disc size (Pearson correlation = 0.68, P < 0.0001). Horizontal and vertical average nerve widths were 0.36 ± 0.05 and 0.28 ± 0.05 mm, respectively. In analyses adjusting for potential confounders, disc area increased significantly with axial length (Ptrend < 0.0001) and refraction (Ptrend = 0.02). Rim area increased only with axial length (Ptrend = 0.01). There were no gender differences, except for average nerve width, marginally greater in boys. Most disc and cup dimensions were significantly larger in East-Asian than European white and Middle Eastern children.

CONCLUSIONS. Disc, cup, and neural rim parameters were generally normally distributed in this young population. Axial length appeared to be a stronger determinant of disc and rim size than refraction. Some ethnic but not gender differences were demonstrated for most parameters.

Although qualitative assessment of the optic nerve head is important in diagnosis of optic neuropathy, a knowledge of how optic disc parameters vary with disc size and other ocular and demographic parameters in the general population is valuable in differentiating healthy from diseased nerves and for identifying optic discs at risk of disease. This is particularly important in children because the associated vision loss can adversely influence their overall development.1

Although normative data on macular thickness in a large sample of 6-year-old children were recently reported,2 childhood normative data on optic disc parameters are limited3 and to our knowledge, there have been no population-based reports. Further, optic nerve head differences between white and East Asian children have not previously been examined, and few studies have controlled for the influence of disc size on measures of neural content.4 5

Optical coherence tomography (OCT) is a promising new technology that allows rapid and reproducible measurement of the retina and optic nerve head.6 OCT has already been incorporated into the management of conditions such as glaucoma, macular edema caused by diabetic retinopathy or uveitis, and other macular diseases, including age-related macular degeneration and central serous chorioretinopathy.7 8 The potential usefulness of this instrument in juvenile glaucoma was recently demonstrated.9

In the present study, we sought to establish a normative database of optic disc, cup, and neural rim parameters and to examine their variation with refraction, axial length, gender, and ethnicity, in a population-based sample of young (predominantly 6-year-old) children.


   Methods
Top
 Abstract
 Methods
Results
 Discussion
 References
 
Study Population
The Sydney Childhood Eye Study comprises population-based surveys designed to examine childhood eye conditions across a range of ages. This article describes data from the Sydney Myopia Study component, which examined school children resident in the metropolitan area of Sydney, Australia, from 2003 to 2004. The study was approved by the Human Research Ethics Committee, University of Sydney and the Department of Education and Training, New South Wales, Australia. It was conducted in accordance with the tenets of the Declaration of Helsinki. We obtained informed written consent from at least one parent of each child, as well as verbal assent from each child. The study protocol adhered to the guidelines set forth in the Declaration of Helsinki.

Detailed study methods have been described elsewhere.10 11 In brief, 34 primary schools in Sydney were identified through random stratified sampling. Stratification of the city was based on socioeconomic status data from the Australian Bureau of Statistics 2001 national census. A proportional mix of public and private or religious schools was included. All children in the first grade of school (mostly aged 6 years) were eligible.

Demographic Data
Demographic data were obtained from a comprehensive questionnaire sent to the parents of the children. The child’s ethnicity was determined from the ethnicity and country of birth of both parents. Ethnic groups represented were white (European), East Asian, Indian, Pakistani, Sri Lankan, African, Melanesian-Polynesian, Middle Eastern, indigenous Australian, South American, and mixed.

Ocular Examination
Axial length was measured before cycloplegia with an optical biometer (IOLMaster; Carl Zeiss Meditec, Inc., Jena, Germany) that used dual-beam partial coherence interferometry (PCI).12 In this instrument, low coherence laser light (wavelength 780 nm) emitted by a superluminescent diode is passed through a Michelson interferometer where it is split into two beams, a reference beam and a second beam directed into the eye. The echo time delay between the reference beam and the second beam, which reflected back from the retinal pigment epithelium (RPE), was used to calculate axial length. The average of five such measurements was used in the analysis.

After instillation of amethocaine 1% (1 drop) to anesthetize the cornea, cycloplegia was induced by instilling cyclopentolate 1% and tropicamide 1% (2 drops each) separated by 5 minutes. Phenylephrine 2.5% was also instilled in a small proportion of children to achieve adequate mydriasis (≥6 mm). Autorefraction (RK-F1 autorefractor; Canon, Tokyo, Japan) was performed 25 to 30 minutes after the last drop. The median of five refractions automatically performed by the instrument was used for analyses. Mydriatic retinal photography was also performed to detect any retinal conditions.

OCT Measurements
Optic disc parameters were measured through dilated pupils using an optical coherence tomographer (StratusOCT, software v.4.0.4; Carl Zeiss Meditec, Inc., Dublin, CA), which used PCI (wavelength 820 nm) to obtain high-resolution (<10 µm) cross-sectional images of the optic disc.13 Measurements were performed using the fast optic disc scanning protocol, which acquired the full scan in 1.92 seconds. Each scan consisted of six 4-mm line scans radially arranged and centered on the optic disc. Each line scan was sampled at 128 points (A-scans), giving a total of 768 A-scans for the whole optic nerve head. Three fast optic disc scans were performed successively without making changes to scan placement, and the measurements were averaged before analysis. Peripapillary retinal nerve fiber layer (RNFL) average thickness was also measured, to examine its relationship with optic disc size. The fast RNFL thickness (3.4) scanning protocol was used, which consists of 256 A-scans along a circular scan path, with a radius of 1.73 mm. The average of three scans was used in the analyses. More than 90% of scans were performed by a single experienced operator. An internal fixation target was used in all scans, with scan placement continuously monitored using an infrared-sensitive video camera (Fig. 1A) . Scans were only accepted if they were complete, free of artifacts, and had signal strengths of at least 5.


Figure 1
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  		FIGURE 1. (A) Infrared-sensitive video view of the right optic disc and scan pattern. The scan pattern consists of 4-mm long intersecting scan lines that are approximately centered on the optic disc. (B) Topographic map showing disc reference points (red circles), the disc margin (red contour), cup margin (green contour), and scan lines (blue and yellow). (C) Profile of the optic nerve head along the vertical meridian showing optic disc reference points at the termination of the retinal pigment epithelium (blue circles), disc line (blue line), cup reference plane (red dotted line) 150 µm anterior to the disc line, nerve width (yellow lines), and rim area (vertical cross-section; red-shaded area).

 
The optic disc margin was automatically defined by the instrument as the termination of the RPE at the optic nerve head (Fig. 1C) . Opposite points across the optic disc were joined by a disc line. A constant reference plane 150 µm anterior to the disc line was used to define the edges of the optic cup. Variables examined included horizontal and vertical cup and disc diameters, cup and disc areas, and cup-to-disc diameter, and area ratios. We also calculated in each child a shape factor, which is the ratio of vertical-to-horizontal disc and cup diameters. The nerve width (nerve fiber layer thickness at the disc) was the shortest distance from the disc margin (termination of RPE) to the internal limiting membrane. The average nerve width was calculated by the instrument as the average of two nerve widths on opposite sides of the disc. Measures of neural rim area included rim area and horizontal integrated rim width. Rim area was defined as the difference between disc and cup areas. Horizontal integrated rim width was a product of the disc circumference and average nerve width. Rim area (vertical cross-section) was the area bounded by the disc line, a line perpendicular to the disc line, and the nerve fiber layer surface. Vertical integrated rim area measured nerve fiber layer volume and was obtained by multiplying the rim area (vertical cross-section) by the disc circumference.

Correction for Magnification
Scans were performed without entering axial length and refraction data, for consistency with usual clinical practice. Transverse disc measurements, however, were affected by magnification when the axial length and refraction of the eye being scanned were different from the default values of 24.46 mm and 0.0 D, respectively (personal communication, Alan Kirschbaum, Carl Zeiss Meditec, Inc., 2006). The magnification of this instrument is given by:

Formula
where h0 and h are uncorrected and corrected transverse lengths, respectively. The same correction was used for areas with one axial dimension. For transverse areas and for volumes, the denominator was squared. Daxial and Drefraction are changes in magnification due to differences from default values of axial length (L) and refraction (Derror), respectively, where Daxial = (24.46 – L)/0.42 and Drefraction = (Derror – Daxial).

Statistical Analysis
Analyses were performed for right eyes on computer (Statistical Analysis System software, ver .9.1; SAS Institute, Cary, NC). We used the Kolmogorov-Smirnov test to check for normality of distributions since the sample size was large. Differences between children with and without OCT performed were examined using {chi}2 tests for categorical variables, and two-sample Student’s t-test for continuous variables. Effects of axial length and refraction on measures were examined in an analysis of trend, whereas gender and ethnic differences were examined using mixed models and generalized estimating equations, with adjustment for multiple variables.


   Results
Top
 Abstract
 Methods
 Results
Discussion
 References
 
Population Characteristics
There were 2238 eligible children, of whom 1765 (78.9%) consented to the study. Twenty-five children were absent from school during the examination period, and 431 had scans with low signal strength, leaving data available for 1309 children (75% of those examined). These children were predominantly European white (n = 866, 66.2%), East Asian (n = 197, 15.1%), and Middle Eastern (n = 53, 4.1%). The remaining 193 (14.7%) children were from six other ethnic groups whose numbers were too small for their data to be meaningful. There was a slightly higher ratio of European white to East Asian and Middle Eastern children among those who had scan data available. There were no significant differences between the two OCT groups for the categorical variables of age and gender, nor for the continuous variables of spherical equivalent, axial length, logarithm of the minimum angle of resolution acuity, height, weight, and body mass index (Table 1) .


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  		TABLE 1. Characteristics of Children with and without OCT Scans of the Optic Disc (24.8%)

 
Distribution
The overall distributions of optic disc, optic cup, and neural rim parameters are shown in Figure 2 and Table 2 . Horizontal and vertical disc diameter and disc area were normally distributed, with mean ± standard deviations of 1.53 ± 0.21 mm, 1.79 ± 0.28 mm, and 2.20 ± 0.39 mm2, respectively. Disc size varied by approximately fourfold between the largest and the smallest discs. The mean disc shape was vertically oval with a shape factor of 1.2 ± 0.2 (range, 0.4–2.8). In 17.0% of the children, the optic disc was horizontally oval (shape factor, <1.0).


Figure 2
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  		FIGURE 2. Frequency distribution of magnification-corrected (A) optic disc horizontal and vertical diameters; (B) optic cup horizontal and vertical diameters; and (C) optic disc, cup, and rim areas.

 

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  		TABLE 2. Overall Distribution of Optic Nerve Head Parameters in Right Eyes

 
The optic cup could not be defined in 97 children (7.4% of those with scans). Among these, 87% were European white, 2% were East Asian, and 4% were Middle Eastern in background. They were slightly older (by 6 weeks, P = 0.01) than the children with definable optic cups. There were no significant differences, however, in gender distribution, spherical equivalent, and axial length. When these children were included in the analysis, mean ± SD horizontal and vertical cup diameter and cup area were 0.65 ± 0.32 mm, 0.68 ± 0.32 mm, and 0.44 ± 0.33 mm2, respectively. Optic cup diameter (horizontal and vertical) and area with these children excluded are presented in Table 2 . Cup diameter and area varied 30- to 300-fold, respectively, between the smallest and largest cup, whereas cup volume varied 1700-fold. Horizontal and vertical cup diameters were normally distributed in these children. The distribution of cup area and volume was slightly positively skewed. Mean cup shape was almost circular with a shape factor of 1.1 ± 0.2 (range, 0.3–3.6). In 35.6% of children, the optic cup was horizontally oval (shape factor, <1.0).

Cup-to-disc diameter and area ratios were normally distributed. Horizontal (0.46 ± 0.16) and vertical (0.42 ± 0.15) cup-to-disc diameter ratios were similar to each other. Cup-to-disc diameter ratio varied 30-fold with a maximum ratio of 0.90 horizontally.

Neural rim area was normally distributed, with similar results between rim area (1.76 ± 0.44 mm2) and horizontal integrated rim width (1.71 ± 0.30 mm2). There were significant correlations (r) of optic disc area with rim (r = 0.68, P < 0.0001) and cup area (r = 0.39, P < 0.0001). Cup and rim areas for specific optic disc sizes are shown in Figure 3 . For the smallest mean optic disc area (1.4 mm2), mean cup area was 0.25 mm2 (95% confidence interval [CI] 0.17–0.33 mm2), and rim area was 1.19 mm2 (95% CI, 1.10–1.27). For the largest mean optic disc area (3.4 mm2), cup area was 0.89 mm2 (95% CI, 0.30–1.48), and rim area was 2.93 mm2 (95% CI, 2.36–3.51).


Figure 3
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  		FIGURE 3. Plot showing the relationship of rim and cup areas with optic disc area. Error bars, 95% CI. Optic disc, cup, and rim areas were corrected for magnification.

 
Peripapillary RNFL average thickness was also positively associated with optic disc area (Ptrend < 0.0001). Mean RNFL average thickness increased from 99.4 µm (95% CI, 98.1–100.7 µm) to 109.4 µm (95% CI, 108.0–110.8 µm) from the lowest (mean, 1.70 mm2) to highest (mean, 2.76 mm2) quintile of optic disc area.

The average nerve width was greater along the vertical (0.36 ± 0.05 mm) than the horizontal meridian (0.28 ± 0.05 mm). There were similar findings for the rim area (vertical cross-section), which was greater for the vertical (0.32 ± 0.19 mm2) than horizontal meridian (0.19 ± 0.13 mm2).

Effects of axial length and refraction were examined in multivariate analyses adjusting for age, gender, ethnicity, and cluster sampling. Optic disc area increased significantly with axial length (Ptrend < 0.0001; Fig. 4A ). For the lowest quintile of axial length (mean 21.63 mm), mean disc area was 2.09 mm2 (95% CI, 2.03–2.14), whereas for the highest quintile (mean 23.54 mm), mean disc area was 2.29 mm2 (95% CI, 2.23–2.34). Rim area was positively associated with axial length (Ptrend = 0.01). Mean rim area was 1.68 mm2 (95% CI, 1.62–1.74) and 1.74 mm2 (95% CI, 1.68–1.81), for the lowest and highest quintiles of axial length, respectively. After further adjusting for disc area, rim area became negatively associated with axial length. Mean rim area was 1.77 mm2 (95% CI, 1.73–1.81) and 1.67 mm2 (95% CI, 1.62–1.72) for the lowest and highest quintiles of axial length, respectively.


Figure 4
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  		FIGURE 4. Plots showing the relationship of magnification-corrected optic disc and rim area with (A) axial length and (B) spherical equivalent refraction. Association with disc area was adjusted for age, gender, ethnicity and cluster sampling, whereas the association with rim area was additionally adjusted for optic disc area. Error bars are 95% CI. The mean and range (in parentheses) of axial length (AL; mm) and spherical equivalent (SE; diopters) for each quintile were (1) AL: 21.63 (19.64–22.04); SE: 0.32 (–4.88–0.75); (2) AL: 22.26 (22.04–22.44); SE: 0.94 (0.76–1.08); (3) AL: 22.62 (22.44–22.78); SE: 1.20 (1.08–1.27); (4) AL: 22.99 (22.78–23.17); SE: 1.47 (1.27–1.66); and (5) AL: 23.54 (23.17–25.35); SE: 2.45 (1.66–8.58).

 
There was a weak association between optic disc area and spherical equivalent (SE, Ptrend = 0.02; Fig. 4B ), with only a marginal decrease in mean disc area (2.20 mm2; 95% CI, 2.15–2.24 versus 2.12 mm2, 95% CI 2.05–2.19) between the lowest (mean SE, +0.32 D) and highest (mean SE, +2.45 D) quintile of spherical equivalent. There were no significant associations of rim area with refraction (Ptrend = 0.9). After including disc area in this model, rim area increased significantly with more hyperopic refraction (Ptrend = 0.008). Mean rim area was 1.67 mm2 (95% CI, 1.63–1.71) and 1.74 mm2 (95% CI, 1.70–1.78) for the lowest and highest quintiles, respectively.

Optic disc and cup parameters by gender, adjusted for age, ethnicity, axial length, and cluster-sampling, are presented in Table 3 . There were generally no gender differences in disc or cup parameters, except for horizontal and vertical average nerve width, measurements of which were slightly larger in the boys than in the girls. Rim area remained nonsignificantly different between boys and girls even after adjustment for optic disc area (P = 0.4).


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  		TABLE 3. Gender-Specific Distribution of Optic Nerve Head Parameters

 
Ethnicity-specific distributions of optic disc and cup parameters, adjusted for age, gender, axial length, and cluster sampling, are detailed in Table 4 . There were no significant differences between the Middle Eastern and the European white children. With the exception of vertical disc diameter, all disc and cup dimensions were significantly larger in the East Asian than in the European white children. Mean disc area was approximately 4% larger, whereas mean cup area and mean cup-to-disc area ratio was approximately 60% larger. Measures of neural rim were correspondingly lower in the East Asian children, with mean rim area being approximately 10% smaller and average nerve width being 7% and 16% lower for the vertical and horizontal meridians, respectively. Vertical integrated rim area and rim area (vertical cross-section) were also lower in the East Asian children. After further adjustment for differences in disc area, rim area remained significantly larger (P < 0.0001) in the European white (1.81 mm2; 95% CI, 1.79–1.83) than the East Asian (1.55 mm2; 95% CI, 1.49–1.60) children, but was similar to that in the Middle Eastern children (mean 1.81 mm2; 95% CI, 1.73–1.89).


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  		TABLE 4. Ethnicity-Specific Distribution of Optic Nerve Head Parameters

 

   Discussion
Top
 Abstract
 Methods
 Results
 Discussion
References
 
In this report, we have provided population norms for optic nerve head parameters in young children, most of whom were aged 6 years. Most parameters were normally distributed. Mean optic disc and cup shape were vertically oval, with large proportions having horizontally oval discs and cups. There was an association of increasing disc area with increasing cup area, neural rim area and RNFL thickness. Average nerve width and rim area (vertical cross-section) were greater along the vertical than horizontal meridians. Axial length was a significant determinant of disc and neural rim area. The East Asian children had larger discs but smaller neural rim areas than did the European white children.

In comparing these results with other studies, it should be noted that optic disc and cup parameters were defined by the OCT, which uses different reference points from other examination methods, including retinal photography, scanning laser polarimetry, and confocal scanning laser ophthalmoscopy (CSLO). Optic disc reference points were defined using the termination of the RPE at the disc, and the entire optic disc margin was interpolated between these disc reference points. In scanning laser polarimetry and photographic methods, the optic disc margin is manually traced. The optic cup was also defined using a fixed reference plane rather than the slope of the retinal surface, so children with photographically shallow cups that did not reach this reference plane would be classified as having no cup. Despite this, the use of a fixed reference plane is likely to reduce variability in measurement. Differences in optical magnification should also be considered. In small studies, measures of disc size using CSLO14 and of cup-to-disc ratio using stereophotographs15 were both larger than when measured using OCT.

Many studies have reported optic nerve head parameters in adults,4 5 16 17 18 19 20 21 22 23 24 25 26 but there have been few in children.3 27 Mansour3 examined stereo-photographs of 66 children aged 2 to 10 years and reported generally larger disc and neural rim areas than we found (Table 5) . Histologic measurements of disc size in adult eyes16 17 are also generally larger, despite tissue shrinkage by 13%16 to 21%17 caused by specimen fixation. Tong et al.27 reported horizontal and vertical cup-to-disc diameter and area ratios of 0.45, 0.38, and 0.19, respectively, in 8- to 13-year-old emmetropic East Asian children (n = 100), although they did not use stereophotographs. These were similar to our findings in European white children, but were slightly smaller than for our East Asian children. In contrast, Mansour3 reported average cup-to-disc diameter ratios that were considerably smaller than those found in our sample (boys 0.30, girls 0.21, European white 0.15).


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  		TABLE 5. Comparison of Optic Disc and Neural Rim Dimensions with Selected Studies

 
The greater average nerve width and rim area (vertical cross-section) along the vertical compared with the horizontal meridian is consistent with previous observations that the peripapillary nerve fiber layer is thicker in the superior and inferior regions than in the temporal or nasal regions.28 29 This configuration also corresponds to regional differences in the size and number of nerve fibers in the optic disc.30 Histologic estimates of nerve fiber layer thickness at the disc margin in adult eyes were limited by small study samples.29 31 Dichtl et al.31 reported values of 313 µm superiorly, 397 µm inferiorly, 131 µm temporally, and 165 µm nasally. Varma et al.29 reported corresponding values of 405, 376, 372, and 316 µm. Because the number of nerve fibers decrease with age,16 32 33 our estimates would be lower than expected, although these marked differences could in part be explained by the different measurement techniques and the greater chance of selection bias in small non–population-based studies. The decreased reflectance of nerve fibers at the optic disc resulting from their sloped orientation relative to the OCT scan beam13 34 may also cause underestimation of this parameter. Another important consideration is that optic disc size significantly influences other parameters, including peripapillary nerve fiber layer thickness and vertical cup-to-disc diameter ratio,4 highlighting the importance of taking optic disc size into consideration when optic disc parameters are measured.

In the present study, optic disc area increased and rim area decreased significantly with axial length. Chihara and Chihara,35 and Miglior et al.18 reported moderate positive correlations with disc (r = 0.6) and rim areas (r = 0.5). No correlation was reported with disc area by Quigley et al.17 and with rim area by Jonas and Gusek,36 although these studies were limited by relatively small selected samples of eye bank eyes17 or clinic subjects.36 Optic disc area increased and rim area decreased only marginally with less hyperopic refractions, despite both being statistically significant. It should be noted, though, that our study sample was predominantly hyperopic, with a myopia (SE ≤ –0.5 D) prevalence of only 1.4%.11 Similar data derived from older children in whom the prevalence of myopia is likely to be higher are needed to reach definitive conclusions regarding the effect of myopia on disc and neural rim area. Previous studies conducted in adults18 22 23 36 and children3 generally reported nonsignificant or only a weak association of disc and neural rim area with refraction. Studies that found an association tended to report slightly larger discs in myopic than nonmyopic eyes.25 35 Several studies also found no significant association of refraction with cup-to-disc area27 and diameter27 37 ratios. Considered together, these findings indicate that eye size has a greater influence on disc and neural rim area than does refraction. This suggests that the observed increased risk of open-angle glaucoma in myopic adults38 could be related to the associated increased eye size rather than myopia per se because axial length in myopic eyes is increased,11 39 40 and disc size has been reported to be slightly larger in open-angle glaucoma.41 42 This is only speculative, though, because most children in our study were hyperopic.

Our data showed a general lack of association of gender with disc, cup, and neural rim parameters after adjusting for potential confounders. Average nerve width was significantly greater in the boys than in the girls, but these differences were only marginal. The lack of association of gender with disc size is consistent with previous reports in children3 and adults.16 22 35 36 Several studies reported larger discs in adult males,17 18 23 including that of Ramrattan et al.,25 who reported data in more than 5000 predominantly white participants. Ramrattan et al.25 also reported slightly larger neural rim area in men, but there were no gender differences in cup area and cup-to-disc ratios. Gender differences in ocular biometry or height could have contributed to these inconsistent findings. Further, gender differences may only become manifest at a later age than that of this population sample.

Numerous studies have found ethnic differences in disc and cup parameters. Most notably, persons of white background have been reported to have smaller discs than those of African-American,3 5 17 43 Asian,23 or Indian (South Asian)23 backgrounds. Cup area5 and cup-to-disc ratio3 19 are also smaller in white than African-American persons, although neural rim area is not significantly different between these two ethnic groups.44 Differences between white and African-American children have not been examined in population-based studies. Our data are probably the first to report that children of European white background have smaller disc and cup size, but larger neural rim area than children of East Asian origin, and that there were no significant differences between children of European white and Middle Eastern origins. These ethnic differences in neural rim area concur with previous findings of ethnic differences in peripapillary nerve fiber layer thickness.33 45 46 Ethnic differences in these parameters probably have a stronger genetic than developmental basis, because they can be demonstrated in children as well as adults. If increased disc size predisposes to glaucoma, and assuming that ethnic differences in optic disc and optic cup parameters are preserved with any growth of the optic nerve with age, then these findings could also help explain observed differences in the prevalence of open-angle glaucoma between East Asian and white populations.47 48 49

Strengths of this study include a large sample size, high response rate, and standardized examination protocol. Multiple measurements of the optic disc, nerve fiber layer, axial length, and refraction, were also performed to reduce measurement error, and optic disc dimensions were adjusted for the magnification of the OCT. The sample also permitted examination of ethnic differences in parameters studied. An important limitation was the exclusion of a significant proportion of children who had scans with low signal strength. It is not clear how this could have affected our results, although the inclusion of poor-quality scans would not be acceptable. The predominantly hyperopic refraction also limited our ability to study the effects of myopia.

In summary, in this OCT study of a large population of 6-year-old children, optic disc and neural rim parameters were normally distributed. Disc and neural rim areas were slightly smaller than found in adult studies. Average nerve width was greater along the vertical than horizontal meridian, consistent with previously observed peripapillary distribution of nerve fiber layer thickness. Optic disc size was a significant determinant of peripapillary nerve fiber layer thickness. Axial length appears to have a stronger influence on disc and rim area than the refraction. Significant differences in disc, cup, and neural rim parameters were also found between East Asian and European white children.


   Footnotes
 
Supported by Australian National Health and Medical Research Council Grant 253732, the Westmead Millennium Institute, University of Sydney, and the Vision Cooperative Research Centre, University of New South Wales, Sydney, Australia.

Submitted for publication January 24, 2006; revised March 18, 2006; accepted June 1, 2006.

Disclosure: S.C. Huynh, None; X.Y. Wang, None; E. Rochtchina, None; J.G. Crowston, None; P. Mitchell, 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: Paul Mitchell, Centre for Vision Research, Department of Ophthalmology, Westmead Hospital, Hawkesbury Road, Westmead NSW 2145, Australia; paul_mitchell{at}wmi.usyd.edu.au.


   References
Top
 Abstract
 Methods
 Results
 Discussion
 References
 

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  2. Huynh SC, Wang XY, Rochtchina E, Mitchell P. Distribution of macular thickness by optical coherence tomography: findings from a population-based study of 6-year old children. Invest Ophthalmol Vis Sci. 2006;47:2351–2357.[Abstract/Free Full Text]
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