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(Investigative Ophthalmology and Visual Science. 2003;44:3369-3373.)
© 2003 by The Association for Research in Vision and Ophthalmology, Inc.
DOI:  10.1167/iovs.02-0975
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Optical Tomography–Measured Retinal Nerve Fiber Layer Thickness in Normal Latinos

Rohit Varma,1,2 Sheila Bazzaz,1 and Mei Lai2

1From the Doheny Eye Institute, Department of Ophthalmology, and the 2Department of Preventive Medicine, Keck School of Medicine, University of Southern California, Los Angeles, California.


   Abstract
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 Abstract
Methods
 Results
 Discussion
 References
 
PURPOSE. To measure retinal nerve fiber layer (RNFL) thickness in normal Latinos using optical coherence tomography (OCT).

METHODS. Three hundred twelve Latino participants, aged 40 years or more, underwent a detailed ophthalmic examination, including measurement of visual acuity, intraocular pressure, visual field perimetry, and stereoscopic optic disc photography. None of the participants had any evidence of ocular hypertension, glaucoma, or other ocular disease. Nine scans were performed on one eye of each participant by optical coherence tomography (OCT Model 2000; Carl Zeiss Meditec, Dublin, CA, software version A 6.1): three circumferential peripapillary scans and six radial scans of the macula. The average RNFL thicknesses in the peripapillary region and macula were measured. A paired t-test and linear regression analyses were used to analyze the data.

RESULTS. The mean age of the participants was 52 years (range, 40–79 years). The average peripapillary RNFL thickness 1.74 mm from the center of the disc was 132.7 ± 14.4 µm, and the average macular RNFL thickness was 44.8 ± 14.8 µm. The average macular retinal thickness was 173 ± 28.5 µm. The average peripapillary RNFL thickness in the four quadrants was as follows: superior 157.7 ± 17.8 µm, nasal 109.3 ± 19.1 µm, inferior 159.8 ± 18.9 µm, and temporal 102.5 ± 19.0 µm. There were no gender-related differences in macular or peripapillary RNFL thickness (P = 0.12 and P = 0.35, respectively). The average macular and peripapillary RNFL thickness was thinner in older Latinos than in younger Latinos (P = 0.04 and P = 0.0001, respectively).

CONCLUSIONS. Regional and age-related differences in the peripapillary and macular RNFL thickness should be considered when diagnosing and monitoring individuals with diseases that affect the RNFL.

Optical coherence tomography (OCT) is a relatively new, noncontact imaging technique that has been developed to assess tissue thickness, such as that of the retinal nerve fiber layer (RNFL). The fundamentals of image acquisition and their interpretation have been described.1 2 3 Briefly, with OCT, a scanning interferometer is used to obtain a cross section of the retina based on the reflectivity of different layers within the retina. OCT has been shown to detect changes in tissue thickness with micrometer scale sensitivity.1 2 OCT has been applied to a multitude of ophthalmologic diseases, including epiretinal membranes,4 age-related macular degeneration,5 diabetic macular edema,6 and central serous chorioretinopathy.7 Direct cross-sectional imaging of the retina with OCT offers the potential for early diagnosis and more sensitive monitoring of various optic nerve and retinal diseases, such as glaucoma, macular degeneration, and macular edema.

In the past, evaluation of the RNFL has depended on good-quality photographs, which require clear media, a dilated pupil, a darkly pigmented fundus, a trained photographer, and most important, an experienced observer. Evaluation of RNFL photographs is subjective, and diffuse RNFL loss may be difficult to assess. Advancement in OCT technology has provided an objective and quantitative method to evaluate RNFL thickness. Previous studies have demonstrated a good correlation between histopathologic and OCT measurements of RNFL thickness.2 The OCT has been shown to localize focal RNFL defects accurately.8 9 In addition, the reproducibility10 11 12 13 and intersession repeatability14 of RNFL thickness measurements obtained with OCT have been established. The purpose of this study was to use OCT to measure RNFL thickness in various parts of the retina in Latinos with no evidence of retinal or optic nerve disease. In addition, we sought to evaluate differences in RNFL thickness related to age and gender.


   Methods
Top
 Abstract
 Methods
Results
 Discussion
 References
 
Three hundred twelve consecutive healthy Latino participants were included. Informed consent was obtained from all participants. The study protocol was approved by the Institutional Review Board at the University of Southern California and followed the recommendations of the Declaration of Helsinki.

Each participant received a detailed ophthalmologic examination, automated perimetry using the Swedish interactive test algorithm (SITA) standard test and/or the 24-2 full threshold test (Carl Zeiss Meditech, Dublin, CA, and simultaneous stereoscopic optic disc photography. The ophthalmologic examination included visual acuity measurement, slit lamp biomicroscopy, applanation tonometry, and dilated direct and indirect fundus examination. Participants were considered to have no evidence of retinal or optic nerve disease if they had no history of ocular disease or surgery, had a reliable SITA standard test or 24-2 full-threshold test with no visual field defect (the pattern standard deviation and the glaucoma hemifield test results were within normal limits), had intraocular pressures less than 21 mm Hg, and had no evidence of any optic nerve or retinal disease based on binocular direct and indirect fundus examination. A normal optic disc included cup-to-disc asymmetry of less than 0.2, a neural rim without generalized or localized thinning, and absence of retinal nerve fiber layer defects, disc hemorrhages, or optic disc pallor. One eye of each subject was selected for study.

OCT is an imaging technique that generates cross-sectional images of ocular microanatomy. Low-coherence light (820 nm wavelength) from a superluminescent diode is projected onto a beam splitter, creating two beams: one directed at the retina and one acting as a reference beam. The amplitude and delay of tissue reflection is determined by an interferometer that combines the electromagnetic beam of the two reflected light beams. The instrument has a tissue resolution of 10 to 20 µm.1 2 In the OCT model 2000 (Carl Zeiss Meditech, Dublin, CA, software version A 6.1), the retina is differentiated from other layers with an algorithm detecting the edge of the retinal pigment epithelium and the photoreceptor layer. Macular retinal thickness is calculated by obtaining the difference between the first signal from the vitreoretinal interface and the signal from the anterior boundary of the retinal pigment epithelium. The nerve fiber layer in the macular and peripapillary region is determined by obtaining the difference in the distance between the vitreoretinal interface and its adjacent highly reflective layer, with the posterior border determined by the computer, based on reflectivities that achieve a certain predefined threshold. The threshold is individually determined for each scan as a multiple of the local maximum reflectance to adjust for variations in optical alignment or drying of the corneal surface or changes in pupil size. An interpolation algorithm is used to correct for any missing boundaries caused by blood vessel shadowing. The nerve fiber layer thickness is calculated as a multiple of the number of pixels between the anterior and posterior edges of the RNFL. The analysis yields a single mean RNFL thickness at the macular or peripapillary retina.

For macular measurements, the OCT generates six linear scans 30° apart, centered on the fovea, consisting of 100 A-scans each. Each scan acquisition time is 1 second. Each linear scan is 5.93 mm in length. The scan length is corrected for magnification based on the refractive error of the eye. The retinal nerve fiber thickness measured over the six linear scans (600 A scans) is then averaged to provide an average for the macular RNFL thickness. Similarly, the retinal thickness over the six linear scans is averaged to provide the average macular retinal thickness. In the circular peripapillary scan around the optic nerve head circumference, the OCT generates 100 A-scans along a 360° circular path. Three circular scans were obtained at the peripapillary retina at a default radius of 1.74 mm from the center of the optic disc, and the measurements were averaged to provide the average peripapillary RNFL thickness. In addition, the peripapillary scan is divided into four equal 90° quadrants (superior, inferior, temporal, and nasal) and RNFL thickness measurements in these four quadrants are also provided.

All imaging studies were performed on the same day of the ophthalmic examination by one experienced technician. All imaging studies were performed after pupillary dilation. An internal fixation point offset nasally from the scan area has previously been shown to lead to lower intrasubject variation11 and was therefore used for image acquisition. After image acquisition, a cross-correlation scan registration program is applied to the images to decrease artifacts in the image caused by a patient’s movement during image acquisition. Image speckle noise is also reduced by a digital filtering program. The placement of each macular and peripapillary scan was performed by the operator, who had a view of the fundus through a video camera that provides an image of the area of the fundus being scanned. The operator had to identify the fovea for the macular scan and the center of the optic disc for the peripapillary scan. The variation in positioning the scan is the primary source of variability in the measurements. Intraobserver (only one observer acquired and analyzed all the images) and interimage reproducibility was examined by determining the coefficient of variation (CoV). The CoV of the three peripapillary scans for each eye was calculated. The mean CoV was calculated from the individual CoV for each individual.

Analyses of variance (ANOVAs) were conducted to compare differences in the RNFL among various age groups, and t-tests were conducted to compare gender-related RNFL thickness differences. All analyses were conducted at the 0.05 significance level, on computer (SAS software; SAS Institute, Cary, NC).


   Results
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 Abstract
 Methods
 Results
Discussion
 References
 
Three hundred twelve eyes of 312 participants were examined from March 2000 to November 2000. The mean age of all participants was 51.9 ± 9.8 years. Participants included 141 men and 171 women. The demographic and clinical characteristics of the participants are presented in Table 1 . The OCT scans were well tolerated by all participants.


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  		TABLE 1. Demographic/Clinical Characteristics of the Study Group

 
The intraobserver and interimage CoV for RNFL thickness measurements were average peripapillary (3.85%), superior (4.72%), inferior (4.59%), nasal (9.36%), and temporal (10.62%).

The mean RNFL thickness (± SD) at the macula was 44.8 ± 14.8 µm, and the mean RNFL thickness in the peripapillary circumference (at a scan diameter of 3.4 mm centered on the optic disc) was 132.7 ± 14.4 µm. The mean RNFL thickness ± SD was 157.7 ± 17.8 µm in the superior peripapillary quadrant, 159.8 ± 18.9 µm in the inferior peripapillary quadrant, 102.5 ± 19.0 µm in the temporal peripapillary quadrant, and 109.3 ± 19.1 µm in the nasal peripapillary quadrant. The regional RNFL thickness in the superior, inferior, temporal, and nasal quadrants were all significantly different from one another (P < 0.0001), except for the difference between the superior and inferior quadrants (P = 0.027).

There were no statistically significant gender-related differences in the macula, peripapillary circumference, and superior, temporal, and nasal peripapillary RNFL thicknesses (P > 0.12). The inferior quadrant RNFL was thicker in women than in men (P = 0.05).

The average macular and peripapillary RNFL thickness was thinner in older Latinos than in younger Latinos (P = 0.04 and P = 0.0001, respectively; Fig. 1 , Table 2 ) In addition, the RNFL was consistently thinner in older individuals than in younger individuals in all four quadrants (Fig. 2 , Table 2 ).



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  		FIGURE 1. Average peripapillary and macular retinal nerve fiber layer thickness in normal Latino eyes by age.

 

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  		TABLE 2. RNFL Thickness in Normal Eyes by Age

 


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  		FIGURE 2. Quadrantic peripapillary RNFL thickness in normal Latino eyes by age.

 
We also obtained retinal thickness measurements in the macular region. Good-quality retinal images were available on 186 eyes to obtain retinal thickness measurements. The mean retinal thickness in the macular region as measured with the linear scans was 173 ± 28 µm. The mean macular retinal thickness was 171, 172, 176, and 180 µm in the 40- to 49-, 50- to 59-, 60- to 69-, and the 70+ year age groups, respectively. There was no significant difference between the macular retinal thickness in older and younger individuals (P = 0.72).


   Discussion
Top
 Abstract
 Methods
 Results
 Discussion
References
 
Recent advances in technology have made it possible to measure the RNFL thickness in an objective, quantifiable, and reproducible fashion.2 10 11 12 13 14 OCT evaluates the reflectance of posterior segment structures and incorporates a mathematical algorithm capable of localizing the anterior and posterior limits of the retinal nerve fiber layer. This study was designed to use OCT to measure RNFL thickness in normal Latino eyes.

Our results agree with previous studies using OCT that report mean peripapillary RNFL thickness in the range of 80 to 150 µm,1 11 12 14 15 16 17 using a default scan diameter of 3.4 mm from the center of the optic disc (Table 3) . The RNFL was thickest in the superior and inferior quadrants and thinner in the temporal and nasal quadrants, which is consistent with previous studies.14 15 Schuman et al.,14 Bowd et al.,15 and Liu et al.17 report a difference between the nasal and temporal RNFL thickness, with the nasal quadrant thinner than the temporal quadrant (Table 3) . However, we found that the temporal RNFL thickness was thinner than the nasal quadrant (P < 0.0001). These observations are supported by previous imaging11 and histologic data.18 Indeed, histologic measurements of the thickness of the RNFL in normal human eyes demonstrate that the superior, inferior, and nasal quadrant RNFL at the disc margin are significantly thicker than the temporal quadrant RNFL at the disc margin.18


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  		TABLE 3. Comparison of OCT-Measured RNFL Thickness in Normal Eyes

 
In the present study, there was no statistically significant difference in RNFL thickness (except in the inferior quadrant) between men and women at the macula, peripapillary circumference, and superior, temporal and nasal quadrants. Previously, Schuman et al.14 and Bowd et al.15 also found no significant gender-related differences in any RNFL parameters. Thus, our results agree with previous reports that have shown no significant gender-related differences in the optic disc and RNFL thickness.19 20

Our study found a significant difference in the peripapillary RNFL thickness in the four quadrants and in the macula between older and younger Latinos. These age-related differences in RNFL thickness have been demonstrated previously, although in fewer participants. Using OCT in 33 subjects, Schuman et al.14 found that older individuals had thinner RNFL than younger individuals (P = 0.03). However, the same study reported no age-related difference in the cup-to-disc ratio and neural rim area. These results are similar to our findings in this and previous studies.19 Histologic data regarding age-related differences in the number of optic nerve fibers are conflicting. In a histologic study of 19 eyes, Repka and Quigley21 found no statistically significant difference in the number of nerve fibers between younger and older individuals. On the other hand, histologic studies by Balazsi et al.22 (studying 16 eyes) and Johnson et al.23 (studying 13 eyes) have reported fewer axons in older individuals compared with younger individuals.

The present study is one of the largest to evaluate normal differences in RNFL thickness. We ensured that no participant had any ocular disease with a thorough ophthalmic examination. In addition, we reduced data collection variability by collecting data in a standardized manner, with only one technician performing all the scans on one instrument.

Although this study was performed in Latinos, we believe that the general patterns of regional and age- and gender-related differences in RNFL thickness are generalizable to all racial and ethnic groups. However, caution should be exercised when generalizing the absolute measurements of RNFL thickness to other ethnic groups. Several studies have shown racial-ethnic differences in the absolute magnitude of optic disc parameters.19 24 25 26 Although absolute measurements of the RNFL thickness may be different across racial or ethnic groups, it is likely that the variation in the thickness of the normal RNFL overlaps among the different groups because of the large interindividual variation within each racial/ethnic group. Therefore, although it is important to establish average racial and ethnic subgroup norms, the value of these norms in distinguishing between normal and abnormal eyes may be limited.

An important limitation of the optical coherence tomograph is the lack of intersession image registration that would allow images to be acquired and analyzed in a standardized manner. Currently, the operator asks the patient to focus on an internal fixation light to stabilize the eye during image acquisition. However, some saccadic movement of the eye may still occur. A solution to this problem would be the use of an eye-tracking system during image acquisition. Although such a system usually adjusts for horizontal and vertical movements, a torsional shift in the eye due to either positioning of the head or to torsional movements of the eye can be adjusted during image acquisition and analysis by locating landmarks on the fundus to register images from multiple sessions. Furthermore, another source of error may be induced by the inability of the operator to center the scan accurately on the disc or the macula. Again, the use of landmarks to center the images would help decrease this source of error. In our study, we used trained operators who had extensive experience with OCT. However, given the limitations of the current hardware and software, we were unable to decrease further the variability in the RNFL measurement. Finally, despite the lack of these possible refinements, the CoV in our measurements was acceptably low (3.8%–10.6%).

In summary, there are regional differences in RNFL thickness when measured with OCT. Also, older individuals have a thinner RNFL than younger individuals. Although the utility of OCT in the diagnosis and clinical management of glaucoma requires further investigation, our results suggest that the regional and age-related differences in RNFL thickness must be taken into consideration when determining optic nerve and macular disease with this instrument.


   Footnotes
 
Supported by the National Eye Institute Grants U10-EY-11753 and EY-03040, and a research grant from Carl Zeiss Meditech.

Submitted for publication September 23, 2002; revised November 7, 2002 and March 10 and 24, 2003; accepted April 6, 2003.

Disclosure: R. Varma, Carl Zeiss Meditech (F); S. Bazzaz, None; M. Lai, 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: Rohit Varma, Doheny Eye Institute, 1450 San Pablo Street, DEI 4803, Los Angeles, CA 90089-9224; rvarma{at}hsc.usc.edu.


   References
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 Abstract
 Methods
 Results
 Discussion
 References
 

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