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 variation
11 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).