Purpose
To determine the cutoffs for the interocular difference in retinal nerve fiber layer (RNFL) thickness measured with Cirrus HD-OCT (Carl Zeiss Meditec, Inc) in normal eyes.
Design
Observational, clinical study.
Methods
Scans were acquired at 7 academic glaucoma clinics from both eyes of 284 normal subjects using the Optic Disc Cube 200 × 200 protocol. The interocular differences in RNFL thickness were calculated, and normal ranges of interocular differences were determined as the 2.5th and 97.5th percentiles.
Results
The average RNFL in the right eye was 0.52 μm thicker than in the left eye; the difference was marginally significant ( P = .049). The temporal, nasal, and inferior quadrants had significantly thicker RNFL in the right eye, whereas the left eye showed thicker RNFL in the superior quadrant. The 2.5th and 97.5th percentile interocular difference tolerance limits for average RNFL thickness were −7.9 μm and 8.8 μm, respectively. Although the difference in average RNFL thickness correlated with differences in axial length, disc area, cup-to-disc ratio, and vertical cup-to-disc ratio, only differences in axial length (β = −0.21; P < .001) and disc area (β = 0.17; P < .001) were associated with an interocular difference of average RNFL thickness after adjustment for each other. The interocular difference remained stable despite significant decrease in RNFL thickness with aging.
Conclusions
An interocular difference in average RNFL thickness exceeding 9 μm when measured with the Cirrus HD-OCT in normal eyes may be considered statistically significant asymmetry and may be indicative of early glaucomatous damage.
Most studies of the relationship between structure and function in glaucoma have shown that structural changes to the retinal nerve fiber layer (RNFL) precede ophthalmoscopically observable changes to the optic disc and visual function loss as tested by automated perimetry. Thus, detecting these structural changes can be important for early diagnosis of the disease.
Because organ pairs are not always perfectly symmetric, studies have been carried out in various medical specialties to assess whether differences between organ pairs can predict or influence the localization of certain diseases. With regard to glaucoma, this approach has been used for cup-to-disc ratio (CDR) because this concept was introduced as a standardized method of assessing the optic nerve head (ONH). Ever since, interocular asymmetry in CDR of the same individual has been shown to be an early sign of glaucomatous damage or a predictor of future damage in patients with ocular hypertension. Similarly, other studies have shown that asymmetry in intraocular pressure (IOP) is related to the presence of glaucomatous visual field (VF) loss. Asymmetry in static perimetry also has been shown to be related to early glaucomatous damage. However, one of the problems with the CDR is its wide physiologic variation in normal individuals, as a result of the variability in disc and cup size, so that the CDR is larger in large discs than in small discs. Additionally, asymmetries in CDR, IOP, and visual fields do not necessarily indicate the extent to which the RNFL is damaged. Because retinal ganglion cell (RGC) death is the key pathologic event in glaucoma, it is essential that methods for early diagnosis focus on the RNFL, which is made up of RGC axons converging to form the optic nerve. The availability of objective and noninvasive imaging methods to measure the RNFL thickness in vivo offers an opportunity to diagnose glaucoma earlier than with conventional techniques. Therefore, it may be appropriate to apply the concept of interocular asymmetry in RNFL thickness for early diagnosis of glaucoma or suspected glaucoma by using these new imaging methods. A few studies have been published on this topic using time-domain optical coherence tomography (OCT) and other imaging technologies. The recent introduction of spectral-domain (SD) OCT has enabled significant improvement in image acquisition speed, rate and resolution, and accuracy in measurements compared with time-domain OCT and other imaging technologies. Therefore, interocular asymmetry in RNFL thickness measured with SD OCT may be different than previously reported. The present study was carried out to determine normal tolerance limits of the amount of asymmetry in peripapillary RNFL thickness measured with the Cirrus HD-OCT (Carl Zeiss Meditec, Inc, Dublin, California, USA) when the values from both eyes are within normal range. Determining these tolerance limits may help identify subjects having early glaucomatous axonal loss when the interocular asymmetry tolerance limit is exceeded.
Methods
Subjects
Male and female normal subjects at least 18 years of age were recruited at 7 academic ophthalmology practices and were invited to participate in the study.
Each subject was informed of the nature of the study, and the subjects’ willingness to participate required a written consent form.
Exclusion criteria included contraindication to dilation or intolerance to topical anesthetics or mydriatics; IOP of 22 mm Hg or more or any type of glaucoma, including normal-tension glaucoma, in either eye; intraocular surgery in the study eye (except cataract or refractive surgery if performed more than 1 year before testing); best-corrected visual acuity worse than 20/40; evidence of diabetic retinopathy, macular edema, or other vitreoretinal disease in either eye; or evidence of optic nerve abnormality in either eye. Subjects with CDR asymmetry of 0.2 or more were excluded to avoid including glaucoma suspects. Subjects also were excluded if their scans showed algorithm failure, or if either eye had a signal strength of less than 6. Each subject underwent a complete ophthalmologic evaluation, including measurement of visual acuity, slit-lamp examination, IOP measurement, and dilated fundus examination.
An exclusionary diagnosis of glaucoma was made by the investigators at the study sites and was based on glaucomatous ONH abnormalities or VF defects as detected by 2 reliable Swedish interactive threshold algorithm standard 24-2 Humphrey Visual Field examinations. Subjects were excluded if their optic nerve showed abnormalities suggestive of glaucoma (cup-to-disc ratio ≥ 0.5 in either eye, cup-to disc ratio asymmetry ≥ 0.2, optic disc hemorrhage, or focal thinning of the rim). A visual field was considered glaucomatous if the glaucoma hemifield test results were outside normal limits, the pattern standard deviation had a P value of less than .05, or if there was a cluster of 3 or more points in the pattern deviation plot in a single hemifield (superior or inferior) with P values less than .05, one or more of which with a P value of less than .01.
Optical Coherence Tomography Scanning Procedure
The pupils of all subjects were dilated with tropicamide 1% and phenylephrine 2.5% drops. The same instrument was used for both eyes of the same subject, and only 1 scan was acquired for each eye. At each participating center, all scans were obtained by the same operator with Cirrus HD-OCT version 3.0 using the Optic Disc 200 × 200 axial scans protocol. This protocol generates a 6 × 6-mm cube of data after a series of 200 B-scans with 200 A-scans per B-scan (40 000 points) in approximately 1.5 seconds (27 000 A-scans/second). The instrument uses the intrinsic algorithms to delineate the boundaries of the RNFL automatically and to calculate the RNFL thickness at each point on the 1.73-mm radius circle around the optic disc. The RNFL thickness measurements (overall average, quadrants, and clock hours) as well as the signal strength scaled from 0 (worst) to 10 (best) are reported on the printout. Only scans with signal strength of 6 or more, without eye movement or blinking artifacts within a 1.73-mm radius around the ONH or without algorithm segmentation failure, were used for analysis. The following ONH parameters were measured automatically by the Carl Zeiss ONH analysis algorithm developed for Cirrus HD-OCT (software version 5.0): disc area, rim area defined as the difference between disc area and cup area, vertical rim thickness (VRT; the total rim thickness, in micrometers, measured in the vertical meridian and corresponds to the summation of the rim width at the superior and inferior positions), cup-to-disc area ratio (CDR; ratio of cup area to disc area), vertical cup-to-disc ratio (VCDR; ratio of vertical line through the cup center to the same vertical line extending to the disc margin), horizontal cup-to disc ratio (HCDR; ratio of the horizontal line through the cup center to the same line extending to the disc margin), and cup volume.
Statistical Methods and Data Analysis
All analyses were performed with SPSS software version 17.0 (SPSS, Inc, Chicago, Illinois, USA). Mean measurements of the 2 eyes were compared using the Student t test for paired samples. Interocular differences were determined by subtracting the measurements of left eyes from those of right eyes. Normal ranges for interocular differences were established as the 2.5th and 97.5th percentiles for average and sectoral (quadrants and clock hours) RNFL thickness. The relationships between interocular difference in average RNFL thickness and age, interocular differences in refraction expressed as spherical equivalent, IOP, central corneal thickness, axial length, disc area, CDR, HCDR, and VCDR were investigated using univariate and multivariate linear regressions. These variables first were fitted in a univariate model, then variables with P values less than .05 were entered in a multivariate analysis to determine the independence of effects. The interocular difference in average RNFL thickness was regressed against age to verify the hypothesis that the symmetry of RNFL thickness measurements varies as a function of age. For all analyses, a P value < .05 was considered statistically significant.
Results
Demographic and Clinical Characteristics
Data for 284 subjects (135 males and 149 females) were included in the analysis. There were 122 (43%) white persons, 35 (12.3%) Hispanic persons, 51 (18%) black persons, 67 (23.7%) Asians, and 6 (2.1%) others. Their mean age was 46.13 ± 16.87 years (range, 18 to 84 years). The main clinical features for right and left eyes are shown in Table 1 . Right eyes (23.96 mm) were slightly longer than left eyes (23.90 mm; P = .002), whereas HCDR was slightly greater in left eyes (0.51) compared with right eyes (0.49; P = .003). No statistically significant interocular differences were observed with regard to refraction, central corneal thickness, IOP, CDR, VCDR, disc area, integrated rim area, and VRT.
| Variables | Right Eye | Left Eye | P Value |
|---|---|---|---|
| Refraction (diopters) a | −0.89 (2.11) | −0.88 (2.06) | .88 |
| CCT (μm) | 550.28 (36.60) | 549.81 (36.35) | .22 |
| IOP (mm Hg) | 14.01 (2.49) | 14.06 (2.42) | .52 |
| Axial length (mm) | 23.96 (1.03) | 23.90 (1.05) | .002 |
| Disc area (mm 2 ) | 1.83 (0.35) | 1.84 (0.36) | .42 |
| CDR | 0.48 (0.17) | 0.49 (0.17) | .08 |
| Vertical CDR | 0.45 (0.16) | 0.46 (0.16) | .32 |
| Horizontal CDR | 0.49 (0.19) | 0.51 (0.19) | .003 |
| Cup volume (mm 3 ) | 0.15 (0.15) | 0.15 (0.14) | .92 |
| IRA (mm 2 ) | 1.32 (0.23) | 1.31 (0.24) | .30 |
| VRT (μm) | 843.42 (224.87) | 842.05 (232.61) | .8 |
Asymmetry in Retinal Nerve Fiber Layer Thickness
Table 2 shows the mean differences in RNFL thickness between right and left eyes. The average RNFL in the right eye was 0.52 ± 4.4 μm thicker than in the left eye; the difference was marginally significant ( P = .049). Similarly, all quadrants showed statistically significant interocular differences in RNFL thickness ( P < .05). The RNFL in the right eye was thicker in the temporal, nasal, and inferior quadrants, whereas the left eye showed thicker RNFL in the superior quadrant. All clock hours revealed statistically significant interocular differences in RNFL thickness, with exception for clock hours 4, 5, and 6. The left eye had thicker RNFL only at clock hours 12 and 1; the RNFL was thicker in the right eye in the remaining 7 clock hours. There was no significant difference in mean signal strength between right eyes (9.201 ± 0.63) and left eyes (9.203 ± 0.68; P = .94). The mean RNFL symmetry score, as provided by the manufacturer’s software, was 0.89 ± 0.06. The percentile distribution of the interocular differences in peripapillary RNFL thickness parameters is displayed in Table 3 , and the mean distribution of interocular differences in average RNFL thickness is shown in Figure 1 . The 2.5th and 97.5th percentiles of interocular difference tolerance limits for average RNFL thickness were −7.9 μm and 8.8 μm, respectively, depending on whether the RNFL was thicker in the left or right eye. Cutoff points for quadrants (10.1 μm and 22.6 μm) and clock-hour sectors (10.8 μm and 39.8 μm) were higher than those of average RNFL thickness.
| Parameters | Right Eye (SD) | Left Eye (SD) | Difference (SD) | P Value |
|---|---|---|---|---|
| Average RNFL | 93.09 (9.33) | 92.57 (9.86) | 0.52 (4.44) | 0.049 |
| TP quadrant | 64.52 (11.67) | 61.92 (10.42) | 2.60 (6.03) | <.001 |
| SP quadrant | 115.73 (14.69) | 119.41 (15.90) | −3.68 (9.07) | <.001 |
| NS quadrant | 70.51 (11.61) | 68.66 (11.47) | 1.85 (7.30) | <.001 |
| IF quadrant | 121.61 (16.05) | 120.29 (16.01) | 1.31 (9.95) | 0.027 |
| Clock-hr 1/11 | 102.20 (20.32) | 113.22 (21.59) | −11.02 (15.40) | <.001 |
| Clock-hr 2/10 | 88.13 (18.76) | 85.58 (18.58) | 2.55 (14.39) | 0.003 |
| Clock hour 3/9 | 57.41 (9.54) | 55.27 (9.13) | 2.14 (8.67) | <.001 |
| Clock hour 4/8 | 66.00 (13.17) | 65.13 (14.00) | 0.87 (9.94) | 0.141 |
| Clock hour 5/7 | 98.54 (20.69) | 99.24 (20.94) | −0.70 (13.48) | 0.386 |
| Clock hour 6 | 134.26 (23.77) | 132.47 (25.05) | 1.79 (18.70) | 0.108 |
| Clock hour 7/5 | 132.01 (23.01) | 129.16 (24.09) | 2.85 (17.67) | 0.007 |
| Clock hour 8/4 | 64.95 (14.77) | 62.03 (14.34) | 2.91 (10.15) | <.001 |
| Clock hour 9/3 | 51.19 (9.22) | 49.74 (8.29) | 1.44 (6.10) | <.001 |
| Clock hour 10/2 | 77.41 (16.07) | 73.99 (13.38) | 3.42 (10.99) | <.001 |
| Clock hour 11/1 | 127.60 (21.59) | 122.88 (22.85) | 4.72 (16.64) | <.001 |
| Clock hour 12 | 117.40 (24.75) | 122.14 (24.68) | −4.74 (17.30) | <.001 |
| Percentiles | ||||
|---|---|---|---|---|
| Parameters | 2.5 | 5 | 95 | 97.5 |
| Average RNFL | −7.9 | −6.7 | 7.2 | 8.8 |
| TP quadrant | −10.1 | −6.9 | 12.2 | 14.9 |
| SP quadrant | −22.6 | −18.4 | 10.7 | 15.4 |
| NS quadrant | −11.9 | −10.9 | 12.4 | 16.4 |
| IF quadrant | −16.2 | −13.2 | 15.7 | 19.1 |
| Clock hour 1/11 | −39.4 | −35.6 | 15.2 | 20.9 |
| Clock hour 2/10 | −26.9 | −21.5 | 25 | 30.4 |
| Clock hour 3/9 | −12.5 | −10.6 | 16 | 19.4 |
| Clock hour 4/8 | −22.5 | −16.7 | 16.1 | 19.1 |
| Clock hour 5/7 | −27.6 | −23.4 | 24.2 | 31.3 |
| Clock hour 6 | −32.4 | −26.0 | 34.6 | 39.2 |
| Clock hour 7/5 | −39.8 | −27.6 | 27.7 | 34.8 |
| Clock hour 8/4 | −21.6 | −14.9 | 18.4 | 21.5 |
| Clock hour 9/3 | −10.8 | −7.7 | 11.3 | 14.4 |
| Clock hour 10/2 | −17.3 | −14.0 | 22.1 | 27.1 |
| Clock hour 11/1 | −26.1 | −23.3 | 33.3 | 38.5 |
| Clock hour 12 | −39.2 | −31.8 | 23.7 | 26.9 |
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