Symmetry between the right and left eyes of the normal retinal nerve fiber layer measured with optical coherence tomography (an AOS thesis)

Abstract

Purpose: To determine the limits of the normal amount of interocular symmetry in retinal nerve fiber layer (RNFL) thickness measurements obtained with third-generation time domain optical coherence tomography (OCT3).

Methods: Both eyes of normal volunteers were scanned using the peripapillary standard and fast RNFL algorithms of OCT3.

Results: A total of 108 volunteers were included in the analysis. The mean +/- standard deviation (SD) of age of the volunteers was 46.0 +/- 15.0 years (range 20-82). Forty-two participants (39%) were male and 66 (61%) were female. Mean RNFL thickness correlated extremely well, with intraclass correlation coefficients of 0.89 for both algorithms (95% confidence interval [CI], 0.84-0.93). The mean RNFL thickness of the right eye measured 1.3 mum thicker than the left on the standard scan (SD 4.7, 95% CI 0.4-2.2, P = .004) and 1.2 mum on the fast scan (SD 5.2, 95% CI 0.1-2.2, P = .026). The 95% tolerance limits on the difference between the mean RNFL thicknesses of right minus left eye was -10.8 and +8.9 mum with the standard scan algorithm and -10.6 and +11.7 mum with the fast scan algorithm.

Conclusions: Mean RNFL thickness between the 2 eyes of normal individuals should not differ by more than approximately 9 to 12 mum, depending on which scanning algorithm of OCT3 is used and which eye measures thicker. Differences beyond this level suggest statistically abnormal asymmetry, which may represent early glaucomatous optic neuropathy.

FIGURE 1

Schematic drawing of how interferometry works. A light source sends a light wave to a beam splitter that splits the beam in two, one going to the eye and one to a reference mirror. Each beam is reflected back to the detector, and the time delay between the arrival of the two waves is translated into distance between tissues.

FIGURE 2

Stereoscopic optic disc photographs of the right (upper) and left (lower) eyes of a glaucoma suspect. Intraocular pressures ranged between 14 and 20 mm Hg in the right eye and 16 and 32 mm Hg in the left eye before treatment. On medical therapy, pressures ranged between 10 and 18 mm Hg in the right eye and 13 and 20 mm Hg in the left eye. The patient has been followed for more than 15 years while on therapy without progression. Note the mild cup-disc asymmetry with the left cup being slightly larger than the left despite equal optic disc sizes.

FIGURE 3

Right (upper) and left (lower) visual fields of patient described in Figure 2. There is a 1-dB difference in mean deviation (MD), with the left eye being worse than the right, yet both within the normal range. The Glaucoma Hemifield Test (GHT) and Pattern Standard Deviation (PSD) are normal in the left eye. This same 1- to 1.5-dB asymmetry in mean deviation has been present throughout the patient’s course.

FIGURE 4

Peripapillary fast Stratus OCT scan of the right and left eyes of the patient described in Figures 2 and 3. The Stratus OCT peripapillary retinal nerve fiber layer (RNFL) printout provides a number of representations of RNFL thickness measurements. The TSNIT plot (upper left) is a representation of the patient’s RNFL thickness (black line) in the 3.4-mm-diameter circle around the optic disc, starting in the temporal region (9:00 in the right eye and 3:00 in the left eye), then moving toward the superior region (12:00 in both eyes), then nasally (3:00 right eye, 9:00 left eye), then inferiorly (6:00 both eyes), and then winding up directly temporally. When the black line is in the green area of the graph, the RNFL thickness is normal in that location compared to normal age-matched controls. When the line dips into the yellow area, the patient’s RNFL thickness is considered thinner than the thinnest 5% of normal age-matched controls. When the black line dips into the red area, the patient’s RNFL thickness is considered thinner than the thinnest 1% of normal age-matched controls. Moving clockwise on the printout, the circle representation of the RNFL thickness is divided into clock-hour pictures, with RNFL averages for each of 12 clock-hours displayed in green, yellow, or red and interpreted in the same manner as compared to normal age-matched controls. Below the clock-hour circle is the quadrant circle, which is the average measurements in each of the 4 quadrants, color-coded and interpreted in the same manner. To the right of the circle representations is a black and white photograph of the optic disc and peripapillary region taken immediately after scanning is completed to document where around the optic disc the scan is actually being taken. The table of numbers in the lower right portion of the printout provides a variety of comparisons between upper and lower hemifields and between eyes, but the most interesting for our purposes are the mean RNFL values for right and left eyes given at the bottom of the table. And in the lower left are the TSNIT plots for both right and left eyes superimposed on each other. Note in this patient that the left mean retinal nerve fiber layer thickness is 20 μm thinner than the right and that the superimposed right/left TSNIT plots show mild thinning in the superior and inferior areas of the left eye. The 6:00 clock-hour in the left eye is shaded yellow, indicating that the patient’s average RNFL thickness measurement is thinner than 5% of that of age-matched controls. Also, the inferior quadrant of the left eye is shaded yellow. This corresponds well with the asymmetric intraocular pressures, mild C:D asymmetry, and 1-dB difference in mean deviation on the visual field between eyes. The OCT is the only test with a frankly abnormal result, the mild thinning inferiorly in the left eye.

FIGURE 5

Histogram showing the frequency distribution of interocular differences (OD – OS) in spherical equivalent refractive error for each subject. Most subjects had less than 1 diopter difference between eyes.

FIGURE 6

Histogram showing the frequency distribution of the differences in retinal nerve fiber layer (RNFL) thickness between right and left eyes measured by the standard scan algorithm of the Stratus OCT. The histogram is slightly skewed to the left, with more subjects having a slight positive difference in the right minus left interocular differences than those having a negative difference.

FIGURE 7

Histogram showing the frequency distribution of the differences in mean retinal nerve fiber layer (RNFL) thickness between right and left eyes measured by the fast scan algorithm of the Stratus OCT. Similar to Figure 4, there is a slight left skew to the curve owing to the fact that more subjects had a positive difference between right and left eyes, with more right eyes having a thicker RNFL than left.

FIGURE 8

This graph shows the relationship between interocular difference in fast mean retinal nerve fiber layer (RNFL) thickness (y-axis) and interocular difference in spherical equivalent of the refractive error (x-axis). The R² value of 0.011 suggests that very little of the differences in interocular asymmetry between individuals can be explained by differences in refractive error.

FIGURE 9

This graph shows the relationship between interocular difference in fast mean retinal nerve fiber layer (RNFL) thickness (y-axis) and interocular difference in intraocular pressure (x-axis). Although the relationship is statistically significant (P = .021), this relationship accounted for only 6% of the variability in interocular differences between individuals (R² = 0.062).

FIGURE 10

This graph shows the relationship between interocular difference in fast mean retinal nerve fiber layer (RNFL) thickness (y-axis) and interocular difference in vertical C:D ratio (x-axis). Although the relationship is statistically significant (P = .005), this relationship accounted for only 8% of the variability in interocular differences between individuals (R² = 0.081).

FIGURE 11

This graph shows the relationship between interocular difference in fast retinal nerve fiber layer (RNFL) thickness (y-axis) and age (x-axis). Although the relationship approaches statistical significance (P = .07), this relationship accounted for only 3% of the variability in interocular differences between individuals (R² = 0.031), suggesting that very little of the variability in interocular differences is attributable to increasing age.

FIGURE 12

Results of the analysis performed in 47 unique normal subjects using Cirrus OCT compared to the fast scan results of the Stratus OCT from the larger cohort that is the primary subject of this thesis. The graph shows similar interocular differences for mean RNFL, quadrants, and clock-hours, both in direction and magnitude. Since scan circle placement by the operator is not necessary in Cirrus OCT, it would appear that interocular differences are not a function of scan circle placement.