In this study, we investigated whether changing the refractive error of an eye at the corneal plane has an effect on RNFL thickness measurement using OCT. The refractive error was varied by using CLs with different powers. This is similar to changes encountered after and refractive surgery and possibly after cataract surgery, or if a patient wears CLs for one measurement but not for the next. The CL power was varied by 20 D, which is clinically a large range. To assess repeatability, we performed two separate measurements without CL, one in the beginning and one at the end of the session. There was very good agreement between the two measurements (in only one subject was the difference for global RNFL thickness greater than 10 μm, the threshold for clinical significance in this study).
Appropriate OCT signal strength is necessary to achieve valid and reliable RNFL measurements.
9 All scans met our minimum criterion (signal strength = 6), therefore signal strength is unlikely to have significantly affected our results. Although the difference in signal strength was significant betweens scans obtained with −10 D CL and no CL, no other comparisons showed significant differences.
Several studies have found that RNFL thickness measured with OCT is affected by the refractive error of the eye. Salchow et al.
4 calculated from their cohort of normal children that an increase in spherical equivalent by 1 diopter was associated with an increase in RNFL thickness of 1.7 μm. A similar effect was found in a large population-based study on children in Australia.
5 Both studies are cross-sectional in design; although they suggest an effect of refractive error on RNFL thickness, the refractive error was not varied in a given eye to investigate its effect on OCT measurements of RNFL thickness.
Longitudinal measurements on eyes after changes in refractive error have been published. Sharma et al.
10 measured RNFL thickness using OCT in eyes with mild to moderate myopia before and after LASIK and LASEK. Mean average (global) RNFL thickness was 98.2 ± 5.6 μm before and 98 ± 6.01 μm after LASIK, and 98.5 ± 5.9 μm before and 99.1 ± 6.2 μm after LASEK; the differences were minor and statistically insignificant. Another study on RNFL thickness after LASIK supported these findings.
11 El-Ashry et al.
12 measured RNFL thickness in patients before and after cataract surgery. They found that after cataract surgery, the RNFL was thicker by 8.5 ± 8.2 μm, which was statistically significant. The largest differences were noted in the nasal followed by the inferior quadrant. The authors attributed the differences to a better scan quality after cataract surgery. Since our subjects were young adults and did not have any disturbances of the refractive media, this potential source of error does not apply to them.
Bayraktar, Bayraktar, and Yilmaz
6 found that the diameter of the circular OCT scan, used to measure peripapillary RNFL thickness, was affected by axial length but not by refractive error. Although it is held that magnification does not affect the axial scans (in the
z-axis), it will likely affect the horizontal (
x- and
y-axes) dimension of an OCT scan, in our case the diameter of the circular scan. In fact, Bayraktar et al.
6 found that the radius of the circular scan, which was set to 1.73 mm, actually varied from 1.51 to 1.87 mm. On average, the difference between the preset value and the actual radius was small (0.05 ± 0.09 mm), but the range was significant. For each 1 mm increase in axial length, the actual projected scan radius increased by approximately 0.06 mm or 3.5%. Furthermore, when the examiner adjusted the scan radius to 1.73 mm, global RNFL thickness was significantly different compared to the unadjusted scan. When no manual adjustment of the actual projected scan radius was done, thinner RNFL thickness measurements were found for longer eyes, and thicker measurements for shorter eyes. This was attributed to differences in scan radius, since larger radii yielded thinner RNFL thickness measurements, compared to smaller radii.
6
To correct for potential effects of magnification on OCT measurements of the RNFL, El-Dairi and coworkers
13 adjusted their measurements for axial length. There are several ways to calculate magnification of ocular fundus structures,
14 but these are rarely used in clinical practice. For this reason, we did not enter axial length or refractive error into the OCT machine, and only adjusted the focus to ensure that the fixation light could be seen clearly by the subject, and that the scan circle was centered well on the optic nerve. To overcome the effect of magnification when comparing scans in different persons, the cross-sectional area of ganglion cell axons converging onto the optic disc may be calculated by multiplying the magnification-corrected scan circumference with the RNFL average thickness.
5,6 Since our goal was to investigate the effect that changing the refractive error of a given eye at the corneal plane has on OCT measurements of RNFL thickness, we did not incorporate axial length into our measurements and analysis.
Youm et al.
15 found that global RNFL was thicker when measured with CL compared to without (their study did not examine the effect of CL power on the measurement, but rather asked whether wearing a CL itself affects the measurement). Even though these differences reached statistical significance, they were insignificant in absolute numbers (99.4 ± 9.7 μm with CL versus 100.8 ± 10.3 μm without CL in CL wearers, and 102.8 ± 10.8 μm with CL versus 105.3 ± 9.9 μm without CL in non-CL wearers.
15 In our study, global RNFL measurements did not consistently demonstrate this pattern; measurements with −10 D CL yielded thicker RNFL measurements than those without a CL and with +10 D CL. Moreover, there was no consistent trend in our results toward thicker or thinner RNFL with positive or negative CL power.
A large study on normal children found the greatest variation of RNFL thickness in the 12 o'clock and 6 o'clock sectors.
5 In a series on healthy 12-year old children, Wang et al.
16 also found reproducibility to be lowest in the inferior quadrant. These factors may partially explain our observation that RNFL thickness measurement with −10 D CL were significantly different in the inferior quadrant, but not in other quadrants. We did observe that fixation was difficult for the study subjects when wearing the −10 D CL. This could have led to decentration of the scan and further contributed to the differences observed in the inferior quadrant. Finally, CL centration on the cornea may have been less optimal with the thicker −10 D CL compared to thinner CLs, causing optical aberrations.
Huynh et al.
5 found that RNFL thickness decreased with increasing axial length, but increased with more positive spherical equivalent. Rauscher et al.
17 found that moderately myopic subjects tended to have relatively thin peripapillary RNFL when measured with an OCT (Stratus OCT; Carl Zeiss Meditec). This was most pronounced in the superior and inferior quadrants. In that study, global RNFL thickness decreased by 7 μm for every mm of axial length, and by 3 μm for every diopter of myopia. Both effects were statistically significant, though the effect of axial length on RNFL thickness was stronger. It therefore appears that anatomic properties influence OCT measurements of RNFL thickness more than optical properties of the eye. Our findings support this, as we did not find CL power to affect RNFL thickness measurements using OCT in a systematic way.
In clinical practice, changing the refractive error of an eye by 20 D represents a major change. This would be encountered if, for example, a patient with pathologic myopia wore a CL for one OCT measurement but not for the next. After cataract extraction and intraocular lens implantation or after refractive surgery, differences of this magnitude are rarely observed. Finally, one should keep in mind that our results do not apply to differences in refractive error caused by axial length.
We interpret our results that refractive error (changed at the corneal plane with contact lenses) does not significantly affect RNFL thickness measurements using OCT. Caution is warranted in the inferior quadrant, where variations may occur. For clinical practice, RNFL thickness measurements with and without CL should be comparable, and this may also apply to measurements before and after cataract or refractive surgery.
Supported in part by a departmental Challenge Grant from Research to Prevent Blindness, Inc., New York, NY, to the Department of Ophthalmology and Visual Science, Yale University School of Medicine.