The optical coherence tomographer is a modern imaging device designed to measure the RNFL and ONH parameters in a noncontact and noninvasive manner. RNFL measurements have been reliable and reproducible, and newer versions of optical coherence tomographers based on spectral domain technology that provide higher resolution and faster scanning speeds have been developed
It has been reported that many factors, including refractive error, axial length, myopic optic disc tilt, eccentric scan location, and head tilt during the examination can affect the OCT measurements
[10–13]. Lee et al. reported that refractive error changes induced by wearing soft contact lenses of eight diopters without astigmatic power could affect RNFL thickness measured by a Cirrus HD OCT. They considered the RNFL thickness was underestimated in eyes with increasing negative refractive error, while it was overestimated with increasing positive refractive error. Therefore we hypothesize that, not only spherical refractive error, but also cylindrical refractive error can affect OCT measurements.
Our study showed that CA influenced spectral-domain OCT measurements of both RNFL thickness and ONH parameters. Eyes with higher CA had a larger disc area and rim area, thinner temporal RNFL thickness and farther temporally positioned superotemporal and inferotemporal peak locations of RNFL thickness. The high CA group had significantly thinner RNFL thickness than the normal astigmatism group in the 2 o’clock, 9 o’clock and 10 o’clock sectors (Figure
Our results showed an intriguing finding that had not been reported previously. To date, the mechanism for changes in RNFL thickness and ONH parameters induced by astigmatism is not clear, however, possible explanations are as the followings: In high myopes, the optic disc is usually inserts obliquely. Once the optic disc tilts temporally, the nasal half of the optic disc elevates anteriorly, and the temporal half of the optic disc depresses posteriorly
[15–17]. The CA may enhance the magnification effect among high myopes, which may be result in the disc area and distance from the disc rim border to the disk front surface were exaggerated. Such changes can lead to differences in reflectivity or backscatter detected by the OCT and subsequent differences in the RNFL thickness measurements.
These findings are ascribable to CA induced ocular magnification. The relationship between the measurement of the OCT image and the size of the actual fundus dimension can be expressed as t = p ·q ·s according to the Littmann formula
, Where t is the actual fundus dimension, p is the camera magnification factor in the OCT imaging system, q is a magnification factor related to the eye, and s is the measurement in OCT. Various methods have been introduced to estimate factor q based on the ametropia, keratometry, and or axial length
. Although one can input the patient’s axial length and spherical equivalent in OCT, the effect of astigmatism has not been considered. Hwang et al suggested that when the degree or axis of astigmatism changes, RNFL thickness measurement can be affected by changing the scan distance from the optic disc. All subjects in this study were WTR CA and the plus cylinder axis was within 30° of deviation from the 90° meridian. The maximum power was in the vertical meridian, the result for in the optic disc was vertically oval, and the scan circle was farther from the optic disc in the horizontal meridian. Thus, the measurement of RNFL thickness between two groups, using the same-sized scan circle, might be misleading because the RNFL thickness decreases at increasing distances from the optic disc
. There was a tendency for the RNFL thickness in the temporal and nasal regions to become thinner, even though the RNFL thickness of the nasal region was not statistically different between two groups (P = 0.067).
In this study, the sample size may be inadequate to reveal a statistically significant correlation between total astigmatism and the temporal / nasal quadrant average RNFL thickness. Further studies are needed to clarify this point.
In conclusion, we found that high corneal astigmatism with the rule influences the measurements of both RNFL thickness and ONH parameters by the Cirrus HD OCT. Eyes with higher corneal astigmatism had a larger disc area and rim area, thinner temporal RNFL thickness and farther temporally positioned superotemporal and inferotemporal peak locations of the RNFL in high myopes. Therefore, the degree of corneal astigmatism with the rule influences should be considered when interpreting the ONH parameters and peripapillary RNFL thickness measured by the Cirrus HD OCT in high myopes.