The effect of optical magnification during retinal nerve fiber layer thickness measurement in different degrees of myopia by using frequency domain OCT

Objective

To investigate the effect of optical magnification on retinal nerve fiber layer (RNFL) thickness in different degrees of myopia by using spectral domain optical coherence tonmography (SD-OCT).

Methods

A retrospective case control study was designed.Eighty-five healthy people who underwent myopia correction and physical examination in the ophthalmology department of the Second Affiliated Hospital of Zhengzhou University from September to December in 2017 were selected, one eye was randomly selected from each subject.All the subjects were divided into 20 cases of emmetropic group, 21 cases of low myopia group, 20 cases of moderate myopia group, and 24 cases of high myopia group according to diopter.The visual acuity, best corrected visual acuity, slit lamp microscopy, fundoscopy, intraocular pressure and axial measurement, SD-OCT and visual field examination were performed on all the subjects.The difference of the mean RNFL and the thickness of the peripapillary quadrants among different groups, and relationship between RNFL and length of eye axis or diopter were analyzed before and after the correction of optical magnification.This study followed the Declaration of Helsinki.

Results

Before optical magnification correction, the average thickness of average, upper, lower and nasal quadrants RNFL were negatively correlated with the length of ocular axis (r=-0.595, -0.493, -0.639, -0.500; all at P=0.000), positively correlated with the diopter (r=0.005, 0.565, 0.600, 0.464; all at P=0.000); the thickness of temporal quadrant RNFL was positively correlated with the length of ocular axis (r=0.683, P=0.000), negatively correlated with the diopter (r=-0.730, P=0.000). After optical magnification correction, the thickness of average, upper, lower and nasal quadrants RNFL had no correlation with the length of ocular axis and diopter (all at P>0.05); the thickness of temporal quadrant RNFL was positively correlated with the length of ocular axis (r=0.840, P=0.000), negatively correlated with the diopter (r=-0.855, P=0.000). Before optical magnification correction, the thickness of average, upper, lower and nasal quadrants RNFL in emmetropic group were significantly higher than those of the other three groups (all at P<0.05). The thickness of temporal quadrant RNFL was significantly lower than those in the other three groups (all at P<0.05). The thickness of average, upper, lower and nasal quadrants RNFL in high myopia group were significantly lower than those in low myopia group and moderate myopia group (all at P<0.05), while the thickness of temporal quadrant RNFL was significantly higher than those in low myopia group and moderate myopia group (all at P<0.05). There was no significant difference in RNFL thickness between low myopia group and moderate myopia group (all at P>0.05). After optical magnification correction, the thickness of average, upper, lower and nasal quadrants RNFL showed no significant differences among the 4 groups (all at P>0.05). The thickness of temporal quadrant RNFL was significantly different among the 4 groups (F=58.313, P=0.000).

Conclusions

When measuring RNFL thickness in myopic eyes by SD-OCT, the longer the axial length, the more obvious the optical magnification effect.The thickness of temporal quadrant RNFL increases in high myopia patients, so glaucoma should be vigilant when the thickness of temporal quadrant RNFL decreases.