Journal of Clinical Ophthalmology and Research

ORIGINAL ARTICLE
Year
: 2014  |  Volume : 2  |  Issue : 3  |  Page : 131--136

Peri-papillary retinal nerve fiber layer thickness profile in subjects with myopia measured using optical coherence tomography


Ajay R Kamath, Lakshey Dudeja 
 Department of Ophthalmology, Kasturba Medical College, Mangalore, Karnataka, India

Correspondence Address:
Dr. Ajay R Kamath
1402, Westwind Apartments, Collector«SQ»s Gate, Balmatta, Mangalore - 575 002, Karanataka
India

Abstract

Aims: To evaluate the effect of myopia on peri-papillary retinal nerve fiber layer (RNFL) thickness in various quadrants and clock hour positions. Also, to evaluate the effect of myopia on the location of superotemporal and inferotemporal peak positions of peri-papillary RNFL. Setting and Design: Observational cross-sectional study from November 2011 to March 2013. Myopic patients between age group of 20-30 years were evaluated by spectral domain optical coherence tomography (SD-OCT). Materials and Methods: Myopic eyes were classified into three groups based on refractive error - Group 1: myopia up to 3 D (Diopter, D), Group 2: myopia of 3-6 D, and Group 3: Myopia > 6 D. OCT scan was done to measure peri-papillary RNFL and to locate superotemporal and inferotemporal peak positions. Statistical Analysis Used: Data was analyzed using one way ANOVA and post hoc analysis using Tukey«SQ»s test. Results and Conclusion: A total of 118 myopic eyes were evaluated. Eyes in Group 3 show significant thinning in 360 degree average RNFL thickness and in all quadrants except temporal as compared to Group 1 and 2. Correspondingly, in eyes with myopia > 6 D (as compared to Group 1 and 2), decrease in RNFL thickness is seen in all clock hour positions except 4, 8, 9, and 10. Also, in high myopia (>6 D), there is a significant shift in inferotemporal RNFL peak to temporal side while no significant change is noticed in superotemporal RNFL peak when compared to Groups 1 and 2. No significant difference was noted between Groups 1 and 2 in RNFL thickness profile and RNFL peak positions. While analyzing RNFL thickness in subjects with highly myopic eyes, this difference in topographic profile of RNFL thickness should be taken into consideration.



How to cite this article:
Kamath AR, Dudeja L. Peri-papillary retinal nerve fiber layer thickness profile in subjects with myopia measured using optical coherence tomography.J Clin Ophthalmol Res 2014;2:131-136


How to cite this URL:
Kamath AR, Dudeja L. Peri-papillary retinal nerve fiber layer thickness profile in subjects with myopia measured using optical coherence tomography. J Clin Ophthalmol Res [serial online] 2014 [cited 2022 Aug 19 ];2:131-136
Available from: https://www.jcor.in/text.asp?2014/2/3/131/138853


Full Text

Myopic eyes are at a two-three times higher risk of developing glaucoma than emmetropic eyes. [1],[2] Optical coherence tomography (OCT) is widely used for pre-perimetric diagnosis of glaucoma. Although thinning of retinal nerve fiber layer (RNFL) is a sign of glaucoma, it is uncertain whether RNFL thickness varies with myopia. To date, there are several reports in literature concerning the relationship between myopia and RNFL thickness, with various results. [1],[2],[3],[4],[5],[6],[7],[8],[9],[10],[11],[12],[13],[14],[15] Although Hoh et al. [1] reported that the mean peri-papillary RNFL thickness does not vary with myopia, others have stated that OCT may not be reliable in the analysis of highly myopic eyes, based on observations of thinner RNFLs measured by OCT. [5],[6],[7],[8],[9],[12],[13] Another issue is the topographic profile of the RNFL in subjects with highly myopic eyes. Such eyes are often classified outside normal limits in the nasal sectors [3],[8] or superior and inferior sectors [9],[11] when evaluated with OCT. This suggests that highly myopic eyes might have a topographic profile of the RFNL that differs from that of non-myopic eyes, with more pronounced thinning in the certain sectors.

The purpose of this study was to investigate whether there is any correlation between peri-papillary RNFL thickness and myopia using OCT, with emphasis on the topographic profile. Also, the effect of myopia on the location of superotemporal RNFL and inferotemporal RNFL peak positions was analyzed.

 Materials and Methods



It was an observational cross-sectional study carried out from November 2011-March 2013. The study was started after approval from institutional ethical committee. Myopic patients between 20-30 years of age attending outpatient department of ophthalmology at a tertiary care centre were included in the study. Patients diagnosed with glaucoma or with intraocular pressure >21 mmHg or optic disc changes suggestive of glaucoma or other optic neuropathies were excluded from the study. Also, patients with history of previous intraocular or refractive surgery were excluded. Neurologic diseases like Alzheimer's disease, [14] multiple system atrophy, [15] Parkinson's disease, [15] and multiple sclerosis [16] can also affect RNFL thickness; hence, such subjects were also excluded. A total of 118 myopic eyes were included after ruling out exclusion criteria (with 95% confidence interval, 90% power of study). Using Snellen's chart, visual acuity was assessed and the refractive error noted and adjusted to spherical equivalent. Patients whose refraction measured ≤ −3 D (Diopter, D) were classified as Group 1 (low myopia), those between −3 to −6 D were be classified as Group 2 (moderate myopia), and those with more than −6 D were classified as Group 3 (high myopia). Intraocular pressure (IOP) was measured for every individual, and patients with IOP > 21 mmHg were excluded from the study. Fundus examination was performed by direct ophthalmoscopy and slit lamp biomicroscopy in all subjects after pupillary dilatation. The thickness of the peri-papillary RNFL was measured through the dilated pupil using spectral domain optical coherence tomography (Cirrus 4000 HD OCT system, Version: 5.1.1.4). After proper alignment, three 200 × 200-cube optic disc scans were obtained per eye by centering a circle of fixed diameter (1.73 mm) on the disc. Scans with signal strength below 6 were discarded, and the scan with the highest signal strength and least eye movement was selected. All scans were done by same operator. The 360 degree average thickness, the average thickness in the four sectors - superior, inferior, temporal, and nasal, and the average thickness in all clock hour positions was noted. Clock hour positions were followed according to right eye: Considering 12 o'clock superiorly, 3 o'clock nasally, 6 o'clock inferiorly, and 9 o'clock temporally. Also, the location of superotemporal and inferotemporal peak positions was noted.

Data was collected and analyzed. The 360 degree average, quadrant-wise, and clock hour RNFL thickness values, and location of superotemporal and inferotemporal peak positions were compared between groups using one way analysis of variance (ANOVA). Post hoc analysis using Tukey's test was done. Statistical analysis was performed with SPSS 17 and P < 0.05 was considered statistically significant.

 Results



A total number of 118 myopic eyes of patients between 20-30 years of age were evaluated after ruling out exclusion criteria. Of these, Group 1 had 39 eyes, Group 2 had 39 eyes, and Group 3 had 40 eyes. Mean age in Group 1 was 23.4 years (Standard deviation, SD = 2.02), in Group 2 was 23.2 (SD = 1.91), and in Group 3 was 24.3 (SD = 2.55), which had no statistically significant difference. Thirty percent of patients were males in Group 1, 47.5% were males in Group2 while 62.5% were males in Group 3. Mean refractive error (in spherical equivalent) in Group 1 was 1.644 (SD 0.866), in Group 2 was 4.431 (SD 0.772), and in Group 3 was 8.531 (SD 3.274). The 360-degree average peri-papillary RNFL thickness decreases as the degree of myopia increases, and this decrease is significant in all quadrants except temporal quadrant [Table 1] and [Table 2]. Correspondingly, significant decrease in thickness occurs in all clock hour positions except 4, 8, 9, and 10 o'clock positions [Table 3]. Post hoc test using Tukey's test compared the three groups amongst each other and showed that in most quadrants and clock hour positions significant difference is noted between myopic eyes < 6 D and myopic eyes > 6 D [Table 4] and [Table 5]. No significant difference was noted between Groups 1 and 2 in peri-papillary RNFL thickness profile. Comparison of location of superotemporal and inferotemporal RNFL peak positions showed significant shift of inferotemporal peak to temporal side in Group 3 as compared to Group 1 or 2, while no significant change was noted in the superotemporal peak [Table 6] and [Table 7]. [Figure 1] and [Figure 2] show an example of change in peri-papillary thickness profile and in location of peak positions with change of degree of myopia.{Figure 1}{Figure 2}{Table 1}{Table 2}{Table 3}{Table 4}{Table 5}{Table 6}{Table 7}

 Discussion



In our study, the 360-degree average peri-papillary RNFL thickness was found to decrease as degree of myopia increased, and the decrease was significant in high myopia group than in low and moderate myopia groups. Peri-papillary RNFL thickness decreased in superior, inferior, and nasal quadrants in eyes with myopia > 6 D but the decrease was not significant in temporal quadrant. Correspondingly, significant decrease in peri-papillary RNFL thickness was noted in eyes with high myopia in 1, 2, 3, 5, 6, 7, 11, and 12 o'clock hour positions, with an insignificant change in 4, 8, 9, and 10 o'clock hour positions. Another point to be noted is that the decrease in nasal quadrant is lesser than in other quadrants (as P value is more than in other groups). Location of supero-temporal peak did not show significant change as degree of myopia increased while inferotemporal peak showed a significant shift to temporal side in high myopia group as compared to low and moderate myopia groups.

Amongst studies with SD-OCT, similar results were noted by Kang et al., [11] Wang et al., [13] and Savini et al. [17] who noted that peri-papillary RNFL thickness decreases with increase in the degree of myopia. Although Kang et al. [11] and Wang et al. [13] found significant decrease only in superior and inferior quadrants, insignificant change in nasal quadrant, and thickening in temporal quadrant, we noted a significant decrease in superior and inferior quadrant (P value < 0.001) and in nasal quadrant (P value = 0.002) while change was insignificant in temporal quadrant (P value = 0.98). Savini et al. [17] also noted decrease in RNFL thickness in all quadrants with weakest correlation of decrease in temporal quadrant. This points towards temporal dragging of RNFL as pointed by Hong et al. [18] as well.

Although magnification factor has been used in SD-OCT - based studies done earlier, it has been identified that the actual scanning radius in eyes with greater axial length (myopic eyes) could be longer than 1.73 mm due to the magnification effect. Thus, using same-sized scan circle for measuring RNFL in patients with different degrees of myopia might be misleading because the RNFL thickness decreases with increasing distances from the optic disc. However, the potential larger diameter of the scan circle due to the magnification effect in high myopic eyes does not necessarily mean that the margins of scanning radius will fall outside actual disc margins as the size of optic disc also increases with myopia. [19],[20] Although the radius of the scan circle can be adjusted manually by correcting length-related ocular magnification, we did not correct the scanning radius in the present study. We were concerned about over estimation of RNFL thickness due to decrease in the distance between the scanning circle and the optic disc margin in myopic eyes. [10]

Amongst studies based on stratus OCT, our findings show similarity with findings of a study in 48 Korean subjects by Kim MJ et al. [10] in 2008. They found 360 degree average RNFL to be significantly lower in high myopia [mean 107.4 μm (SD = 7.6)] than in low myopia group [mean 115.8 μm (SD = 8.5)] (P = 0.029 by post hoc test). In quadrant-wise measurements, RNFL thickness was thicker in low myopia group than in moderate or high myopia group for superior, inferior, and nasal quadrants (all P values ≤ 0.020). However, temporal quadrant showed thinning in low myopia group than in moderate or high myopia groups. (P = 0.001) Also, there were significant differences in superotemporal (P = 0.002) and inferotemporal (P < 0.001) peak point locations between low myopia and high myopia groups (one-way ANOVA). The superotemporal and inferotemporal peaks were closer to temporal horizon in high myopia group. Similar results were seen in studies by Choi SW et al., [5] Budenz et al., [12] Kang SH et al., [11] Leung CK et al., [4] and Vernon SA et al. [8]

In contrast, Hoh et al. [1] suggested that the peri-papillary RNFL thickness does not vary with degree of myopia or axial length. These differences can be attributed to the different OCT imaging equipment used. They used OCT-1, which obtains only 12 RNFL thickness values from each scan, while rest of the studies, including ours, used Stratus or Cirrus OCT. It is likely that the values obtained in OCT-1 scans may be insufficient to pick up differences due to axial length or degree of myopia.

We speculate that the insignificant change in RNFL thickness in the temporal quadrant despite the lower 360-degree average thickness among subjects with highly myopic eyes suggests that a redistribution of the RNFL with increase of myopia. This might be due to dragging of retina towards temporal horizon and compression of bundles originating from opposite hemisphere at the horizontal raphe, causing thinning of RNFL thickness in all quadrants except temporal. This is supported by closer locations of the peak RNFL thicknesses to the temporal horizon. Although significant shift of inferotemporal peak to temporal side was noted in our study, similar change in superotemporal peak was not seen (unlike in a study by Kim MJ et al. [10] ).

As compared to the previous studies which were performed mainly in Caucasians, [8] Koreans, [5],[10],[11] Chinese, [4] or Italian subjects, our study was performed in Indian subjects using SD-OCT. The limitations of our study include the relatively small number of eyes in each of the myopia categories, not including emmetropes or hypermetropes in the study and not considering correction for magnification factor for increased axial length. In addition, we excluded glaucoma suspects and subjects with peri-papillary atrophy extending to area where OCT samples are taken, which may have introduced some selection bias.

To summarize, we have demonstrated that subjects with highly myopic eyes (myopia > 6 D) had thinner RNFLs than did subjects with low or moderate myopia. Moreover, they showed a different topographic profile. No significant difference was noted in RNFL thickness profile between low (myopia < 3 D) and moderate myopia group (myopia = 3-6 D). The subjects with highly myopic eyes had significantly thinner RNFLs in the non-temporal sectors compared with the low and moderate myopia group but RNFL thickness did not show a significant decrease in the temporal quadrant. This profile should be taken into consideration while analyzing RNFL thickness in subjects with highly myopic eyes, aiming to diagnose glaucoma.

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