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ORIGINAL ARTICLE
Year : 2017  |  Volume : 30  |  Issue : 3  |  Page : 966-970

Association of retinal nerve fiber layer thickness and degree of myopia using spectral-domain optical coherence tomography


Department of Ophthalmology, Faculty of Medicine, Menoufia University, Shebeen El-Kom, , Shebeen El-Kom, Egpyt

Date of Submission20-Dec-2015
Date of Acceptance02-Feb-2016
Date of Web Publication15-Nov-2017

Correspondence Address:
Ahmed A Salama
Department of Ophthalmology, Faculty of Medicine, Menofia University, El-Shohadaa, Shebeen El-Kom
Egpyt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/mmj.mmj_539_15

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  Abstract 

Objective
The objective of this study was to assess the effect of myopia on the peripapillary retinal nerve fiber layer (RNFL) thickness by spectral-domain (SD) optical coherence tomography (OCT).
Background
Myopia is the most common ocular abnormality worldwide, and its high subtype is among the leading causes of blindness. Several studies proposed it as a potential risk for glaucoma. However, it is unclear how both are related. RNFL thickness, currently used for glaucoma monitoring, can be the missed link. The aim of this study is to assess the effect of myopia on the peripapillary RNFL thickness by SD-OCT, thus correlating it to glaucoma.
Patients and methods
Eighty-six eyes of 86 participants were included in this cross-sectional observational study and categorized into low, moderate, and high myopia groups. Study participants underwent thorough ophthalmological examination, autorefractometer, axial length measurement, and perimetry. Peripapillary RNFL thickness was measured in each quadrant using Spectralis SD-OCT. The effects of spherical equivalent, axial length, age, and sex on peripapillary RNFL thickness were evaluated using Pearson's correlation test.
Results
The mean age was 38.81 ± 7.76 years (range: 25–55 years), mean spherical equivalent was –5.31 ± 3.34 D (range: −0.41 to −12.5 D), with an average axial length of 25.73 ± 1.14 mm (range: 24.0–28.2 mm). The mean RNFL thickness was thinner in highly and moderately myopic eyes (81.77 ± 6.09 and 85.10 ± 5.10 mm, respectively, P = 0.0001) compared with low myopic eyes (98.14 ± 5.59). A significant correlation was noted between spherical equivalent and axial length with peripapillary RNFL thickness (P < 0.05). However, no correlation was noted between age and sex with peripapillary RNFL thickness (P > 0.05).
Conclusion
High myopia should be considered in the interpretation of OCT data because of thinning of RNFL thickness, and normative database corrected for refractive error and axial length should be incorporated.

Keywords: myopia, optical coherence tomography, retinal nerve fiber layer


How to cite this article:
Said-Ahmed KE, A. Ibrahem AM, Salama AA. Association of retinal nerve fiber layer thickness and degree of myopia using spectral-domain optical coherence tomography. Menoufia Med J 2017;30:966-70

How to cite this URL:
Said-Ahmed KE, A. Ibrahem AM, Salama AA. Association of retinal nerve fiber layer thickness and degree of myopia using spectral-domain optical coherence tomography. Menoufia Med J [serial online] 2017 [cited 2019 Dec 10];30:966-70. Available from: http://www.mmj.eg.net/text.asp?2017/30/3/966/218297


  Introduction Top


Myopia is the most common ocular abnormality worldwide, with prevalence in adults estimated to be 22.7 and 26.2% in the Baltimore Eye Survey and the Beaver Dam Study, respectively. Myopia and its ocular morbidity represent a major concern regarding its clinical and socioeconomic perspectives considering its high prevalence and increasing rate. High myopia, defined as refractive error more than −6.00 D and axial length more than 26 mm, is among the leading causes of visual impairment in young population [1],[2].

Among its potential morbidity, myopia was proposed to be a risk for glaucoma as stated by many studies [3]. Nevertheless, this relationship remains obscure. Retinal nerve fiber layer (RNFL) thickness is an important parameter to follow the extent of glaucomatous damage [4],[5]. However, it remains uncertain whether RNFL thickness varies with the refractive state of the eye or not. Therefore, it is important to investigate whether any correlation exists between RNFL thickness and axial length or refractive state of the eye [6].

Optical coherence tomography (OCT) is an objective and accurate method for RNFL thickness measurement. Compared with other RNFL analyzers, such as scanning laser polarimetry and Heidelberg retinal tomography, OCT provides high-resolution images with a quantitative analysis of the retinal features. With the recent technology of spectral-domain (SD)-OCT, ultra-high-resolution three-dimensional images of the retina and optic disc can be obtained with ultra-high speed [6],[7].

This study was carried out to evaluate the peripapillary RNFL thickness in myopic eyes using SD-OCT. We aim at elucidating the relationship between myopia RNFL thickness relying on the OCT measurements. Thorough understanding of this critical issue is mandatory to provide adequate care for myopic patients.


  Patients and Methods Top


This cross-sectional study included 86 eyes of 86 participants (one eye of each participant was selected as the study eye based on alternating assignment) recruited from the ophthalmology clinic at the Menoufia University Hospital from July 2014 to May 2015. The study was carried out in accordance with the declaration of Helsinki and was approved by the local clinical research ethics committee.

All patients between 20 and 55 years old, with spherical equivalent less than −0.5, coming to the outpatient clinic at Menoufia university hospitals were considered eligible for this study. Those with intraocular pressure more than 21 mmHg, ocular media opacities, uveitis, abnormal optic disc (e.g., tilted disc or peripapillary atrophy), previous intraocular surgery, systemic diseases such as diabetes mellitus, or coexisting retinal disease were excluded from the study.

All study participants were interviewed for full history taking followed by thorough ophthalmological assessment including, but not limited to, best corrected visual acuity, anterior segment evaluation with a slit-lamp biomicroscopy, goldmann applanation tonometry, gonioscopy, dilated fundus examination, and automated static perimetry using Octopus perimetry (Octopus 101; Haag-Streit Inc., Koeniz, Switzerland).

Autorefraction (Topcon, Tokyo, Japan) was done and the result was refined with subjective refraction, and then spherical equivalent was calculated. Axial length in each eye was measured by A-scan ultrasound biometry (Sonomed PacScan 300; Sonomed Inc., Lake Success, New York, USA). The average of five measurements with a SD up to 0.1 mm was taken as the axial length of the eye.

The participants were categorized into three groups: low (SE between −0.5 and −3.0 D), moderate (SE between −3.1 and −5.99 D), and high myopia (SE less than −6.0 D).

Peripapillary RNFL thickness measurements were obtained by Spectralis Spectral domain Optical Coherence Tomography with Spectralis software, version 4.0 (Heidelberg Engineering, Heidelberg, Germany). Three scans were obtained for each test eye, and the mean of these scans was used for the analysis. The Spectralis OCT software calculates the RNFL thickness for the four quadrants (superior, inferior, nasal, temporal, each 90°) and also for the average thickness (overall global thickness). The signal strength (range: 0–40 db) of each scan was reviewed and scans with signal strength of less than 15 db were excluded from the analysis.

Statistical analysis was performed using SPSS, version 20.0 (SPSS Inc., Chicago, Illinois, USA). P value less than 0.05 was considered to be statistically significant.


  Results Top


Eighty-six eyes of 86 participants were included in this study. The mean age was 38.81 ± 7.76 (range: 25–55) years including 46 (53.5%) men and 40 (46.5%) women. The mean spherical equivalent was −5.31 ± 3.34 D (range: −0.41 to −12.5 D) and the mean axial length was 25.73 ± 1.14 mm (range: 24.0–28.2 mm).

The mean peripapillary RNFL thickness was 101.7 ± 10.9 μm in the superior quadrant, 110.2 ± 12.7 μm in the inferior quadrant, 60.75 ± 7.98 μm in the nasal quadrant, and 78.44 ± 5.33 μm in the temporal quadrant with an average RNFL thickness of 88 ± 8.97 μm.

Statistically significant difference was found between spherical equivalent and axial length of the three different groups. However, no significant statistical difference of age or sex was found [Table 1].
Table 1: Statistical analysis of age, spherical equivalent, axial length, and sex in the three different groups

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RNFL thickness was lower in the high and moderate myopic eyes compared with low myopic eyes in all quadrants except in the temporal quadrant (P = 0.05) [Table 2] and [Figure 1].
Table 2: Retinal nerve fiber layer thickness in different groups of myopia

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Figure 1: Representative Retinal Nerve Fiber Layer scans from normal subjects with myopia.

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Our results reported a significant positive correlation between spherical equivalent and RNFL thickness. In addition, there was a significant negative correlation between axial length and RNFL thickness. A strong correlation was noted in all quadrants except in the temporal quadrant, which is consistent with the finding that RNFL thickness in the temporal quadrant was not affected by the degree of myopia [Table 3] and [Figure 2] and [Figure 3].
Table 3: Correlation between spherical equivalent, axial length, and retinal nerve fiber thickness

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Figure 2: Correlation between spherical equivalent and average retinal nerve fiber thickness.

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Figure 3: Correlation between axial length and average retinal nerve fiber thickness.

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No correlation was found between age and peripapillary RNFL thickness (r=−0.082, P = 0.456).


  Discussion Top


RNFL thickness is an important parameter in early detection and follow-up of glaucomatous damage. Association of myopia with RNFL thickness has been extensively studied to elucidate this relationship. Prevalence of glaucoma is higher in myopic patients; thus, myopia may be a confounder in addition to being a risk factor.

Several explanations for RNFL thinning in myopia have been reported. Myopic globe elongation may serve to stretch and thin the RNFL because of mechanical forces [8]. Myopic retinal degeneration may account for a decreased RNFL thickness. A larger myopic disc with a normal number of ganglion cells may provide a greater surface area for axon distribution [9]. RNFL thinning may also be the result of prenatal retinal ganglion cell regression [10]. In contrast to the earlier studies that used time domain OCT for RNFL thickness measurement, in this study we used SD-OCT, which uses a scanning beam similar to that in time domain OCT, but it has a static reference mirror that allows higher scanning speed and more images to be taken in a single pass. Therefore, the SD-OCT provides better axial resolution and faster scanning of a wider area. In our study, RNFL measurements were lower in the high and moderate myopic eyes compared with low myopic eyes in all quadrants except in the temporal quadrant, which was not affected by the degree of myopia. This finding is in agreement with Mohammed Salih [11] who reported significantly low RNFL thickness in participants with high (SE more than −6) and moderate myopia (SE between −3.1 and −5.99) compared with those with low myopia (SE more than −3) in all quadrants except the temporal one (P = 0.68).

In our study, a significant positive correlation was detected between spherical equivalent and RNFL thickness in the superior (r = 0.906, P = 0.001), inferior (r = 0.908, P = 0.001), nasal (r = 0.894, P = 0.001), temporal quadrants (r = 0.453, P = 0.001), and the average RNFL thickness (r = 0.870, P = 0.001). A strong correlation was noted in all quadrants except the temporal quadrant, which showed weak correlation, and this finding is consistent with the finding that RNFL thickness in the temporal quadrant is not affected by the degree of myopia.

These findings were consistent with the results from previous studies. Mohammed Salih [11] and Rauscher et al. [12] reported a significant positive correlation between the SE and the RNFL thickness in the superior and, inferior quadrants and the average RNFL thickness, but not the nasal and temporal quadrants. Leung et al. [8] reported a significant positive correlation between SE and RNFL thickness in each clock hour except in the temporal sector.

In contrast to this, Hoh et al. [13] and Bowd et al. [6] reported no correlation between the mean peripapillary RNFL thickness and SE.

A single common explanation for this discrepancy is that the latter studies may have been limited by the poorer resolution of the earlier generation OCT and confocal laser devices and thus lower sensitivity.

In this study, we found a strong significant negative correlation between axial length and RNFL thickness in all quadrants except in the temporal quadrant, which showed weak correlation [Table 3]. This is consistent with the finding that RNFL thickness in the temporal quadrant was not affected by the degree of myopia.

Many studies reported negative correlation between axial length and RNFL thickness. Leung et al. [8] reported a significant negative correlation between axial length and RNFL thickness in each clock hour except in the temporal sector. Budenz et al. [14] reported decreasing RNFL measurements with increasing axial length (2.2 μm/mm) in the normative database of Stratus OCT.

In contrast, Hoh et al. [13] reported no correlation between AL and RNFL thickness for the 3.40 mm (r = −0.04, P = 0.62) and for the 4.5 mm (r = 0.03, P = 0.75) scan diameters.

As regards the age, it remains unclear whether age could affect the RNFL thickness. In our study, as well as in that by Leung et al. [8], no correlation was detected between RNFL thickness and age, which could be explained by the similar and narrow age range in both studies (mean age was 38.81 and 36 years, respectively). Some studies have found significant correlations between age and the average RNFL thickness. In the normative database of stratus OCT, Budenz et al. [14] found a small but significant correlation with age (2 μm of RNFL thinning/decade) with an age range from 18 to 85 years (mean age 47 years).

Regarding sex, in our study no difference was detected in sex between different myopia groups. To our knowledge, no significant sex differences in RNFL thickness have been found until now.

In summary, we found that the RNFL thickness may be affected by the degree of myopia; RNFL measurements were lower in the high and moderate myopic eyes compared with low myopic eyes.

Therefore, high myopia should be considered in the interpretation of OCT data because of thinning of RNFL thickness and normative database corrected for refractive error and axial length should be incorporated. It is difficult for OCT to discriminate between high myopes with and without glaucoma. Therefore, clinical signs of glaucomatous nerve fiber damage are very important.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
  References Top

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    Figures

  [Figure 1], [Figure 2], [Figure 3]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]



 

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